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                                  THE

                         PHILOSOPHY OF HEALTH;

                                  OR,

                             AN EXPOSITION

                                OF THE

                   PHYSICAL AND MENTAL CONSTITUTION
                                OF MAN,

                    WITH A VIEW TO THE PROMOTION OF

                    HUMAN LONGEVITY AND HAPPINESS.

                                  BY

                        SOUTHWOOD SMITH, M.D.,
  _Physician to the London Fever Hospital, to the Eastern Dispensary,
                      and to the Jews’ Hospital_.

                       IN TWO VOLUMES. VOL. II.

                           _THIRD EDITION._

                                LONDON:

               C. COX, 12, KING WILLIAM STREET, STRAND.

                                 1847.


     London: Printed by WILLIAM CLOWES and SONS, Stamford Street.




                         CONTENTS OF VOL. II.


 CHAPTER VIII.

 OF THE FUNCTION OF RESPIRATION.

 Respiration in the plant; in the animal—Aquatic and aërial
 respiration—Apparatus of each traced through the lower to the
 higher classes of animals—Apparatus in man—Trachea, Bronchi, Air
 Vesicles—Pulmonary artery—Lungs—Respiratory motions: inspiration;
 expiration—How in the former air and blood flow to the lungs; how
 in the latter air and blood flow from the lungs—Relation between
 respiration and circulation—Quantity of air and blood employed in each
 respiratory action—Calculations founded on these estimates—Changes
 produced by animal respiration on the air: changes produced by
 vegetable respiration on the air—Changes produced by respiration on
 the blood—Respiratory function of the liver—Uses of respiration  Page 1


 CHAPTER IX.

 OF THE FUNCTION OF GENERATING HEAT.

 Of the temperature of living bodies—Temperature of plants—Power
 of plants to resist cold and endure heat—Power of generating
 heat—Temperature of animals—Warm-blooded and cold-blooded
 animals—Temperature of the higher animals—Temperature of the different
 parts of the animal body—Temperature of the human body—Power of
 maintaining that temperature at a fixed point, whether in intense
 cold or intense heat—Experiments which prove that this power is a
 vital power— Evidence that the power of generating heat is connected
 with the function of respiration—Analogy between respiration and
 combustion—Phenomena connected with the functions of the animal body,
 which prove that its power of generating heat is proportionate to
 the extent of its respiration—Theory of the production of animal
 heat—Influence of the nervous system in maintaining and regulating the
 process—Means by which cold is generated, and the temperature of the
 body kept at its own natural standard during exposure to an elevated
 temperature                                                    Page 120


 CHAPTER X.

 OF THE FUNCTION OF DIGESTION.

 Process of assimilation in the plant; in the animal—Digestive
 apparatus in the lower classes of animals; in the higher
 classes; in man—Digestive processes—Prehension, Mastication,
 Insalivation, Deglutition, Chymification, Chylification, Absorption,
 Fecation—Structure and action of the organs by which these operations
 are performed—Ultimate results—Powers by which those results are
 accomplished—Two kinds of digestion, a lower and a higher; the former
 preparatory to the latter                                      Page 159


 CHAPTER XI.

 OF THE FUNCTION OF SECRETION.

 Nature of the function—Why involved in obscurity—Basis of the
 apparatus consists of membrane—Arrangement of membrane into elementary
 secreting bodies—Cryptæ, follicles, cæca, and tubuli—Primary
 combinations of elementary bodies to form compound organs—Relation of
 the primary secreting organs to the blood-vessels and nerves—Glands,
 simple and compound—Their structure and office—Development of glands
 from their simplest form in the lowest animals to their most complex
 form in the highest animals—Development in the embryo—Number and
 distribution of the secreting organs—How secreting organs act upon
 the blood—Degree in which the products of secretion agree with, and
 differ from, the blood—Modes in which modifications of the secreting
 apparatus influence the products of secretion—Vital agent by which the
 function is controlled—Physical agent by which it is effected  Page 279


 CHAPTER XII.

 OF THE FUNCTION OF ABSORPTION.

 Evidence of the process in the plant, in the animal—Apparatus
 general and special—Experiments which prove the absorbing power of
 blood-vessels and membrane—Decomposing and analysing properties
 of membrane—Endosmose and exosmose—Absorbing surfaces, pulmonary,
 digestive, and cutaneous—Lacteal and lymphatic vessels—Absorbent
 glands—Motion of the fluid in the special absorbent vessels—Discovery
 of the lacteals and lymphatics—Specific office performed by the
 several parts of the apparatus of absorption—Condition of the system
 on which the activity of the process depends—Uses of the function
                                                                Page 332


 CHAPTER XIII.

 OF THE FUNCTION OF EXCRETION.

 In what excretion differs from secretion—Excretion in the
 plant—Quantity excreted by the plant compared with that excreted
 by the animal—Organs of excretion in the human body—Organization
 of the skin—Excretory processes performed by it—Excretory
 processes of the lungs—Analogous processes of the liver—Use of the
 deposition of fat—Function of the kidneys—Function of the large
 intestines—Compensating and vicarious actions—Reasons why excretory
 processes are necessary—Adjustments                            Page 369


 CHAPTER XIV.

 OF THE FUNCTION OF NUTRITION.

 Composition of the blood—Liquor sanguinis—Recent account of the
 structure of the red particles—Formation of the red particles in
 the incubated egg—Primary motion of the blood—Vivifying influence
 of the red particles—Influence of arterial and venous blood on
 animal and organic life—Formation of human blood—Course of the new
 constituents of the blood to the lungs—Space of time required for the
 complete conversion of chyle into blood after its first transmission
 through the lungs—Distribution of blood to the capillaries when
 duly concentrated and purified—Changes wrought upon the blood while
 it is traversing the capillaries—Evidence of an interchange of
 particles between the blood and the tissues—Phenomena attending the
 interchange—Nutrition, what, and how distinguished from digestion—How
 the constituents of the blood escape from the circulation—Designation
 of the general power to which vital phenomena are referrible—Conjoint
 influence of the capillaries and absorbents in building up
 structure—Influence of the organic nerves on the process—Physical
 agent by which the organic nerves operate—Conclusion           Page 422




                                  THE

                         PHILOSOPHY OF HEALTH.




 CHAPTER VIII.

 OF RESPIRATION.

 Respiration in the plant; in the animal—Aquatic and aërial
 respiration—Apparatus of each traced through the lower to the
 higher classes of animals—Apparatus in man—Trachea, Bronchi, Air
 Vesicles—Pulmonary artery—Lung—Respiratory motions: inspiration;
 expiration—How in the former air and blood flow to the lung; how
 in the latter air and blood flow from the lung—Relation between
 respiration and circulation—Quantity of air and blood employed in each
 respiratory action—Calculations founded on these estimates—Changes
 produced by animal respiration on the air: changes produced by
 vegetable respiration on the air—Changes produced by respiration on
 the blood—Respiratory function of the liver—Uses of respiration.


313. No organized being can live without food and no food can nourish
without air. In all creatures the necessity for air is more urgent than
that for food, for some can live days, and even weeks, without a fresh
supply of food, but none without a constant renewal of the air.

314. The food having undergone the requisite preparation in the
apparatus provided for its assimilation, is brought into contact with
the air, from which it abstracts certain principles, and to which
it gives others in return. By this interchange of principles the
composition of the food is changed: it acquires the qualities necessary
for its combination with the living body. The process by which the air
is brought into contact with the food, and by which the food receives
from the air the qualities which fit it for becoming a constituent part
of the living body, constitutes the function of respiration.

315. In the plant, the air and the food meet in contact and re-act
on each other in the leaf. The crude food of the plant having in its
ascent from the root through the stalk, received successive additions
of organic substances, by which its nature is assimilated to the
chemical condition of the proper nutritive fluid of the plant (320
and 325), undergoes in the leaf a double process; that of Digestion
and that of Respiration. The upper surface of the leaf is a digestive
apparatus, analogous to the stomach of the animal; the under surface
of the leaf is a respiratory apparatus, analogous to the lung of the
animal. For the performance of this double function, incessantly
carried on by the leaf, its organization is admirably adapted.

[Illustration: Fig. CXXII.

 View of the net-work which forms the solid structure of the leaf, and
 which consists partly of woody fibres, and partly of spiral vessels.
 1. Vessels of the upper surface; 2. vessels of the under surface; 3.
 distribution of the vessels through the substance of the leaf; 4.
 interspaces between the vessels occupied by parenchyma or cellular
 tissue.]

316. The solid skeleton of the leaf consists of a net-work composed
partly of woody fibres and partly of spiral vessels which proceed
from the stem, and which are called veins (fig. CXXII. 1, 3). In
the interstices between the veins is disposed a quantity of cellular
tissue, termed the parenchyma of the leaf (fig. CXXII. 4): the whole is
enveloped in a membrane, called the cuticle (fig. CXXIII. 1), which is
furnished with apertures denominated stomata, or stomates (fig. CXXIV.).

[Illustration: Fig. CXXIII.

 Vertical section of the leaf as it appears when seen highly magnified
 under the microscope. 1. Cells of the cuticle filled with air; 2.
 double series of cylindrical cells occupying the upper surface of
 the leaf filled with organic particles; 3. irregular cells forming
 a reticulated texture occupying the under surface of the leaf; 4.
 interspaces between the cells, termed the intercellular passages or
 air chambers.]

317. The cuticle consists of a layer of minute cellules, colourless,
transparent, without vessels, without organic particles of any kind,
and probably filled with air (fig. CXXIII. 1). These cellules open
externally, at certain portions of the cuticle, by apertures or
passages which constitute the stomates (fig. CXXIV.), and which present
the appearance of areolæ with a slit in the centre (fig. CXXIV.).
They form a kind of oval sphincters, which are capable of opening
or shutting, according to circumstances, and they are disposed on
both surfaces of the leaf, but most abundantly on the under surface,
excepting in leaves which float on water, in which they are always on
the upper surface only.

[Illustration: Fig. CXXIV.

 View of the stomata of a leaf, some of them represented as open and
 others as closed.]

318. The cellular tissue or parenchyma, immediately beneath the
cuticle, when examined in thin slices, and viewed under a microscope
with a high magnifying power, presents a regular structure disposed
in perfect order. It consists, on the upper surface, of a layer, and
sometimes of two and even three layers, of vesicles of an oblong or
cylindrical form, placed perpendicularly to the surface of the leaf,
set close to each other (fig. CXXIII. 2), and filled with organic
particles constituting the green matter which determines the colour
of the leaf. On the under surface, on the contrary, the vesicles,
which are larger than the cylindrical, are of an irregular figure,
and are placed in an horizontal direction, at such distances as to
leave wide intervals between each other (fig. CXXIII. 3); yet uniting
and anastomosing together, and thus forming a reticulated tissue,
presenting the appearance of a net with large meshes (fig. CXXIII. 3).

319. A leaf, then, consists of a double congeries of vesicles
containing organic particles, penetrated by woody fibre and air vessels
(which is probably the true nature of the spiral vessels), the whole
being enclosed within a hollow stratum of air-cells.

320. The crude sap, composed principally of water, holding in solution
carbonic acid, acetic acid, sugar, and a matter analogous to gum,
is transmitted through the leaf-stalk to the cylindrical vesicles
of the upper surface of the leaf (fig. CXXIII. 2). These vesicles
exhale a large proportion of the water; the evaporation of which is
so powerfully assisted by the action of the sun’s rays, that it would
probably become excessive, were it not for the perpendicular direction
of the cylindrical vesicles (fig. CXXIII. 2); but in consequence of
their being disposed perpendicularly to the surface of the leaf, their
ends only are presented towards the heavens (fig. CXXIII. 2), and thus
the main part of their surface is protected from the direct influence
of the solar rays. The primary effect of the evaporation carried on in
the cylindrical vesicles, is the condensation of the organic matters
contained in the sap.

321. At the same time that the cylindrical vesicles pour the
superfluous water of the sap into the surrounding atmosphere, they
abstract from the atmosphere in return carbonic acid, which, together
with that already contained in the sap, is decomposed. The oxygen is
evolved; the carbon is retained. The physical agent by which this
chemical change, which constitutes the digestive process of the plant,
is effected, is the solar ray; hence the vesicles which contain the
fluid to be decomposed, are placed on the upper surface of the leaf,
where their contents are fully exposed to the action of the sun;
and hence also this process takes place only during the day, and
most powerfully under the direct solar ray: but although the direct
influence of the sun be highly conducive to the process, yet it is
not indispensable to it; for it goes on in daylight although there be
no sunshine. Light, then, would appear to be the physical agent which
effects on the crude food of the plant a change analogous to that
produced on the crude food of the animal by the juices of the stomach.

322. After the sap has been elaborated in the cylindrical vesicles,
by the exhalation of its watery particles, by the condensation of its
organic matter, by the retention of carbon and the evolution of oxygen,
it is transmitted to the reticulated vesicles of the under surface of
the leaf (fig. CXXIII. 3), These vesicles, large, loose, and expanded,
as they have an opposite function to perform, are arranged in a mode
the very reverse of the cylindrical: in such a manner as to present
the greatest possible extent of surface to the surrounding air (fig.
CXXIII. 3): at the same time the broad interspaces between them (fig.
CXXIII. 4) are so many cavernous air-chambers into which the air is
admitted through the stomates (fig. CXXIV.). The cylindrical vesicles,
exposed to the direct rays of the sun, are protected by the closeness
with which they are packed; and by the small extent of surface they
present to the heavens: the reticulated vesicles, whose function
requires that they should have the freest possible exposure to the
surrounding air, are protected from the solar ray, first by their
position on the under surface of the leaf; and, secondly, by the dense
and thick barrier formed by the stratum of cylindrical vesicles (fig.
CXXIII. 2).

323. In the cylindrical vesicles carbonic acid is decomposed; in the
reticulated vesicles, on the contrary, carbonic acid is re-formed. The
oxygen required for this generation of carbonic acid is abstracted
partly from the surrounding air; the carbon is derived partly,
perhaps, from the air, but chiefly from the digested sap, and the
carbonic acid, formed by the union of these elements, is evolved into
the surrounding atmosphere.

324. This operation, which is strictly analogous to that of respiration
in the animal, in which carbonic acid is always generated and expired,
is carried on chiefly in the night. In this manner, under the influence
of the solar light, the leaf decomposes carbonic acid; retains the
carbon and returns the greater part of the oxygen to the air in a
gaseous form. At night, in the absence of the solar ray, the leaf
absorbs oxygen, combines this oxygen with the materials of the sap to
produce carbonic acid, which, as soon as formed, is evolved into the
surrounding air. The carbonic acid gas exhaled during the night is
re-absorbed during the day and oxygen is evolved; and this alternate
action goes on without ceasing; whence the plant deteriorates the
air by night, by the abstraction of its oxygen and the exhalation of
carbonic acid; and purifies it by day by the evolution of oxygen and
the abstraction of carbonic acid.

325. The result of these chemical actions is the conversion of
the crude sap into the proper nutritive juice of the plant. When
it reaches the cylindrical vesicles, the sap is colourless, not
coagulable, without globules, composed chiefly of water holding in
solution carbonic and acetic acids, sugar, gum, and several salts;
when it leaves the reticulated vesicles it is a greenish fluid,
partly coagulable and abounding with organic particles under the
form of globules. Its chemical composition is now wholly changed; it
consists of resinous matter, starch, gluten, and vegetable albumen. It
is now thoroughly elaborated nutritive fluid; the proper food of the
plant (cambium); rich in all the principles which are fitted to form
vegetable secretions: it is to the plant what arterial blood is to
the animal, and like the vital fluid formed in the lung, the cambium
elaborated in the leaf, is transmitted to the different parts and
organs of the plant to serve for their nutrition and development.

326. The formation of this nutritive fluid by the plant is a vital
process, as necessary to the continuance of its existence, as the
process of sanguification is necessary to the maintenance of the life
of the animal. If the plant be deprived of its leaves, if the cold
destroy, or the insect devour them, the nutrition of the plant is
arrested; the development of the flowers, the maturation of the fruit,
the fecundation of the seeds, all are stopped at once, and the plant
itself perishes.

327. The proper nutritive juice of the plant, completed by the process
of respiration, is formed by the elaboration of organic combinations
of a higher nature than those afforded by the sap. Acid, sugar, gum
(325) are converted into the higher organic compounds, resin, gluten,
starch, albumen, probably by chemical processes, the result of which
is the inversion of the relative proportions of oxygen and carbon. In
the organic matters contained in the sap, the proportion of oxygen,
compared with that of carbon, is in excess; on the contrary, in the
higher compounds contained in the cambium, the carbon preponderates: by
the inversion of the relative proportions of these two elements, the
organic compounds of a lower nature, appear to be changed into those of
a higher; to be brought into a chemical condition nearer to that of the
proper substance of the plant; a condition in which they receive the
last degree of elaboration preparatory to their conversion into that
substance.

328. In the process of respiration in the animal, as in the plant,
parts of the digested aliment mix with the air; parts of the air mix
with the digested aliment; and by this interchange of principles, the
chemical composition of the aliment acquires the closest affinity to
that of the animal body; is rendered fit to combine with it; fit to
become a constituent part of it.

329. The extent and complexity of the respiratory apparatus in the
animal, is in the direct ratio of the elevation of its structure and
the activity of its function, to which the quantity of air consumed by
it is always strictly proportionate.

330. The process of respiration in the animal is effected by two
media, air and water; but the only real agent is the air; for the
water contributes to the function only by the air contained in it.
Respiration by water is termed aquatic, that by the atmosphere,
atmospheric or aërial respiration.

331. The quantity of air contained in water being small, aquatic
is proportionally less energetic than aërial respiration; and,
accordingly, the creatures placed at the bottom of the animal scale,
having the simplest structure and the narrowest range of function, are
all aquatic.

332. Whatever the medium breathed, respiration in the animal is
energetic in proportion to the extent of the respiratory surface
exposed to the surrounding element. As the water-breathing animals
successively rise in organization, their respiratory surface becomes
more and more extended, and a proportionally larger quantity of water
is made to flow over it. It is the same in aërial respiration: the
higher the animal, the greater the extent of its respiratory surface;
and the larger the bulk of air that acts upon it.

333. Whatever the medium breathed, respiration is effected by the
contact of fresh strata of the surrounding element with the respiratory
surface. The mode in which this constant renewal of the strata is
effected, is either by the motion of the body to and fro in the
element; or by the creation of currents in it, which flow to the
respiratory surface. A main part of the apparatus of respiration
consists of the expedients necessary to accomplish these two objects;
and that apparatus is simple, or complex, chiefly according to the
extent of the mechanism requisite to effect them.

334. Whatever the medium breathed, the organic tissue which constitutes
the essential part of the immediate organ of respiration is the skin.
The primary tissue of which the skin is composed is the cellular (23
et seq.), which, organized into mucous membrane (33 et seq.), forms
the essential constituent of the skin (34). In all animals the skin
covers both the external and the internal surfaces of the body (34).
When forming the external envelop, this organ commonly retains the
name of skin; when forming the internal lining, it is generally called
mucous membrane; and in all animals, from the monad to man, either in
the form of an external envelop, or an internal lining, or by both in
conjunction, or by some localization and modification of both, the
skin constitutes the immediate organ of respiration. In different
classes of animals it is variously arranged, assumes various forms,
and is placed in various situations, according to the medium breathed,
and the facility of bringing its entire surface into contact with the
surrounding element; but in all, the organ and its office are the
same: it is the modification only—that modification being invariably
and strictly adaptation, which constitutes the whole diversity of the
immediate organ of respiration.

335. At the commencement of the animal scale, in the countless tribes
of the polygastrica (vol. i. p. 34, et seq.), respiration is effected
through the delicate membrane which envelops the soft substance of
which their body is composed. The air contained in the water in which
they live, penetrating the porous external envelop, permeates every
part of their body; aërates their nutritive juices; and converts them
immediately into the very substance of their body. They are not yet
covered with solid shells, nor with dense impervious scales, nor with
any hard material which would exclude the general respiratory influence
of water, or render necessary any special expedient to bring their
respiratory surface into contact with the element.

336. But in some tribes even of these simple creatures there is visible
by the microscope an afflux of their nutritive juices to the delicate
pellicle that envelops them, in the form of a vascular net-work, in
which there appears to be a motion of fluids, probably the nutritive
juices flowing in the only position of the body in which they could
come into direct contact with the surrounding element. In some more
highly advanced tribes, as in wheel animalcules, there is an obvious
circulating system in vessels near the surface of the skin. In other
tribes, the internal surface constituting the alimentary canal, is of
great extent and width, and forms numerous cavities which are often
distended with water. In this manner a portion of the internal, as
well as the external surface is made contributary to the function
of respiration, and this extended respiration is conducive to their
great and continued activity, to their rapid development, and to the
extraordinary fertility of their races.

[Illustration: Fig. CXXV.—_Medusa._

 1. The mouth; 2. the stomach; 3. large canals going from the stomach;
 4. smaller canals which form; 5. a plexus of vessels at the margin of
 the disc serving for respiration; 6. margin of the disc.]

337. In creatures somewhat higher in the scale, a portion of the
external surface is reflected inwards in the form of a sac, with an
external opening (fig. CXXV. 1). In some medusæ there are numerous
sacs of this kind, which pass inwards until they are separated only
by thin septa from the cavities of the stomach. The water permeating
and filling these sacs comes into contact with an interior portion
of the body, not to be reached through the external surface. At the
margin of the disk (fig. CXXV. 6) there is spread out a delicate
net-work of vessels (fig. CXXV. 5); these vessels communicate with
small canals (fig. CXXV. 4) which open into larger canals (fig. CXXV.
3) that proceed directly from the stomach (fig. CXXV. 2). As the
aliment is prepared by the stomach, it is transmitted thence by these
communicating canals to the exterior net-work of vessels where it is
aërated.

338. As organization advances, as the component tissues of the body
become more dense, and are moulded into more complex structures, when,
moreover, these structures are placed deep in the interior of the body,
far from the external envelop, and proportionally distant from the
surrounding element, the respiratory apparatus necessarily increases
in complexity. The first complication consists in the formation of
minute, delicate, transparent tubes (fig. CXXVI. 5), which communicate
with the external surface by a special organ (fig. CXXVI. 4) that
conveys water into the interior of the body (fig. CXXVI. 5). By means
of these ramifying water-tubes, upon the delicate walls of which the
blood-vessels are spread out in minute and beautiful capillaries, the
water is brought into immediate contact with the vascular system.

[Illustration: Fig. CXXVI.—_Holothuria._

 1. Mouth; 2. salivary sacs; 3. intestine; 4. cloaca; 5. ramified
 tubes, conveying water for respiration into the interior of the body.]

339. Next, in the ascending scale, the external envelop of the body is
extended into a distinct additional or supplemental organ, by which
the function of the skin is assisted. This additional organ is called
branchia or gill. The simplest form of branchia consists of folds
or duplicatures of skin, forming ramified tufts (fig. CXXVII. 1),
which in general have a regular and often a symmetrical disposition
on the external surface (fig. CXXVII. 1). Sometimes, as in the water
breathing annelides, these tufts form a fan-like expansion around the
head; but at other times they are disposed in regular series along the
whole extent of the body.

[Illustration: Fig. CXXVII.—_Lumbricus Marinus._

 1. Respiratory tufts. 2. Artery and vein, supplying the respiratory
 apparatus. 3. Dorsal vessel.]

340. Instead of branchiæ in the form of ramified tufts, the ascending
series of animals, namely, the higher crustacea, possess branchiæ
composed of numerous, delicate, thin laminæ or leaves, divided from
each other, yet placed in close proximity, like the teeth of a
fine comb, whence this arrangement is termed pectinated. Over the
blood-vessels of the system spread out on these delicate, fringed,
pectinated leaves, the water is driven in constant streams.

341. Still higher in the scale, as in molluscous animals, an internal
sac is formed to which are sometimes attached numerous tufts; but which
at other times is itself plaited into beautifully disposed regular
folds, crowded with blood-vessels and constantly bathed with fresh
currents of water.

[Illustration: Fig. CXXVIII.

 Trichoda showing the form and a frequent arrangement of Cilia.]

342. In all these water-breathing creatures, respiration is effected,
either by the progressive motion of the body through the water, or by
the creation of currents which bring fresh strata of the fluid into
contact with the respiratory surfaces. Both objects are effected by the
same instruments, namely, minute fibres having the appearance of fine
hairs or bristles. These fibres which are called cilia, have in general
an elongated, flattened, thin, and tapering form (fig. CXXVIII). Their
number, position, and arrangement, are infinitely various. Sometimes,
as in the poriferous animals, they are so minute that they cannot
be rendered visible to the eye even by the microscope, although the
evidence of their existence and action is indubitable. Sometimes they
are of great size and strength, attached by distinct ligaments to the
body and moved by powerful muscles, as in wheel animalcules. Sometimes,
as in polypiferous animals, they are disposed around the orifice of the
polypes or upon the sides of the tentacula, the instruments by which
the animal seizes its prey. Sometimes they are symmetrically disposed
in longitudinal series along the surface of the body, as in the Beroe
pileus; at other times they are arranged in circles; whenever there
are branchiæ, they are disposed around the margin of the branchial
apertures, and always on the margins of the minute meshes which compose
the branchiæ themselves.

343. In some cases the number of these cilia is immense. Each polype,
for example, has usually twenty-two tentacula, and there are about
fifty cilia on each side of a tentaculum, making two thousand two
hundred cilia on each polype. As there are about one thousand eight
hundred cells in each square inch of surface, and the branches of an
ordinary specimen present about ten square inches of surface, we may
estimate that an ordinary specimen of this zoophite presents more
than eighteen thousand polypes, three hundred and ninety-six thousand
tentacula, and thirty-nine million six hundred thousand cilia. But
other species contain more than ten times these numbers. Dr. Grant has
calculated that there are about four hundred million cilia on a single
Flustra foliacea.

344. The motions of these cilia are regular, incessant, and when in
full activity far too rapid to be distinguished by the eye even when
assisted by the microscope. They are generally to be perceived only
when their motions are comparatively feeble. They produce two effects.
In animals capable of progressive motion, they transport the body
through the water, while they constantly bring new strata of water into
contact with the respiratory surface. In this case they are partly
organs of locomotion, and partly organs subservient to respiration.
On the other hand, in animals which are not capable of moving from
place to place, they create currents by which the respiratory surface
is constantly bathed with fresh streams of water. These currents
are regular, constant, unceasing. Like some physical phenomena not
depending on vitality, it is a continued stream as regular as the
motions of rivers from their source to the ocean, or any other
movements depending on the established order of things. Dr. Grant, to
whom we are indebted for our knowledge of the true nature of these
currents, as well as of the instruments by which they are effected,
gives the following account of the observation which led to the
discovery:—“I put,” says he, “a small branch of the spongia coalita,
with some sea water into a watch-glass, under the microscope, and on
reflecting the light of a candle through the fluid, I soon perceived
that there was some intestine motion in the opaque particles floating
through the water. On moving the watch-glass, so as to bring one of the
apertures on the side of the sponge fully into view, I beheld, for the
first time, the splendid spectacle of this living fountain, vomiting
forth from a circular cavity an impetuous torrent of liquid matter,
and hurling along in rapid succession opaque masses which it strewed
everywhere around. The beauty and novelty of such a scene in the animal
kingdom long arrested my attention, but after twenty-five minutes of
constant observation, I was obliged to withdraw my eye from fatigue,
without having seen the torrent for one instant change its direction,
or diminish in the slightest degree the rapidity of its course. I
continued to watch the same orifice, at short intervals, for five
hours, sometimes observing it for a quarter of an hour at a time, but
still the stream rolled on with a constant and equal velocity.”

[Illustration: Fig. CXXIX.—_Diagram of the Apparatus of the Circulation
and Respiration in the Fish._
 1. Auricle (Single) of the heart. 2. Ventricle (single) of the heart.
 3. Trunk of the branchial artery. 4. Division of the branchial
 artery going to the branchiæ or gills. 5. Leaves of the branchiæ. 6.
 Branchial veins, which return the blood from the branchiæ, and unite
 to form. 7. the aorta, by the division of which the aërated blood is
 carried out to the system.]

345. The simple expedients which have been described suffice for
carrying on the function of respiration in the water-breathing
invertebrata; but in creatures that possess a vertebral column, and the
more perfect skeleton of which it forms a part, there is a prodigious
advancement in the organization of the whole body, of the nervous and
muscular systems especially, the organs of the animal, as well as in
all the organs of the organic life. A corresponding development of
the function of respiration is indispensable. Accordingly, a sudden
and great development in the apparatus of this function is strikingly
apparent in fishes, the lowest order of the vertebrata, in which the
branchiæ, though still preserving the same form as in the animals
below them, are large and complex organs. The branchiæ of fishes
still consist of fringed folds of membrane disposed, as in the
preceding classes, in laminæ or leaves (fig. CXXIX. 5); but there are
now commonly four series of these leaves, on each side of the body,
placed in close approximation to each other, the several leaves being
divided into minute fibres, which are set close like the barbs of a
feather, or the teeth of a fine comb (fig. CXXIX. 5). Each leaf rests
either on a cartilaginous or a bony arch, which exactly resembles the
rib of the more perfect skeleton, and performs a strictly analogous
function; for these arches are capable of alternately separating from,
and of approximating to, each other, and these alternate motions are
effected by appropriate muscles. As these movements of separation or
approximation take place, the branchiæ are either opened or closed,
and their surface proportionally expanded or contracted. Upon these
leaves (fig. CXXIX. 5) the veins (347) of the system (fig. CXXIX. 4)
are spread out in a state of capillary division of extreme minuteness,
forming a net-work of vessels of extreme tenuity and delicacy. So
prodigiously is the surface increased for the expansion of these
vessels by the leaf-like disposition of the branchiæ, that it is
computed that the branchial surface of the skate is at least equal to
the surface of the whole human body.

346. Through this extended surface the whole blood of the system
must circulate, and every point of it must be unceasingly bathed
with fresh streams of water. To generate the force necessary for
the accomplishment of these objects, an increase of power must be
communicated both to the circulating and to the respiratory apparatus.
Neither the contractile power of the vessels by which in some of
the simpler animals the nutritive fluid is put in motion, nor the
contraction of the rudimentary heart by which in creatures somewhat
higher in the scale a more decided impulse is given to the blood, are
sufficient. A muscular heart, capable of acting with great power, is
now constructed, which is placed in such a position as to enable it to
propel with velocity the whole blood of the body through the myriads
of capillary vessels that crowd every point of the surface of the
branchial leaflets. To bring the water with the requisite degree of
force into contact with this flowing stream, the apparatus of cilia
is wholly inadequate. The water entering by the mouth, is driven with
force, by the powerful muscles of the thorax, through apertures that
lead to the branchial cavities. At the instant that the branchial
leaves receive the currents of water through the appropriate apertures,
the cartilaginous or bony arches which sustain the leaves, separate to
some distance from each other, and to that extent expand the leaves and
proportionally increase the surface exposed to the water: at the same
time, the rush of water through the leaves unfolds and separates each
of the thousand minute filaments of which they are composed, so that
they all receive the full action of the fluid as it flows over them.

347. After the venous blood of the system has been thus exposed to the
action of the respiratory medium, it is taken up by the vessels called
the branchial veins (fig. CXXIX. 6), which for the reason assigned
(372) are functionally arteries, as the branchial artery (fig. CXXIX.
4) is functionally a vein. The branchial veins uniting together form
the great arterial trunk of the system, (fig. CXXIX. 7) by which the
aërated blood is carried out to every part of the body.

348. But as if even this extent of apparatus were insufficient to
afford the amount of respiration required by the system of the fish,
the entire surface of its body, which in general is naked and highly
vascular, respires like the branchiæ. Moreover, many fishes swallow
large draughts of air, by which they aërate the mucous surface of
their alimentary canal, which also is highly vascular; and still
further, numerous tribes of these animals are provided with a distinct
additional organ, a bag placed along the middle of the back filled
with air. Commonly this air bag communicates with some part of the
alimentary canal near the stomach, by means of a short wide canal
termed the ductus pneumaticus, but sometimes it forms a simple shut
sac without any manifest opening; at other times it is divided and
subdivided in a perfectly regular manner, forming extended ramified
tubes; while at other times its ramifications present the appearance of
so many pulmonary cells. It is the rudiment of the complex lung of the
higher vertebrata, and it assists respiration; although since in some
tribes it contains not atmospheric air but azote, it is without doubt
subservient to other uses in the economy of the animal.

349. In water-breathing animals, from the lowest to the highest, it
is then manifest that a special apparatus is provided for, constantly
renewing the streams of water that are brought into contact with their
respiratory surface.

[Illustration: Fig. CXXX.—_Tracheæ._

 1. Integument or skin of the body. 2. Spiracula opening on the
 external surface of the skin. 3. Tracheæ, or air tubes, proceeding in
 form of radii from the spiracles to 4. the alimentary canal.]

350. It is the same in aërial respiration. In the simplest form of
aërial respiration the apparatus consists of minute bags or sacs,
placed commonly in pairs along the back, which open for the admission
of the air on the external surface, by small orifices called spiracula
or spiracles (fig. CXXX. 2), at the sides of the body. In the common
earth-worm there are no less than one hundred and twenty of these
minute air vesicles, each of which is provided with an external opening
placed between the segments of the body. In the leech, the number is
reduced to sixteen on each side, which open externally by the same
number of minute orifices. Over the internal surface of these air
vesicles the blood of the system is distributed in minute and delicate
capillaries; and is capable of being aërated by whichever medium may
pass through the external orifices, whether water or air.

351. In this simple apparatus is apparent the rudiment of the more
perfect aërial respiration by the organs termed tracheæ, minute air
tubes which ramify like blood-vessels through the body (fig. CXXX. 3).
These air tubes open on the external surface by distinct apertures
termed _spiracula_ or _spiracles_ (fig. CXXX. 2), which are commonly
placed in rows on each side of the body (fig. CXXX. 2), with distinct
prominent edges (fig. CXXX. 2), often surrounded with hairs; sometimes
guarded by valves to prevent the entrance of extraneous bodies, and
capable of being opened and closed by muscles specially provided for
that purpose. These tubes, as they proceed from the spiracles to
be distributed to the different organs of the body, often present
the appearance of radii (fig. CXXX. 3), and when traced to their
terminations are found to end in vesicles of various sizes and
figures, but commonly of an elongated and oblong form. These minute
vesicles, when examined by the microscope, are seen to afford still
minuter ramifications, which are ultimately lost in the tissues of the
body.

352. The tracheæ are composed of three tunics, the external dense,
white and shining; the internal soft and mucous, between which is
placed a middle tunic, dense, firm, elastic, and coiled into a
spiral. By this arrangement the tube is constantly kept in a state of
expansion, and is therefore always open to the access of air. A great
part of the blood of the body, in the extensive class of creatures
provided with this form of respiratory apparatus, including the almost
countless tribes of insects, is not contained in distinct vessels, but
is diffused by transudation through the several organs and tissues of
the body. All the creatures of this class live in air, and possess
great activity; they therefore require a high degree of respiration;
yet they are commonly small in size, and often some portions of
their body consist of exceedingly dense and firm textures; hence to
have localized the function of respiration, by placing the seat of
it in a single organ, would have been impossible, on account of the
disproportionate magnitude which such an organ must have possessed; in
this case it was easier to carry the air to the blood, than the blood
to the air, and accordingly the air is carried to the blood, and, like
the blood in creatures of higher organization, is diffused through
every part of the system.

[Illustration: Fig. CXXXI.—_Respiratory Organs of the Scorpion._

 1. Spiracles. 2. Integument of one half of the body turned back.
 3. Branchial organs. 4. Cells or pouches in which they are lodged.
 _a._ One of the respiratory organs removed and magnified, showing
 its resemblance to the branchial leaflets, and presenting the
 pectinated appearance described in the text.]

[Illustration: Fig. CXXXII.—_Apparatus of Respiration in the Frog._

 1. Trachea. 2. Vesicular lungs. 3. Stomach.]

353. The next advancement in the ascending scale is, by a step which
obviously connects this higher class with the classes below and above
it. It consists of distinct cells, termed pulmonic cavities (fig.
CXXXI. 4), which communicate externally by spiracula (fig. CXXXI.
1), like tracheæ (351), but which are lined internally by a soft and
delicate membrane plaited into folds, disposed like the teeth of a
comb (pectinated) (fig. CXXXI. _a_), presenting a striking analogy
to the structure of gills (345), and therefore called by the French
writers pneumo-branchiæ. These cavities have the internal form of an
aquatic organ, but they perform the function of air-breathing sacs. In
scorpions (fig. CXXXI. 1) and spiders, this form of the apparatus is
seen in its simplest condition; in the slug and snail it is more highly
developed: for in these latter animals a rounded aperture, placed
near the head, and guarded by a sphincter muscle, that alternately
dilates and contracts, leads to a single cavity, which is lined with a
membrane delicately folded, and overspread with a beautiful net-work of
pulmonary blood-vessels.

354. Passing from this to the lowest order of the air-breathing
vertebrata (fig. CXXXII.), the apparatus is perfectly analogous,
but more developed. In the reptile, this air-breathing sac, which
now constitutes a true and proper lung, instead of being simple
and undivided, is formed by numerous septa, which traverse each
other in all directions, into vesicles or cells (fig. CXXXII. 2),
which proportionally enlarge the surface for the distribution of
blood-vessels. In the Batrachian reptile, as the frog, salamander,
newt, &c. (fig. CXXXII.), the vesicles, comparatively few in number,
are of large size, and as thin and delicate as soap-bubbles. In the
ophidian reptile, as the serpent, the sac is large and elongated, but
divided only in the upper and back part into vesicles; while in the
Saurian reptiles, as the crocodile, lizard, chamelion, &c., the sac is
comparatively small, but subdivided into very minute vesicles, bearing
a close analogy to the more perfectly organized lung of the higher
animals.

[Illustration: Fig. CXXXIII.—_Respiratory Apparatus of the Bird, as
seen in the Swan._
 1. The Trachea. 2. The lungs. 3. Apertures through which air passes
 into, 4. Air cells of the body. 5. A bristle passed from one of the
 air cells of the body, to the cavity containing the lungs. 6. A
 bristle passed from the cavity of the thigh-bone into another air cell
 of the body.]

355. In birds, the next order of vertebrata (fig. CXXXIII.), as in
insects, the class of invertebrated animals which are formed for flight
(352), the respiratory organs extend through the greater part of
the body (fig. CXXXIII. 4). The lungs (fig. CXXXIII. 2), which still
consist of a single pulmonic sac on each side (fig. CXXXIII. 2), are
divided into cells, minute compared with those of the reptile, yet
large compared with those of the quadruped; at the same time numerous
air sacs, similar in structure to those of the lungs, but of larger
size, are distributed over different parts of the body (fig. CXXXIII.
4), which communicate with the air cells of the lungs (fig. CXXXIII.
3); while of these larger sacs, several communicate also with the bones
(fig. CXXXIII. 6), so as to fill with air those cavities which in other
animals are occupied with marrow.

356. In the mammalia, the highest order of the vertebrata, respiration
is less extended through the system, and is concentrated in a single
organ, the lung, which, though comparatively smaller in bulk than in
some of the lower classes, is far more developed in structure. The lung
in this class consists of a membranous bag, divided into an immense
number of distinct vesicles or cells, in the closest possible proximity
with each other, yet not communicating, and presenting, from their
minuteness, a vast extent of internal surface. This bag is confined to
a distinct cavity of the trunk, the thorax (fig. CXXXIV.), completely
separated from the abdomen by the muscular partition, the diaphragm
(fig. CXXXIV. 10). This organ no longer sends down cells into the
abdomen, nor membranous tubes into the bones; but is concentrated
within the thorax along with the heart (fig. CXXXIV. 2, 3, 8). In
all the orders of this class, the development and concentration of
the organ are in strict proportion to the perfection of the general
organization.

[Illustration: Fig. CXXXIV.—_View of the Respiratory Apparatus in Man._

 1. The Trachea. 2. The right lung. 3. The left lung. 4. Fissures,
 dividing each lung into, 5. Large portions termed lobes. 6. Smaller
 divisions termed lobules. 7. Pericardium. 8. Heart. 9. Aorta. 10.
 Diaphragm separating the cavity of the thorax from that of the
 abdomen.]

357. In man there are two pulmonary bags (fig. CXXXIV. 2, 3), of nearly
equal size, which, together with the heart, completely fill the large
cavity of the thorax (fig. CXXXIV.), their external surface being
everywhere in immediate contact with the thoracic walls. One of these
bags is placed on the right side of the body, constituting the right
lung (fig. CXXXIV. 2), and the other on the left, constituting the left
lung (fig. CXXXIV. 3). Each lung is divided by deep fissures, into
large portions called lobes (figs. CXXXIV. 4, and CXXXV. 6), of which
there are three belonging to the right, and two to the left lung. Each
lobe is subdivided into innumerable smaller parts termed lobules (figs.
CXXXIV. 6, and CXXXV. 6), while the lobules successively diminish in
size until they terminate in minute vesicles that constitute the great
bulk of the organ (fig. CXXXV. 8).

358. The complete centralization of the respiratory function which thus
takes place in man, renders the apparatus exceedingly complex both on
account of the expedients which are necessary to obtain the requisite
extent of surface, in the small allotted space, and to bring into
contact within that space the fluids that are to act on each other.

[Illustration: Fig. CXXXV.—_View of the Air Tubes and Lung._

 1. The larynx. 2. Trachea. 3. Right bronchus. 4. Left bronchus. 5.
 Left lung; the fissures denoted by the two lines which meet at 6,
 dividing it into three lobes, and the smaller lines on its surface
 marking the division of the lobes into lobules. 7. Large bronchial
 tubes. 8. Minute bronchial tubes terminating in the air cells or
 vesicles.]

359. The apparatus consists of a vessel to carry the air to the blood;
a vessel to carry the blood to the air; an organ in which the air and
the blood meet; and an organization by which both fluids are put in
motion. The vessel that carries the air to the blood is the windpipe
(fig. CXXXV. 1, 2); the vessel that carries the blood to the air is
the pulmonary artery (fig. CXL. 7); the organ in which the blood and
the air meet is the lung (fig. CXXXV. 5); the organization which puts
the air in motion, is the structure of bones, cartilage and muscles,
called the thorax (figs. CXLI. and CXLVI.), and the engine that
communicates motion to the blood is the right ventricle of the heart
(fig. CXL. 5).

360. The windpipe is a tube which extends from the mouth and nostrils
to the lung (figs. CLIII. 1, 9, and CXXXV. 2, 5). It is attached to
the back part of the tongue (fig. CLII. 2, 9), and passes down the
neck immediately before the esophagus, or the tube which leads to the
stomach (fig. CLIII. 9, 12).

361. In the different parts of its course the windpipe is differently
constructed, performs different offices, and receives different names
according to the diversity of its structure and function. The first
division of it is called the larynx (fig. CXXXV. 1.), the second the
trachea (fig. CXXXV. 2), the third the bronchi (figs. CXXXV. 3, 4, 7,
and CXXXVII.), and the fourth the air vesicles or cells (figs. CXXXV.
8, and CXXXVIII. 2).

[Illustration: Fig. CXXXVI.—_Posterior View of the Larynx and Trachea._

 1. The os hyoides. 2. Thyroid cartilage. 3. Cricoid cartilage. 4.
 Arytenoid cartilages, separated from each other. 5. Epiglottis. 6.
 Opening of the glottis. 7. Termination of the cartilaginous rings of
 the trachea. 8. The ligamentous portion of the trachea. 9. Trachea
 laid open, showing its internal mucous surface and follicles, with the
 anterior portion of the cartilaginous rings appearing through it.]

362. The first portion of the windpipe called the larynx (figs. CXXXV.
and CXXXVI), constitutes the organ of the voice. It is situated at
the upper and fore part of the neck (fig. CLIII. 7, 9), immediately
under the bone to which the root of the tongue, called the os hyoides
(figs. CLIII. 6, and CXXXVI. 1), is attached. The larynx forms a
very complex structure, and is composed of a variety of cartilages,
muscles, ligaments, membranes, and mucous glands (fig. CXXXVI. 2, 3,
4, 5). At its upper part is a narrow opening of a triangular figure
called the glottis (fig. CXXXVI. 6), by which air is admitted to and
from the lung. Immediately above this opening is placed the cartilage,
which obtains its name from its situation, _epiglottis_ (fig. CXXXVI.
5), which is attached to the root of the tongue (fig. CLIII. 6, 7), and
which may be distinctly seen in the living body by pressing down the
tongue.

363. The Epiglottis is highly elastic, and is an agent of no
inconsiderable importance in respiration, deglutition, and speaking.
In respiration it breaks the current of air which rushes to the lungs
through the mouth and nostrils, and prevents it from flowing to the
delicate air cells with too great a degree of force. During the
action of deglutition the epiglottis is carried completely over the
glottis (fig. CLIII. 6, 7, 8), partly because it is necessarily forced
backwards, when the tongue passes backwards in delivering the food to
the pharynx (fig. CLIII. 6, 7, 8, 10), partly because it is carried
backwards by certain minute muscles which act directly upon it, and
perhaps also partly in consequence of its own peculiar irritability.
The moment the action of deglutition has been performed the epiglottis
springs from the aperture of the glottis, partly by its own elasticity,
and partly by the return of the tongue to its former position. During
the act of speaking the column of air which is expelled from the lung,
which rushes through the glottis, and which thus forms the voice,
strikes against the epiglottis, and the voice becomes thereby in some
degree modified.

[Illustration: Fig. CXXXVII.

 View of the trachea, showing, first, the division of the tube into
 the right and left bronchus, and the subdivision of the bronchi into
 the bronchial tubes; and secondly, the membranous and cartilaginous
 tissues of which the organ is composed.]

364. The second portion of the windpipe termed the trachea (fig. CXXXV.
2), commences at the under part of the larynx (fig. CXXXV. 1), and
extends as far as the third dorsal vertebra, opposite to which it
divides into two branches which are termed the bronchi (fig. CXXXV. 3,
4, and CXXXVII.). One of these branches, called the right bronchus,
goes to the right lung; the other branch, called the left bronchus,
goes to the left lung (fig. CXXXV. 3, 4).

365. The trachea of man, like the tracheæ of the air-breathing insect
(351), is composed of three tissues. These tissues differ essentially
from each other in nature, and are widely different in form and
arrangement. They consist of membrane, muscle, and cartilage.

366. The membranous portion of the human trachea consists of
three coats, the cellular (fig. CXXXVII.), the ligamentous (fig.
CXXXVI. 8), and the mucous (fig. CXXXVI. 9). From the cellular and
ligamentous coats the tube receives its strength, and in some degree
its elasticity; and the mucous coat constitutes the chief seat of
the respiratory function. Between the ligamentous and mucous coats
are placed two sets of muscular fibres; the first, the external set,
passes in a circular direction around the tube; the second set,
placed immediately beneath the circular, is disposed longitudinally,
and collected into bundles. The office of the circular fibres is to
diminish the calibre of the tube, and that of the longitudinal is to
diminish its length.

367. As the tracheæ of the insect are kept constantly open for the
free admission of air by their middle membranous tunic, dense, firm,
elastic, and coiled into a spiral (351), so, for the accomplishment
of the same purpose, there are placed between the membranous coats
of the human trachea delicate rings of the more highly organized
substance, cartilage (35). These cartilaginous rings amount in the
entire course of the tube to sixteen or eighteen in number (fig. CXXXV.
2); each cartilage being about a line in breadth, and the fourth of a
line in thickness. They never form complete circles, but only a large
segment of a circle (fig. CXXXVI. 7); the circle is incomplete behind
(fig. CXXXVI. 7, 9), because there the esophagus is in direct contact
with the trachea (fig. CLIII. 9, 12), and instead of dense and firm
cartilage, a soft and yielding substance is placed in this situation,
in order that there may be no impediment to the free dilatation of the
esophagus during the passage of the food.

368. The point at which the bronchi enter the substance of the lung is
called the root of the lung (fig. CXXXV. 3, 4). As soon as the bronchi
begin to divide and ramify within the lung each cartilage, instead of
preserving its crescent shape, is divided into two or three separate
pieces, which nevertheless are still so disposed as to keep the tube
open. With the progressive diminution in the size of the bronchial
branches, their cartilages become less numerous, and are placed at
greater distances from each other, until at length as the bronchi
terminate in the vesicles, the cartilages wholly disappear; and with
the decreasing number and size of the cartilages, the thickness of the
cellular, ligamentous, and muscular coats of the bronchi also lessens,
until at the points where the cartilages disappear, the muscular and
mucous tunics, now reduced to a state of extreme tenuity, alone remain.
The essential constituent of the air vesicles, then, is the mucous
membrane; but there is reason to suppose that the muscular tunic is
likewise continued over these vesicles.

369. It has been stated that the tracheæ of the insect terminate in the
different tissues of its body by minute vesicles of an oblong form.
The termination of the bronchi in the human lung presents a strikingly
analogous appearance. Malpighi, who with extraordinary talent and
success devoted his life to the investigation of the minute structures
of the various organs of the human body, represents the mucous membrane
of the bronchial tubes as terminating in minute vesicles of unequal
size: and Reisseissen, who has more recently resumed the inquiry and
examined this structure with extreme care, agrees with Malpighi in
stating that the bronchial tubes at their terminal points expand into
minute, delicate, membranous vesicles of a cylindrical and somewhat
rounded figure (fig. CXXXVIII. 2). The bronchial tubes do not divide
to any great degree of minuteness (fig. CXXXVIII. 1), but terminate
somewhat abruptly in the vesicles (fig. CXXXVIII. 2), which though
minute are large enough to be visible to the naked eye (fig. CXXXVIII.
2). Viewed in connexion with the bronchial tubes at their terminal
points, the vesicles present a clustered appearance, not unlike
clusters of currants attached to their stem (fig. CXXXVIII. 2).

[Illustration: Fig. CXXXVIII.—_View of the Bronchial Tubes terminating
in Air vesicles._
 Fig. 138.
 Fig. 139.
 External view.—1. Bronchial tube. 2. Air vesicles. Fig. 139. The same
 laid open.]

370. In the insect, for the reason assigned (351), these vesicles are
diffused over the system, aërating every point of the body; in man
they are concentrated in the lung; yet by their minuteness, and by
the mode in which they are arranged, they present in the small space
occupied by this organ, so extended a surface that Hales, representing
the size of each vesicle at the 100dth part of an inch in diameter,
estimates the amount of surface furnished by them collectively at
20,000 square inches. Keil estimating the number of the vesicles at
174,000,000, calculates the surface they present, at 21,906 square
inches. Leiberkuhn at 150 cubic feet; and, according to Monro, it is
thirty times the surface of the human body.

[Illustration: Fig. CXL.

 1. The trachea. 2. The right and left bronchus; the left bronchus
 showing its division into smaller and smaller branches in the lung,
 and the ultimate termination of the branches in the air vesicles. 3.
 Right auricle of the heart. 4. Left auricle. 5. Right ventricle. 6.
 The aorta arising from the left ventricle, the left ventricle being in
 this diagram concealed by the right. 7. Pulmonary artery arising from
 the right ventricle and dividing into, 8. The right, and 9. The left
 branch. The latter is seen dividing into smaller and smaller branches,
 and ultimately terminating on the air vesicles. 10. Branches of one of
 the pulmonary veins proceeding from the terminations of the pulmonary
 artery on the air vesicles, where together they form the net-work of
 vessels termed the Rete Mirabile. 11. Trunk of the vein on its way to
 the left auricle of the heart. 12. Superior vena cava. 13. Inferior
 vena cava. 14. Air vesicles magnified. 15. Blood-vessels distributed
 upon them.]

371. Such is the structure of the vessel that carries the air to the
blood, and such is the mode of its distribution.

The vessel that conveys the blood to the air is the pulmonary artery,
the great vessel which springs from the right ventricle of the heart
(fig. CXL. 5).

The pulmonary artery soon after it issues from the right ventricle of
the heart divides into two branches (fig. CXL. 7, 8, 9), one for each
lung (fig. CXL. 8, 9). Each branch of the pulmonary artery as soon as
it enters its corresponding lung (fig. CXL. 9) divides and ramifies
through the organ in a manner precisely similar to the bronchial
tubes. Every branch of the artery is in contact with a corresponding
branch of the bronchus (fig. CXL. 2), divides as it divides, and
accurately tracks its course throughout (fig. CXL. 2), until the
ultimate divisions of the artery at length reach the ultimate vesicles
of the bronchus (fig. CXL. 2, 10), upon the delicate walls of which
the capillary arteries rest, expand, and ramify, forming a net-work of
vessels, so complex that the anatomist who first observed it, named it
the _Rete Mirabile_, the wonderful net-work, and it is still called
the _Rete Mirabile Malpighi_, or the _Rete Vasculosum Malpighi_ (fig.
CXL. 2, 9, 10).

372. The blood which has finished its circulation through the system,
returned by the great systemic veins (fig. CXL. 12, 13), to the right
side of the heart (fig. CXL. 3), is driven by the right ventricle (fig.
CXL. 5), into the pulmonary artery (fig. CXL. 7); by the branches of
which (fig. CXL. 8, 9) it is distributed to the air vesicles of the
lungs: consequently the right heart of man bears precisely the same
relation to the lungs, that the single heart of the fish bears to the
branchiæ; the former is a pulmonic, as the latter is a branchial heart;
one half of the double heart of the more highly organized creature is
employed to circulate the venous blood of the system through the lungs,
as the whole of the single heart of the less highly organized animal,
is employed to propel the blood through the branchiæ (368). From the
capillary branches of the pulmonary artery in the Rete Mirabile (fig.
CXL. 9), arises another set of vessels termed the pulmonary veins
(fig. CXL. 10), which receive the blood from the venous vessels spread
out on the air vesicles: for the pulmonary artery is functionally a
vein, since it contains venous blood, though it is nominally an artery
because it carries blood from the heart (269); and in like manner the
pulmonary veins are functionally arteries since they contain arterial
blood, though they are nominally veins because they carry blood to the
heart (272). The branches of the pulmonary arteries are larger in size
and greater in number than those of the pulmonary veins, the reverse of
what is observed in any other part of the body; because the pulmonary
artery contains the blood which is to be acted upon by the air, while
the pulmonary veins merely receive the blood which has been acted upon
by the air, and the former ramifies more minutely than the latter, in
order that the air may act on a larger surface of blood.

373. In the Rete Mirabile the junction of the air-vessel with the
blood-vessel is accomplished. The combination of these two sets of
vessels constitutes the lung; for the lung is composed of air-vessels
and blood-vessels united, and sustained by cellular tissue, and
inclosed in the thin but firm membrane called the pleura (104 and 105).

374. Such is the arrangement of that part of the respiratory apparatus
which contains the fluids that are to act on each other. The object
of the remaining portion of it is to produce the movements which are
necessary to bring the fluids into contact. This is accomplished by
the mechanism and action of the thorax and diaphragm (figs. CXLI. and
CXXXIV. 10).

375. These organs, which invariably act in concert, are so constructed
and disposed, that when in action they give to the chest two alternate
motions, one that by which its capacity is enlarged; and the other that
by which it is diminished. These alternate movements are called the
motions of respiration. The motion by which the capacity of the chest
is enlarged is termed the action of inspiration, and that by which it
is diminished the action of expiration.

376. The action of inspiration, or that by which the capacity of the
chest is enlarged, is effected by the combined movements of the thorax
and diaphragm; by the ascent of the thorax and by the descent of the
diaphragm.

377. The osseous portion of the thorax, which has been fully described
(69 _et seq._), consists of the spinal column (fig. CXLI. 1), the
ribs with their cartilages (fig. CXLI. 2, 3), and the sternum (fig.
CXLI. 4). The soft portion of the thorax consists of muscles and
membrane (figs. CXLII., CXLVI., and CXLVII.), together with the common
integuments of the body. The chief boundaries of the cavity of the
thorax before, behind, and at the sides, are osseous, being formed
before by the sternum and the cartilages of the ribs (fig. CXLI. 4,
3); behind by the spinal column and the necks of the ribs (fig. CXLI.
1,2); and at the sides by the bodies of the ribs. Below the boundary is
muscular, being formed by the diaphragm (fig. CXLIII. 3).

378. Externally the thorax is convex and enveloped by muscle and skin;
internally it is concave (fig. CXLIII. 1), and lined by a continuation
of the same membrane which envelops the lungs, the pleura (104). But
that portion of the pleura which lines the internal wall of the thorax
is called the costal pleura (pleura costalis), in contradistinction to
that which envelops the lungs, which is termed the pulmonary pleura,
or pleura pulmonalis (104). By the costal pleura, a thin but firm and
strong membrane, smooth, polished, and like all the membranes of its
class (serous membrane 30, _et seq._), kept in a state of perpetual
moisture and suppleness, by a fluid secreted at its surface, the
movements of the thorax are facilitated, at the same time that they are
prevented from injuring the delicate organs contained in it.

379. The moveable parts of the osseous portion of the thorax are the
ribs and sternum. The ribs, though by one extremity tied with exceeding
firmness to the spinal column by ligaments specially constructed, and
admirably adapted for that purpose (figs. LVI. 1, and LVII. 1), and
though attached at their other extremity by their cartilages to the
sternum (fig. LVIII.), are capable of three motions, an upward, an
outward, and a downward motion.

[Illustration: Fig. CXLI.—_View of the osseous portion of the Thorax._

 1. Spinal column. 2. Ribs. 3. Cartilages of ribs. 4. Sternum.]

380. The ribs form a series of moveable arches, the convexity of the
arches being outwards, and the whole being disposed in an oblique
direction (fig. CXLI. 2). The first rib springs from the vertebral
column at nearly a right angle (fig. CXLI. 2); the acuteness of this
angle increases in succession as the ribs descend from the first to
the last (fig. CXLI. 2); in this manner each rib is inclined obliquely
outwards and downwards, and the obliquity thus given to the general
direction of the ribs augments progressively from above downwards (fig.
CXLI. 2).

381. In consequence of this conformation and arrangement of the ribs,
every degree of motion which is communicated to them, necessarily
influences the capacity of the space they enclose. If they are moved
upwards they must enlarge that space at the sides, because the
intervals between each other will be increased (fig. CXLI. 2); and from
behind forwards, because the distance between the spinal column and the
sternum (the sternum being protruded forwards with their cartilaginous
extremities) (fig. CXLI. 3, 4), will be increased. If, on the other
hand, they are moved downwards, the capacity of the thorax will be
proportionally diminished in every direction (fig. CXLI.).

[Illustration: Fig. CXLII.

 View of the intercostal muscles which fill up the interspaces between
 the ribs. These muscles consist of a double layer of fibres, the
 external and the internal, which cross or intersect each other.]

382. One part of the action of inspiration consists, then, of this
ascent of the ribs. The ascent of the ribs is effected by the
contraction of a double layer of muscles called the intercostal (fig.
CXLII.), placed in succession between each rib; and which communicate
this motion in the following mode. The first rib is fixed; the second
rib is moveable, but less moveable than the third, the third than the
fourth, and so on through the series: consequently the contraction of
the intercostal muscles (figs. CXLII. and CXLVI. 2) must elevate the
whole series, because the upper ribs afford fixed points for the action
of the muscles; and so, when all these muscles contract together, they
necessarily pull the more moveable arches upwards towards the more
fixed (figs. CXLI. and CXLVI. 2).

383. But from the oblique direction of the ribs, they cannot ascend
without at the same time protruding forwards their anterior extremities
(fig. CXLI.). Those extremities being attached to the sternum, which
forms the anterior wall of the thorax, they cannot be protruded
forwards without at the same time carrying the sternum forwards with
them (fig. CXLI.). Thus, by this two-fold motion of the ribs, an upward
and consequently an outward motion, the capacity of the thorax is
increased from behind forwards, that is, in its small diameter.

384. Such is the part of the action, in inspiration, performed by the
motion of the ribs. The remaining part of that action, by far the most
important, consists of the enlargement of the capacity of the thorax
from above downwards, or in its long diameter. This is effected by the
descent of the diaphragm (fig. CXLIII.).

385. The diaphragm is a circular muscle, forming a complete but
moveable partition between the thorax and the abdomen (figs. CXXXIV.
10, and CXLIII. 3). When not in action its upper surface forms an
arch (figs. CXLIII. 4, and CXLV. 1), the convexity of which is towards
the thorax (figs. CXLIII. 4, and CXLV. 1), and reaches as high as
the fourth rib (fig. CXLV. 1); its under surface, or that towards
the abdomen, is concave (figs. CXXXIV. 10, and CXLV. 1). Its central
portion is tendinous (fig. CXLIII. 4). This central tendinous portion
of the diaphragm, which is in apposition with the heart (fig. CXXXIV.
8), and firmly attached to the pericardium (fig. CXXXIV. 7), is nearly
if not quite immoveable: it is only the lateral or muscular portions
(fig. CXLIII. 4) that are capable of motion. Its central portion is
constructed of dense and firm tendon, and is immoveable, primarily, in
order to afford one of the two fixed points (the ribs affording the
other fixed point), essential to the action of the muscular fibres that
constitute its lateral or moveable portions; and secondarily, in order
to afford a support to the heart, which rests upon this central tendon.
Thus, in consequence of this tendon being rendered absolutely fixed,
the motions of the diaphragm are completely prevented from incommoding
the motions of the heart; the function of respiration from interfering
with the function of the circulation.

[Illustration: Fig. CXLIII.—_View of the Diaphragm._

 1. Cavities of the thorax. 2. Portion of cavity of the abdomen. 3.
 Lateral or muscular and moveable portions of the diaphragm. 4. Central
 or tendinous and fixed portion of the diaphragm.]

386. During the action of inspiration the muscular or lateral portions
of the diaphragm contract (fig. CXLIII. 3); its muscular fibres
shorten themselves, and are approximated towards the central tendon
(fig. CXLIII. 2); the consequence is that the whole muscle descends
(fig. CXLIV. 1); passes from the fourth to below the seventh rib
(fig. CXLIV.), loses its arched form and presents the appearance of
an oblique plane (fig. CXLIV.). At the same time the muscles of the
abdomen are protruded forwards (fig. CXLIV. 2), and the viscera
contained in its cavity are pushed downwards. The result of these
movements is, that the capacity of the thorax is enlarged by all the
space that intervenes between the fourth rib (fig. CXLV. 1), and the
lowest point of the oblique plane formed by the diaphragm (fig. CXLIV.
1), together with all that gained by the protrusion of the walls of the
abdomen and the descent of its viscera (fig. CXLIV. 2).

[Illustration: _Views of the Diaphragm in the different states of
Respiration._

Fig. CXLIV.

Fig. CXLV.

 Fig. 144.—1. Diaphragm in its state of greatest descent in
 inspiration. 2. Muscles of the abdomen, showing the extent of their
 protrusion in the action of inspiration. Fig. 145.—1. Diaphragm in the
 state of its greatest ascent in expiration. 2. Muscles of the abdomen
 in action forcing the viscera and diaphragm upwards.]

387. By the action of the intercostal muscles, then, the capacity of
the thorax is enlarged at the sides and from behind forward, or in its
short diameter; by the action of the diaphragm, the capacity of the
thorax is enlarged from above downwards, or in its long diameter; by
the combined action of both, the capacity of the thorax is enlarged in
every direction, and thus the motion of inspiration is completed.

388. Expiration, the respiratory motion which alternates with that of
inspiration, consists of the diminution of the capacity of the thorax,
which is effected by the converse motions of the same organs; that is,
by the descent of the ribs and the ascent of the diaphragm.

389. By the descent of the ribs, the capacity of the thorax is
diminished in its short diameter, because by this motion, the oblique
arches of the ribs are approximated to each other and to the spinal
column, and the sternum is also approximated to the spinal column.
The descent of the ribs is effected first by the elasticity of their
cartilages (fig. CXLI. 2). When the intercostal muscles relax, the
force which raised the ribs ceases to be applied, and that moment the
elasticity of the cartilages comes into play, and carries the ribs down
wards. Secondly, by the contraction of the abdominal muscles (figs.
CXLV. 2, and CXLVI. 6, 7, 8), the direct effect of which is to pull the
ribs downwards (fig. CXLVI. 6, 7, 8).

390. By the ascent of the diaphragm the capacity of the thorax is
diminished in its long diameter (fig. CXLV. 1). When the diaphragm
ascends, it changes from the figure of an oblique plane (fig. CXLIV.
1), re-assumes its arched form (fig. CXLV. 1), and reaches as high as
the fourth rib (fig. CXLV. 1). At the same time the abdominal muscles
contract (fig. CXLV. 2), and are carried inwards towards the spinal
column (fig. CXLV. 2). The result of these movements is, that the
capacity of the thorax is diminished by all the space that intervenes
between the lowest point of the oblique plane formed by the diaphragm
and the fourth rib (fig. CXLV. 1), and by all the abdominal space lost
by the contraction of the muscles of the abdomen (fig. CXLV. 2).

[Illustration: Fig. CXLVI.—_View of the principal external Muscles of
Respiration._

 1. The muscle called the Scalenus. 2. The muscles called the
 Intercostals. 3. Subclavius. 4. The bone called the Clavicle. 5. The
 muscle called the Serratus Magnus Anticus. 6. Obliquius Externus. 7.
 Rectus. 8. Obliquius Internus.]

391. The first step necessary to the ascent of the diaphragm is the
relaxation of its muscular fibres. As soon as these fibres are in a
state of relaxation, that is, when the organ has changed from an active
to a completely passive state, the powerful muscles of the abdomen
(fig. CXLVI. 6, 7, 8) contract, and push the abdominal viscera and
the diaphragm with them upwards towards the cavity of the chest (fig.
CXLV. 2); and thus, by the descent of the ribs and the ascent of the
diaphragm, the capacity of the thorax is diminished in every direction,
and the motion of expiration is completed.

392. Such is the mechanism by which the capacity of the thorax is
alternately enlarged and diminished in the two alternate states of
inspiration and expiration, and the mechanism thus adjusted works in
the following mode.

393. Expiration succeeding to the state of inspiration, the ribs
descend, the diaphragm ascends, the capacity of the thorax lessens,
and the compressed lungs are forced within the smallest possible
space. Then, inspiration, succeeding to the state of expiration, the
ribs ascend and the diaphragm descends; the capacity of the thorax is
enlarged, and the lungs freed from their pressure expand and fill the
greater space obtained. In about a second and a half after the state
of inspiration has been induced, that of expiration recommences; the
motion of inspiration occupying about double the time of the motion
of expiration, and these alternate conditions succeed each other in a
regular and uniform course, day and night, during our sleeping and our
waking hours to the end of life.

394. As long as the function is performed in a perfectly natural
manner, a given number of these alternate movements takes place
in a certain time, constituting what is termed the rhythm of the
respiratory motions. These motions perfectly regular in number and
time, are likewise, in the natural state of the function, performed
only with a certain degree of energy; but they are variously modified
at the command of the will; in obedience to numerous sensations and
emotions; in the performance of a great variety of complex actions,
and in different states of disease. These modifying circumstances may
cause the action of inspiration to be more full and deep, and that of
expiration to be more forcible and complete than natural; or they may
cause both movements to be shorter and quicker than common: hence the
distinction of respiration into ordinary and extraordinary.

395. In ordinary respiration, that is, when the respiratory motions
are perfectly calm and easy, the ascent and descent of the ribs are
scarcely perceptible; the action is confined almost exclusively to the
ascent and descent of the diaphragm. In this condition the only action
of the intercostal muscles is to fix the ribs, and thus to afford one
of the two fixed points which have been shown (385) to be essential
to the action of the diaphragm. But in extraordinary respiration,
that is, when circumstances happen in the economy which require that
those motions should be extended, auxiliary sources can be put in
requisition. There are many powerful muscles situated about the breast,
shoulder and back (fig. CXLVI. and CXLVII.); which are capable of
elevating the ribs and protruding the sternum to a very considerable
extent (figs. CXLVI. 1, 2, 3, 5; and CXLVII. 1, 2, 3). Where, for
example, the fullest inspiration which it is possible to take is
required, the bones of the shoulder and shoulder-joint are firmly
fixed by resting the hands upon the knees, and then every muscle which
has the slightest connexion with the thorax, either before or behind,
capable of raising the ribs, is added to the inspiratory apparatus
(figs. CXLIV. and CXLVII.); at the same time that the abdominal muscles
are relaxed to the utmost degree, in order to facilitate the ascent of
the ribs and the descent of the diaphragm (figs. CXLIV. 2, and CXLVI.
6, 7, 8). If, on the contrary, the fullest possible expiration is
required, the abdominal muscles contract most forcibly (fig. CXLV. 2),
and every other muscle which is capable of still farther depressing the
ribs and of elevating the diaphragm (fig. CXLVI. 6, 7, 8) is called
into intense action. By these forcible and extraordinary efforts the
thorax may be enlarged or diminished double its ordinary capacity.

[Illustration: Fig. CXLVII.—_View of Muscles which are capable of
assisting in elevating the Ribs and protruding the Sternum, in states
of extraordinary respiration._

 1. The muscle called the Great Pectoral. 2. The Small Pectoral. 3. The
 Serratus Magnus.]

396. Such are the mechanism and action of the powers which communicate
to the thorax, the motions by which its capacity is alternately
enlarged and diminished, and by which the requisite impulse is
communicated to the fluids which flow to and from the lungs in the
different states of respiration; that is, by which air and blood flow
to the lungs in the action of inspiration, and from the lungs in the
action of expiration.

397. The mode in which air is transmitted to the lungs by the
dilatation of the thorax, in the action of inspiration, is the
following. The lungs are in direct contact with the inner surface of
the thorax, and follow passively all its movements. When the volume
of the lungs is reduced to its minimum by the diminished capacity of
the thorax, in the state of expiration, they still contain a certain
bulk of air. As their volume increases with the enlarging capacity of
the thorax in the state of inspiration, this bulk of air having to
occupy a greater space expands. By this expansion of the air in the
interior of the lungs, it becomes rarer than the external air. Between
the rarified air within the lungs, and the dense external air, there
is a direct communication by the nostrils, mouth, trachea, larynx, and
bronchi. In consequence of its greater weight, the dense external air
rushes through these openings and tubes to the lungs and fills the
air vesicles, the current continuing to flow until an equilibrium is
established between the density of the air within the lungs and the
density of the external air; and thus there is established the flow of
a current of fresh air to the air vesicles.

398. The external air which, in obedience to the physical law that
regulates its motion, thus rushes to the lung in order to fill the
partial vacuum created by the dilatation of the thorax in inspiration,
produces, in passing to the air vesicles, a peculiar sound. When the
lungs are perfectly healthy, and the respiration is performed in a
natural manner, if the ear be applied to any part of the chest, a
slight noise can be distinguished both in the action of inspiration
and that of expiration. A soft murmur, somewhat resembling the sound
produced by the deep inspirations occasionally made by a person
profoundly sleeping. This sound, though appreciable even by the naked
ear, and though produced many times every minute, in every healthy
human being from the first moment of the existence of the first man,
had never been heard, or at least never attended to, until about twenty
years ago, when it was observed by accident. A physician, Dr. Laennec,
of Paris, having occasion to examine a young female labouring under,
as he supposed, some disease of the heart, and scrupling to follow
his first impulse to apply his ear to the chest, chanced to recollect
that solid bodies have the power of conducting sounds better than the
air. Thereupon he procured a quire of paper, rolled it up tightly,
tied it, and then applied one extremity to the patient’s chest and the
other to his ear. Profiting by the result, which was, that he could
hear the beating of the heart infinitely more distinctly than he
could possibly feel it by the hand, he substituted for this first rude
instrument a wooden cylinder, which he called a stethescope or chest
inspector. The attentive and practised use of this instrument is found
to be capable of revealing to the ear all that is passing in the chest
almost as clearly and certainly as it would be visible to the eye,
were the walls of the chest and the tissues of its organs transparent.
Besides the entrance of the air into the lung in inspiration, and its
exit in expiration, even the motion of the blood in the heart, and in
the great blood-vessels, are rendered by this instrument distinctly
manifest to sense; and as the ear which has once become familiar with
the natural sounds produced by these operations in the state of health,
can detect the slightest deviation occasioned by disease, the practical
application of this discovery has already effected for the pathology
of the chest, what the discovery of the circulation of the blood has
accomplished for the physiology of the body.

399. At the instant that the expanding lung admits the current of air,
it receives a stream of blood. The air rushes through the trachea to
the air vesicles impelled by its own weight; the blood flows through
the trunks of the pulmonary artery to its capillary branches, spread
out on the walls of the air vesicles, driven by the contraction of
the right ventricle of the heart. A current of air and a stream of
blood are thus brought into so close an approximation that nothing
intervenes between the two fluids, but the fine membranes of which
the air vesicles and the capillary branches of the pulmonary artery
are composed, and these membranes being pervious to the air, the air
comes into direct contact with the blood; the two fluids re-act on each
other, and in this manner is accomplished the ultimate object of the
action of inspiration.

400. On the other hand, by the action of expiration, the bulk of the
lung is diminished; the air vesicles are compressed, and a portion of
the air they contained, forced out of them by the collapse of the lung,
is received by the bronchi, transmitted to the trachea, and ultimately
conveyed out of the system by the nostrils and mouth.

401. At the same instant that a portion of air is thus expelled from
the lung and carried out of the system, a stream of blood, namely,
blood which has been acted upon by the air, arterial blood, is
propelled from the lung and is borne by the pulmonary veins to the
left side of the heart, to be transmitted to the system (fig. CXL. 10,
11, 4). In this manner, by the simultaneous expulsion from the lung of
a current of air and a stream of blood is accomplished the ultimate
object of the action of expiration.

402. That blood flows to the lung during the action of inspiration, and
is expelled from it during the action of expiration, is established by
direct experiment.

403. If the great vessel which returns the blood from the head to the
heart, called the jugular vein, be exposed to view in a living animal,
it is seen to be alternately filled and emptied according to the
different states of inspiration and expiration.

It becomes nearly empty at the moment of inspiration, because at that
moment the venous stream is hurried forward to the right chambers
of the heart, which in consequence of the general dilatation of the
chest are now expanded to receive it. This may be rendered still more
strikingly manifest to the eye. If a glass tube, blown at the middle
into a globular form, be inserted by its extremities into the jugular
vein of a living animal in such a manner that the venous stream must
pass through this globe, it is found that the globe becomes nearly
empty during inspiration, and nearly full during expiration; empty
during inspiration, because, during this action the blood flows
forwards to the right chambers of the heart; full during expiration,
because during this action the venous stream, retarded in its passage
through the lung, its motion becomes so slow in the jugular vein that
there is time for its accumulation in the glass globe. In the artery,
on the contrary, in which the course of the current is the reverse
of that in the vein, the opposite result takes place. In the carotid
artery the stream is seen to be feeble and scanty during inspiration,
but forcible and full during expiration, and if the artery be divided
the jet of blood that issues from it absolutely stops during the
action of inspiration; and the fuller and deeper the inspiration the
longer is the interval between the jets, while it is during the action
of expiration that the jet is full and strong.

404. In the course of some experiments performed by Dr. Dill and myself
with a view to ascertain with greater precision the relation between
respiration and circulation, we observed a phenomenon which places
these points in a still more clear and striking light. We happened to
divide a jugular vein. We saw that the vessel ceased to bleed during
inspiration, and that it began to bleed copiously the moment expiration
commenced; the reverse of what uniformly happens in the entire state
of the vessel. The reason is, that the division of the vein cuts off
its communication with the lung, removes it from the influence of
respiration, brings it under the influence, the sole influence of the
powers that move the arterial current, and consequently reverses its
natural condition, and so reverses the manner in which its current
flows; affording a beautiful illustration of the influence of the two
actions of respiration on the two sets of blood-vessels concerned in
the function.

405. It is then the venous system that is immediately related to
inspiration, and the arterial to expiration. Each respiratory action
exerts a specific influence over its own sanguiferous system, and
the influence of the one action is the reverse of that of the
other, as the two currents they work flow in opposite directions.
The lungs, in inspiration, expand and receive the venous stream; in
expiration, collapse and expel the arterial stream. The expansion of
the lungs in inspiration is thus simultaneous with the dilatation of
the heart: during the inspiratory action both organs receive their
blood. The collapse of the lungs in expiration is simultaneous with
the contraction of the heart: during the expiratory action both organs
expel their blood.

406. We are thus enabled to form a clear and exact conception of the
mechanism and action of both parts of this complicated function. Almost
all the points connected with the systemic circulation were established
upwards of three hundred years ago (279), but many points connected
with the pulmonic circulation have been established only recently. Our
knowledge of the phenomena of both, and of their mutual relation and
dependence, has been slowly increasing, and is at length tolerably
complete; and now that we understand the exact office and working of
each, we see that the action of the one is not only in harmony with
that of the other, but co-operates with it, and renders it perfect.

407. But although the main points relative to the influence of
inspiration and expiration over the pulmonary circulation may be said
to be universally admitted, still physiologists are not agreed as to
the relative quantities of blood which are transmitted through the
lungs during these different respiratory states. All are agreed that
the state of inspiration is favourable to the passage of the blood
through the lungs: some maintain that this expansion of the lungs in
inspiration is essential to the pulmonary circulation. There is the
like general consent that the state of expiration retards the flow of
blood through the lungs; by many it is conceived that it completely
stops the current. By these physiologists it is supposed that, during
the action of expiration, the lungs are in a state of collapse; that
they contain a comparatively small portion of air; that in this state
the air vesicles are so compressed, and the pulmonary blood-vessels so
coiled up, that the lungs are absolutely impermeable, and consequently,
that when the blood arrives at the right chambers of the heart, it is
incapable of making its way to the left. This, according to a prevalent
theory, is the immediate cause of death in asphyxia, the state of the
system induced by suspended respiration, as in drowning, hanging, and
suffocation. Death takes place in this condition of the system, it is
argued, because the circulation of the blood is arrested at the right
side of the heart, cannot permeate the lungs, and consequently cannot
reach the left ventricle, to be sent out to supply the organs of the
body.

408. This opinion, which appears at first view to be favoured by
numerous observations and experiments, has been shown to be fallacious
by a series of decisive experiments, performed by Dr. Dill and myself,
undertaken, as has been stated (404), with the object of ascertaining,
in a more exact manner than had hitherto been done, the relation
between the circulation and respiration. The previously ascertained
fact that the heart continues to beat and the blood to flow several
minutes after the complete suspension of the respiration, or after
apparent death, afforded us the means of pursuing our research. The
details of these experiments are given elsewhere: it is sufficient to
state in this place the main results.

409. As a standard of comparison, the quantity of blood which flows
through the lungs after apparent death, when the lungs remain in a
perfectly natural state, was previously ascertained. It was found,
after death produced in an animal by a blow on the head, that blood
continued to be transmitted through the lungs for the space of
twenty-five minutes after the complete cessation of respiration. There
passed through the lungs in all five ounces and two drachms of blood.

410. Respiration was now suspended the instant after a perfectly
natural and easy _inspiration_; there flowed through the lungs four
ounces and five drachms of blood.

411. Respiration was next suspended the instant after a perfectly
natural and easy _expiration_; there flowed through the lungs two
ounces and seven drachms of blood.

412. When the trachea of an animal is closed by the pressure of a cord
in suspension, or when an animal is immersed under water, it makes a
succession of violent expirations, by which a large quantity of air is
forced out of the lungs. Hence, when the lungs of an animal that has
perished by hanging or drowning, are examined, they are always found
much reduced in bulk; so much reduced in bulk as to have suggested the
theory that the extreme collapse of the lungs and their consequent
impermeability, is the cause of death in this condition of the system.
On bringing this theory to the test of experiment, it was found that
blood continued to flow through the lungs after apparent death from
suspension, for the space of eleven minutes, and that there passed
through in all five ounces of blood. The comparatively larger quantity
transmitted in this case than when the inspiration and expiration were
perfectly natural, was owing to the larger size of the animal. In the
experiments made with a view to ascertain the relative proportions
of blood transmitted through the lungs in the states of natural
inspiration and expiration, the animals were chosen as nearly as
possible of the same size, and were much smaller than the former.

413. On examining the quantity of blood that passed through the lungs
after death from submersion, it was found to be very nearly the same
as that which was transmitted after death from suspension.

414. But the lungs may be brought to a much greater degree of collapse
than that to which they are reduced in hanging and drowning. By
introducing an exhausting syringe into the trachea, a much larger
quantity of air may be drawn out of the lungs than they are capable
of expelling by the most violent efforts of expiration. When, in this
mode, the lungs had been reduced to the greatest possible degree of
collapse, and had been exhausted of all the air that could be drawn out
of them, there flowed through them two ounces of blood.

415. Such are the results when the lungs are reduced successively
from the moderate degree of collapse incident to a perfectly natural
expiration, to the great degree of collapse incident to suspension
and submersion, and the most extreme degree of collapse which it is
possible to induce by exhaustion.

416. When the phenomena that take place in the opposite condition of
the lungs were investigated, results were obtained which present a
striking contrast to those which have been stated. On forcing into
the lungs the largest quantity of air which they are capable of
containing without the rupture of the air vesicles, and in this manner
communicating to them the greatest degree of dilatation compatible
with their integrity, it was found that in this state there passed
through them _only three drachms of blood_.

417. But on fully distending the lungs with water instead of air, the
pulmonary circulation was instantaneously and completely arrested; they
were incapable of transmitting a single drop of blood. On cutting the
aorta across, as in all the preceding experiments, not a particle of
blood was obtained, excepting what issued at a single jet, and which
consisted only of the blood contained in the vessel at the moment the
respiration was stopped.

418. From these experiments it follows—

1. That the state of inspiration is favorable to the passage of
the blood through the lungs. In the dilatation of inspiration they
transmitted nearly double the quantity that passed in the collapse of
expiration; or, as four ounces and five drachms are to two ounces and
seven drachms (410 and 411).

2. That no degree of collapse to which the lungs can be reduced is
capable of wholly stopping the flow of the blood through them. In the
collapse of suspension and submersion they transmitted as much blood,
with the exception of two drachms, as when death was produced by a blow
on the head (412 and 409). In the greatest degree of collapse capable
of being produced by an exhausting syringe, they transmitted half as
much as in the collapse of suspension and submersion (414 and 412).

3. That it is only a moderate degree of dilatation that is favorable
to the transmission of the blood through the lungs. When the lungs
are over-distended with air, they are capable of transmitting only
an exceedingly small quantity of blood (416); when they are fully
distended with water, they are incapable of transmitting a single drop
of blood (417). In fact they can contain only a certain quantity of
air and blood; and when either of these fluids preponderates, it can
only be by the proportionate exclusion of the other. It will appear
hereafter that these results are capable of applications of the highest
interest and importance in the explanation of numerous phenomena of
health and of disease.

419. Physiologists have laboured with great diligence to determine the
exact quantity of air and blood which enters and which flows from the
lung at each of the actions of respiration, and they have succeeded in
obtaining tolerably precise results.

420. The quantity of air capable of being received into the lungs of
an adult man, in sound health, at an inspiration, is determined with
correctness by an instrument constructed by Mr. Green, analagous to
one suggested by Mr. Abernethy. It consists of a tin trough, about a
foot square, and six inches deep, three parts of which are filled
with water. Into this trough is placed a three-gallon glass jar, open
at the bottom, and graduated at the side into pints, half-pints, &c.
To the upper end of the jar a flexible tube is affixed, having at its
connexion a stop-cock. The lungs being emptied, as in the ordinary
action of expiration, and the mouth applied to the end of the flexible
tube, the nostrils being closed by the pressure of the fingers, the
air is drawn out of the jar into the lungs by the ordinary action of
inspiration. When as much air is thus drawn into the lungs as the air
vesicles will hold, the stop-cock is closed, and the quantity of air
inspired is ascertained by the rise of the water, the level of the
water corresponding with the indications marked on the side of the jar.

421. The quantity of air which a person by a voluntary effort can
inspire at one time is found, as might have been anticipated, to be
different in every different individual. These varieties depend, among
other causes, on the greater or less development of the trunk, on the
presence or absence of disease in the chest, on the degree in which the
lung is emptied of air by expiration previously to inspiration, and
on the energy of the inspiratory effort. The greatest volume of air
hitherto found to have been received by the lung, on the most powerful
inspiration, is nine pints and a quarter. The average quantity which
the lungs are capable of receiving in persons in good health, and free
from the accumulation of fat about the chest, appears to be from five
to seven pints. The latter is about the average quantity capable of
being inspired by public singers.

422. But these measurements relate to the greatest volume of air which
the lungs are capable of receiving, on the most forcible inspiration
which it is possible to make, after they have been emptied by forcible
expiration, and consequently express the quantity received in
extraordinary, not in ordinary inspiration. The quantity received at an
inspiration easy, natural, and free from any great effort, may be two
pints and a half, but the quantity received at an ordinary inspiration,
made without any effort at all, is, according to former observations
which referred to Winchester measure, about one pint.

423. The quantity of air expelled from the lung by an ordinary
expiration is probably a very little less than that received by an
ordinary inspiration (456).

424. No one is able by a voluntary effort to expel the whole contents
of the lungs. Observation and experiment lead to the conclusion that
the lungs, when moderately distended, contain at a medium about twelve
pints of air. As one pint is inhaled at an ordinary inspiration,
and somewhat less than the same volume is expelled at an ordinary
expiration (456), there remain present in the lungs, at a minimum,
eleven pints of air. There is one act of respiration to four pulsations
of the heart; and, as in the ordinary state of health there are
seventy-two pulsations, so there are eighteen respirations in a minute,
or 25,920 in the twenty-four hours.

425. About two ounces of blood are received by the heart at each
dilatation of the auricles; about the same quantity is expelled from
it at each contraction of its ventricles; consequently, as the heart
dilates and contracts seventy-two times in a minute, it sends thus
often to the lungs, there to be acted upon by the air, two ounces of
blood. It is estimated by Haller that 10,527 grains of blood occupy
the same space as 10,000 grains of water, so that if one cubic inch of
water weigh 253 grains, the same bulk of blood will weigh 266⅓ grains.

426. It is ordinarily estimated that on an average one circuit of the
blood is performed in 150 seconds; but it is shown (451 and 452) that
the quantity of air always present in the lungs contains precisely a
sufficient quantity of oxygen to oxygenate the blood, while flowing
at the ordinary rate of 72 contractions of the heart per minute, for
the exact space of 160 seconds. It is therefore highly probable that
this interval of time, 160 seconds, is the exact period in which the
blood performs one circuit, and not 150 seconds, as former observations
had assigned. If this be so, then 540 circuits are performed in the
twenty-four hours; that is, there are three complete circulations of
the blood through the body in every eight minutes of time.

427. But it has been shown (425) that the weight of the blood is to
that of water as 1.0527 is to unity, and that consequently 10,527
grains of blood are in volume the same as 10,000 grains of water.

428. From this it results that if in the human adult two ounces of
blood are propelled into the lungs at each contraction of the heart,
that is, 72 times in a minute, there are in the whole body precisely
384 ounces, or 24 pounds avoirdupois, which measure 692.0657 cubic
inches, or within one cubic inch of 20 imperial pints, which measure
693.1847 cubic inches.

429. By an elaborate series of calculations from these data Mr.
Finlaison has deduced the following general results:—

1. As there are four pulsations to one respiration (424), there are 8
ounces of blood, measuring 14.418 cubic inches, presented to 10.5843
grains of air, measuring 34.24105 cubic inches.

2. The whole contents of the lungs is equal to a volume of very nearly
411 cubic inches full of air, weighing 127 grains, of which 29.18132
grains are oxygen.

3. In the space of five-sixth parts of one second of time, two ounces,
or 960 grains weight of blood, measuring 3⅗ or 3.60451 cubic inches,
are presented for aëration.

4. Therefore the air contained in the lungs is 114 times the bulk of
the blood presented, while the weight of the blood so presented is 7½
times as great as the weight of the air contained.

5. In one minute of time the fresh air inspired amounts to 616⅓ cubic
inches, or as nearly as may be 18 pints, weighing 190½ grains.

6. In one hour the quantity inspired amounts to 1066⅔ pints, or 2
hogsheads, 20 gallons, and 10⅔ pints, weighing 23¾ ounces and 31 grains.

7. In one day it amounts to 57 hogsheads, 1 gallon, and 7¼ pints,
weighing 571½ ounces and 25 grains (454).

8. To this volume of air there are presented for aëration in one minute
of time 144 ounces of blood, in volume 259½ cubic inches, which is
within 18 cubic inches of an imperial gallon.

9. In one hour 540 pounds avoirdupois, measuring 449¼ pints, or 1
hogshead and 1¼ pints;—and

10. In the twenty-four hours, in weight 12,960 pounds; in bulk 10,782½
pints, that is, 24 hogsheads and 4 gallons.

11. Thus, in round numbers, there flow to the human lungs every minute
nearly 18 pints of air (besides the 12 pints constantly in the air
vesicles) and nearly 8 pints of blood; but in the space of twenty-four
hours, upwards of 57 hogsheads of air and 24 hogsheads of blood.

430. Provision cannot have been made for bringing into contact such
immense quantities of air and blood, unless important changes are to be
produced in both fluids; and accordingly it is found that the air is
essentially changed by its contact with the blood, and the blood by its
contact with the air.

431. Chemistry has demonstrated the changes effected in the air.
Common atmospheric air is a compound body, consisting of pure air and
of certain substances diffused in it. Pure air is composed of two
gases, azote and oxygen, always combined in fixed proportions. The
substances diffused in pure air, and which are in variable quantity,
are aqueous vapour and carbonic acid gas. These latter substances form
no part of the chemical agents essentially concerned in the process of
respiration. The only constituents of the air which are essentially
concerned in the process of respiration are the two gases, azote and
oxygen, the union of which, in definite proportions, constitutes pure
air. But of these two gases each does not perform the same part in the
function of respiration, nor is each equally necessary to the support
of life.

432. If a living animal be placed in a vessel full of atmospheric
air, and if all communication of the atmosphere with the vessel be
prevented, the animal in a given time perishes. If an animal be placed
in a vessel full of azote, after a given time it equally perishes;
but if an animal be placed in a vessel full of oxygen, not only is
the function of respiration carried on with far greater energy than
in atmospheric air, but the animal lives a much longer time than in
the same bulk of the latter fluid. If twenty cubic inches of pure
oxygen be capable of sustaining the life of an animal for the space of
fourteen minutes, it can support life in the same bulk of atmospheric
air only six minutes; and if its respiration be confined to either of
these gases, after they have been already respired by another animal
of the same species, the former will live only four minutes; that is,
not longer than when entirely deprived of air. It follows that the gas
which gives to atmospheric air its chief power of sustaining life is
oxygen.

433. Accordingly it is proved that no animal, from the lowest to the
highest, is capable of sustaining life unless a certain proportion of
oxygen be present in the fluid which it respires. Whether it breathe
by the skin, by gills, or by lungs, whether the respiratory medium be
water or air, the presence of oxygen is alike indispensable. Yet the
life of no animal can be sustained by pure oxygen. If azote be not
mixed with oxygen, evils are produced in the economy which sooner or
later prove fatal. On the other hand, if the proportion of oxygen
be diminished beyond a certain point, drowsiness, torpor, and death
result. Not oxygen alone, then, but oxygen combined with azote, in the
proportion in which nature has united these two fluids to form the
atmosphere of the globe, is indispensable to animal existence.

434. When the same portion of atmospheric air is repeatedly respired
by an animal, the oxygen contained in it gradually disappears, the gas
lessening with every successive respiration, until at last so small a
quantity remains that it is no longer capable of sustaining the life of
an animal of that class. When respiration has deprived the air of its
oxygen to such an extent, that it can no longer support animal life,
the air is said to be consumed; but, correctly speaking, it is merely
changed in composition, in the proportions in which its constituents
are combined; consequently the effect of respiration is to alter the
chemical composition of the air.

435. The essential change that takes place consists in the diminution
of the oxygen and the increase of the carbonic acid. When inspired,
atmospheric air goes to the lungs loaded with oxygen; when expired, it
returns loaded with carbonic acid. That the air which returns from the
lungs is loaded with carbonic acid, may be rendered manifest even to
the eye. If a person breathe through a tube into water holding lime in
solution, the carbonic acid contained in the expired air will unite
with the lime and form a white powder analogous to chalk (carbonate of
lime), which being insoluble, becomes visible.

436. On the other hand, the diminution of oxygen is demonstrated by
chemical analysis. If 100 parts of atmospheric air be successively
respired, until it is no longer capable of supporting life, and if
it be then subjected to analysis, it is found that in place of being
composed of 79 parts azote, 21 oxygen, and a variable quantity of
carbonic acid, sometimes amounting to half a grain per cent., it
consists of 77 parts azote, and 23 carbonic acid. The oxygen is gone,
and is replaced by 23 parts of carbonic acid; at least this is the
ordinary estimate; but different experimentalists differ somewhat in
their account of the absolute quantity of oxygen that disappears, and
of carbonic acid that is generated.

437. Whatever estimates of the oxygen consumed, and of the carbonic
acid generated, be adopted, they can be taken only as medium
quantities. Dr. Edwards has demonstrated that the absolute quantity
of oxygen consumed in a given time is constantly varying, not only
in animals of different species, but even in the same animal under
different circumstances; insomuch, that there are scarcely two hours
in the day in which the same individual expends precisely the same
quantity. The nature and degree of the exercise taken during the
observation, the condition of the mind, the state of the health,
the kind of food, the temperature of the air, and innumerable other
causes materially influence the quantity of oxygen consumed. When,
for example, the hourly consumption of oxygen, at the temperature
of 54° Fahrenheit, amounted to 1345 cubic inches,[1] it fell, at
the temperature of 79°, to 1210 cubic inches. During the process of
digestion more is consumed than when the stomach is empty; more is
required when the diet is animal than when it is vegetable, and more
when the body and mind are active than when at rest.

  [1] The ordinary consumption of oxygen is, for an adult, 1905 cubic
  inches per hour (444).

438. With regard to the carbonic acid, Dr. Prout has recently made the
remarkable discovery, not only that the generation of this gas differs
according to different circumstances, and more especially according
to particular states of the system; but that the quantity of it which
is produced regularly varies at particular periods of the day. The
quantity generated is always more abundant during the day than during
the night. About daybreak it begins to increase; continues to do so
until noon, when it comes to its maximum, and then decreases until
sunset. The maximum quantity generated at noon exceeds the minimum by
about one-fifth of the whole. If from any cause the relative quantity
be either increased or diminished above or below the ordinary maximum
or minimum, it is invariably diminished or increased in an equal
proportion during some subsequent diurnal period. The absolute quantity
generated is materially diminished by the operation of any debilitating
cause, such as low diet, protracted fasting, or long-continued
exercise, depressing passions and the like. Few circumstances of any
kind increase the quantity produced, and those only in a slight degree.

439. The changes produced by respiration on the other constituent of
the air, azote, appear at first view to be extremely variable. By
numerous and accurate experiments it is established that the quantity
of this gas is at one time increased; at another diminished, and at
another unchanged. It is probable that there is a constant absorption
and exhalation of it; and that the apparent irregularity is the result
of the preponderance of the one process over the other. When absorption
preponderates, a smaller quantity is found in the air expired than
in that inspired: when exhalation preponderates, a larger quantity
is expired than inspired; and when the absorption and exhalation are
equal, just as much is expired as inspired, and consequently there
appears to be no absorption at all.

440. Such are the phenomena of respiration, as far as the labours of
physiologists has succeeded in ascertaining them, up to the present
time. But as the estimates of the quantity of air and blood contained
in the lungs were rather matters of conjecture than of demonstration,
and as the quantity of oxygen consumed, of carbonic acid generated, and
of azote absorbed, appeared still not to be determined with exactness,
I requested Mr. Finlaison to apply his power of calculation to the
investigation of this subject, taking as the basis of his calculations
the facts positively and precisely ascertained by experiment and
analysis. This he has done with great care, and has obtained the
following results.

441. It was formerly estimated that the weight of pure atmospheric air
is 305,000 grains troy for one million of cubic inches; but the latest
authorities assign it to be 310,117 grains. Of this weight of one
million of cubic inches of pure air,

  The weight of the oxygen is          71,809.3
  The weight of the azote is          238,307.7
                                      —————————
                 Total                310,117.0

442. But common atmospheric air in its ordinary state contains in 1000
cubic inches,

  Of pure air                    989
  Of the vapour of water          10
  Of carbonic acid gas             1

Ten inches of pure air are equal in weight to nine of oxygen.

Eight inches of azote are equal in weight to seven of oxygen.

The specific gravity of carbonic acid is to pure air at the rate of
15,277 to 10,000.

The specific gravity of the vapour of water is to pure air as 6,230
to 10,000. It follows that a million of cubic inches of air in its
ordinary state weigh 309,111½ grains.

Carbonic acid gas is composed of oxygen and pure carbon in the
proportion of eight grains of oxygen to three of carbon out of every
eleven grains of carbonic acid.

443. Though during particular portions in the twenty-four hours, under
circumstances which influence variously the actions of life (437 and
438), the quantity of the oxygen consumed, of carbonic acid generated,
and of azote absorbed, vary (436 to 439), yet it is probable that
the daily consumption, reproduction, and absorption of these gases,
is pretty much the same one day with another. The experiments of
Dr. Edwards clearly show that while these quantities vary to such
an extent, when the observation embraces only a short interval, as
to be scarcely ever the same hour by hour, yet that they lessen as
the interval extends, until at length a nearly exact equilibrium is
established.

444. Experimental philosophers have not obtained precisely the
same results as to the quantities consumed and reproduced of these
respective gases. At present, therefore, we can only approximate to
the exact amount by taking the average of their observations. The
following are the results of the principal experiments which have
been instituted. The quantity of oxygen consumed by an adult man in
twenty-four hours is, according to

  Menzes                      51,840
  Lavoisier                   46,048
  Davy                        45,504
  Allen and Pepys             39,534

The mean of all which is, 45,731.5 inches.

445. In like manner the quantity of carbonic acid generated in the same
time is, according to

         Davy                     38,304 cubic inches.
         Allen and Pepys          38,232      “
  The mean of which is,           38,268      “

The weight of 38,268 inches of carbonic acid gas is 18,130.1474 grains
troy; and the weight of 45,731½ inches of oxygen is 15,757.9131 grains
troy.

Now this weight of oxygen must have been derived from the decomposition
of 221,882 cubic inches of common atmospheric air.

446. It has been shown that, in the state of health, one contraction
of the heart propels to the lungs two ounces of blood; that this
action of the heart is repeated 72 times in one minute; that to every
four actions of the heart there is one action of respiration; that
consequently there are 18 respirations in a minute, and 25,920 in the
twenty-four hours.

447. From these premises it results that at each action of the heart
there is decomposed of the air inspired, 8.5603 cubic inches, that is,
a quarter of a pint within one-tenth of a cubic inch,—the quarter of a
pint imperial measure being 8.6648 cubic inches.

448. Previous observation had assigned one pint as the volume of
air ordinarily inhaled at a single inspiration. We now see that the
quantity decomposed is a quarter of a pint. It is, then, an absolute
truth, that of the whole volume of air inspired, one-fourth part only
is decomposed, and that three-fourths, after having been diffused
through the air vesicles of the lungs, are expired without change.

449. Observation had also assigned 12 pints
  of air as the volume constantly present in the
  lungs,—that is,                 415.9108 cubic inches.
  The truth seems to be,
  that forty-eight times the
  quantity decomposed is
  constantly present, namely,      410.8926 cubic inches.
  The difference is only             4.0182 cubic inches,
  which difference weighs less than  1¼ grains troy.

450. It is then concluded that the real contents of the lungs is a
volume of 410.8926 cubic inches, which is exactly the 540th part of
221,882 cubic inches, being the whole volume decomposed in twenty-four
hours. But 160 seconds is also exactly the 540th part of the number of
seconds in twenty-four hours.

451. Of the whole weight of oxygen consumed
  in twenty-four hours                   15,757.9131 grains,
  the 540th part, or the proportion
  of 160 seconds, is                        29.18132  “
  and 410.8926 cubic inches of
  atmospheric air, which, as
  above, is the contents of the
  lungs, contain of oxygen the
  same weight                               29.18132  “

452. Then, if respiration were suddenly stopped, provision is made by
the quantity of air always retained in the lungs for the oxygenation of
the blood while flowing at the ordinary rate of 72 strokes per minute,
for the exact space of 160 seconds, and for not one instant longer.

453. This interval of time, then, as has been stated (426), is very
probably the time in which the blood performs one circuit, not 150
seconds. Then 540 circuits are performed in the twenty-four hours, or
3 circuits in every eight minutes. From this estimate has been deduced
the quantity of blood contained in the whole body of the human adult
(428).

454. The air inspired in twenty-four hours contains as under:—

                            Bulk in        Weight in      Ingredients.
                          cubic inches.   grains troy.

  Undecomposed, and to be
   returned unchanged       665,646        205,758.833,    Common air,

  To be decomposed,
   containing in solution

  { Pure atmospheric air    219,441       { 15,757.913,   Oxygen,
  {                                       { 52,294.509,   Azote,
  { Vapour of water           2,219            428.726,   Vapour,
  { Carbonic acid gas           222            105.130,   Carbonic acid,
                 Total      887,528        274,345.111,   Of all kinds.

This is, in bulk, 25,607¼ imperial pints, or 57 hogsheads, 1 gallon,
and 7¼ pints, and in weight 571½ ounces and 25 grains.

455. Now, although the air expired, in consequence of its
recomposition, may have undergone changes in bulk, yet it seems
agreeable to all analogy to suppose that its weight will remain the
same as the weight inhaled. This, however, is not asserted as a truth,
but only assumed, in order to show the result of such a theory.

456. Then the air expired in twenty-four hours will be as follows:—

                             Bulk in         Weight in
                           cubic inches.   grains troy.
  Given out undecomposed
    as before                 665,646        205,758.833
  Recomposed carbonic
    acid gas                   38,268         18,130.147
  Azote liberated             165,927         50,027.405
  Vapour of water as before     2,219            428.726
                              ———————        ——————————-
        Total                 872,060        274,345.111

weighing as before, but less in bulk by 446¼ pints: so that for every
100,000 inches expired there were inspired 101,774 cubic inches.

  457. When from the weight of
  carbonic acid gas thus expired, viz.,      18,130.147
  we deduct the small portion inhaled

  in solution with the air                      105.130
                                             ——————————
        The remainder is                     18,025.017

  The constituent parts of which are,
  oxygen derived from the air                 13,109.104
                                              ——————————
  And pure carbon derived from the
  blood being the difference                   4,915.913

Thus in the compass of twenty-four hours the blood has produced 10
ounces and 116 grains very nearly of pure carbon.

  458. Now, from the oxygen consumed              Grains.
  in twenty-four hours as above                15,757.913

  Deduct the weight restored in the
  form of carbonic acid gas                    13,109.104
                                               ——————————
  The remainder must have been absorbed
  into the blood                                2,648.809

  But the weight of carbon given out
  being as above                                4,915.913
                                               —————————
  There is still an excess given outweighing    2,267.104

459. Some azote, however, is absorbed into the blood (439) as well as
the above ascertained quantity of oxygen.

  The weight of azote so absorbed must
  be precisely                                  2,267.104

  if the theory be true, that equal weights
  are expired and inspired. In
  which case, as the weight of the
  azote of the air inspired was, as
  shown above                                  52,294.509

  While the azote expired could only
  have weighed                                 50,027.405
                                               —————————-
  The difference would have been absorbed       2,267.104

And thus the weight of carbon discharged by the blood is precisely
compensated by the united weight of the oxygen and azote which it has
absorbed.

460. Since it appears to be a general truth that one quarter of the air
respired is decomposed, and that the volume of air continually present
in the lungs is sufficient for that consumption of oxygen which is
requisite in 160 seconds of time, _if that volume be_, as is apparent,
48 _times the quantity decomposed_ out of a single respiration, no
error in the quantity of oxygen consumed in the twenty-four hours,
which we have assumed, will affect the time of 160 seconds. For there
being 18 × 60 × 24 respirations, and 60 × 60 × 24 seconds of time in
the twenty-four hours, the 48th part of the first, and the 160th part
of the last product is equally the 540th part of the whole, whatever it
may be.

461. But if the time in which a circuit of the blood is performed
be, as is most evident, identical with the time in which the whole
volume of air in the lungs is decomposed, and if such period of time
were, as the old observers have assigned, 150 seconds, then it would
follow that only 45 times the quantity of air decomposed at a breath
is present in the lungs, amounting to 385¼ cubic inches, and that the
whole blood in the body is 24 ounces less than on the supposition
of 160 seconds, that is to say, only 360 ounces, or 22½ pounds
avoirdupois. Because the 45th part of 18 × 60 × 24 is the same as the
150th part of 60 × 60 × 24; in each it is the 567th part of the whole.

462. From the whole of these observations and calculations the
following general results are deduced:—

1. The volume of air ordinarily present in the lungs is very nearly
twelve pints (449).

2. The volume of air received by the lungs at an ordinary inspiration
is one pint (422).

3. The volume of air expelled from the lungs at an ordinary expiration
is a very little less than one pint (456).

4. Of the volume of air received by the lungs at one inspiration, only
one-fourth part is decomposed at one action of the heart (447).

5. The fourth part of the volume of air received by the lungs at
one inspiration, and decomposed at one action of the heart, is so
decomposed in the five-sixth parts of one second of time (429.3).

6. The time in which a circuit of blood is performed is identical with
the time in which the whole volume of air in the lungs is decomposed
(461).

7. The whole volume of air decomposed in twenty-four hours is 221,882
cubic inches, exactly 540 times the volume of the contents of the
lungs; 160 seconds being also exactly the 540th part of the number of
seconds in twenty-four hours (450).

8. The quantity of the blood that flows to the lungs to be acted upon
by the air at one action of the heart is two ounces (425).

9. This quantity of blood is acted upon by the air in the five-sixth
parts of one second of time (429.3).

10. One circuit of the blood is performed in 160 seconds of time. Three
circuits are performed every eight minutes; 540 circuits are performed
in the twenty-four hours (453).

11. The quantity of blood in the whole body of the human adult is 24
pounds avoirdupois, or 20 pints imperial measure (428).

12. In the space of twenty-four hours, 57 hogsheads of air flow to the
lungs (429.7).

13. In the same space of time 24 hogsheads of blood are presented in
the lungs to this quantity of air (424.10).

14. In the mutual action that takes place between these quantities of
air and blood, the air loses 15,757.9131 grains, or 328¼ ounces of
oxygen, and the blood 10 ounces and 116 grains of carbon (445).

15. The blood, while circulating through the lungs, permanently retains
and carries into the system—of oxygen, 2,648,809 grams; and of azote,
2,267,104 grains (458).

16. The ultimate results are two:—

1st. While the chemical composition of the blood is essentially
changed, its weight amidst all these complicated actions is maintained
steadily the same; for the weight of carbon which is discharged by the
blood is precisely compensated by the united weight of the oxygen and
azote which it absorbs (459).

2ndly. The distribution of quantities is universally by proportions
or multiples. Thus, of the air inspired, one measure is decomposed
and three measures are returned unchanged: of the air decomposed at a
single inspiration, there are always in store in the lungs precisely
forty-eight measures; and so on in many other cases. The proportions
are not arithmetical, but geometrical. When we compare arithmetical
quantities with each other, we say that one quantity is by so much
greater than another; when we compare geometrical quantities, we say
that one quantity is so many times greater than another. From this
adoption in the distribution of quantities of geometrical proportions
it results that whatever be the size of the animal the ratios remain
uniformly the same, and that thus one and the same law is adapted to
the vital agencies of living beings under every possible diversity of
magnitude and circumstance.

463. Such are the interesting and important properties and relations
deducible from the phenomena of respiration. The disappearance of
oxygen and azote from the air inspired, and the replacement of the
oxygen that disappears by the production of carbonic acid, and of the
azote by the exhalation of azote, in which, as we have seen, the great
changes wrought by respiration on the air consist, are essentially the
same in all animals, whatever the medium breathed, and whatever the
rank of the animal in the scale of organization. In all, the proportion
of the oxygen of the inspired air is diminished;—in all, carbonic acid
gas is produced. Comparing, then, the ultimate result of the function
of respiration in the two great classes of living beings, it follows
that the plant and the animal produce directly opposite changes in the
chemical constitution of the air. The carbonic acid produced by the
animal is decomposed by the plant, which retains the carbon in its own
system and returns the oxygen to the air. On the other hand, the oxygen
evolved by the plant is absorbed by the animal, which in its turn
exhales carbonic acid for the re-absorption of the plant.

464. Thus the two great classes of organized beings renovate the air
for each other, and maintain it in a state of perpetual purity. The
plant, it is true, absorbs oxygen during the night as well as the
animal; but the quantity which it gives off in the day more than
compensates for that which it abstracts in the absence of light. This
interesting fact has been recently established by an extended series
of experiments instituted by Professor Daubeney[2] for the express
purpose of investigating this point.

  [2] On the Action of Leaves upon Plants, and of Plants upon the
  Atmosphere, by Charles Daubeney, M.D. F.R.S., Professor of Chemistry
  and Botany in the University of Oxford. Philosophical Transactions of
  the Royal Society of London, for the year 1836. Part I. succession:
  the amount of oxygen now evolved was increased from twenty-one to
  thirty-nine per cent., and probably had not even then attained the
  limit to which the increase of this constituent might have been
  brought. From the proportions of the constituent elements of carbonic
  acid gas (442) it necessarily follows that, by the mere process of
  decomposition, out of every eleven grains of carbonic acid gas eight
  grains of oxygen must be liberated, three grains of carbon being
  retained by the plant, and consequently that eight grains of oxygen
  must be restored to the atmosphere, less only by so much as the plant
  itself may absorb. How great, then, must be the production of oxygen
  by an entire tree under favourable circumstances; that is, when animal
  respiration and animal putrefaction present to it an abundant supply
  of carbonic acid on which to act!

465. From the general tenor of these experiments, it appears that,
in fine weather and as long as the plant is healthy, it adds to the
atmosphere an amount of oxygen not only sufficient to compensate for
the quantity it abstracts in the absence of light, but to counterpoise
the effects produced by the respiration of the whole animal kingdom.
The result of one of these experiments will convey some conception
of the amount of oxygen evolved. A quantity of leaves about fifty in
number were enclosed in a jar of air; the surface of all the leaves
taken together was calculated at about three hundred square inches; by
the action of these leaves on the carbonic acid introduced into the
jar, there was added to the air contained in it no less than twenty-six
cubic inches of oxygen. As there was reason to conclude that the
evolution of oxygen, in the circumstances under which this experiment
was performed, was considerably less than it would have been in the
open air, several plants were introduced into the same jar of air in
pretty quick

466. This influence, says Professor Daubeney, is not exerted
exclusively by plants of any particular kind or description. I have
found it alike in the monocotyledonous and dycotyledonous; in such as
thrive in sunshine and those which prefer the shade; in the aquatic
as well as in those of a more complicated organization. How low in
the scale of vegetable life this power extends is not yet exactly
ascertained; the point at which it stops is probably that at which
there ceases to be leaves.

467. From the whole, then, it appears that the functions of the plant
have a strict relation to those of the animal; that the plant, created
to afford subsistence to the animal, derives its nutriment from
principles which the animal rejects as excrementitious, and that the
vegetable and animal kingdoms are so beautifully adjusted, that the
very existence of the plant depends upon its perpetual abstraction of
that, without the removal of which the existence of the animal could
not be maintained.

468. The changes produced upon the blood by the action of respiration
are no less striking and important than those produced upon the air.
The blood contained in the pulmonary artery, venous blood (fig.
140-7.), is of a purple or modena red colour: the moment the air
transmitted to the blood by the bronchial tubes comes into contact with
it, in the rete mirabile (fig. 140-10.), this purple blood is converted
into blood of a bright scarlet colour. Precisely the same change is
produced upon the blood by its contact with the air out of the body.
If a clot of venous blood be introduced into a vessel of air, the clot
speedily passes from a purple to a scarlet colour; and if the air
contained in the vessel be analyzed, it is found that a large portion
of its oxygen has disappeared, and that the oxygen is replaced by a
proportionate quantity of carbonic acid. If the clot be exposed to pure
oxygen, this change takes place more rapidly and to a greater extent;
if to air containing no oxygen, no change of colour takes place.

469. The elements of the blood upon which a portion of the air exerts
its action are carbon and hydrogen. The oxygen of the air unites with
the carbon of the blood and forms carbonic acid, and this gas is
expelled from the system by the action of expiration. The constituent
of the blood which affords carbon to the air would appear to be chiefly
the red particles. The other portion of the oxygen of the air unites
with the hydrogen which is expelled with the carbonic acid in the form
of aqueous vapour. The direct and immediate effect of the action of
respiration upon the blood is then to free it from a quantity of carbon
and hydrogen.

470. Physiologists are not agreed whether the union of the oxygen of
the air with the carbon of the blood takes place in the lungs or in the
system. Some experimentalists maintain that the oxygen which disappears
from the air, and that which is contained in the carbonic acid, are
exactly equivalent, so that no oxygen can be absorbed. According to
this view, which has been clearly shown to be incorrect (459), the
effect of respiration is merely to burn the carbon of the blood, just
as the oxygen of the air burns wood in a common fire, the result
of this combustion being the generation of carbonic acid, which is
expelled from the system the moment it is formed.

471. The theory of Dr. Crawford is essentially the same, which supposes
that venous blood contains a peculiar compound of carbon and hydrogen,
termed _hydro-carbon_, the elements of which unite in the lungs with
the oxygen of the air, forming water with the one and carbonic acid
with the other. Mr. Cooper, for many years past, has taught the same
doctrine in his lectures, without any knowledge of the fact that
Crawford had suggested a similar modification of his theory.

472. It is now established that more oxygen disappears than is
accounted for by the amount of carbonic acid that is generated. The
experiments of Dr. Edwards had already shown this in so decisive
a manner that physiologists almost universally admitted it as an
ascertained fact. The calculations of Mr. Finlaison, to whom the
opinions of physiologists on this point were unknown, have now
determined the precise amount of oxygen (444 _et seq._), and the
probable amount of azote (459) absorbed. By many physiologists it is
supposed that the oxygen retained by the lungs, as long as it remains
in this organ, enters only into a state of loose combination with the
blood; that in this state of loose combination, it is carried from
the lungs into the general system; and that it is only in the system
that the union becomes intimate and complete. According to this view,
the lungs are merely the portal by which the substances employed in
respiration are received and discharged, the essential changes induced
taking place in the system. That it is through the lungs that the
oxygen required by the system is received, is an opinion founded on
experiments no less exact than decisive; it is in accordance with the
most probable theory of the production and distribution of animal heat
(chap. ix.); and the preponderance of evidence in its favour is so
great that, in the present state of our knowledge, it may be considered
as established; but it will appear hereafter that the lungs are by no
means passive in the process, and that, physiologically considered,
they as truly constitute a gland secreting carbonic acid gas as the
liver is a gland secreting bile.

473. Such are the main facts which have been ascertained relative to
respiration, as far as this function is performed by the lungs. But
the liver is a respiratory organ as well as the lungs. It decarbonizes
the blood. It carries on this process to such an extent, that some
physiologists are of opinion that the liver is the chief organ by
which the decarbonization of the blood is effected. The following
considerations show that whatever be the relative amount of its action,
the liver powerfully co-operates with the lungs in the performance of a
respiratory function.

1. The liver, like the lungs, is a receptacle of venous blood; blood
loaded with carbon. The great venous trunk which ramifies through the
lungs is the pulmonary artery, containing all the blood which has
finished its circuit through the system. The great venous trunk which
ramifies through the liver is the vena portæ, containing all the blood
which has finished its circuit through the apparatus of digestion. The
liver is a secreting organ, distinguished from every other secreting
organ by elaborating its peculiar secretion from venous blood. Carbon
is abstracted from the venous blood that flows through the lungs in the
form of carbonic acid; carbon is abstracted from the venous blood that
flows through the liver in the form of bile.

2. All aliment, but more especially vegetable food, contains a large
portion of carbon, more it would appear than the lungs can evolve. The
excess is secreted from the blood by the liver, in the form of resin,
colouring matter, fatty matter, mucus, and the principal constituents
of the bile. All these substances contain a large proportion of carbon.
After accomplishing certain secondary purposes in the process of
digestion, these biliary matters, loaded with carbon, are carried out
of the system together with the non-nutrient portion of the aliment.
In the decarbonizing process performed by the lungs and the liver,
the chief difference would seem, then, to be in the mode in which the
carbon that is separated is carried out of the system. In the lungs
it is evolved, as has been stated, in union with oxygen in the form
of carbonic acid; in the liver, in union with hydrogen in the form of
resin and fatty matter.

3. Accordingly, in tracing the organization of the animal body from the
commencement of the scale, it is found that among the distinct and
special organs that are formed, the liver is one of the very first. It
would appear to be constructed as soon as the economy of the animal
requires a higher degree of respiration than can be effected by the
nearly homogeneous substance of which, very low down in the scale,
the body is composed. Invariably through the whole animal series, the
magnitude of the liver is in the inverse ratio to that of the lungs.
The larger, the more perfectly developed the lungs, the smaller the
liver; and conversely, the larger the liver the smaller and the less
perfectly developed the lungs. This is so uniform that it may be
considered as a law of the animal economy. In the highly organized
warm-blooded animal, with its large lungs, divided into numerous lobes,
and each lobe composed of minute vesicles respiring only air, the
magnitude of the liver compared with that of the body is small. In the
less highly organized animal of the same class, with its smaller and
less perfectly developed lung, respiring partly air and partly water,
the liver increases as the lung diminishes in size. In the reptile
with its little vesicular lung, divided into large cells, the liver is
proportionally of greater magnitude. In the fish which has no lung,
but which respires by the less highly organized gill, and only in the
medium of water, the proportionate size of the liver is still greater;
but in the molluscous animal, in which the lung or the gill is still
less perfectly developed, the bulk of the liver is prodigious.

4. In all animals the quantity of venous blood which is sent to the
liver increases, as that transmitted to the lung diminishes. In the
higher animal the great venous trunk which ramifies through the liver
(the vena portæ) is formed by the veins of the stomach, intestines,
spleen, and pancreas, which are the only organs that transmit their
blood to the liver. In the reptile, besides all these organs, the hind
legs, the pelvis, the tail, the intercostal veins forming the vena
azygos and in some orders of this class, even the kidneys also send
their blood to the liver; but in the fish, in addition to all the
preceding organs, the apparatus of reproduction likewise transmits its
blood to the liver. The very formation of the venous system in the
different classes of animals seems thus to point to the liver as a
compensating and supplementary organ to the lung.

5. The permanent organs in the lower animal are a type of the
transitory forms through which the organs of the higher animal pass
in the progress of their growth. Thus the liver of the human fœtus is
of such a disproportionate size, as to approximate it closely to that
of the fish or of the reptile. After the birth of the human embryo,
respiration is effected in part by the lung; but before birth the lung
is inactive, no air reaches it; it contributes nothing to respiration;
the decarbonizing action of the blood is accomplished, not by the lung,
but by the liver; hence the prodigious bulk of the fœtal liver and its
activity in the secretion of bile, and especially towards the latter
months of pregnancy, when all the organs are greatly advanced in size
and completeness.

6. Pathology confirms the evidence derived from comparative anatomy and
physiology. When the function of the lung is interrupted by disease,
the activity of the liver is increased. In inflammation of the lung
(pneumonia); in the deposition of adventitious matter in the lung
(tubercles), by which the air vesicles are compressed and obliterated,
the lung loses the power of decarbonizing the blood in proportion to
the extent and severity of the disease with which it is affected. In
this case the secretion of bile is increased. In diseases of the heart
the liver is enlarged. In the morbus cæruleus (516) the liver retains
through life its fœtal state of disproportion.

7. In the last place, there is a striking illustration of the
respiratory action of the liver, in the vicarious office which it
performs for the lung, during the heat of summer in cold, and all the
year round in hot climates. In the heat of summer, and more especially
in the intense and constant heat of a warm climate, in consequence of
the rarefaction of the air, respiration by the lung is less active
and efficient than in the winter of the cold climate. During the
exposure of the body to this long-continued heat, there is a tendency
to the accumulation of carbon in the blood. An actual accumulation is
prevented, by an increased activity in the secretion of bile, to which
the liver is stimulated by the heat. In order to obtain the material
for the formation of this unusual quantity of bile, it abstracts
carbon largely from the blood; to this extent it compensates for the
diminished efficiency of the lung, and thus removes through the vena
portæ that superfluous carbon which would otherwise have been excreted
through the pulmonary artery.

474. Taking life in its most extended sense, as comprehending both the
circles it includes, the organic and the animal (vol. i. chap. 2), it
may be said to have three great centres, of which two relate to the
organic, and the third to the animal life (vol. i. chap. 2). The two
centres which relate to the organic life are the systems of respiration
and circulation; the third, which relates to the animal life, is the
nervous system. Of the organic life, the lungs and the heart are the
primary seats; of the animal, the brain and the spinal cord. Between
each the bond of union is so close, that any lesion of the one
influences the other, and neither can exist without the support of
all. They form a triple chain, the breaking of a single link of which
destroys the whole.

475. But of these three great centres of life, upon which all the
other vital phenomena depend, the most essential is respiration; hence,
to consider the relation of this function to the others, is to take the
most comprehensive view of the uses which respiration serves in the
economy.

476. The first and most important use of the function of respiration
is to maintain the action of the organs of the animal life. It has
been shown (vol. i. chap. 2) that the organic is subservient to the
animal life, and that to build up the apparatus of the latter, and to
maintain it in a condition fit for performing its functions, is the
final end of the former. The direct and the immediate effect of the
suspension of respiration is the abolition of both functions of the
animal life—sensation and voluntary motion. If a ligature be placed
around the trachea of a living animal so as completely to exclude all
access of air to the lungs, and if the carotid artery be then opened,
and the blood allowed to flow, the bright scarlet-coloured blood
contained in the artery is observed gradually to change to a purple
hue. The exact point of time at which this change begins may be noted.
It is seen to assume a darker tinge at the end of half a minute; at
the end of one minute its colour is still darker, and at the end of
one minute and a half, or at most two minutes (426), it is no longer
possible to distinguish it from venous blood. As soon as this change of
colour begins to be visible the animal becomes uneasy; his agitation
increases as the colour deepens; and when it becomes completely dark,
that instant the animal falls down insensible. If in this state of
insensibility air be readmitted to the lungs, the dark colour of the
blood rapidly changes to a bright scarlet, and instantly sensation
and consciousness return. But if, on the contrary, the exclusion of
the air be continued for the space of three minutes from the first
closing of the trachea, the animal not only remains to all appearance
dead, but in general no means are capable of recovering him from the
state of insensibility; and if the exclusion of the air be protracted
to four minutes, apparent passes into real death, and recovery is no
longer possible. It follows that one of the conditions essential to the
exercise of the function of the brain is, that this organ receive a due
supply of arterial blood.

477. The second use of the function of respiration is to afford
blood capable of maintaining the muscles in a condition fit for the
performance of their peculiar office, that of contractility. The
closure of the trachea not only abolishes sensation, but the power
of voluntary motion: sensation and motion are lost at once: on the
re-admission of air to the lungs, both functions are regained at
once: it follows that the process of respiration is as essential
to the action of the muscle as to that of the brain. “By arterial
blood,” says Young, “the muscles are furnished with a store of that
unknown principle by which they are rendered capable of contracting.”
“The oxygen absorbed by the blood,” says Spalanzani, “unites with
the muscular fibres and endows them with their contractility.” It
is more correct to say, respiration takes carbon from the blood and
gives it oxygen, and by this means endows the blood with the power of
maintaining the contractility of the muscular fibre.

478. But respiration is as essential to the action of the organs of
the organic life as to those of the animal. In a short time after
the respiration ceases, the circulation stops. When the blood is no
longer changed in the lungs, it soon loses all power of motion in the
system; because venous blood paralyses the muscular fibres of the heart
as of the arm. When the left ventricle of the heart sends out venous
blood to the system, it propels it into its own nutrient arteries,
as well as into the other arteries of the body; into the coronary
arteries, as well as into the other branches of the aorta; the heart
loses its contractility, for the same reason as every muscle under
the like privation; because venous instead of arterial blood flows in
its nutrient arteries; and the circulation stops when the heart is no
longer contractile, because the engine is destroyed that works the
current.

479. Venous blood consists of chyle, the nutritive fluid formed from
the aliment; of lymph, a fluid composed of organic particles, which
having already formed an actual part of the solid structures of the
body, are now returning to the lungs to receive a higher elaboration;
and of blood which, having completed its circuit through the system,
and there given off its nutrient and received excrementitious matter,
is now returning to the lungs for depuration and renovation. These
commingled fluids, on parting in the lungs with carbonic acid and
water, and on receiving in return oxygen and azote, are converted into
arterial blood; that is, blood more coagulable than venous, and richer
in albumen, fibrin, and red particles, the proximate organic principles
of all animal structures. The rich and pure stream thus formed is sent
out to the various tissues and organs, from which, as it flows to
them, they abstract the materials adapted to their own peculiar form,
composition, and vital endowments. By the reception of these materials
the organs are rendered capable of performing the vital actions which
it is their office to accomplish. And thus the processes of digestion,
absorption, secretion, nutrition, formation, reproduction, all the
processes included in the great organic circle, no less than muscular
action and nervous energy, depend on receiving a due supply of arterial
blood. All these actions, like the faculties of the animal life, cease
totally and for ever in a few minutes after the formation of this vital
fluid has been stopped by the suspension of respiration.

480. In the last place, the depurating process effected by respiration
is necessary to prevent the decomposition of the blood, and eventually
that of the body. The first step in the spontaneous decomposition
of animal matter consists in the loss of a portion of its carbon,
which, uniting with the oxygen of the atmosphere, forms carbonic
acid; precisely the same thing that takes place in the process of
respiration. The bodies of all animals, of worms, insects, fishes,
birds, and mammalia, deoxidate the air and load it with carbonic
acid after death, some of them nearly as much as during life; and
this before any visible marks of decomposition can be traced. It is
probable that the cause which more immediately operates in preventing
the decomposition of the body is the abstraction of a part of the
carbon of the blood; that were these carbonaceous particles allowed
to accumulate, they would produce a tendency to decomposition, which
would terminate in complete disorganization; and consequently, that one
main object of the process of respiration is to afford blood not only
capable of nourishing and sustaining the organs, but of maintaining
their integrity, by removing noxious matter, the presence of which
would subvert their composition and lead to their entire decomposition.

481. The ultimate object of respiration, then, is to prepare and to
preserve in a state of purity a fluid capable of affording to all the
parts of the body the materials necessary to maintain their vital
endowments. By the exhalation of oxygen and water, and the absorption
of carbon, under the agency of light, the plant elaborates such a fluid
from its nutritive sap, and out of this elaborated sap forms terniary
combinations, the organic elements of all vegetable solids. By the
absorption of oxygen and azote, and the exhalation of carbonic acid
and water, probably under the influence of electricity, conducted and
regulated by the nervous system, the animal elaborates such a fluid
from its aliment, and out of this elaborated fluid forms quaternary
combinations, albumen, and fibrin, the organic elements of all animal
solids.




CHAPTER IX.

 Of the temperature of living bodies—Temperature of plants—Power
 of plants to resist cold and endure heat—Power of generating
 heat—Temperature of animals—Warm-blooded and cold-blooded
 animals—Temperature of the higher animals—Temperature of the different
 parts of the animal body—Temperature of the human body—Power of
 maintaining that temperature at a fixed point whether in intense
 cold or intense heat—Experiments which prove that this power is a
 vital power—Evidence that the power of generating heat is connected
 with the function of respiration—Analogy between respiration and
 combustion—Phenomena connected with the functions of the animal body,
 which prove that its power of generating heat is proportionate to
 the extent of its respiration—Theory of the production of animal
 heat—Influence of the nervous system in maintaining and regulating the
 process—Means by which cold is generated, and the temperature of the
 body kept at its own natural standard during exposure to an elevated
 temperature.


482. Closely connected with the function of respiration, is the power
which all living beings possess of resisting within a certain range
the influence of external temperature. The plant is warmer than the
surrounding air in winter, and colder in summer. A thermometer placed
at the bottom of a hole bored into the centre of a living tree,
precaution being taken to keep off as much as possible all external
influence either of heat or cold, does not rise and fall according to
the changes of external temperature; but rises when the external air
is cold, and falls when it is warm. Thus, in a cold day in spring, the
wind being north, at six o’clock in the evening, the temperature of
the external air being 47°, that of a tree was 55°. On another cold
day in the same month, there being snow and hail, and the wind in the
north-east, at six o’clock in the evening, the external temperature
being 39°, that of the tree was 45°. On the contrary, in one
experiment, when the temperature of the air was 57½°, that of the tree
was only 55°; and when the temperature of the air was 62°, that of the
tree was 56°.

483. These experiments afford an explanation of circumstances familiar
to common observation. Every one has noticed that the snow which
falls on grass and trees melts rapidly, while that on the adjoining
gravel walks often remains a long time unthawed. Moist dead sticks are
constantly found frozen hard in the same garden with tender growing
twigs, which are not in the least degree affected by the frost. Every
winter in our own climate tender herbaceous plants resist degrees of
cold which freeze large bodies of water.

484. But the colder, and the warmer the climate, the more strikingly
does the plant exemplify the power with which it is endowed of
resisting external temperature. In the northern parts of America the
temperature is often 50° below zero; yet, though exposed to this
intense degree of cold, the spruce fir, the birch, the juniper, &c.
preserve their vitality uninjured. From numerous experiments which
have been performed expressly with a view to ascertain this point, it
is found that a plant which has been once frozen is invariably dead
when thawed. It is also proved by direct experiment, that if the sap
be removed from its proper vessels, it freezes at 32°, the ordinary
freezing point. In the northern parts of America, then, the plant must
preserve in its living vessels its sap from freezing, when exposed to
a temperature of 50° below zero; which sap out of these vessels would
congeal at the ordinary freezing point; that is, the plant of this
climate is endowed with the power of resisting a degree of cold ranging
from the ordinary freezing point to 50° below zero; a property which
can be referred only to a vital power, by the operation of which the
plant generates within itself a degree of heat sufficient to counteract
the external cold.

485. The opposite faculty of resisting the influence of external heat
is exemplified by the trees and shrubs of tropical climates, often
surrounded by a temperature of 104°, which they resist just as the
plant of the northern clime resists the intense degrees of cold to
which it is exposed.

486. That the plant is endowed with the power of generating heat is
demonstrated by the phenomena which attend the performance of some of
its vital processes, such as those of germination and flowering. During
the germination of barley, the thermometer was observed to rise in the
course of one night to 102°. The bulb of a thermometer applied to the
surface of the spadix of an arum maculatum, indicated a temperature
7° higher than that of the external air; but in an arum cordifolium,
at the Isle of France, a thermometer placed in the centre of five
spadixes stood at 111°; and in the centre of twelve at 121°, though the
temperature of the external air was only 66°.

487. Animals indicate in a still more striking degree the power of
generating heat. The lower the animal in the scale of organization,
indeed, the nearer it approaches to the plant in the comparative
feebleness of this function. The heat of worms, insects, crustacea,
mollusca, fishes, and amphibia, is commonly only two or three degrees
above that of the medium in which they are immersed. Absolutely
colder than the higher animals, they are at the same time incapable
of resisting any considerable changes in the temperature of the
surrounding medium, whether from heat to cold or from cold to heat.
The higher animals, on the contrary, maintain their heat steadily at a
fixed point, or very nearly at a fixed point, however the temperature
of the surrounding medium may change. Hence animals are divided
into two great classes, the cold-blooded and the warm-blooded. The
temperature of the cold-blooded is lower than that of the warm-blooded,
and it varies with the heat of the surrounding medium; the temperature
of the warm-blooded is higher than that of the cold-blooded, and
it remains nearly at the same fixed point, however the heat of the
surrounding medium may change.

488. The temperature natural to the higher animals differs somewhat
according to their class. The temperature of the bird is the highest,
and is pretty uniformly about 103° or 104°; that of the mammiferous
quadruped is 100 or 101°; that of the human species is 97° or 98°.

489. The temperature of the animal body is not precisely the same in
every part of it. The ball of the thermometer introduced within the
rectum of the dog stood at 100½; within the substance of the liver at
100¾; within the right ventricle of the heart at 101°, and within the
cavity of the stomach at 101°. In the brain of the lamb it stood at
104°; in the rectum at 105°; in the right ventricle of the heart, and
in the substance of the liver and of the lungs, at 106°; and in the
left ventricle of the heart at 107°.

490. The temperature natural to the human body is 98°. When the human
body is surrounded by an atmosphere at the temperature of 30°, it
must have its heat rapidly extracted by the cold medium; yet the
temperature of the body, however long it remain exposed to such a
degree of cold, does not sink, but keeps steadily at its own standard.
But animals which inhabit the polar regions are often exposed to a cold
40° below zero. The temperature of Melville Island is so low during
five months of the year that mercury congeals, and the temperature is
sometimes 46° below zero; yet the musk oxen, the rein deer, the white
hares, the polar foxes, and the white bears which abound in it maintain
their temperature steadily at their own natural standard.

491. The power which the higher animal possesses of resisting heat
is still more remarkable than its power of resisting cold. On taking
rabbits and guinea-pigs from the temperature of 50°, and introducing
them very rapidly to the temperature of 90°; it was found that the
animals acquired only two or three degrees of heat. How different
the result when the cold-blooded animal is subjected to the same
experiment! The temperature of the surrounding air being 45°, a
thermometer introduced into the stomach of a frog rose to 49°. The
frog being then put into an atmosphere made warm by heated water, and
allowed to stay there twenty minutes, the thermometer on being now
introduced into the stomach rose to 64°.

492. But the human body may be actually placed in a temperature of 60°
above that of boiling water, not only without sustaining the slightest
injury, but without having its own temperature raised excepting by two
or three degrees. The attention of physiologists was first directed
to this curious fact by some remarkable circumstances related by the
servants of a baker at Rochefoucault, who were in the habit of going
into the heated ovens in order to prepare them for the reception of
the loaves. In performing this service, the young women were sometimes
exposed to a temperature as high as 278°. It was stated that they could
endure this intense heat for twelve minutes, without any material
inconvenience, provided they were careful not to touch the surface
of the oven. Subsequently Drs. Fordyce, Blagden, and others, with a
view to ascertain the exact facts, entered a chamber, heated to a
temperature much above that of boiling water, and some of the phenomena
observed during these experiments are highly curious.

493. In the first room entered by these experimentalists, the highest
thermometer varied from 132° to 130°; the lowest stood at 119°. Dr.
Fordyce having undressed in an adjoining cold chamber, went into the
heat of 119°; in half a minute the water poured down in streams over
his whole body, so as to keep that part of the floor where he stood
constantly wet. Having remained here fifteen minutes, he went into the
heat of 130°; at this time the heat of his body was 100°, and his pulse
beat 126 times in a minute. While Dr. Fordyce stood in this situation a
Florence flask was brought in by his order, filled with water heated
to 100°, and a dry cloth with which he wiped the surface of the flask
quite dry; but it immediately became wet again, and streams of water
poured down its sides, which continued till the heat of the water
within had risen to 122°, when Dr. Fordyce went out of the room, after
having remained fifteen minutes in a heat of 130°: just before he left
the room his pulse made 129 beats in a minute; but the heat under his
tongue and in his hand did not exceed 100°.

494. In a subsequent experiment the chamber was entered when the
thermometer stood above 211°. The air heated to this degree, says
Dr. Blagden, felt unpleasantly hot; but was very bearable. Our most
uneasy feeling was a sense of scorching in the face and legs; our legs
particularly suffered very much, by being exposed more fully than any
other part to the body of the stove, heated red hot by the fire within.
Our respiration was not at all affected; it became neither quick nor
laborious; the only difference was a want of that refreshing sensation
which accompanies a full inspiration of cool air. But the most striking
effects proceeded from our power of preserving our natural temperature.
Being now in a situation in which our bodies bore a very different
relation to the surrounding atmosphere from that to which we had been
accustomed, every moment presented a new phenomenon. Whenever we
breathed on a thermometer, the quicksilver sank several degrees. Every
expiration, particularly if made with any degree of violence, gave a
very pleasant impression of coolness to our nostrils, scorched before
by the hot air rushing against them whenever we inspired. In the same
manner our now cold breath agreeably cooled our fingers whenever it
reached them. Upon touching my side, it felt cold like a corpse; and
yet the actual heat of my body, tried under my tongue, and by applying
closely the thermometer to my skin, was 98°, about a degree higher than
its ordinary temperature. When the heat of the air began to approach
the highest degree which this apparatus was capable of producing, our
bodies in the room prevented it from rising any higher; and when it
had been previously raised above that point, invariably sunk it. Every
experiment furnished proofs of this. Mr. Banks and Dr. Solander each
found that his single body was sufficient to sink the quicksilver very
fast, when the room was brought nearly to its maximum of heat.

495. In a third series of experiments the temperature of the chamber
was raised to the 260th degree. At this time, continues Dr. Blagden,
I went into the room, with the addition to my common clothes of a
pair of thick worsted stockings drawn over my shoes, and reaching
some way above my knees. I also put on a pair of gloves, and held a
cloth constantly between my face and the stove (necessary precautions
against the scorching of the red-hot iron). I remained eight minutes in
this situation, frequently walking about to all the different parts of
the room, but standing still most of the time in the coolest spot near
the lowest thermometer. The air felt very hot, but by no means so as to
give pain. I had no doubt of being able to bear a much greater heat;
and all who went into the room were of the same opinion. I sweated, but
not very profusely. For seven minutes my breathing remained perfectly
good; but after that time, I began to feel an oppression in my lungs,
attended with a sense of anxiety; which gradually increasing for the
space of a minute, I thought it most prudent to end the experiment.
My pulse, counted as soon as I came into the cool air, for the uneasy
feeling rendered me incapable of examining it in the room, beat at
the rate of 144 pulsations in a minute, which is more than double its
ordinary quickness. In the course of this experiment, and others of
the same kind by several of the gentlemen present, some circumstances
occurred to us which had not been remarked before. The heat, as might
have been expected, felt most intense when we were in motion; and on
the same principle, a blast of the heated air from a pair of bellows
was scarcely to be borne: the sensation in both these cases exactly
resembled that felt in our nostrils on inspiration. It was observed
that our breath did not feel cool to our fingers unless held very
near the mouth; at a distance the cooling power of the breath did
not sufficiently compensate the effect of putting the air in motion,
especially when we breathed with force.

496. On going undressed into the room, the impression of the air was
much more disagreeable than before; but in five or six minutes, a
profuse sweat broke out, which instantly relieved me. During all the
experiments of this day, whenever I tried the heat of my body, the
thermometer always came very nearly to the same point (the ordinary
standard), not even one degree of difference, as in our former
experiments.

497. To prove that there was no fallacy in the degree of heat shown
by the thermometer, but that the air which we breathed was capable of
producing all the well-known effects of such heat on inanimate matter,
we put some eggs and a beef steak upon a tin frame, placed near the
standard thermometer, and farther distant from the stove than the wall.
In about thirty minutes the eggs were taken out roasted quite hard. In
about forty-seven minutes the steak was not only dressed, but almost
dry. Another beef steak was rather overdone in thirty-three minutes. In
the evening when the heat was still greater, we blew upon a third steak
with the bellows, which produced a visible change on its surface, and
hastened its dressing; the greatest part of it was pretty well done in
thirteen minutes.

498. The human body, then, may be exposed to a temperature 50° below
zero, without having its own heat appreciably diminished; it may be
exposed to a temperature 60° above that of boiling water, without
having its own heat increased beyond two or three degrees; or, as
appears from experiments subsequently performed expressly to ascertain
this point, from three to five degrees. In the former case, the body
must generate a degree of heat sufficient to compensate the great
quantity of caloric which is every moment abstracted from it by the
intensely-cold surrounding medium. In the latter case it must generate
a degree of cold sufficient to counteract the great quantity of
caloric which is every moment communicated to it by the intensely-hot
surrounding medium.

499. Powers so wonderful and so opposite appeared to the physiologists
of former times to be involved in such profound mystery, that they
did not even attempt to investigate their nature, or trace their mode
of operation; but satisfied themselves with referring them to some
innate quality of the body, and with considering them as essential
attributes of life. And difficulties connected with the subject still
remain, which the present state of knowledge does not permit us wholly
to surmount; but we are able at least to refer these powers to their
proper seat, and to trace some steps of the processes by which they
produce results so wonderful and beautiful.

500. It is certain that whatever be the ultimate physical processes by
which the generation of heat and the production of cold are effected
in the animal body, the phenomena are dependent on the condition of
life. No such phenomena take place excepting in living bodies. This is
illustrated in a striking manner by a series of experiments performed
by Mr. Hunter. A part of the living human body was immersed in water
gradually made warmer and warmer from 100° to 118°; precisely the same
part of the body, dead, was immersed in the same water, and both parts,
the living and the dead, were continued in this heat for some minutes.
The dead part raised the thermometer to 114°; the living part raised it
to no higher than 102¼°. On applying the thermometer to the sides of
the living part, the quicksilver immediately fell from 118° to 104°;
on applying it close to the dead part, the thermometer did not fall
above a single degree; the living part actually produced a cold space
of water around it. Hence in bathing in water, whether colder or warmer
than the heat of the body, the water soon acquires the same temperature
with that of the body; and, consequently, in a large bath the patient
should move from place to place, and in a small one there should be a
constant succession of water of the intended heat.

501. A fresh, that is, a living egg was put into cold water at about
zero, frozen, and then allowed to thaw. By this process its vitality
was destroyed, and consequently its power of resisting cold and heat
lost. This thawed egg was next put into a cold mixture with an egg
newly laid: the time required for freezing the fresh egg was seven
minutes and a half longer than that required for freezing the thawed
egg.

502. A new-laid egg was put into a cold atmosphere fluctuating between
17° and 15°; it took about half an hour to freeze; but when thawed and
put into an atmosphere at 25° (10° warmer), it froze in half the time.

503. A fresh egg and one that had been frozen and thawed were put into
a cold mixture at 15°; the thawed one soon came to 32°, and began
to swell and congeal; the fresh one sunk to 29½, and in twenty-five
minutes after the dead one, it rose to 32°, and began to swell and
freeze.

504. The result of this experiment upon the fresh egg was similar to
that of analogous experiments made upon the frog, eel, snail, &c. where
life allowed the heat to be diminished 2° or 3° below the freezing
point, and then resisted all further decrease; but the powers of life
having been expended by this exertion, the parts then froze like any
other dead animal matter.

505. The heat of the bird is increased somewhat when it is prepared
for incubation. Some eggs were taken from under a sitting hen whose
temperature was 104°, at the time when the chick was about three-parts
formed. A hole was broken in the shell and the bulb of a thermometer
introduced; the quicksilver rose to 99½°; but in some eggs that were
addled it was proved that their heat was not so high by two degrees, so
that the life of the living egg assisted to support its own temperature.

506. These facts sufficiently show the dependence of the faculty of
generating heat and of producing cold on the powers of life. But the
processes by which, under the agency and control of the vital powers,
these different results are effected, are various, and even opposite.

507. The power of generating heat is connected in the closest manner
with the function of respiration, and is directly dependent upon it.
The evidence of this is indubitable. For—

508. i. Respiration is combustion, and, like ordinary combustion, is
attended with the production of heat. In ordinary combustion oxygen
disappears, and a new compound is formed, consisting of oxygen combined
with the combustible matter; that is, an oxidized body is generated. On
burning a piece of iron wire in oxygen, the oxygen disappears, and the
iron increases in weight. The oxygen combines with the iron, forming
a new product, oxide of iron, and the weight of this new substance
is found on examination to be exactly equal to the weight of the
wire originally employed, added to the quantity of oxygen which has
disappeared.

509. It is precisely the same in respiration. In this process oxygen
combines with combustible matter, carbon: the oxygen disappears, and a
new body, carbonic acid, is generated.

510. ii. One phenomenon which invariably accompanies the combination of
oxygen with combustible matter is the extrication of heat. Whenever a
substance passes from a rarer into a denser state; when, for example, a
gas is converted into a liquid or solid, or when a liquid solidifies,
heat is evolved; because, according to the ordinary theory of
combustion, the denser substance has a less capacity for caloric than
the rarer, and consequently in passing from a rare into a dense state,
a quantity of caloric previously combined or latent within it is set
free. The combined or latent caloric contained in a body is termed its
specific caloric; the caloric which is evolved on its change of state
is named free or sensible caloric.

511. The combination of oxygen with carbon, as in the combination
of oxygen with combustible matter in every other instance, must
be attended with the evolution of heat. Though the product of the
combustion, in the present case, be a gaseous body, carbonic acid,
still, according to the ordinary theory of combustion, carbonic
acid has less specific caloric, or less capacity for caloric, than
oxygen; and therefore in combining with carbon, a portion of its
specific caloric becomes free or sensible, that is, heat is evolved.
But whatever theory of combustion be adopted, the fact is certain,
that whenever oxygen combines with carbon to form carbonic acid,
heat is evolved; not only in the rapid union which takes place in
ordinary combustion, but also in the slow combination which occurs in
fermentation, putrefaction, and germination; in the latter of which
processes, as in the malting of barley, the temperature rises as high
as 10°. The union of oxygen with carbon in the lungs during respiration
must therefore necessarily produce heat, just as it does in a charcoal
fire, or in any other natural process in which this combination takes
place.

512. iii. Numerous phenomena connected with the animal body show that
its temperature is in strict proportion to the quantity of oxygen which
is consumed in respiration, and to the quantity of carbonic acid which
is formed by the union of oxygen and carbon during the process.

513. In all animals whose respiratory organs are so constructed, that
the consumption of oxygen and the consequent generation of carbonic
acid is minute in quantity, the production of heat is proportionably
small. It has been shown (337 _et seq._), that in almost the entire
class of the invertebrata, the respiratory apparatus is comparatively
minute and imperfect; accordingly, in these animals the power of
generating heat is at the minimum. In the fish, though the respiratory
apparatus be large, and though all the blood of the body circulate
through it (345 _et seq._), yet only a small quantity of air is brought
into contact with the respiratory organ, merely the air contained in
water. In the reptile, though it possess a true and proper lung, and
respire air, yet only one half of the blood of its body circulates
through the comparatively small, imperfectly divided, and simply
constructed air bag, which constitutes its respiratory organ (354).
Hence, the striking contrast exhibited between the temperature of these
cold-blooded creatures and that of the mammiferous quadruped, whose
lung, comparatively large, and composed of innumerable minute and
closely-set air vesicles (fig. CXXXIV. and CXXXV.), presents to the air
an immense extent of surface (370), and the whole mass of whose blood
incessantly traversing this surface, comes at every point into contact
with the air (399).

514. In the various tribes of warm-blooded animals, the elevation
and uniformity of the temperature is strictly proportionate to the
comparative magnitude of the lungs; to the complexity of their
structure; to the minuteness and number of the air vesicles; and,
consequently, to the quantity of oxygen consumed, and of carbonic acid
generated.

515. In all animals with red blood there is a strict relation between
the temperature of the body and the lightness or depth of the colour
of the blood; invariably the deeper the colour, the higher the
temperature. Thus, the blood of the fish and of the reptile is of a
light, and that of the bird of an intense red colour. It has been shown
(229) that the lightness or deepness of the colour of the blood depends
on the quantity of red particles which it contains, and the chemical
action between the air and the blood is carried on chiefly through the
medium of the red particles.

516. Even in the same animal, the temperature differs at different
times, according to the energy with which the process of respiration
is carried on. When the circulation of the blood is sluggish and the
respiration slow and feeble, the quantity of oxygen consumed is small,
and the temperature low; when, on the contrary, the circulation is
rapid, and the respiration energetic, the quantity of oxygen consumed
is large, and the temperature proportionably high. Whatever diminishes
the quantity of air that flows to the lungs, and the quantity of blood
that circulates through them, diminishes the temperature. Malformation
of the heart, in consequence of which a quantity of blood is sent to
the system without passing through the lungs, as in the individuals
termed Ceruleans: disease of the lungs, by which the access of air
to the air vesicles is obstructed, as in asthma, are morbid states
invariably attended with a diminution of the temperature.

517. When a warm-blooded animal is placed in an elevated temperature,
its consumption of oxygen is comparatively small; when it is placed
in a cold atmosphere, and the production of a large quantity of heat
is necessary to maintain its temperature at its natural standard, its
consumption of oxygen is proportionably large; accordingly, it is
established by direct experiment that the same animal consumes a much
larger quantity of oxygen in winter than in summer.

518. Due allowance being made for the difference in their bulk, young
animals consume less oxygen than adults; and they have a less power of
generating heat. Different species of young animals differ from each
other in their power of generating heat, and the closest relation is
observable between the difference in their power of consuming oxygen
and that of generating heat. Puppies and kittens require so small
a quantity of oxygen for supporting life, that they may be wholly
deprived of this gas for twenty minutes, without material injury, while
adult animals of the same species perish when deprived of it only for
four minutes. As long as these young creatures retain the power of
sustaining life for so protracted a period without oxygen, they are
wholly incapable of maintaining their own temperature; on free exposure
to air, even in summer, the heat of their body sinks rapidly, and if
this exposure be continued long, they perish of cold. In like manner,
young sparrows and other birds which are naked when hatched, consume
little oxygen, and are incapable of maintaining their temperature; but
can support life when deprived of oxygen much longer than adult birds
of the same species; while young partridges which are able to retain
their own temperature at the period of quitting the shell, die when
deprived of oxygen as rapidly as the adult bird.

519. The state of hybernation illustrates in the same striking manner
the relation between respiration and the generation of heat. One of the
most remarkable phenomena connected with this curious state, is the
reduction, sometimes even the apparent suspension, of respiration; and
in all cases of hybernation, the respiratory function is performed in
a feeble manner, and only at distant intervals. Exactly in proportion
to the diminution of the respiration, is the reduction of the power of
generating heat; so that when the state of hybernation is established,
the temperature of the external parts of the body sinks nearly to that
of the surrounding medium; while the internal parts, the blood, and
the vital organs are only a degree or two higher. In experiments made
to reduce an hybernating animal to a torpid state by cold artificially
produced, De Saissy found that he could not bring on the state of
hybernation by the reduction of temperature alone, without also
constraining the respiration.

520. These and other analogous facts abundantly establish the relation
between the function of respiration and that of calorification, and
lead to the general conclusion that the generation of animal heat is
in the direct ratio of the quantity of air and blood which are brought
into contact, and which act on each other in a given time. Yet an
attempt has recently been made by an ingenious physiologist[3] to
disturb this induction, and to show that the production of animal heat
is not in the direct ratio of the quantity of oxygen inhaled, but in
the inverse ratio of the quantity of blood exposed to this principle.
This position is maintained on the following grounds:—

  [3] An Experimental Inquiry into the Laws which regulate the Phenomena
  of Organic and Animal Life. By George Calvert Holland, M.D. and more
  complete than the expirations; it is a state of continual sighing. In
  like manner, in certain diseases, such as asthma, the inspirations
  greatly preponderate both in frequency and energy over the
  expirations. In such conditions of the system the blood accumulates
  in preternatural quantity in all the internal organs; but more
  especially in the lungs; and two consequences follow: first, there
  is a remarkable diminution in the energy of all the vital actions;
  and secondly there is a proportionate diminution in the production of
  animal heat.

521. Inspiration favours the flow of blood to the lungs; expiration
retards it: consequently, if from any causes the inspirations
preponderate in number and proportion over the expirations, a greater
quantity of blood than usual will be accumulated in the lungs. There
are conditions of the system in which this preponderance of the
inspirations actually takes place; when the mind is under the influence
of certain emotions, for example, as when it is depressed by anxiety
and fear. In this state the inspirations are more frequent

522. On the contrary, as it is the effect of inspiration to facilitate
the motion of the blood through the lungs, so it is the effect of
expiration to retard it; hence, when the expirations preponderate the
opposite state of the system is induced; all the vital actions are
performed with increased energy; the heart beats with unusual vigor;
the pulse becomes quick and strong; a larger quantity of blood is
determined to the surface of the body, and this excited state of the
system is always attended with an augmentation of the temperature.

523. As in the first state there is a greater and in the second a
smaller quantity of blood than natural contained in the lungs, the
inference deduced by Dr. Holland is, that the production of animal heat
is in the inverse ratio of the quantity of blood exposed to oxygen. But
this inference is neither logical nor sound.

524. If, as a comparison of all the phenomena of respiration exhibited
throughout the entire range of the animal kingdom, shows the production
of animal heat to be in the direct ratio of the quantities of air and
blood which are brought into contact, and which re-act on each other,
every phenomenon of respiration must be in harmony with this law, and,
accordingly, when really understood, it is found to be so.

525. Inspiration, by the dilatation of the thorax, and consequently of
the lungs incident to that action, is favorable to the flow of blood to
the lungs. But it is only a certain degree of dilatation of the lungs
that is favorable to the flow of blood through them (407 _et seq._).
If the dilatation be carried beyond a certain point, the quantity of
blood transmitted through the pulmonary tissue is diminished (406);
if the dilatation be carried farther, the transmission of the blood
may be wholly stopped (417). The quantity of the blood which flows to
the lungs, and the quantity which circulates through them, are not
then identical. So large a quantity may flow to them as to impede or
retard or wholly stop the pulmonary circulation. In proportion to the
accumulation of blood in the lung must necessarily be the distension of
the pulmonary tissue; in that proportion the lung must be approximated
to its condition in the experiment in which it was distended with water
(417), when it did not transmit a single particle of blood. Further,
in proportion to the preternatural distension of the pulmonary tissue
with blood must be the exclusion of air from the air vesicles for the
lungs can contain only a certain quantity of blood and air (418.3), so
that the blood can preponderate only by the exclusion of the air.

526. In those states of the system, then, in which the preponderance
of the inspirations induces a preternatural accumulation of blood in
the lungs, the production of animal heat is diminished for a two-fold
reason; first, because the distension of the pulmonary tissue with
blood retards the pulmonary circulation, and proportionally lessens
the quantity of blood which is brought into contact with the air;
and, secondly, because the distended blood-vessels compress the air
vesicles, and so diminish the quantity of air which is brought into
contact with the blood.

527. It follows that the diminution of temperature which takes place in
this condition of the system is not because the production of animal
heat is in the inverse ratio of the quantity of blood which is exposed
to oxygen; but because from a two-fold operation there is a diminution
of the quantity of blood and of oxygen which are brought into contact.

528. The reason is equally obvious why there is an increase of
the temperature in those conditions of the system in which the
expirations preponderate over the inspirations. Expiration, it is
true, somewhat retards the circulation of the blood through the lungs,
but the preponderance of this respiratory action does not raise the
temperature by the retardation of the flow of blood through the lungs,
and the consequent diminution of the quantity transmitted in a given
time; for though expiration somewhat retards the circulation of the
blood through the branches of the pulmonary artery, it promotes its
circulation through the branches of the pulmonary veins (fig. CXL. 10).
It is indeed by the action of expiration that the aërated blood is
transmitted from the lungs to the left heart to be sent out renovated
to the system. Expiration has no influence whatever over the aëration
of the blood. Before the action of expiration takes place, the blood
is already aërated. The office of expiration is to remove from the
system the air which has served for respiration, and to transmit to the
system the blood which has been subjected to respiration. Consequently,
in those states of the system in which the expirations preponderate,
the temperature is increased, not because the expiratory actions, by
lessening the quantity of blood in the lungs, diminish the quantity
exposed to oxygen, but because they transmit to the system oxygenated
blood as rapidly as it is formed, that is, blood which either produces
animal heat in the act of its formation, or which generates it as it
flows through the system.

529. These conditions establish the conclusion deduced, as has
been stated, from the comparison of the phenomena of respiration
exhibited throughout the entire range of the animal kingdom. But if
the production of animal heat be really the result of combustion,
if that combustion take place in the lung, and if the lung be thus
the focus whence the heat radiates to every other part of the body,
why is not the heat of this organ and of the parts in its immediate
neighbourhood higher than the temperature of the rest of the body?
Some of the internal organs are indeed a degree or two hotter than the
general mass of the circulating blood (469), and among these the lung
is admitted to rank perhaps the very highest. But how can a quantity of
caloric sufficient to maintain the heat of the body in a temperature
of forty degrees below zero radiate from an organ the temperature of
which is only two or three degrees above that of the body itself? It
is estimated that, in every minute, during the calm respiration of a
healthy man of ordinary stature, 26·6 cubic inches of carbonic acid,
at the temperature of 50° Fahr. are emitted, and that an equal volume
of oxygen is withdrawn from the atmosphere. From these data it is
calculated that, in an interval of twenty-four hours, not less than
eleven ounces of carbon are consumed. Why is the lung, the seat of this
combustion, not only not greatly warmer than any other organ; but why
is it not even consumed by the fire which is thus incessantly burning
within it?

530. It has been shown (468 and 469) that when the carbon of the
blood unites in the lung with the oxygen of the air, the nature of
the blood, in consequence of the abstraction of carbon, undergoes an
essential change, passing from venous into arterial. By an elaborate
series of experiments, conducted with extraordinary care and skill,
it would appear that arterial has a greater capacity for caloric than
venous blood, in the proportion of 114·5 to 100. In consequence of this
difference in the constitution of the two kinds of blood, the heat
generated in the lung by the combustion of carbon, instead of being
evolved or becoming sensible (510. ii.), and so raising the temperature
of the organ, goes to satisfy the increased capacity for caloric of
arterial blood, is spent, not in rendering the fluid sensibly warmer,
but in augmenting its specific caloric (510. ii.). Arterial blood is
not increased in temperature,[4] but with its absolute quantity of
caloric augmented, flows from the lung to the left heart (fig. CXL.
10), and thence to the system (fig. CXL. 6). In the system, in every
organ, at every point of the component tissue of every organ and at
every moment of time, the blood repasses from the arterial to the
venous state: by this transition its capacity for heat is diminished;
the venous cannot retain in it the same quantity of caloric as the
arterial blood, consequently a portion of caloric is extricated; that
which was latent becomes sensible, and caloric being set free the
temperature is raised. In this process the lung is not burnt, it is
only rendered just sensibly warmer than any other part of the body,
though it be the organ by which the whole mass of blood receives its
caloric, because it is only in the capillary part of the systemic
circulation, when the arterial blood again passes into the venous
state, that the caloric acquired is liberated. In this manner, gently,
steadily, uninterruptedly, an abundant, unceasing, and equable current
of heat is distributed to every part and particle of the system.

  [4] It is not a perfectly accurate statement that the temperature of
  venous and arterial blood is precisely the same. The latest and best
  experiments concur in showing that arterial blood, at least in the
  heart and the great arterial trunks, is one or two degrees warmer than
  venous blood. The weight of evidence from experiment is also in favour
  of the opinion, that the different parts of the body are _somewhat_
  less warm as they recede from the lungs and heart; but the difference
  is so slight that it may be disregarded in the general argument.

531. Such is the celebrated theory of animal heat suggested by Dr.
Crawford, of which it has been justly said, that it affords one of the
most beautiful specimens of the application of physical and chemical
reasoning to the animal economy that has ever been presented to the
world.

532. The main position on which this theory rests—that arterial
possesses a greater capacity for caloric than venous blood—professes
to be founded on experiments which, though of a delicate and complex
nature, are nevertheless uniform and decisive in their results.
In consequence of their extreme interest and importance, these
experiments have been subjected, by different physiologists, to rigid
examination, with a somewhat conflicting result. The greater number
of experimentalists maintain that Crawford’s experiments are correct
in all the essential points, and that the objections which have been
urged against them do not really affect them; while others are of
opinion that, even although it must, upon the whole, be admitted that
the specific heat of arterial is greater than that of venous blood;
yet that the excess is so small as to be inadequate to account for
the effects attributed to it. Dr. Davy’s experiments, which of all
that have been instituted are generally conceived to be the most
unfavourable to the theory of Crawford, do not afford uniform results.
Three experiments out of four indicate a greater capacity in arterial
than in venous blood; in those in which the experimentalist himself
places the most confidence, in the relative proportion of 913 to 903;
while, according to Crawford, the relative proportion is 114·5 to 100.

533. But when this subject is closely considered, the discrepancy
in question turns out to be of no real consequence. There is a
modification of the theory, which removes every difficulty, and
dispenses with the necessity of any regard whatever to the point in
dispute.

534. It has been shown (444 _et seq._), that during the process of
respiration more oxygen disappears than is accounted for by the
carbonic acid that is generated; that this excess of oxygen is absorbed
by the blood; and that in the lung the oxygen merely enters into a
state of loose combination with the blood, the union being intimate
and complete only in the system. The complete chemical combination
of the oxygen with the carbon takes place, then, not in the lungs,
but in the capillary arteries of the system; consequently it is only
while flowing in capillary arteries that carbonic acid is formed; that
is, it is only in these vessels that the arterial combustion takes
place: of course, therefore, it is only in these vessels that heat
is extricated, and only from them that it can be communicated to the
adjacent parts. According to this view, wherever there is a capillary
artery, the combustion of carbon incessantly goes on, and there caloric
is as incessantly set free; but since there is not a point of any
tissue, in which there are not capillary arteries, there is not a point
from which caloric does not radiate. As soon as formed, carbonic acid
passes from the capillary arteries into the capillary veins; by the
veins it is transmitted to the lungs; and by the lungs it is expelled
from the system. The real operations carried on in the lungs, then,
are the transmission of oxygen and the extrication of carbonic acid;
but this organ is not the seat of the essential and ultimate part
of the function; it is merely the portal through which the elements
employed in the process have their entrance and exit. Thus the question
concerning the greater capacity of arterial blood for caloric is of
no importance whatever: the phenomena may be equally accounted for,
whatever be, in this respect, the constitution of the blood.

535. The result of the whole is, the complete establishment of the
fact, that the production of heat in the animal body is a chemical
operation, dependent on the combination of oxygen with carbon in the
capillary arteries of the system; that is, it is the result of the
burning of charcoal at every point of the body.

536. The agent which maintains and regulates this internal fire is the
nervous system. There is, indeed, reason to suppose that the nervous
system, in some mode or other, contributes to the actual production
of animal heat. It is established by direct experiment, that the
quantity of carbonic acid formed in the system is inadequate to the
supply of the caloric expended by it; that in a given time more heat
is abstracted from the body by the surrounding medium, than can be
accounted for by the consumption of the amount of carbonic acid thrown
off by the lungs during the same interval. There is evidence that the
source of this additional heat is the nervous system.

537. The influence exerted by the nervous system over the production
of animal heat, is demonstrated by the fact, established by numerous
observations and experiments, that whatever weakens the nervous power,
proportionally diminishes the capacity of producing heat. For,

1. The destruction of a portion of the spinal cord diminishes the
temperature of an animal without, as far as is ascertained, the
disturbance of any other function.

2. The privation of the heart and blood-vessels of the nervous
influence, as by decapitation, though the passage of the blood through
the lungs and its ordinary change from the venous to the arterial state
be maintained by artificial respiration, greatly diminishes, if it do
not altogether suspend, the generation of animal heat.

3. The abolition of sensibility by the administration of a narcotic
poison, artificial respiration being maintained, as effectually
disturbs the generation of animal heat as decapitation; while the power
of generating heat is restored, in the exact proportion to the return
of the sensibility by the cessation of the action of the poison.

4. The temperature of an organ is found, by direct experiment, to be
diminished by the division of the nerves that supply it with nervous
influence. The nerves that supply the horn were divided on one side
of the body in a young deer; the other horn was left entire. The
temperature of the horn—the nerves of which had been divided—was
found, after some hours, to be considerably diminished, and it
continued diminished for several days; at length its temperature was
restored. On examining the horn about ten days after the operation
had been performed, the divided nerves were found to be connected by
a newly-formed substance; thus apparently accounting for the loss of
temperature in the first instance, and for its subsequent restoration.

538. But although these and other analogous facts prove, beyond all
question, the important influence of the nervous system over the
development of animal heat, yet the mode in which that influence
operates is not ascertained. Its action may be either direct or
indirect. The nerves may possess some specific power of generating
heat,—extricating it immediately from the blood by a process analogous
to secretion,—or they may evolve it indirectly by other operations, as
by some of the processes of nutrition. Each hypothesis is maintained
by able physiologists; but the balance of evidence (as will appear
hereafter) is greatly in favour of the opinion that the influence
of the nervous system over this process is altogether indirect. A
beautiful illustration of this is afforded in the following operation,
which is going on, without ceasing, every instant during life.

539. The skin which forms the external covering of the body is composed
essentially of gelatin. No gelatin is contained in the blood; but the
albumen of the blood is capable of being converted into gelatin by the
addition of oxygen. Albumen is received by the capillary artery of the
skin; the blood, of which albumen forms so important a constituent,
contains a quantity of oxygen which it receives at the moment of
inspiration, and which it retains in a state of loose combination
(470 _et seq._). Under the influence probably of the organic nerve,
the capillary artery chemically combines a portion of the free oxygen
with the albumen of the blood, and gelatin is the result. In this
process the albumen gives off carbon; the blood affords oxygen; the
two elements unite; carbonic acid is formed; and, as in every other
instance in which carbonic acid is formed, heat is evolved. In this
manner a fire is kindled, and is kept constantly burning, where it
is most needed to counteract the influence of external cold, at the
external surface of the body.

540. Such are the main points which have been established in relation
to the production and distribution of animal heat. But it has been
shown that the living body is capable of bearing without injury a
temperature by which it is rapidly consumed when deprived of life. By
what means does the vital power enable the body to resist the influence
of such intense degrees of heat?

541. Two circumstances are observable when the body is placed in a
temperature greatly higher than its own. First, it can endure such
a temperature only in the medium of air. Air can easily be borne at
the temperature of 260°; aqueous vapour at the temperature of 130°
few Europeans are capable of enduring longer than twelve minutes; the
peasants of Finland appear to be able to sustain it, for the space of
half an hour, as high as 167°; but the hottest liquid water-bath which
any one seems to have been able to bear for the space of ten minutes,
is the hottest spring at Barêges, the temperature of which is 113°.
But in heated air the quantity of heat in actual contact with the body
is much less than in the other media; because in proportion as the
air is heated it is expanded, and in proportion as it is expanded the
particles are diminished that come into contact with the body.

542. In the second place, the afflux of the colder fluids from the
central parts of the system to the surface may for a time exert some
influence in keeping down the temperature of the body. But above all
this, in the third place, a two-fold provision is made in the body
itself for the reduction of its temperature when exposed to intense
degrees of heat; by the one, the power with which it is endowed
of producing heat is diminished; by the other, cold is positively
generated.

543. It has been shown (517) that in proportion to the elevation
of the temperature to which the body is exposed the blood becomes
less venalized, and in the proportion in which the blood retains its
arterial character the consumption of oxygen is diminished. Venous
blood contains an excess of carbon, arterial blood an excess of oxygen.
Consequently in proportion as the blood retains its arterial character
it affords less carbon for the combination of oxygen, that is less
inflammable matter. At an elevated temperature therefore there must, of
necessity, be a diminished production of heat within the body, since
the blood contains a diminished quantity of combustible material.

544. Moreover, in proportion to the elevation of the temperature to
which the body is exposed, evaporation takes place from the entire
surface of the pulmonary vesicles. No experiments have been performed
which enable the physiologist to ascertain precisely the quantity of
vapour exhaled from the lungs in a given time, when the body is exposed
to a given degree of heat; but both observation and experiment show
that it is very great. The blood pours out upon the whole surface of
the air vesicles a quantity of moisture in the form of water: by the
surrounding air this water is converted into vapour: by the conversion
of a fluid from the state of a liquid into that of vapour caloric is
absorbed: by the absorption of caloric cold is generated, and that to
such a degree that fluids exposed to the influence of evaporation may
be frozen in the intensest heat of summer. The very process by which
art, aided by science, affords to the inhabitants of warm climates the
luxury of ice, is that by which nature generates cold in the human
lungs when the body is exposed to a temperature above its own. Not
only, then, is the lung the instrument by which the body acquires the
power of evolving heat in greater or less quantity in proportion to
the demands of the system, but this very same organ, under a change
of circumstances, produces the directly contrary effect, and actually
generates cold.

545. In the process of producing cold the skin is a powerful auxiliary
to the lungs. More fluid is, indeed, evaporated from the surface of the
skin in the form of perspiration, than from the lungs in the form of
vapour; the cutaneous, like the pulmonary evaporation, increases in the
ratio of the temperature, and both co-operate in abstracting the excess
of caloric.

546. Finally, in proportion to the elevation of the temperature is
the acceleration of the circulation; the pulse is augmented in power,
and doubled or trebled in frequency (495); but in proportion to
the rapidity of the circulation is the increase of the quantity of
evaporable matter which is transmitted to the evaporating surfaces.

547. From the whole it appears that by the combination of carbon and
oxygen provision is made for the production of the greatest quantity
of caloric that can at any time be required for the wants of the
system; that when a decreased evolution of heat is necessary a smaller
quantity of carbon and oxygen is brought into union, and that when,
from exposure to intense degrees of heat, it is requisite for the
maintenance of the temperature of the body at its own standard, that
it should actually generate cold, it accomplishes this object by the
evaporation of water.




CHAPTER X.

OF THE FUNCTION OF DIGESTION.

 Process of Assimilation in the plant; in the animal—Digestive
 apparatus in the lower classes of animals; in the higher
 classes; in man—Digestive processes—Prehension, Mastication,
 Insalivation, Deglutition, Chymification, Chylification, Absorption,
 Fecation—Structure and action of the organs by which these operations
 are performed—Ultimate results—Powers by which those results are
 accomplished—Two kinds of digestion, a lower and a higher; the former
 preparatory to the latter.


548. Digestion is the function by which the aliment is converted into
nutriment. No food can nourish until it be converted into a fluid
analogous in chemical composition to that of the body by which it is
assimilated. The conversion of the crude aliment into such a fluid is
effected by a vital power peculiar to living beings, by which they
subvert the constitution of other organized bodies, and cause them to
assume their own. They accomplish this change by the agency of certain
secretions which they elaborate in their own organs, and which they
add to the substances they receive as aliment. By the action of these
secretions, the chemical composition of the aliment is brought into a
close affinity to that of the body which it nourishes.

549. This change in the chemical composition of the aliment, by means
of fluids secreted by the living bodies which receive it, is manifest
in the plant as well as in the animal. The sap, as it issues from the
root, is a colourless and limpid fluid; it has a specific gravity
a little greater than that of water; it has a sweetish taste; it
contains an acid which is sometimes free, and is either the carbonic
or the acetic; but more commonly it is combined with lime or potass.
To this crude sap, in this the first stage of its formation, vegetable
secretions, sugar and mucus, assimilative substances, are superadded,
probably by the fibres of the root.

550. As the sap ascends in the stalk, a greater quantity and a greater
number of these vegetable secretions are poured into it. In the ratio
of its elevation it acquires sugar, mucus, albumen, and an azotized
substance analogous to gluten. By the admixture of these assimilative
secretions, the crude sap is progressively assimilated nearer and
nearer to the chemical composition of the proper nutritive fluid of the
plant. Thus prepared, the sap passes to the leaf, in the upper surface
of which it undergoes a process analogous to that of digestion in the
animal (315), and is converted into proper nutrient matter.

551. The plant can only take up, by absorption, liquid food; it never
receives solid substances as aliment: it therefore needs no apparatus
for the division, solution, and fluidification of its food; its sole
work of assimilation consists in changing the innate affinities
of liquid aliment. But animals which live on vegetable and animal
substances have to modify, by their digestive juices, the affinities of
organic solids: hence assimilation in the animal must necessarily be a
more complex operation than it is in the plant.

552. Fixed immovably to the soil by its roots, the nutritive apparatus
of the plant is always in contact with its food, which is slowly but
unceasingly absorbed according to the wants of its system. But the
animal endowed with the faculty of locomotion receives its aliment into
the interior of its body, that it may transport its food along with it
in all its changes of place; and that, as in the plant, its food may be
always in contact with its nutritive apparatus. The interior nutrition
of the animal and the convergence of its nutritive apparatus to the
centre of its system, and the exterior nutrition of the plant and the
divergence of its nutritive apparatus to the peripheral extremity of
its body, are differences in their mode of nutrition, connected with
essential differences in the mode of life peculiar to the two beings.

553. Plant-like animals have a plant-like mode of nutrition. The
transition from the one class to the other is so gradual as to be
almost insensible. Fixed to the same spot in the ocean as the tree to
the land, the nutritive surface of the poriferous animal is always in
contact with the water, as the soil is with the external surface of
the plant. The cellular substance of which the bag of the poriferous
animal is composed is permeated in all directions by ramifying and
anastomosing canals, which, beginning by minute pores placed on the
external surface, terminate in larger orifices, termed vents, which
are fecal openings. These internal canals are incessantly traversed by
streams of water, which enter through the minute, and are discharged
through the larger orifices. By these currents the nutrient matter
contained in the water is conveyed to every part of the body, and
the streams that issue from the fecal orifices abound with minute
flocculent particles, the residue of the digested matter. No separate
part of the body is appropriated to the function of digestion any
more than in the plant; there is merely a general absorbent surface;
the water is to this animal what the soil is to the plant; its whole
surface is a root; every point of that surface is constantly in contact
with its food, and every point is absorbent.

554. In the class above the porifera, the margins of the superficial
pores are merely lengthened out into minute sacs, irritable and
sentient, surrounded with vibratile cilia (342). These sacs, which are
termed polypi, are so many little stomachs, which select, seize, and
digest the food brought to them in the currents of water created by the
action of the cilia (344).

[Illustration: Fig. CXLVIII.—_Hydra Viridis._

 1. The Hydra with its tentacula expanded. 2. The tentacula. 3. The
 body of the Hydra. 4. Disc for attachment. 5. The Hydra in the act of
 creeping. 6. The Hydra with an animalcule in its digestive cavity.]

555. The fresh-water polype, the little hydra (fig. CXLVIII. 1), is one
of these minute sacs detached and endowed with the power of locomotion
(fig. CXLVIII. 5), a sentient, self-moving digestive bag. Capable of
swallowing animals many times its own size, as the red-blooded worm,
this little creature stretches its whole body like a thin elastic
membrane over its prey, so as completely to alter its own shape, and
the membranous substance of which it is composed becoming transparent
by the distention, allows the subsequent process to be distinctly seen.
The red fluid of the worm, as the process of digestion advances, is
slowly diffused over every part of the internal surface of the polype.
The whole internal surface of this minute self-moving bag is digestive;
a true and proper stomach (fig. CXLVIII. 6). By dexterous manipulation,
this internal surface may be rendered external, and the animal turned
completely inside out. Then the external begins to perform the office
of the internal surface, carrying on the function of digestion,
just as well as that which was primitively formed for it; while the
originally digestive becomes the generative surface, for the creature
buds from this surface, now the outer one; a striking and instructive
illustration of the analogy between the external covering of the animal
body or the skin, and its internal lining, or the mucous surface.

[Illustration: Fig. CXLIX.

 Group of Monades; the dark spots in the interior of their bodies
 representing their digestive sacs.]

556. In the monades (fig. CXLIX.), and in all the lower animalcules,
the digestive apparatus, instead of forming the entire internal
surface of the body, consists of numerous sacs, which constitute so
many separate stomachs, whence the name of the class, _polygastrica_.
When empty, or when filled with water, these digestive sacs cannot
be distinguished from the common cellular tissue of the body; but on
feeding the animals with coloured organic matter, minutely diffused in
water, the coloured particles readily enter the digestive sacs, and
render apparent their form and arrangement. In the minutest animal
hitherto appreciable, the monas termo, the 2000th part of a line
in diameter, four rounded sacs have been seen filled with coloured
particles (fig. CXLIX.). Each of these sacs, about the 6000th part of
a line in diameter, opens by a narrow neck into a funnel-shaped mouth,
surrounded with a single row of long vibratite cilia, by the action
of which the floating organic particles are brought within the reach
of the mouth. In general, even in this class, an alimentary canal
traverses the whole extent of the body, into which all the different
stomachs open. Sometimes numerous branches proceed from the main trunks
of the alimentary canal, bearing the nutritive matter to the different
parts of the body (fig. CL. 2). Often, in order to extend the digestive
surface, the alimentary canal is produced, forming rounded enlargements
called cœcal appendages, all of which act as so many additional
stomachs (fig. CLI. 3). In some individuals, observed under favourable
circumstances, nearly 200 of these cœcal stomachs, filled with coloured
matter, have been counted, and there may have been many more unseen,
because empty and collapsed. In the lowest tribes of this class
there is but one orifice to the alimentary canal, the oral; the food
entering, and the fecal matter passing out of the system by the same
aperture; but in the higher orders there is both an oral and an anal
orifice, and the mouth and the anus are placed at opposite extremities
of the body, as in the higher animals.

[Illustration: Fig. CL.—_Fasciola Hepatica._

 1. Mouth. 2. Alimentary tubes. 3. Sucker.]

557. Up to this point in the animal series the digestive sacs and the
alimentary canal are merely cavities formed in the common cellular
tissue of the body, without any lining membrane, without teeth, or
without any instruments for dividing and preparing the aliment, and
without a single gland, as far as has been ascertained, to assist the
digestive process. All the assimilative functions, the respiratory as
well as the digestive, appear to be performed by this single surface.
But in the ascending scale not only is an apparatus appropriated to
digestion, perfectly distinct from that assigned to respiration,
but even the stomach and the alimentary canal are separate organs,
distinguished from each other, both in structure and function.
Still higher in the scale new organs are successively added, as the
process becomes more complex and refined, in order to assist the main
operations carried on in particular parts of the apparatus; and as
that apparatus approaches its highest degree of perfection, not only
do the several parts of which it is composed increase in number and
complexity, but each part becomes more and more isolated from the rest,
a specific office being assigned to each in the division of labour
that is made. Viewing, however, the digestive apparatus as a whole,
whether simple or complex, whether consisting of a single uninterrupted
surface, or divided into many separate portions, its nature is
universally and invariably the same, and from the monad to man is
endowed with analogous vital energies.

[Illustration: Fig. CLI.—_Aphrodita Aculeata._

 1. Proboscis in a retracted state. 2. Interior of digestive cavity.
 3, 3. Cœcal appendages opening into it.]

558. Comparative anatomy, which has succeeded in tracing through the
different classes, orders, genera, and countless tribes of animals,
the modifications in form and structure of the digestive apparatus,
has shown that those modifications are invariably in strict adaptation
to the kind of food on which the apparatus is destined to act and to
the extent of the elaboration requisite to convert crude aliment into
proper animal substance. To trace this adaptation through the rising
and ever-varying series, is a most interesting and instructive study,
not only exhibiting, in the very organs that elaborate its food, the
physical and even the mental qualities assigned by the hand of nature
to each individual, but oftentimes shedding a clear and bright light on
the complex structures of the highest and most perfect organization.
Striking and beautiful illustrations are afforded by these
investigations of the principle formerly insisted on (vol. i. chap. i.
p. 28, 3), that the communication of the higher faculties exalts the
apparatus even of the very lowest processes, that the latter may work
in harmony with the former. In conformity with this principle, as the
nobler endowments exalt the animal in the scale of organization, so
even its very digestive apparatus becomes extended, isolated, complex
and refined.

559. The highest and most perfect form of the digestive apparatus is
that which is disposed in a series of chambers in free communication
with each other. In these chambers the food undergoes a succession of
changes, by which it is progressively assimilated to the nature of
animal substance. This assimilation, however, is never effected by the
sole agency of the chambers themselves; it is accomplished, to a great
extent, by the influence of special organs placed in the neighbourhood
of the digestive chambers. In the lowest animal there is but one
substance and one surface for every function; in the highest, even for
the performance of the lowest function, there is the combination of
many substances which are arranged in complex modes.

560. In man, the digestive chambers are five; the auxiliary organs are
many.

The first of these chambers is the cavity called the mouth; the
second is the bag termed the pharynx; the pharynx communicates by the
esophagus with the third chamber, the stomach; the fourth chamber
consists of the convoluted tubes named the small intestines, and the
fifth consists of the larger tubes, denominated the large intestines.
The assistant organs are, first, numerous appendages to the mouth,
namely, the tongue, the teeth, the salivary glands, and the muscles
that work the jaws; and, secondly, certain appendages to the small
intestines, namely, the pancreas, the liver, the mesenteric glands, and
the lacteal vessels.

561. By the mouth the food is softened and reduced to a pulp; by the
tongue, materially aided by the soft palate, this pulp, when duly
prepared, is transmitted to the pharynx; received by the pharynx, it
is sent on to the esophagus; by the esophagus, it is conveyed to the
stomach; in the stomach, it is converted into a peculiar substance
called chyme; the chyme, passing from the stomach into the first
portion of the small intestines, is there converted into the substance
called chyle; the chyle, carried slowly along the remaining portion of
the small intestines, is successively absorbed by the lacteals; by the
lacteals, it is conveyed through the mesenteric glands to the thoracic
duct, and by the thoracic duct it is poured into the venous blood close
to the heart. By the large intestines the refuse matter is conveyed out
of the system.

562. The function of digestion consists, then, of the following
processes:—

1. Prehension. 2. Mastication. 3. Insalivation. 4. Deglutition.
5. Chymification. 6. Chylification. 7. Absorption. 8. Fecation.

563. Prehension is the reception of the aliment; mastication is the
mechanical comminution of it; insalivation is the admixture of it with
certain juices poured into the mouth; deglutition is the transmission
of it, when duly moistened and divided, into the stomach; chymification
is the conversion of it into chyme; chylification is the conversion of
the chyme into chyle; absorption is the assumption of the chyle by the
lacteals and the transmission of it into the blood, and fecation is
the separation and discharge of the refuse matter. Each part of this
extended apparatus is modified in structure so as specially to fit it
for the performance of the office which is appropriated to it.

564. The mouth is not merely the opening between the two lips, but
consists of an oval chamber, bounded above by the upper jaw and the
palate; below by the tongue and the lower jaw; laterally by the cheeks;
behind by the soft palate; and before by the lips.

565. The upper and lower jaw, the palate bones, and the teeth,
constitute the hard or the bony parts of the mouth. The soft parts
consist of the lips, the cheeks, the soft palate, the tongue, and the
mucous membrane which lines the whole.

566. The lips and cheeks are composed principally of muscles, covered
on the outside by the skin, and lined on the inside by the mucous
membrane of the mouth. In the interspaces between the muscles is
disposed a quantity of fat, which gives form to the face, facilitates
the movements of the muscles, and protects the glands, blood-vessels,
and nerves, with which all these organs are most abundantly supplied.

567. The roof of the mouth, called the palate, consists partly of bony
and partly of membranous substance. The bony part of the palate forms
an arch in the upper jaw, the position of which in the erect posture is
horizontal: the membranous part of the palate consists of the mucous
membrane of the mouth, which affords a covering to the bony part of the
palate.

[Illustration: Fig. CLII.—_View of the Mouth, showing particularly the
Soft Palate, Tonsils, and Tongue._

 1. Anterior arch of the soft palate. 2. Posterior arch. 3. Tonsils or
 amygdalæ. 4. Uvula. 5. Communication between the mouth and pharynx.
 6. The tongue. 7. Anterior or nervous papillæ. 8 and 9. The upper and
 lower turbinated bones dividing the nostrils into (10) chambers.]

[Illustration: Fig. CLIII.—_A side view of the Mouth, Pharynx, Nose,
&c._

 1. Mouth. 2. Tongue. 3. Section of the lower jaw. 4. Submaxillary
 gland. 5. Sublingual gland. 6. Hyoid bone. 7. Thyroid cartilage. 8.
 Thyroid gland. 9. Trachea. 10. Interior of the pharynx. 11. Section
 of the soft palate. 12. The esophagus. 13. The interior of the nose.
 14. The two spongy bones dividing it into three chambers. 15. The
 posterior communication with the upper part of the pharynx.]

[Illustration: Fig. CLIV.—_Posterior view of the Nose, Mouth, Larynx,
and Pharynx laid open._

 1. Posterior openings of the nose, communicating with the upper part
 of the pharynx. 2. Posterior surface of the soft palate. 3. The uvula.
 4. Back part of the mouth communicating with the pharynx. 5. The
 tonsils. 6. Back part or root of the tongue. 7. Posterior surface of
 the epiglottis. 8. The larynx. 9. The opening of the larynx into the
 pharynx. 10. Cut edges of the pharynx. 11. Esophagus, the continuation
 of the pharynx. 12. The Trachea, continuation of the larynx. 13.
 Muscles acting on the pharynx.]

568. From the posterior part of the bony arch of the palate is
suspended, transversely, a moveable partition, called the soft palate
(fig. CLII. 1 and 2), which is composed of muscular fibres enclosed
in the mucous membranes of the mouth. No less than ten distinct
muscles enter into the composition of the soft palate. These muscles
are disposed in such a manner that they render the organ capable of
descending and of applying itself against the tongue (fig. CLII. 6), so
as completely to close the passage between the mouth and the pharynx
(figs. CLII. 5, and CLIV. 1), and of ascending and carrying itself
obliquely backwards towards the posterior wall of the pharynx, so as
completely to close the passage between the pharynx and the nose (fig.
CLIV. 2, 1); hence this moveable partition performs the office of a
double valve, closing the passage from the mouth to the pharynx, and
from the pharynx to the nose.

569. From the centre of the soft palate hangs pendulous the
conical-shaped body called the uvula (fig. CLII. 4), which consists of
a small muscle enveloped in the mucous membrane of the mouth. The uvula
assists in completing the valve formed by the soft palate (fig. CLIV.
2, 3); it is also an important organ in the modulation of the voice.
When destroyed by disease, both the deglutition of the food and the
sound of the voice become imperfect.

570. The lateral edges of the soft palate separate into two layers,
which enclose between them the bodies called the tonsils (fig. CLII.
3), two glands commonly about the size of an almond. These organs
co-operate with other glands in secreting the fluids of the mouth.

571. The tongue (figs. CLII. 6, and CLIII. 2) is composed of six
distinct muscles enveloped in the mucous membrane of the mouth. The
fibres of these muscles are so interwoven with each other as to form
an intricate net-work; and their number, arrangement, and exquisite
organization render the organ capable of executing a surprising variety
of movements with the most perfect precision, and with a velocity
of which the mind can scarcely form a conception: some of these
movements being requisite to bring the aliment under the operation of
mastication, and others to produce articulate speech.

572. The tongue divided into base, apex, and dorsum, is supported by a
bone called the hyoid bone (os hyoides) (figs. CXXXVI. 1, and CLIII.
6), which, unlike any other bone of the body, is placed at a distance
from the general skeleton, and completely imbedded in muscles. This
singularly posted and delicately constructed bone is not only connected
with the tongue, but with many other highly important muscles, to which
it affords a support and a lever.

573. Each jaw is provided with sixteen teeth (fig. CLV.), arranged with
perfect uniformity, eight on each side of each jaw (fig. CLV.); those
of the one side exactly corresponding with those of the other (fig.
CLV.). The teeth, from the differences they present in their size,
form, mode of connection with the jaw, and use, are divided into four
classes, namely, on each side of each jaw, two incisors (figs. CLVI.
and CLVII. 1, 2); one cuspid (figs. CLVI. and CLVII. 3); two bicuspid
(figs. CLVI. and CLVII. 4, 5); and three molars (figs. CLVI. and CLVII.
6, 7, 8).

[Illustration: Fig. CLV.

 A lateral view of the whole series of the teeth, in situ, showing the
 relative situation of those of the upper with those of the lower jaw.
 This figure and the following figures to 159, are copied from Mr. T.
 Bell’s scientific and instructive work on the Anatomy, Physiology, and
 Diseases of the Teeth.]

574. The incisor, or cutting teeth, are situated in the front of the
jaw; that directly in the centre is called the central; and the next
to it the lateral incisor (fig. CLV.). Their office, as their name
imports, is to cut the food, which they do, on the principle of shears
or scissors.

575. Standing next to the lateral incisor is the cuspid, canine, or
eye-tooth (figs. CLV., CLVI., and CLVII.). It is the longest of all the
teeth. Its office is to tear such parts of the food as are too hard to
be readily divided by the incisors.

576. Next the cuspid are the bicuspid, two on each side (fig. CLV.,
CLVII.), so named from their being provided with two distinct
prominences or points. Their office is to tear tough substances
preparatory to their trituration by the next set.

[Illustration: Fig. CLVI.

 Front or external view of the upper teeth. 1. The central incisor.
 2. The lateral incisor. 3. The cuspid. 4. The first bicuspid. 5. The
 second bicuspid. 6. The first molar. 7. The second molar. 8. The third
 molar, or dens sapientiæ.]

577. The molars, or the grinders, three on each side (fig. CLVI.
and CLVII.), provided with four or five prominences on the grinding
surface, with corresponding depressions, which are so arranged that
the elevations of those of the upper are adapted to the concavities of
those of the lower jaw, and the contrary.

[Illustration: Fig. CLVII.

 Front view of the lower teeth. 1. The central incisor. 2. The lateral
 incisor. 3. The cuspid. 4. The first bicuspid. 5. The second bicuspid.
 6. The first molar. 7. The second molar. 8. The third molar, or dens
 sapientiæ.]

578. From the incisor to the molar teeth there is a regular gradation
in size, form, and use, the cuspid holding a middle place between the
incisor and the bicuspid, and the bicuspid being in every respect
intermediate between the cuspid and the molar. Thus the incisor are
adapted only for cutting, the cuspid for tearing, the bicuspid partly
for tearing and partly for grinding, and the molar solely for grinding.
The incisor has only a single root, which is nearly round, and quite
simple (fig. CLVII. 1, 2); the cuspid has only a single root, but this
is flattened and partially grooved (fig. CLVII. 3); even the bicuspid
has only a single root, but this is commonly divided at its extremity,
and is always so much grooved as to have the appearance of two fangs
partially united, the body having two points instead of one, thus
approaching it to the form of the molar (fig. CLVII. 4, 5); and these
last have always two, sometimes three, occasionally four roots, and
their body is greatly increased in size, and has a complete grinding
surface (fig. CLVII. 6, 7, 8).

579. In some animals whose food and habits require the utmost extension
of the office of a particular class of teeth, a corresponding
development of that class takes place. Thus in the carnivora, as is
strikingly seen in the tiger and the polar bear, the cuspid or canine
teeth are prodigiously elongated and strengthened, in order to enable
them to seize their food, and to tear it in pieces. On the other hand,
in the rodentia, or gnawing animals, as in the beaver, the incisors
are exceedingly elongated; while in the graminivora, and especially in
the ruminantia, the molar teeth are by far the most developed. In each
case the other kinds of teeth are of little comparative importance;
sometimes they are even altogether wanting. Thus the shark has only one
kind of tooth, the incisor; but of these there are several rows, and
all of them the creature has the power of erecting at will.

580. So intimately are these organs connected with the kind of food
by which life is sustained, and the kind of food with the general
habits of the animal, that an anatomist can tell the structure of
the digestive organs, the kind of nervous system, the physical and
even the mental endowments; that is, the exact point in the scale of
organization to which the animal belongs, merely by the inspection of
the teeth.

581. In man, the several classes of the teeth are so similarly
developed, so perfectly equalized, and so identically constructed, that
they may be considered as the true type from which all the other forms
are deviations.

582. For the accomplishment of their office the teeth must be endowed
with prodigious strength: for the fulfilment of purposes immediately
connected with the apparatus of digestion, it is necessary that they
should be placed in the neighbourhood of exceedingly soft, delicate,
irritable, and sentient organs. That they may possess the requisite
degree of strength, they are constructed chiefly of bone, the hardest
organized substance. Bone, though not as sensible as some other parts
of the body, is nevertheless sentient. The employment of a sensitive
body in the office of breaking down the hard substances used as food
would be to change the act of eating from a pleasurable into a painful
operation. It has been shown (vol. i. p. 84) that provision is made
for supplying to the animal a never-failing source of enjoyment in the
annexation of pleasurable sensations with the act of eating, and that,
taking the whole of life into account, the sum of enjoyment secured by
this provision is incalculable. But all this enjoyment might have been
lost, might even have been changed into positive pain, nay, must have
been changed into pain, but for adjustments numerous, minute, delicate,
and, at first view, incompatible.

583. Had a highly-organized and sensitive body been made the instrument
of cutting, tearing, and breaking down the food, every tooth, every
time it comes in contact with the food, would produce the exquisite
pain now occasionally experienced when a tooth is inflamed. Yet a
body wholly inorganic and therefore insensible, could not perform the
office of the instrument; first, because a dead body cannot be placed
in contact with living parts without producing irritation, disease, and
consequently pain; and, secondly, because such a body being incapable
of any process of nutrition, must speedily be worn away by friction,
and there could be no possibility of repairing or of replacing it. The
instrument in question, then, must possess hardness, durability, and,
to a certain extent, insensibility; yet it must be capable of forming
an intimate union with sentient and vital organs, must be capable of
becoming a constituent part of the living system.

584. To communicate to it the requisite degree of hardness, the hard
substance forming its basis is rendered so much harder than common bone
that some physiologists have even doubted whether it be bone, whether
it really possess a true organic structure. That there is no ground for
such doubt the evidence is complete. For,

1. The tooth, like bone in general, is composed partly of an earthy
and partly of an animal substance; the earthy part being completely
removable by maceration in an acid, and the animal portion by
incineration, the tooth under each process retaining exactly its
original form.

2. The root of the tooth is covered externally by periosteum; its
internal cavity is lined by a vascular and nervous membrane, and both
structures are intimately connected with the substance of the tooth. If
these membranes really distribute their blood-vessels and nerves to the
substance of the tooth, which there is no reason to doubt, the analogy
is identical between the structure of the teeth and that of bone.

3. Though the blood-vessels of the teeth are so minute that they do
not, under ordinary circumstances, admit the red particles of the
blood, and though no colouring matter hitherto employed in artificial
injections has been able, on account of its grossness, to penetrate
the dental vessels, yet disease sometimes accomplishes what art is
incapable of effecting. In jaundice the bony substance of the teeth is
occasionally tinged with a bright yellow colour; and in persons who
have perished by a violent death, in whom the circulation has been
suddenly arrested, it is of a deep red colour. Moreover, when the
dentist files a tooth, no pain is produced until the file reaches the
bony substance; but the instant it begins to act upon this part of the
tooth, the sensation becomes sufficiently acute.

585. These facts demonstrate that the bony matter of the tooth, though
modified to fit the instrument for its office, is still a true and
proper organized substance.

586. Each tooth is divided into body, neck, and root (fig. CLVIII. 1,
2, 3). The body is that part of the tooth which is above the gum, the
root that part which is below the gum, and the neck that part where the
body and the root unite (fig. CLVIII.). The body, the essential part,
is the tooth properly so called, the part which performs the whole work
for which the instrument is constructed, to the production and support
of which all the other parts are subservient.

[Illustration: Fig. CLVIII.

 Views of different kinds of teeth, showing their anatomical division
 into, 1. The body or crown. 2. The fang or root. 3. The neck.]

[Illustration: Fig. CLIX.—Sections of Teeth, exhibiting their Structure.

 1. The bony substance. 2. The enamel. 3. The internal cavity. 4. The
 foramen, or hole at the extremity of the root.]

587. When a vertical section is made in the tooth, it is found to
contain a cavity of considerable size (fig. CLIX, 3), termed the dental
cavity, which, large in the body of the tooth, gradually diminishes
through the whole length of the root (fig. CLIX. 3). The dental cavity
is lined throughout with a thin, delicate, and vascular membrane,
continued from that which lines the jaw. It contains a pulpy substance.
This pulp, highly vascular and exquisitely sensible, is composed almost
entirely of blood-vessels and nerves, and is the source whence the bony
part of the tooth derives its vitality, sensibility, and nutriment. The
blood-vessels and nerves that compose the pulp enter the dental cavity
through a minute hole at the extremity of the root (fig. CLIX. 4). The
membrane which lines the dental cavity is likewise continued over the
external surface of the root, so as to afford it a complete envelope.

588. Provision having been thus made for the organization of the tooth,
for the support of its vitality, and for its connexion with the living
system, over all that portion of it which is above the gum, and which
constitutes the essential part of the instrument, there is poured a
dense, hard, inorganic, insensible, all but indestructible substance,
termed enamel (fig. CLIX. 2); a substance inorganic, composed of earthy
salts, principally phosphate of lime with a slight trace of animal
matter: a substance of exceeding density, of a milky-white colour,
semi-transparent, and consisting of minute fibrous crystals. The manner
in which this inorganic matter is arranged about the body of the tooth
is worthy of notice. The crystals are disposed in radii springing from
the centre of the tooth (fig. CLX. 3); so that the extremities of the
crystals form the external surface of the tooth, while the internal
extremities are in contact with the bony substance (fig. CLX. 3). By
this arrangement a two-fold advantage is obtained; the enamel is less
apt to be worn down by friction, and is less liable to accidental
fracture.

[Illustration: Fig. CLX.

 Magnified section of a tooth, to illustrate the arrangement of the
 fibrous crystals composing the enamel. 1. Cavity of the tooth. 2. Bony
 substance. 3. Enamel, showing the crystals disposed in radii.]

589. In this manner an instrument is constructed possessing the
requisite hardness, durability, and insensibility; yet organized,
alive, as truly an integrant portion of the living system as the eye or
the heart.

590. No less care is indicated in fixing than in constructing the
instrument. It is held in its situation not by one expedient, but by
many.

1. All along the margin of both jaws is placed a bony arch, pierced
with holes, which constitute the sockets, called alveoli, for the teeth
(fig. CLXI.). Each socket or alveolus is distinct, there being one
alveolus for each tooth (fig. CLXI.). The adaptation of the root to
the alveolus is so exact, and the adhesion so close, that each root is
fixed in its alveolus just as a nail is fixed when driven into a board.

[Illustration: Fig. CLXI.

 Upper jaw, showing the alveoli.]

2. The roots of the tooth, when there are more than one, deviate
from a straight line (fig. CLVI. 6, 7, 8); and this deviation from
parallelism, on an obvious mechanical principle, adds to the firmness
of the connexion.

3. Adherent by one edge to the bony arch of the jaw, and by the other
to the neck of the tooth, is a peculiar substance, dense, firm,
membranous, called the gum, less hard than cartilage, but much harder
than skin, or common membrane; abounding with blood-vessels, yet but
little sensible; constructed for the express purpose of assisting to
fix the teeth in their situation.

4. The dense and firm membrane covering the bony arch of the jaw is
continued into each alveolus which it lines; from the bottom of the
alveolus this membrane is reflected over the root of the tooth, which
it completely invests as far as the neck, where it terminates, and
where the enamel begins: this membrane, like a tense and strong band,
powerfully assists in fixing the tooth.

5. Lastly, the vessels and nerves which enter at the extremity of the
root, like so many strings, assist in tying it down; hence, when in the
progress of age, all the other fastenings are removed, these strings
hold the teeth so firmly to the bottom of the socket, that their
removal always requires considerable force.

591. But a dense substance like enamel, acting with force against so
hard a substance as bone, would produce a jar which, propagated along
the bones of the face and skull to the brain, would severely injure
that tender organ, and effectually interfere with the comfort of eating.

592. This evil is guarded against,

1. By the structure of the alveoli (fig. CLXII.), which are composed
not of dense and compact, but of loose and spongy bone (fig. CLXII.).
This cancellated arrangement of the osseous fibres is admirably
adapted for absorbing vibrations and preventing their propagation (90).

2. By the membrane which lines the socket.

3. By the membrane which covers the root of the tooth; and,

4. By the gum.

[Illustration: Fig. CLXII.

 View of the upper and lower teeth in the alveoli; the external
 alveolar plate being cut away to show the cancellated structure of the
 alveoli, and the articulation of the teeth.]

These membranous substances, even more than the cancellated structure
of the alveoli, absorb vibrations and counteract the communication of
a shock to the bones of the face and head when the teeth act forcibly
on hard materials; so many and such nice adjustments go to secure
enjoyment, nay to prevent exquisite pain, in the simple operation of
bringing the teeth into contact in the act of eating.

[Illustration: Fig. CLXIII.—_View of the Muscles of Mastication, which
elevate the lower jaw._

 1. The temporal muscle. 2. Its insertion passing beneath. 3. The
 zygoma. 4. The masseter muscle, its anterior portion reflected to show
 the insertion of the temporal. The action of these powerful muscles
 is to pull the lower jaw upwards with great force against the upper
 jaw, and at the same time to draw it a little forwards or backwards,
 according to the direction of the fibres of the muscles.]

593. The teeth in mastication are passive instruments put in motion
by the jaws. The upper jaw is fixed, the lower only is movable. The
lower jaw is capable of four different motions; depression, elevation,
a motion forwards and backwards, and partial rotation. These simple
motions are capable, by combination, of producing various compound
motions. Numerous muscles, some of them endowed with prodigious power,
are so disposed and combined as to be able, at the command of volition,
to produce any of these motions that may be required, simple or
compound.

[Illustration: Fig. CLXIV.—_Muscles of the Jaw._

 1. Portion of the zygomatic process of the temporal bone. 2. Ascending
 plate of the lower jaw removed to expose, 3. External pterygoid, and,
 4. Internal pterygoid muscles. The action of these muscles is to raise
 the lower jaw, and to pull it obliquely towards the opposite side.
 When both muscles act together, they bring the lower jaw forwards, so
 as to make the fore-teeth project beyond those of the upper jaw.]

594. By the combination, succession, alternation, and repetition of
these motions, the lower is made to produce upon the upper jaw all the
variety of pressure necessary for the mastication of the food. In this
process the muscles of the tongue perform scarcely a less important
part than the muscles of the lower jaw. Some of its muscular fibres
shorten the tongue, some give it breadth, others render it concave,
and others convex: so ample is the provision for moving this organ to
different parts of the mouth and fauces, whether to bruise the softer
parts of the aliment against the palate, to mix it with the saliva, or
to place it under the pressure of the teeth.

595. By the combined action of the muscles of the lower jaw and tongue,
and that of the teeth, the food is bruised, cut, torn, and divided
into minute fragments. This operation is of so much importance that
the whole process of digestion is imperfect without it. It is proved
by direct experiment that the stomach acts upon the aliment with a
facility in some degree proportionate to the perfection with which it
is masticated. If an animal swallow morsels of food of different bulks,
and the stomach be examined after a given time, digestion is found to
be the most advanced in the smallest pieces, which are often completely
softened, while the larger are scarcely acted upon at all.

596. At the same time that, by the operation of mastication, the
aliment undergoes mechanical division, it imbibes a quantity of fluid
derived from various sources.

1. From the membrane which lines the internal surface of the mouth, and
which affords a covering to all the parts contained in it.

2. From numerous minute glands placed in clusters about the cheeks,
gums, lips, palate, and tongue. Each of these glands is furnished
with its own little duct, which, piercing the mucous membrane, opens
into the cavity of the mouth. From these glands is derived the fluid
with which the interior of the mouth is lubricated. It consists of a
glutinous, adhesive, transparent fluid, of a light grey tint, salt
taste, and slightly alkaline nature, termed mucus.

[Illustration: Fig. CLXV.—_View of the Parotid Gland with the Muscles
of the Face._

 1. Parotid gland. 2. Parotid duct. 3. Masseter muscle. 4. Buccinator.
 5. Triangularis, or depressor of the angle of the mouth. 6. Depressor
 of the lower lip. 7. Orbicularis, or circular muscle of the mouth. 8.
 Great zygomatic, or the distorter of the mouth, as in laughing. 9.
 Elevator of the angle of the mouth. 10. Elevator of the upper lip,
 and wing of the nose. 11. Compressor of the cartilage of the nose.
 12. Orbicularis, or circular muscle of the eyelids. 13. Occipito
 frontalis; elevator of the eyelids; motor of the scalp, &c., an
 important muscle of expression. 14. Tendinous portion of the occipito
 frontalis. 15. Elevator of the ear.]

3. From six large glands placed symmetrically, three on each side,
termed the salivary glands, namely, the parotid (fig. CLXV. 1),
situated before the ear; the submaxillary (fig. CLIII. 4), situated
beneath the lower jaw; and the sublingual (fig. CLIII. 5), situated
immediately under the tongue. Each of these glands is provided with a
duct (figs. CLXV. 2, and cliii. 4, 5), by which it pours the fluid it
elaborates, called saliva, into the mouth.

597. The other fluids of the mouth are always mixed with the saliva,
and are all commonly included under that name. The secretion of these
fluids is unceasing, and they pass into the stomach by successive acts
of deglutition at nearly regular intervals; so that the stomach, after
it has been some time without food, contains a considerable quantity
of these fluids. But they are chiefly needed during the operation of
mastication, and two provisions are made for securing their flow in the
greatest abundance at that time.

598. First, the situation of the glands is such that they are all
exposed to the action of the muscles of mastication (figs. CLXIII. and
CLXIV.), by which action the glands are excited, a large quantity of
blood is determined to them, and the quantity of fluid they secrete
is proportionate to the quantity of blood they receive. Secondly, the
glands are placed under the influence of the mind, so that the very
thought, and still more the taste, of grateful food, acting upon them
as an additional stimulus, causes an additional secretion. The quantity
of fluid formed from these different sources, and mixed with the food
during the mastication of an ordinary meal, is estimated at half a
pint. It must commonly be more than this, because, in a case described
by Dr. Gairdner, of Edinburgh, in which the esophagus had been cut
through, it was observed that from six to eight ounces of saliva were
discharged during a meal, which consisted merely of broth injected
through the divided esophagus into the stomach.

599. Saliva is a frothy, watery fluid, in its healthy state nearly
insipid, and of a slightly alkaline nature. It is composed of water,
a peculiar animal substance called salivary matter, mucus, osmazome,
a little albumen, and several salts. It produces important changes
on the food. By the water, and the salts contained in it, it softens
and dissolves the food; and thus, while it renders it easier to
be swallowed, it prepares it for the subsequent changes it is to
undergo. To this latter object, the assimilation of the food, it
seems to communicate the first tendency by the azotized substances,
the salivary, and the albuminous matter which it adds to it. From
this, the commencement of the assimilative process to its completion,
animalized substances are successively added to the food which have the
property of converting the food more and more into the nature of animal
substance.

600. Comminuted by the teeth, and softened by the saliva, the food is
reduced to a pulp. In this pulp there is a complete admixture of all
the alimentary substances with the assimilative matter secreted from
the blood, into the nature of which it is to be ultimately changed. The
mass is at the same time brought to the temperature of the blood.

601. As long as the operations of mastication and insalivation go on,
the mouth forms a closed cavity from which the food cannot escape; for
the lips enclose it before, the cheeks at the sides, the tongue below,
and the soft palate behind, the inferior edge of which being applied
in close and firm contact with the base of the tongue, prevents all
communication between the mouth and the pharynx.

602. When, by mastication, the food is sufficiently divided, and by
insalivation softened and animalized to fit it for the future changes
it is to undergo, it is collected by the tongue, and carried by that
organ to the back part of the mouth. The soft palate (fig. CLII. 1),
obedient to the stimulus of the duly prepared food, rises the instant
it is touched by it, and affords it a free passage to the pharynx
(figs. CLIII. 10, and CLIV. 10).

603. The pharynx (fig. CLIII. 10), a muscular bag, immediately
continuous with the mouth (fig. CLIII. 1), is a vestibule into which
open several highly important organs. Before is the entrance to the
windpipe, termed the glottis (fig. CLIV. 9), leading directly to the
larynx (fig. CLIV. 8); at the sides are the mouths of two ducts, termed
the Eustachian tubes, which lead to the internal part of the organ of
hearing; above are two entrances to the nose (fig. CLIV. 1); and below
is the passage to the stomach (fig. CLIII. 12).

604. Were the food to enter the Eustachian tubes or the nose, it would
occasion great inconvenience; were it to enter the glottis, it would
cause death. It is prevented from entering the Eustachian tubes and the
nose by the soft palate (fig. CLII. 1 and 2), which by the very act of
rising to afford an opening from the mouth to the pharynx, is carried
over the other apertures so as completely to close them. By the varied
direction of the muscular fibres which enter into the composition of
this organ, it is enabled to execute the different and even opposite
motions required in the performance of its important office.

605. The food is prevented from entering the glottis partly by a
cartilaginous valve (fig. CLIV. 7), termed the epiglottis, placed
immediately above the glottis, and attached to the root of the tongue
(fig. CLIV. 6). In delivering the food to the pharynx the tongue passes
backwards (fig. CLIV. 6). In passing backwards it pushes in the same
direction the epiglottis which is attached to it, and so necessarily
carries it over the glottis, completely closing the aperture (fig.
CLIV. 9). At the same time the opening is still more securely closed
by the glottis itself, in consequence of the powerful and simultaneous
contraction of the muscles that act upon it in the production of the
voice. It is proved, by direct experiment, that the spontaneous closure
of the glottis is a more powerful agent in excluding the food from the
larynx even than the depression of the epiglottis; but both organs
concur in producing the same result; and a double security is provided
against an event which would be fatal.

606. It is deeply interesting to observe the part performed in these
operations by sensation and volition, and the boundary at which their
influence terminates and consciousness itself is lost. Mastication, a
voluntary operation, carried on by voluntary muscles, at the command
of the will, is attended with consciousness, always in the state of
health of a pleasurable nature. To communicate this consciousness, the
tongue, the palate, the lips, the cheeks, the soft palate, and even
the pharynx, are supplied with a prodigious number of sentient nerves.
The tongue especially, one of the most active agents in the operation,
is supplied with no less than six nerves derived from three different
sources. These nerves, spread out upon this organ, give to its upper
surface a complete covering, and some of them terminate in sentient
extremities visible to the naked eye. These sentient extremities,
with which every point of the upper surface, but more especially the
apex, is studded, constitute the bodies termed papillæ, the immediate
and special seat of the sense of taste. This sense is also diffused,
though in a less exquisite degree, over the whole internal surface
of the mouth. Close to the sense of taste is placed the seat of the
kindred sense of smell. The business of both these senses is with the
qualities of the food. Mastication at once brings out the qualities
of the food and puts the food in contact with the organs that are to
take cognizance of it. Mastication, a rough operation, capable of
being accomplished only by powerful instruments which act with force,
is carried on in the very same spot with sensation, an exquisitely
delicate operation, having its seat in soft and tender structures,
with which the appropriate objects are brought into contact only with
the gentlest impulse. The agents of the coarse and the delicate, the
forcible and the gentle operations are in close contact, yet they
work together not only without obstruction, but with the most perfect
subserviency and co-operation.

607. The movements of mastication are produced, and, until they have
accomplished the objects of the operation, are repeated by successive
acts of volition. To induce these acts, grateful sensations are excited
by the contact of the food with the sentient nerves so liberally
distributed over almost the whole of the apparatus. To the provision
thus made for the production of pleasurable sensation, is superadded
the necessity of direct and constant attention to the pleasure
included in the gratification of the taste. It is justly observed by
Dr. A. Combe, that without some degree of attention to the process of
eating, and some distinct perception of its gratefulness, the food
cannot be duly digested. When the mind is so absorbed as to be wholly
unconscious of it, or even indifferent to it, the food is swallowed
without mastication; then it lies in the stomach for hours together
without being acted upon by the gastric juice, and if this be done
often, the stomach becomes so much disordered as to lose its power of
digestion, and death is the inevitable result: so that not only is
pleasurable sensation annexed to the reception of food, but the direct
and continuous consciousness of that pleasurable sensation during the
act of eating is made one of the conditions of the due performance of
the digestive function.

608. With the operation of mastication and one part of the process of
deglutition, immediately to be noticed, the agency of volition and
sensation cease. Beyond this the function of digestion is wholly an
organic process. In addition to the reasons assigned (vol. i. p. 55)
why all the organic processes are placed alike beyond the cognizance of
sense and the control of the will, there is this special reason why, in
the function of digestion, they cease at the exact boundary assigned
them.

609. Every time the act of deglutition is performed the openings to the
windpipe and to the nose are closed, so that during this operation all
access of air to the lungs is stopped, consequently it is necessary
that the passage of the food through the pharynx should be rapid.
Mastication, a voluntary process, may be performed slowly or rapidly,
perfectly or imperfectly, without serious mischief; but life depends on
the passage of the food through the pharynx with extreme rapidity and
with the nicest precision. It is therefore taken out of the province
of volition and entrusted to organs which belong to the organic life,
organs which carry on their operations with the steadiness, constancy,
and exactness of bodies whose motions are determined by a physical law.

610. No sooner does the duly-prepared food touch the soft palate than
the whole apparatus of deglutition is instantly in motion. This movable
partition suddenly rises to afford to the food a free passage to the
pharynx. The pharynx itself, at the same instant, rises to receive the
morsel thrust towards it by the pressure of the tongue; and one muscle,
the stylo-pharyngeus, which concurs in producing this movement, seems
specially intended, in addition, to expand the pharynx. Three muscles
throw their fibres around the pharynx, termed its upper, middle, and
lower constrictors, which, the moment the morsel reaches the pharynx,
contract upon it, and embrace it firmly. At the same instant the
larynx, closing its aperture, springs forward towards the base of the
tongue, under which it is in a manner concealed, the additional shield
of the epiglottis being simultaneously thrown over the glottis. By this
movement of the larynx, upwards and forwards, the course of the morsel
across the dangerous passage is shortened. All these motions take
place with such rapidity that Boerhaave said the action is convulsive.
And now the food, firmly pressed by the pharynx, cannot return to the
mouth, for the root of the tongue is there stopping up the passage; it
cannot enter the Eustachian tubes or the nose, for the soft palate is
there closing the apertures; it cannot enter the larynx, for a double
guard is placed upon the glottis securing its firm closure. The food
can advance in one direction only, the direction required, that which
leads to the esophagus. Well, therefore, on the contemplation of these
complex structures and the consent and harmony with which they act,
might Paley say, “In no apparatus put together by art do I know such
multifarious uses so aptly contrived as in the natural organization of
the human mouth and its appendages. In this small cavity we have teeth
of different shape; first, for cutting; secondly, for grinding; muscles
most artificially disposed for carrying on the compound motions of the
lower jaw by which the mill is worked; fountains of saliva springing up
in different parts of the cavity for the moistening of the food while
the mastication is going on; glands to feed the fountains; a muscular
contrivance in the back part of the cavity for the guiding of the
prepared aliment into its passage towards the stomach, and for carrying
it along that passage. In the mean time, and within the same cavity,
is going on other business wholly different, that of respiration and
of speech. In addition, therefore, to all that has been mentioned,
we have a passage opened from this same cavity of the mouth into the
lungs for the admission of air, for the admission of air exclusively
of every other substance; we have muscles, some in the larynx, and,
without number, in the tongue, for the purpose of modulating that air
in its passage, with a variety, a compass, and a precision of which no
other musical instrument is capable; and, lastly, we have a specific
contrivance for dividing the pneumatic part from the mechanical, and
for preventing one set of functions from interfering with the other.
The mouth, with all these intentions to serve, is a single cavity; is
one machine, with its parts neither crowded nor confined, and each
unembarrassed by the rest.” It should be added, the mouth is also the
immediate seat of one of the senses, and is in intimate communication
with a second sense; both these senses are always excited while the
principal business performed by the machine is carried on, and are
necessarily excited by the very working of the machine, and the
sensations induced in the natural and sound state of the apparatus are
invariably pleasurable.

611. The food is delivered by the pharynx to the esophagus (fig.
CLIII. 12), a tube composed partly of membrane and partly of muscle.
Its muscular fibres consist of a double layer, an external and an
internal layer; the external has a longitudinal direction; the internal
describes portions of a circle around the tube. By the contraction
of the longitudinal fibres the length, and by the contraction of the
circular fibres, the diameter of the tube is diminished. Cellular
membrane envelops these layers of fibres externally, and mucous
membrane covers them internally. When the tube is contracted, the
mucous membrane is disposed in folds, which disappear when it is
dilated, and these folds allow of the expansion of the tube without
injury to the delicate tissue that lines it. The food passes slowly
along the esophagus urged towards the stomach, not by its own gravity,
but by a force exerted upon it by the tube itself, chiefly by the
contraction of its circular fibres. Delivered at length to the
stomach, the food is incapable of returning into the esophagus in
consequence of the oblique direction in which the esophagus enters the
stomach, the obliquity of its entrance serving the office of a valve.

[Illustration: Fig. CLXVI.—_View of the Stomach with its Muscular Coats
displayed._

 1. The esophagus terminating in the stomach. 2. The cardiac orifice.
 3. The pylorus. 4. The commencement of the duodenum. 5. The large
 curvature of the stomach. 6. The small curvature. 7. The large
 extremity. 8. The small extremity. 9. The longitudinal muscular
 fibres. 10. The circular muscular fibres.]

612. The stomach is a bag of an irregular oval shape (fig CLXVI.),
capable, in the adult, of containing about three pints. It is placed
transversely across the upper part of the abdomen (fig. LX. 7). It
occupies the whole epigastric (fig. CV. 3), and the greater part of
the left hypochondriac regions (fig. CVII. 3). Above, it is in contact
with the diaphragm, the arch of which extends over it (fig. LX. 7, b);
below with the intestines (fig. LX. 8, 9), on the right side with the
liver (fig. LX. 6), and on the left side with the spleen (fig. CLXVIII.
5).

[Illustration: Fig. CLXVII. _Internal View of the Stomach and Duodenum._

 1. Mucous membrane, forming the rugæ. 2. Pyloric orifice opening into
 the duodenum. 3. Duodenum. 4. Interior of the duodenum, showing the
 valvulæ conniventes. 5. Termination of, 6. The biliary or choledoch
 duct. 7. Pancreatic duct, terminating at the same point as the
 choledoch duct. 8. Gall-bladder removed from the liver. 9. Hepatic
 duct proceeding from the liver. 10. Cystic duct proceeding from the
 gall-bladder, forming by its union with the hepatic, a common trunk,
 the choledoch.]

613. Into the left extremity, which is much larger and considerably
higher than the right (fig. CLXVI. 7), the esophagus opens by an
aperture called the cardiac orifice (fig. CLXVI. 2). At the right
extremity, a second aperture called the pyloric orifice (fig. CLXVII.
2), leads into the first intestine.

614. Between the cardiac and the pyloric orifices are two curvatures,
one above, called the smaller (fig. CLXVI. 6), the other below, termed
the larger curvature (fig. CLXVI. 5).

615. Like the esophagus, the stomach is composed of two layers of
muscular fibres, the external longitudinal (fig. CLXVI. 9), the
internal circular (fig. CLXVI. 10). By the contraction of the first the
extent of the stomach, from extremity to extremity, is diminished, or
the organ is shortened; by the contraction of the second the extent of
the stomach, from curvature to curvature, is diminished, or the organ
is narrowed. During digestion, by the contraction of these muscular
fibres, the capacity of the stomach is changed alternately in both
directions, whence a gentle motion is communicated to the aliment,
which is thus brought in succession under the influence of the agent
that acts upon it.

616. A thin but strong membrane, derived from the peritoneum, the
membrane that lines the general cavity of the abdomen, forms the
external tunic of the stomach; hence its outer covering is called the
peritoneal coat.

617. The inner or mucous coat (fig. CLXVII. 1), a direct continuation
of the lining membrane of the esophagus, is sometimes called also
villous, on account of the minute bodies termed villi, with which every
point of its internal surface is studded. It is these villi which give
to this surface its pilous or velvety appearance,

[Illustration: Fig. CLXVIII.—_View of the Vascular connexion between
the Stomach, Liver, Spleen, and Pancreas._

 1. Stomach raised to exhibit its posterior surface. 2. Pylorus. 3.
 Duodenum. 4. Pancreas. 5. Spleen. 6. Undersurface of the liver. 7.
 Gall-bladder, in connexion with the liver. 8. Large vessels proceeding
 from. 9. A common trunk to supply the liver, gall-bladder, stomach,
 duodenum, pancreas, and spleen.]

618. The mucous coat is far more extensive than the other two, in
consequence of its being plaited into a number of folds (fig. CLXVII.
1), termed rugæ, which are so disposed as to present the appearance
of a net-work. The object of the rugæ is to enlarge the space for the
expansion of blood-vessels and nerves, and to admit of the occasional
distension of the organ without injury to the delicate tissues of which
it is composed.

619. Immediately beneath the mucous coat are the mucous follicles
which secrete the mucous fluid by which the inner surface of the organ
is defended. These glandular bodies are extremely numerous, and vary
considerably in diameter. The largest are towards the great extremity,
the smaller towards the pylorus.

620. Altogether different from the mucous secretion is another fluid,
which also flows from the mucous surface, termed the gastric or the
digestive juice, from its being the principal agent in the digestive
process. By some anatomists the gastric juice is supposed to be
secreted by minute glands placed between the mucous and the muscular
coats, provided with ducts which pierce the mucous coat, and which bear
their fluid into the stomach precisely as the salivary glands carry the
saliva into the mouth. It is certain that this is the case with some
animals, as in certain birds, the ostrich for example, in which glands
of considerable magnitude, with ducts large enough to be visible, are
seen to pour the digestive fluid into the stomach. But as no such
glands have been discovered in the human stomach, it is generally
conceived that in man the gastric juice is secreted by minute arteries
expanded upon the villi.

621. All around the pyloric orifice (fig. CLXVII. 2) is placed a
thick, strong, and circular muscle (fig. CLXVII. 2), termed, from its
office, pylorus. It is about four times the thickness of the muscular
coat of the stomach, and presents the appearance of a prominent and
even projecting band (fig. CLXVII. 2). From the frequent action of its
fibres, the pylorus often looks as if a piece of packthread had been
tied around it (fig. CLXVI. 3). Its office is, by the contraction of
its fibres, to guard and close the opening from the stomach until the
aliment has been duly acted upon by the digestive fluid.

[Illustration: Fig. CLXIX.

 View of the stomach, showing the number and magnitude of its
 blood-vessels, and the mode of their distribution.]

622. The quantity of blood sent to the stomach is greater than is spent
upon any other organ except the brain. The vessels of the stomach
(fig. CLXIX.) form two distinct layers, of which the external is
distributed to the peritoneal and muscular coats, while the internal,
after ramifying on the fine cellular tissue which unites the muscular
and mucous tunics, penetrates the mucous coat, and is spent upon the
villi, where it forms an exquisitely-delicate net-work. There is,
moreover, an intimate vascular connexion between the spleen, pancreas
and liver, and the stomach (fig. CLXVIII. 8, 9). The arteries which
supply all these organs spring from a common trunk, and there is the
freest communication between them by anastomosing branches.

[Illustration: Fig. CLXX.—_View of the Organic Nerves of the Stomach._

 1. Under surface of the liver turned up, to bring into view the
 anterior surface of the stomach. 2. Gall bladder. 3. Organic nerves
 enveloping the trunks of the blood-vessels. 4. Pyloric extremity of
 the stomach and commencement of the duodenum. 5. Contracted portion
 of the pylorus. 6. Situation of the hour-glass contraction of the
 stomach, here imperfectly represented. 7. Omentum.]

623. Equally abundant is its supply of nerves, some of which are
derived from the organic or non-sentient system, and others from
the animal or sentient system. The organic nerves are spread out in
countless numbers upon the great trunks of the arteries, so as to give
them a complete envelope (fig. CLXX. 3); these nerves, never quitting
the arteries, accompany them in all their ramifications, and the fibril
of the nerve is ultimately lost upon the capillary termination of the
artery. It is by these organic nerves that the stomach is enabled to
perform its organic functions, which, for the reason assigned (vol. i.
p. 82), is placed beyond volition, and is without consciousness. By the
nerves derived from the sentient system which mingle with the organic
(fig. XVI.), the function of nutrition is brought into relation with
the percipient mind, and is made part of our sentient nature. By the
commixture of these two sets of nerves, derived from these two portions
of the nervous system, though we have no _direct_ consciousness of the
digestive process—consciousness ceasing precisely at the point where
the agency of volition stops (vol. i. p. 82, et seq.), yet pleasurable
sensation results from the due performance of the function. Hence
the feeling of buoyancy, exhilaration, and vigour, the pleasurable
consciousness to which we give the name of health, when the action of
the stomach is sound: hence the depression, listlessness, and debility,
the painful consciousness which we call disease, when the action of the
stomach is unsound: hence, too, the influence of the mental state over
the organic process; the rapidity and perfection with which the stomach
works when the mind is happy—when the repast is but the occasion and
accompaniment of the feast of reason and the flow of soul; the slowness
and imperfection with which the stomach works when the mind is harassed
with care struggling against adverse events; or is in sorrow and
without hope; when the friend that sat by our side, and with whom we
were wont to take sweet counsel, is gone, and therefore gone that which
made it life to live.

624. Renovation is the primary and essential office of the stomach,
and its organic nerves enable it to supply the ever-recurring wants of
the system. Gratification of appetite is a secondary and subordinate
office of the stomach, and its sentient nerves enable it to produce
the state of pleasurable consciousness when its organic function is
duly performed. By the double office thus assigned it, the stomach is
rendered what Mr. Hunter named it, the centre of sympathies.

625. From the whole length of the great arch of the stomach, and
partly also from the commencement of the duodenum (fig. CLXX.), the
peritoneal coat of the stomach is produced, forming a thin, delicate
membranous bag, called the omentum, or cawl (fig. CLXX. 7). The omentum
extends from the great arch of the stomach to below the umbilicus,
and completely covers a large portion of the anterior surface of the
abdominal viscera (fig. CLXX. 7). Between the two fine membranous
layers of which it is composed is contained a quantity of fat, of which
substance it serves as a reservoir, and by the transudation of which it
appears to lubricate the intestines, and to assist in preventing their
accretion.

626. The food, on reaching the stomach, does not occupy indifferently
any portion of it, but is arranged in a peculiar manner always in one
and the same part. If the stomach be observed in a living animal, or
be inspected soon after death, it is seen that about a third of its
length towards the pylorus is divided from the rest by the contraction
of the circular fibre called the hour-glass contraction (fig. CLXX.
6). The stomach is thus divided into a cardiac and a pyloric portion
(fig. CLXX. 6). The food, when first received by the stomach, is always
deposited in the cardiac portion, and is there arranged in a definite
manner. The food first taken is placed outermost, that is, nearest the
surface of the stomach; the portion next taken is placed interior to
the first, and so on in succession, until the food last taken occupies
the centre of the mass. When new food is received before the old is
completely digested, the two kinds are kept distinct, the new being
always found in the centre of the old.

627. Soon after the food has been thus arranged, a remarkable change
takes place in the mucous membrane of the stomach. The blood-vessels
become loaded with blood; its villi enlarge, and its cryptæ, the minute
cells between the rugæ, overflow with fluid. This fluid is the gastric
juice, which is secreted by the arterial capillaries now turgid with
blood. The abundance of the secretion, which progressively increases as
the digestion advances, is in proportion to the indigestibility of the
food, and the quietude of the body after the repast.

628. In the food itself no change is manifest for some time; but at
length that portion of it which is in immediate contact with the
surface of the stomach begins to be slightly softened. This softening
slowly but progressively increases until the texture of the food,
whatever it may have been, is gradually lost; and ultimately the most
solid portions of it are completely dissolved.

629. When a portion of food thus acted on is examined, it presents the
appearance of having been corroded by a chemical agent. The white of a
hard-boiled egg looks exactly as if it had been plunged in vinegar or
in a solution of potass. The softened layer, as soon as the softening
is sufficiently advanced, is, by the action of the muscular coat of
the stomach, detached, carried towards the pylorus, and ultimately
transmitted to the duodenum; then another portion of the harder and
undigested food is brought into immediate contact with the stomach,
becomes softened in its turn, and is in like manner detached; and this
process goes on until the whole is dissolved.

630. The solvent power exerted by the gastric juice is most apparent
when the stomach of an animal is examined three or four hours after
food has been freely taken. At this period the portion of the food
first in contact with the stomach is wholly dissolved and detached; the
portion subsequently brought into contact with the stomach is in the
process of solution, while the central part remains very little changed.

631. The dissolved and detached portion of the food, from every
part of the stomach flows slowly but steadily beyond the hour-glass
contraction, or towards the pyloric extremity (626), in which not a
particle of recent or undissolved food is ever allowed to remain. The
fluid, which thus accumulates in this portion of the stomach, is a
new product, in which the sensible properties of the food, whatever
may have been the variety of substances taken at the meal, are lost.
This new product, which is termed chyme, is an homogeneous fluid,
pultaceous, greyish, insipid, of a faint sweetish taste, and slightly
acid.

632. As soon as the chyme, by its gradual accumulation in the pyloric
extremity amounts to about two or three ounces, the following phenomena
take place.

633. First, the intestine called duodenum, the organ immediately
continuous with the stomach, contracts. The contraction of the duodenum
is propagated to the pyloric end of the stomach. By the contraction
of this portion of the stomach, the chyme is carried backwards from
the pyloric into the cardiac extremity, where it does not remain,
but quickly flows back again into the pyloric extremity, which is
now expanded to receive it. Soon the pyloric extremity begins again
to contract; but now the contraction, the reverse of the former,
is in the direction of the duodenum; in consequence of which, the
chyme is propelled towards the pylorus. The pylorus, obedient to the
demand of the chyme, relaxes, opens, and affords to the fluid a free
passage into the duodenum. As soon as the whole of the duly prepared
chyme has passed out of the stomach, the pylorus closes, and remains
closed, until two or three ounces more are accumulated, when the same
succession of motions are renewed with the same result; and again cease
to be again renewed, as long as the process of chymification goes on.

634. When the stomach contains a large quantity of food, these
motions are limited to the parts of the organ nearest the pylorus; as
it becomes empty, they extend further along the stomach, until the
great extremity itself is involved in them. These motions are always
strongest towards the end of chymification.

635. The stomach during chymification is a closed chamber; its cardiac
orifice is shut by the valved entrance of the esophagus, and its
pyloric orifice by the contraction of the pylorus.

636. The rapidity with which the process of chymification is carried
on is different according to the digestibility of the food, the bulk
of the morsels swallowed, the quantity received by the stomach, the
constitution of the individual, the state of the health, and above
all, the class of the animal, for it is widely different in different
classes. In the human stomach in about five hours after an ordinary
meal the whole of the food is probably converted into chyme.

637. The great agent in performing the process of chymification is the
gastric juice. The evidence of this is complete; for,

1. As soon as the food enters the stomach a large quantity of blood is
determined to the arteries, which secrete the gastric juice (627); and
this fluid continues to be poured into the stomach in great abundance
during the whole time the process goes on.

2. The solvent power of this fluid is demonstrated by the fact that it
sometimes dissolves the stomach itself, when death takes place suddenly
during the act of digestion in a sound and vigorous state of the
digestive organs.

3. On introducing into the stomach alimentary substances inclosed in
metallic balls perforated with holes, or in pieces of porous cloth,
it is found, on removing these bodies from the stomach, after a
certain time, that the alimentary substances contained in them are as
completely digested as if they had been in actual contact with the
surface of the stomach; the metallic ball and the cloth remaining
wholly unchanged. This experiment, which has been often performed with
the same uniform result, was the first that led to the discovery of the
true nature of the digestive process.

4. Though it be impossible to imitate out of the stomach all the
circumstances under which the food is placed within it, yet, on
procuring gastric juice from the stomachs of various animals, and
mixing it with different alimentary substances, it is found not only
to dissolve them, but to convert them into a pultaceous mass, closely
resembling chyme. Gastric juice thus procured was put into a glass
tube with boiled beef, which had been masticated; the tube was then
hermetically sealed, and exposed near the fire to a uniform heat: by
the side of this tube was placed another, containing the same quantity
of flesh immersed in water. In twelve hours, the flesh in the tube
containing the gastric juice began to lose its fibrous structure; in
thirty-five hours it had nearly lost its consistence, being reduced to
a soft homogeneous pultaceous mass. It experienced no further change
during the two following days. On the other hand, the flesh that had
been immersed in water was putrid in sixteen hours.

638. Since alimentary substances under the action of the stomach
present precisely the appearance exhibited by bodies exposed to the
influence of chemical agents, it appears that the gastric juice not
only dissolves the food, but dissolves it by a chemical agency. Its
action bears no proportion to the mechanical texture of bodies,
nor to any of their physical properties. It acts upon the densest
membrane, dissolves even bone itself; and yet produces no effect on
other substances of the most tender and delicate texture. On the skin
of fruit, on the finest fibre of flax and cotton, it is incapable of
making the slightest impression. In this selection of substances it
perfectly resembles a chemical agent acting by chemical affinity. On
certain substances its action is unquestionably of a chemical nature.
It occasions the coagulation of albuminous fluids; it prevents the
accession of putrefaction; it stops the process after it has commenced.
From the whole, it follows that the food in the stomach is converted
into chyme by the agency of a fluid secreted by the inner surface of
the stomach, and that this change is effected by a proper chemical
action.

639. It had been long ascertained that the gastric juice contains an
uncombined acid, and that if carbonate of lime be placed in a tube and
introduced into the stomach, the carbonate is dissolved just as if
it were put into weak vinegar. Several years ago, it was discovered
by Dr. Prout that this free acid is muriatic acid. Some time after
the publication of Dr. Prout’s experiments, Chevreul and Leuret and
Lassaigne in France obtained different results; but Tiedemann and
Gmelin, professors in the university of Heidelberg, in an extended
series of experiments, arrived at precisely the same conclusion as the
English physiologist, and apparently without any previous knowledge of
the researches of the latter. Tiedemann and Gmelin state, as the result
of their experiments, that the clear ropy fluid, or the gastric juice
obtained from the stomach some time after it had been without food, is
nearly or entirely destitute of acidity; that the presence of food,
or indeed of any stimulus to the mucous membrane, causes the gastric
juice to become distinctly acid; that this acidity increases according
to the indigestibility of the food; that the quantity of acid poured
out is very copious; that it consists partly of muriatic and partly
of acetic acid; and that both these acids are efficient agents in the
process of digestion. Dr. Prout, who had also recognised the presence
of acetic acid, is of opinion that its formation is an accidental
occurrence not necessary to digestion nor conducive to it; but is
either derived from the aliment, or is the result of irritation or
disease. He contends that the muriatic acid is the efficient digestive
agent.

640. The still more recent experiments of Braconnot appear to have
set this matter at rest, and to have proved, beyond all controversy,
that the stomach, when stimulated by the presence of food or other
foreign agents, possesses the power of secreting free muriatic acid
in great quantity; and that it is by this acid that the solution of
the food is effected. It is even found that muriatic acid is capable
of digesting alimentary substances out of the body. It had been long
known, that if meat and gastric juice be inclosed in a tube and kept
at the temperature of the human body, a product is obtained closely
resembling chyme (637.4). M. Blondelot, a physician at Nancy, has
recently shown that the same result may be obtained by the digestion
of the muscular fibre, in dilute muriatic acid. In both cases the
muscular fibre retains its form and its original fibrous texture; but
on the slightest motion it divides into an insoluble mass, perfectly
homogeneous and similar to the chyme of the stomach;[5] a very close
approximation to the actual digestive process, more especially when it
is considered that it is not possible to imitate out of the stomach
several circumstances materially influencing chemical action under
which the food is placed within the stomach.

  [5] Dr. R. Thomson, British Annals of Medicine, No. 13.

641. Muriatic acid, the chemical agent by which the stomach dissolves
the food, is probably obtained from the muriate of soda (common salt)
contained in the blood. The soda, the basis of the salt, would appear
to be retained in the blood, to preserve the alkaline condition
essential to the maintenance of the sound constitution of the blood,
while the muriatic acid, disengaged from the soda in the process of
secretion, is poured into the stomach to act upon the food.

642. A remarkable confirmation of the correctness of the general
conclusions to which observation and experiment had thus enabled
physiologists to arrive, is afforded by the case of a young soldier
in the American army, of the name of Alexis St. Martin, who received
a wound on the left side by the accidental discharge of a musket. The
charge, which consisted of duck shot, and which was received at the
distance of one yard from the muzzle of the gun, entered the side
posteriorly in an oblique direction, forward and inward; blew off
the integument and muscles to the size of a man’s hand; fractured
and carried away the anterior half of the sixth rib; fractured the
fifth rib; lacerated the lower portion of the left lobe of the lungs;
lacerated the diaphragm, and perforated the stomach.

643. Violent fever and extensive sloughing of the parts injured ensued,
and the life of the young man was often in jeopardy, but he ultimately
recovered. At the distance of about a year from the date of the
accident, the injured parts had all become sound, with the exception
of the perforation into the stomach, which never closed, but left an
aperture permanently open, two inches and a half in circumference.
This aperture was situated about three inches to the left of the
cardia, near the left superior termination of the great curvature.
For some time the food could be retained only by constantly wearing a
compress and bandage; but at length a small fold of the mucous coat
of the stomach appeared, which increased until it completely filled
the aperture and acted as a valve, so as effectually to prevent any
efflux from within, while it admitted of being easily pushed back by
the finger from without: when the stomach was nearly empty, it was easy
to examine its cavity to the depth of five or six inches by artificial
distension; but, when entirely empty, the stomach was always contracted
on itself, and the valve generally forced through the orifice,
together with a portion of the mucous membrane equal in bulk to a hen’s
egg.

644. It chanced that the admirable opportunity thus afforded of
bringing the process of digestion, as far as it is carried on in the
stomach, under the cognizance of sense, occurred to an observant and
philosophical mind, and it was not lost.[6] The following are some of
the curious and instructive phenomena observed.

  [6] Experiments and Observations on the Gastric Juice, and the
  Physiology of Digestion. By W. Beaumont, M.D., Surgeon in the U. S.
  Army. Boston. 1834.

645. The inner coat of the stomach, in its natural and healthy state,
is of a light or pale pink colour, varying in its hues according to its
full, or empty state. It is of a soft or velvet-like appearance (617),
and is constantly covered with a very thin transparent, viscid mucus,
lining the whole interior of the organ (619).

646. Immediately beneath the mucous coat appear small spheroidal, or
oval-shaped glandular bodies, from which the mucous fluid appears to be
secreted (619).

647. By applying aliment or other irritants to the internal coat of
the stomach, and observing the effect through a magnifying glass,
innumerable minute lucid points, and very fine nervous or vascular
papillæ are seen arising from the villous membrane, and protruding
through the mucous coat, from which distils a pure, limpid,
colourless, slightly viscid fluid (620). This fluid, thus excited, is
invariably distinctly acid (639, _et seq._). The _mucus_ of the stomach
is less fluid, more viscid or albuminous, semi-opaque, sometimes a
little saltish, and does not possess the slightest character of acidity
(619). On applying the tongue to the mucous coat of the stomach in its
empty, un-irritated state, no acid taste can be perceived. When food
or other irritants have been applied to the villous membrane and the
gastric papillæ excited, the acid taste is immediately perceptible.
The invariable effect of applying aliment to the internal, but exposed
part of the gastric membrane, is the exudation of the solvent fluid
from the papillæ. Though the aperture of these vessels cannot be seen
even with the assistance of the best microscopes, yet the points from
which the fluid issues are clearly indicated by the gradual appearance
of innumerable very fine lucid specks rising through the transparent
mucous coat, and seeming to burst and discharge themselves upon the
very points of the papillæ, diffusing a limpid thin fluid over the
whole interior gastric surface.

648. The fluid so discharged is absorbed by the aliment in contact; or
collects in small drops, and trickles down the sides of the stomach to
the more depending parts, and there mingles with the food, or whatever
else may be contained in the gastric cavity. This fluid, the efficient
cause of digestion, the true gastric juice is secreted only when it
is needed; it is not accumulated in the intervals of digestion, to
be ready for the next meal; it is seldom if ever discharged from its
proper secreting vessels, except when excited by the natural stimulus
of aliment, the mechanical irritation of tubes, or other excitants.
When aliment is received, the juice is given out in exact proportion
to its requirements for solution, except when more food has been taken
than is necessary for the wants of the system.

649. On collecting this fluid, which it was easy to obtain, it was
found to be transparent, inodorous, saltish, and acidulous to the
taste; it consisted of water, containing free muriatic and acetic
acids, phosphates and muriates, with bases of potass, soda, magnesia,
and lime, together with an animal matter soluble in cold, but insoluble
in hot water.

650. When a portion of liquid aliment, as a few spoonsful of soup,
were introduced into the stomach at the external orifice, the rugæ
(fig. CLXVII. 1) immediately closed gently upon it; gradually diffused
it through the gastric cavity, and prevented the entrance of a second
quantity till this diffusion was effected; then relaxation again took
place, and admitted of a further supply. When solid food was introduced
in the same manner, either in large pieces or finely divided, the same
gentle contraction and grasping motions were excited, and continued
from fifty to eighty seconds, so as to prevent more from being
introduced, without considerable force till the contraction was at an
end.

651. When the position of the body was such that the cardiac portion
of the stomach was brought into view, and a morsel of food was
swallowed in the natural mode, a similar contraction of the stomach,
and closing of its fibres upon the bolus was invariably observed to
take place; and till this was over, a second morsel could not be
received without a considerable effort. Hence, in addition to the other
purposes accomplished by mastication, insalivation, and deglutition,
it is probable that these operations answer the further use of duly
regulating the time for the admission of successive portions of the
food into the stomach.[7]

  [7] See Dr. Andrew Combe on the Physiology of Digestion, in whose
  work a full detail of this instructive case is given. See also Mayo’s
  Outlines of Physiology, 4th Edit. Appendix.

652. On watching the phenomena that take place on the contact of a
portion of food with the stomach, the circumstances described (627)
are seen; the change in the mucous coat from a pale pink to a deep red
colour, in consequence of the enlargement of the blood-vessels and
their admission of a greatly increased number of red particles; the
undulating motion of the stomach, in consequence of the contraction of
its muscular fibres, excited by the stimulus of food; the distillation
of the gastric juice from the enlarged and excited papillæ; the
continuous flow of this fluid until the complete solution of the food,
when food is present; and, on the contrary, the cessation of this
discharge in a short time when it is produced by a mechanical irritant,
as the bulb of a thermometer, although at first the gastric juice
distil from the papillæ, from the contact of such an irritant, just as
when excited by the contact of food.

653. On collecting the gastric juice and placing it in contact with an
alimentary substance out of the stomach, its solution takes place more
slowly, but not less completely, than when retained in the stomach.
An ounce of this fluid was placed in a vial with a piece of boiled,
recently salted beef, weighing three drachms; the vial was then tightly
corked, and immersed in water, raised to the temperature of 100°,
previously ascertained to be the ordinary heat of the stomach. In forty
minutes the process of solution had commenced on the surface of the
beef. In fifty minutes the texture of the beef began to loosen and
separate. In sixty minutes an opaque and cloudy fluid was formed. In
one hour and a half the muscular fibres hung loose and unconnected, and
floated about in shreds in the more fluid matter. In three hours the
muscular fibres had diminished about one half. In five hours only a
few remained undissolved. In seven hours the muscular texture was no
longer apparent; and in nine hours the solution was completed.

654. At the commencement of this experiment a piece of the same beef of
equal weight and size was suspended within the stomach by means of a
string. On examining this portion of beef at the end of half an hour,
it was found to present precisely the same appearance as the piece in
the vial; but on the removal of the string at the end of an hour and
a half the beef had been completely dissolved, and had disappeared,
making a difference of result in point of time of nearly seven hours.
In both, the solution began on the surface, and agitation accelerated
its progress by removing the external coating of chyme as fast as it
was formed.

655. An ordinary dinner having been taken, consisting of boiled salted
beef, bread, potatoes, and turnips, with a gill of pure water for
drink, a portion of the contents of the stomach was drawn off into an
open mouthed vial, twenty minutes after the meal. The vial was placed
in a water-bath, maintained steadily at a temperature of 100°. It
was continued in this temperature for five hours. At the end of that
time the whole contents of the vial were dissolved. On comparing the
solution with an equal quantity of chyme taken from the stomach, little
difference could be distinguished between the two fluids, excepting
that it was manifest that the digestive process had proceeded somewhat
more rapidly in, than out of the stomach. The food, in this experiment,
after having remained in contact with the stomach for the space of
twenty minutes, had imbibed a sufficient quantity of gastric juice to
complete its solution.

656. Fifteen minutes after half a pint of milk had been introduced into
the stomach, it presented the appearance of a fine loosely-coagulated
substance mixed with a semi-transparent whey-coloured fluid. A drachm
of warm gastric juice poured into two drachms of milk at a temperature
of 100°, produced a precisely similar appearance in twenty minutes. In
another experiment, when four ounces of bread were given with a pint of
milk, the milk was coagulated and the bread reduced to a soft pulp in
thirty minutes, and the whole was completely digested in two hours.

657. When the albumen or white of two eggs was swallowed on an empty
stomach, small white flakes began to be seen in about ten or fifteen
minutes, and the mixture soon assumed an opaque whitish appearance. In
an hour and a half the whole had disappeared. Two drachms of albumen
mixed with two of gastric juice out of the stomach underwent precisely
the same changes, but in a somewhat longer time.

658. Dr. Beaumont’s observations are adverse to the opinion, founded
on numerous experiments, that the food is arranged in the stomach in
a definite manner, and that a distinct line of separation exists
between old and new food (626). In the human stomach, according to
the subject of these experiments, the ordinary course and direction
of the food are first from right to left along the small arch, and
thence through the large curvature from left to right. The bolus as
it enters the cardia turns to the left, passes the aperture, descends
into the splenic extremity, and follows the great curvature towards the
pyloric end. It then returns in the course of the smaller curvature,
makes its appearance again at the aperture, in its descent into the
great curvature, to perform similar revolutions. These revolutions
are completed in from one to three minutes. They are probably induced
in a great measure by the circular or transverse muscles of the
stomach (615), as is indicated by the spiral motion of the stem of
the thermometer, both in descending to the pyloric portion, and in
ascending to the splenic. These motions are slower at first than
after chymification has considerably advanced. The whole contents
of the stomach, until chymification be nearly complete, exhibit a
heterogeneous mass of solids and fluids, hard and soft, coarse and
fine, crude and chymified; all intimately mixed, and circulating
promiscuously through the gastric cavity like the mixed contents of a
closed vessel, gently agitated or turned in the hand.

659. In attempting to pass a long glass thermometer through the
aperture into the pyloric portion of the stomach, during the latter
stages of digestion, a forcible contraction is perceived at the point
of the hour-glass contraction of the stomach, and the bulb is stopped.
In a short time there is a gentle relaxation, when the bulb passes
without difficulty, and appears to be drawn quite forcibly, for three
or four inches, towards the pyloric end. It is then released, and
forced back, or suffered to rise again, at the same time giving to the
tube a circular or rather a spiral motion, and frequently revolving it
quite over. These motions are distinctly indicated and strongly felt
in holding the end of the tube between the thumb and finger; and it
requires a pretty forcible grasp to prevent it from slipping from the
hand, and being drawn suddenly down to the pyloric extremity. When the
tube is left to its own direction at these periods of contraction, it
is drawn in, nearly its whole length, to the depth of ten inches; and
when drawn back requires considerable force, and gives to the fingers
the sensation of a strong suction power, like drawing the piston from
an exhausted tube. This ceases as soon as the relaxation occurs, and
the tube rises again, of its own accord, three or four inches, when the
bulb seems to be obstructed from rising further; but if pulled up an
inch or two through the stricture, it moves freely in all directions
in the cardiac portions, and mostly inclines to the splenic extremity,
though not disposed to make its exit at the aperture. These peculiar
motions and contractions continue until the stomach is perfectly
empty, and not a particle of food or chyme remains, when all becomes
quiescent again.

660. The chambers in which the remaining part of the digestive process
is carried on are much less accessible, and no such favourable
opportunity as that enjoyed by Dr. Beaumont has occurred of rendering
their operations manifest to the eye. Nevertheless, the researches of
physiologists have succeeded in disclosing, with almost equal exactness
and certainty, the successive changes which the food undergoes even in
these more hidden organs, that admit of no exposure during life without
extreme danger.

661. The chyme on passing through the pylorus is received into a
chamber (fig. CLXVII. 3) which forms the first portion of the small
intestines. The small intestines, taken together, constitute a tube
about four times the length of the body. This tube is conical, the
base of the cone being towards the pylorus, and its apex at the valve
of the colon, where the small intestines terminate in the large. From
the pylorus to the valve of the colon the small intestines diminish in
capacity, in thickness, in vascularity, in the size of the villi, and
in the depth and number of the valvulæ conniventes.

[Illustration: Fig. CLXXI.

 1. Esophagus. 2. Stomach. 3. Liver raised, showing the under surface.
 4. Duodenum. 5. Small intestines, consisting of—6. Jejunum and ilium.
 7. Colon. 8. Urinary bladder. 9. Gall bladder. 10. Abdominal muscles
 divided and reflected.]

662. The first portion of the small intestine is termed the duodenum
(fig. CLXVII. 3). It is about twelve inches in length, and, unlike the
stomach, which is capable of considerable motion, it is closely tied
down to the back by the peritoneum, which imperfectly covers it. The
rest of the small intestine is divided into two portions—the upper
two-fifths of which are termed jejunum, and the three lower ilium.

663. The duodenum, the chamber which receives the chyme from the
pylorus, is a second stomach, which carries on the process commenced
in the first. It is assisted in the performance of its function by two
organs of considerable magnitude, the pancreas and the liver.

664. The pancreas is a conglomerate gland (fig. CLXXII. 5), of an
elongated form, placed in the epigastric region, lying transversely
across it, immediately behind the stomach (fig. CLXXII. 1), and resting
upon the spinal column (fig. CLXXII. 5). Its right extremity is
attached to the duodenum (fig. CLXXII. 9), and its left to the spleen
(fig. CLXXII. 4). In external appearance it resembles the salivary
glands, but it is of much larger size, and its weight, from four to six
ounces, is three times greater than that of all the salivary glands
together. It secretes a peculiar fluid called the pancreatic juice,
which is carried into the duodenum by a tube named the pancreatic duct
(fig. CLXVII. 7), which opens into the duodenum about four or five
inches from its pyloric end (fig. CLXVII. 2).

[Illustration: Fig. CLXXII.

 1. Stomach raised. 2. Under surface of liver. 3. Gall bladder. 4.
 Spleen. 5. Pancreas. 6. Kidneys. 7. Ureters. 8. Urinary bladder. 9.
 Portion of intestine called duodenum. 10. Portion of intestine called
 rectum. 11. Aorta.]

665. The liver, the largest and heaviest gland in the body, weighing
about four pounds, is placed chiefly in the right hypochondriac region
(fig. CLXXI. 3); but a portion of it extends transversely across the
epigastric, into the left hypochondriac region (figs. CV. and CVII.
3). Its upper surface is in contact with the diaphragm (fig. LX. 6, b);
its under surface with the pyloric extremity of the stomach (fig. LX.
7), and its margin can be felt under the edges of the ribs of the right
side.

666. It has been stated (473, 1.) that the fluid secreted by the liver,
unlike that formed by any other organ of the body, is elaborated from
venous blood, derived from the veins of the digestive organs, and
that these veins uniting together, form a common trunk called the
vena portæ, which penetrates the liver and ramifies through it in the
manner of an artery. Galen long ago compared this venous system to a
tree whose roots are dispersed in the abdomen, and its branches spread
out through the liver. Two comparatively small arteries, called the
hepatic, nourish the liver; the ultimate divisions of these arteries
likewise terminate in the vena portæ. The ultimate branches of the
vena portæ terminate partly in a system of veins, called the hepatic,
which like ordinary veins return the blood to the right side of the
heart; and partly in a system of tubes, termed the biliary ducts,
which contain the fluid secreted by the capillary branches of the vena
portæ. This fluid is the bile. The biliary ducts uniting from all
parts of the liver by innumerable branches, at length form a single
trunk termed the hepatic duct (fig. CLXVII. 9), which carries the bile
partly to the gall bladder (fig. CLXVII. 8) by a duct called the
cystic (fig. CLXVII. 10), and partly to the duodenum (fig. CLXVII. 3)
by a duct named the choledoch (fig. CLXVII. 6), a common trunk formed
by the union of the cystic with the hepatic (fig. CLXVII. 10 and 9).
The choledoch duct opens into the duodenum at the same point as the
pancreatic (fig. CLXVII. 7), and generally by a common orifice.

667. The duodenum, on receiving the chyme from the stomach, transmits
it slowly along its surface. The kind of motion by which the chyme is
borne along the surface of the duodenum is perfectly analogous to that
by which it is transmitted from the stomach to the duodenum, irregular,
sometimes in one direction, and sometimes in another, at one time
commencing in one part of the organ, at another time in another, always
slow, but ultimately progressive.

668. As the chyme slowly advances through the upper part of the
duodenum, the biliary and the pancreatic juices slowly distil into
the lower portion of the organ. The bile is seen to exude from the
choledoch duct, not continually, but at intervals, a drop appearing at
the orifice, and diffusing itself over the neighbouring surface, about
twice in a minute, while the flow of the pancreatic juice is still
slower.

669. No appreciable change takes place in the chyme until it reaches
the orifice of the choledoch duct; but as soon as it comes in contact
with this portion of the duodenum, the chyme suddenly loses its own
sensible properties, and acquires those of the bile, especially its
colour and bitterness. But these properties are not long retained; a
spontaneous change soon takes place in the compound. It separates into
a white fluid and into a yellow pulp. The white fluid is the nutritive
part of the aliment; the yellow pulp is the excrementitious matter.

670. This white fluid, the proper product of the digestive process,
as far as it has yet advanced, is called chyle. If any portion of oil
or fat have been contained in the food, the chyle is of a milk-white
colour; if not, it is nearly transparent. It is of the consistence
of cream, and it bears a close resemblance to cream in its sensible
properties. It differs from chyme in being of a whiter colour, more
pellucid, and of a thicker consistence: it differs also in its chemical
nature, for, whereas chyme is acid, chyle is alkaline.

671. Three fluids are mixed with the chyme in the duodenum, each of
which contributes to the conversion of the chyme into chyle. First, the
secretion of the duodenum itself, a solvent analogous to the gastric
juice. Secondly, the secretion of the pancreas, a watery fluid holding
in solution highly important principles, namely, a large quantity of
albumen, a matter resembling casein, osmazome, and different salts.
Thirdly, the secretion of the liver, a compound fluid, consisting of
water, mucus, and several peculiar animal matters, namely, resin,
cholesterine, picromel, cholic acid, a colouring matter, probably
salivary matter, osmazome, casein, and many salts.

672. There cannot be a question that the secretion of the duodenum has
a solvent power over the chyme analogous to that of the gastric juice.
Some physiologists indeed maintain that the juice poured out from the
inner surface of the duodenum is as powerful a solvent as the gastric
juice. It is certain that substances which have escaped chymification
in the stomach undergo that process in the duodenum, and that there is
the closest analogy between the action of the duodenum on the chyme and
that of the stomach on the crude food.

613. The pancreatic secretion adds to the chyme richly azotized animal
substances, albumen, casein, osmazome (671), by which it is brought
nearer the chemical composition of the blood, and prepared for its
complete assimilation into it. The first addition of such assimilative
matter, it has been shown, is communicated by the salivary glands, but
far more important additions are now supplied from the pancreas. Hence
the larger size of the pancreas and the more copious secretion of the
pancreatic fluid, in herbivorous than in carnivorous animals; hence the
change produced in the size of the pancreas by a long continued change
in the habits of an animal; hence the smaller size of the pancreas in
the wild cat, which lives wholly on animal food, than in the domestic
cat, which lives partly on animal and partly on vegetable food.

674. The bile, the most complex secretion in the body, accomplishes
manifold purposes.

1. Like the pancreatic secretion, it communicates to the chyle richly
azotized animal substances, picromel, osmazome, and cholic acid (671);
by the combination of which with the chyme, it is brought still nearer
the chemical composition of the blood. These principles are manifestly
united with the chylous portion of the chyme, since they are not
discoverable in its excrementitious matter.

2. Bile has the property of dissolving fat; consequently, when oily
or fatty matters are contained in the food, it powerfully assists in
converting these substances into chyle.

3. The excrementitious portion of the bile is highly stimulant. The
contact of its bitter resin with the mucous membrane of the intestines
excites the secretion of that membrane; hence the extreme dryness of
the excrementitious matter when the choledoch duct of an animal has
been tied; and hence the same dryness of this matter in jaundice, when
the bile, instead of being conveyed by its appropriate duct into the
duodenum, is taken up by the absorbents, poured into the blood, and
distributed over the system.

4. The bitter resin of the bile stimulates to contraction the fibres
of the muscular tunic of the intestines: by the contraction of these
fibres the excrementitious matter is conveyed in due time out of the
body; hence the constipated state of the bowels invariably induced when
the secretion of the bile is deficient, or when its natural course into
the intestines is obstructed.

5. The excrementitious portion of the bile exerts an antiseptic
influence over the excrementitious portion of the food during its
passage through the intestines. In animals in which the choledoch duct
has been tied, the excrementitious portion of the food is invariably
found much further advanced in decay than in the natural state. This
is also uniformly the case in the human body in proportion as the
secretion of the bile is deficient, or its passage to the intestine is
obstructed.

675. Such appear to be the real purposes accomplished by the bile
in the process of digestion. Several uses have been assigned to
it, in promoting this process, which it does not serve. Seeing the
instantaneous change wrought in the chyme on its contact with the
bile, it was reasonable to suppose that the main use of the bile
was to convert chyme into chyle, a purpose apparently of sufficient
importance to account for the immense size of the gland constructed
for its elaboration. The soundness of this conclusion appeared to be
established by direct experiment. Mr. Brodie placed a ligature around
the choledoch duct of an animal: after the operation the animal ate
as usual: on killing the animal some time after it had taken a meal,
and examining the body immediately after death, it was clear that
chymification had gone on in the stomach just as when the choledoch
duct was sound, but no chyle appeared to be contained either in the
intestines or in the lacteals. In the lacteals there was found only a
transparent fluid, which was supposed to consist of lymph and of the
watery portion of the chyme. Mr. Brodie’s experiments seemed to be
confirmed by those of Mr. Mayo, who arrived at the conclusion, that
when the choledoch duct is tied, and the animal is examined at various
intervals after eating, no trace whatever of chyle is discoverable in
the lacteal vessels. But these experimentalists inferred that no chyle
existed in the intestines or lacteals, because there was present no
fluid of a milk-white colour, a colour not essential to chyle, but
dependent on the accident of oily or fatty matter having formed a
portion of the food. These experiments have been repeated in Germany
by Tiedemann and Gmelin, and in France by Leuret and Lassaigne, who
have invariably found, after tying the choledoch duct, nearly the same
chylous principles, with the exception of those derived from the bile,
as in animals perfectly sound; and the English physiologists have since
admitted that their German and French colaborateurs have arrived at
conclusions more correct than their own.

676. The bile consists then of two different portions; a highly
animalized portion, which combines with the chyme and exalts its
nature by approximating it to the condition of the blood; and an
excrementitious portion, which, after accomplishing certain specific
uses, is carried out of the system with the undigested matter of the
food. The excrementitious portion of the bile, namely, the resin, the
fat, the colouring principle, the mucus, the salts, constitute by far
the largest portion of it. These constituents of the bile for the most
part contain a very large proportion of carbon and hydrogen, and the
reasons have been already fully stated (473, _et seq._) which favour
the conclusion that the elimination of these substances under the form
of bile is one most important mode of maintaining the purity of the
blood, and that the liver is thus a proper respiratory organ, truly
auxiliary to the lungs. It is a beautiful arrangement, and like one
of the adjustments of nature, that the bile, the formation of which
abstracts from the blood so large a portion of carbon and hydrogen as
to maintain the purity of the circulating mass and to counteract its
putrescent tendency, acts on the excrementitious portion of the food,
always highly putrescent, as a direct and powerful antiseptic; that
the very matter which is eliminated on account of the putrid taint
it communicates to the blood, on its passage out of the body, stops
the putrefaction of the substances which have been ministering to the
replenishment of the blood.

677. The chyle, thick, glutinous, and adhesive, attaches itself
with some degree of tenacity to the mucous surface of the duodenum.
Nevertheless, by the successive contractions of the muscular fibres of
the duodenum the fluid is slowly but progressively propelled forwards.
The separation of the excrementitious matter becomes more complete,
and consequently the chyle more pure as it advances, until, having
traversed the course of the duodenum, it enters the second portion of
the small intestines, the jejunum.

678. The jejunum, so called because it is commonly found empty, and the
ilium, named from the number of its convolutions, on account of their
great length, are provided with a distinct membrane to support them,
and to retain them in their situation, termed the mesentery.

679. The mesentery is a broad membrane composed of two layers of
peritoneum. Between these two layers, at one extremity of the
duplicature, is placed the intestines, while the other extremity is
attached to the spinal column. The mesentery being much shorter than
the intestines, the intestines are gathered or puckered upon the
membrane, by which beautiful mechanical contrivance they are held in
firm and close contact with each other, yet their convolutions cannot
be entangled, nor can they be shaken from their place by the sudden
and often violent movements of the body. It sometimes happens, in
consequence of disease, that the convolutions of the intestines are
glued together by the effusion of lymph, and then the most trifling
causes are capable of producing the severest symptoms of obstruction in
the bowels.

680. The internal surface of the small intestines is distinguished,

1. By the number of the mucous glands, which may be seen by a
magnifying glass to consist partly of a prodigious number of the
minutest follicles, not collected in groups, but equally scattered
throughout; and partly of glands of a larger dimension, disposed in
groups at particular parts of the canal.

2. By the increase in the number and size of the villi, of which there
are about four thousand to the surface of a square inch. Like those of
the stomach, the villi of the small intestine are composed of arteries,
veins, nerves, and mucous ducts; but to the villi of the small
intestine, in length about one-fourth of a line, there is added a new
vessel, the absorbent of the chyle, the lacteal (figs. 175 and 176), so
named from the milk-like chylous fluid which it contains.

3. By the great extension of the mucous coat obtained by the
disposition of the membrane into the folds called valvulæ conniventes
(fig. CLXXIII.). These folds, which rarely extend through the whole
circle of the intestine, are often joined by communicating folds (fig.
CLXXIII.). The folds are broadest in the middle, and narrowest at the
extremities (fig. CLXXIII.). In general, they are about a line and a
half broad. One edge of the fold is loose, but the other is fixed to
the intestine (fig. CLXXIII.). The office of these folds is, first,
without increasing space, to extend surface for the distribution of the
villi; and, secondly, to retard the flow of the chyle, by opposing to
its descent valves so constructed and disposed as, without arresting
its progress, to moderate and regulate its course, in order that time
may be allowed for its absorption.

[Illustration: Fig. CLXXIII.

 Internal view of a portion of the jejunum, showing the arrangement of
 the mucous membrane into valvulæ conniventes.]

[Illustration: Fig. CLXXIV.—_View of the Outer Coats of the Small
Intestine._

 1. Peritoneal coat reflected off. 2. Muscular Coat consisting of—_a._
 longitudinal fibres. _b._ Circular fibres.]

681. The onward flow of the chyle through the course of the small
intestines is effected by the action of the double layer of muscular
fibres, the circular and the longitudinal fasciculi which compose
its muscular coat (fig. CLXXIV.). The disposition of the muscular
fibres of the alimentary canal in general, and of this part of it in
particular, deserves special notice. The ordinary arrangement and
action of muscular fibres would not have produced in this case the kind
and degree of motion required. The muscular fibres that compose the
ventricles of the heart are so accumulated and disposed, that their
contraction originates, and communicates energetic impulse. The muscles
of the arm are so accumulated and disposed that their contraction
originates the like energetic impulse. Muscles so accumulated in the
alimentary canal would have produced motion, indeed, but motion not
only not accomplishing the end in view, but directly defeating it. In
order to obtain the kind and degree of motion in this case required,
the firm and thick muscle is attenuated into minute, delicate, and
thready fibres, not concentrated in a bulky mass, so as to obtain by
their accumulation a great degree of force; but spread out in such
a manner as to form a thin and almost transparent coat. The tender
fibres composing this delicate coat, by their contraction, produce two
alternate, gentle, almost constant motions, called the peristaltic,
from its resemblance to the motion of the earth-worm, and the
antiperistaltic. By the peristaltic action motion is begun at once in
several parts of the canal. Whenever the chyle is applied in a certain
quantity to any part of the intestines, that part contracts, and makes
a firm point, towards which the portions both above and below are
drawn, by means of the longitudinal fibres which shorten the canal, and
at the same time dilate the under part. By the antiperistaltic action,
which is the exact reverse of the former, the chyle is turned over and
over, and exposed to the orifices of the lacteal vessels; while, by the
motion of the chyle forwards and backwards, and backwards and forwards,
produced by these two actions constantly alternating with each other,
its slow, gentle, but ultimately progressive course is secured.

682. The chyle thus gently moved along the extended surface of the
jejunum and ilium, and still in its course acted upon in some degree
by the secretions poured out upon the mucous membrane, successively
disappears, until at the termination of the ilium (fig. CLXXI. 5) there
is scarcely any portion of it to be perceived. It is taken up by the
vessels termed lacteals.

683. The lacteal vessels (figs. 175 and 176), take their origin on
the surface of the villi, by open mouths, too minute to be visible
to the naked eye, but distinguishable under the microscope. These
minute, pellucid tubes, wholly countless in number, are composed of
membranous coats so thin and transparent that the milky colour of their
contents, from which they derive their name, is visible through them,
and yet they are firm and strong. They present a jointed appearance
(figs. CLXXVI. 4, and CLXXVII. 7). Each joint denotes the situation
of the valves with which they are provided, and which are placed at
regular distances along their entire course (fig. CXCII. 1 and 2).
These valves, which are generally placed in pairs (fig. CXCII. 2),
consist of a delicate fold of membrane of a semilunar form, one edge of
which is fixed to the side of the vessel, while the other lies loose
across its cavity (fig. CXCII. 2). So firm is this membrane, and so
accurately does it perform the office of a valve, that even after death
it is capable of supporting a column of mercury of considerable weight
without giving way, and of preventing a retrograde course of the fluid.
The lacteals are nourished by blood-vessels, and animated by nerves,
and it is conceived that they must be provided with muscular fibres, or
some analogous tissue, for they are obviously contractile, and it is
by this contractile power that their contents are moved. The delicacy
and transparency of the vessels, however, render it impossible to
distinguish the different tissues which compose their walls.

[Illustration: Fig. CLXXV.

 View of the inner surface of the ilium as it appears some hours after
 a meal. 1. The smaller branches of the lacteals, turgid with chyle,
 covering the surface of the intestine. 2. Larger branches of the
 lacteals formed by the union of the smaller branches.]

[Illustration: Fig. CLXXVI. _View of the course of the Lacteals._

 1. The aorta. 2. Thoracic duct. 3. External surface of a portion of
 small intestine. 4. Lacteals appearing on the external surface of
 the intestine after having perforated all its coats. 5. Mesenteric
 glands of the first order. 6. Mesenteric glands of the second order.
 7. Receptacle for the chyle. 8. Lymphatic vessels terminating in the
 receptacle of the chyle, or commencement of the thoracic duct.]

684. If the mucous coat of the small intestines be examined some hours
after a meal, the lacteals are seen turgid with chyle, covering its
entire surface (fig. CLXXV. 1). These vessels, which are sometimes
of such magnitude and in such numbers as entirely to conceal the
ramifications of the blood-vessels, unite freely with each other, and
form a net-work, from the meshes of which proceed branches which,
successively uniting, form branches of a larger size (fig. CLXXV. 2).
These larger branches perforate the mucous coat and pass for some way
between the mucous and the muscular tunics: at length they perforate
both the muscular and the peritoneal coats, when, from having been
on the inside of the intestine, they get on the outside of it (fig.
CLXXVI. 3, 4), and are included, like the intestine itself, between the
layers of the mesentery. All the different sets of lacteals converging
and uniting together, form an exceedingly complicated plexus of vessels
within the fold of the mesentery. Radiating from this plexus, the
lacteals advance forwards until they reach the glands, called, from
their being placed between the fold of the mesentery, the mesenteric
(figs. CLXXVI. 5 and 6, and clxxvii. 2 and 3); rounded, oval,
pale-coloured bodies, consisting of two sets, arranged in a double
row (figs. CLXXVI. 5 and 6, and CLXXVII. 2 and 3); the set nearest
the intestine (fig. CLXXVII. 2) being considerably smaller than the
succeeding set (fig. CLXXVII. 3).

[Illustration: Fig. CLXXVII.

 View of the course of the Thoracic Duct from its origin to its
 termination. 1. Lacteal vessels emerging from the mucous surface of
 the intestines. 2. First order of mesenteric glands. 3. Second order
 of mesenteric glands. 4. The great trunks of the lacteals emerging
 from the mesenteric glands, and pouring their contents into—5. The
 receptacle of the chyle. 6. The great trunks of the lymphatic or
 general absorbent system terminating in the receptacle of the chyle.
 7. The thoracic duct. 8. Termination of the thoracic duct at—9. The
 angle formed by the union of the internal jugular vein with the
 subclavian vein.]


685. On reaching the first series of glands (fig. clxxvii. 2), the
lacteals penetrate the substance of the gland, in the interior of
which they communicate with each other so freely, and form such
innumerable windings, that the gland seems to consist of a congeries
of convoluted lacteals. Emerging from the first series of glands, the
lacteals proceed on their course to the second series (fig., CLXXVII.
3), which they penetrate, and in the interior of which they present
the same convoluted appearance as in the first set. On passing out of
this second series of glands, the lacteals unite together, and compose
successively larger and larger branches, until at length they form two
or three trunks (fig. CLXXVII. 4), which terminate in the small oval
sac (fig. CLXXVII. 5), termed the receptacle of the chyle (receptaculum
chyli).

686. In this oval sac or receptacle of the chyle (fig. CLXXVII.
5), which rests upon the second or the first lumbar vertebra, also
terminate the trunks of the general absorbent vessels of the system
(fig. clxxvii. 6), called from the _lymph_ or the pellucid fluid which
they contain, lymphatics, as the lacteals are named from the lactitious
or milky appearance of their contents.

687. The receptacle of the chyle produced forms the thoracic duct
(fig. CLXXVII. 7), a canal about three lines in diameter. This tube
rests upon the spinal column, ascends on the right side of the aorta,
passes through the aortic opening in the diaphragm (fig. CXXXIV. 9,
10), and enters into the chest. Here it forms a transparent tube about
the size of a crow-quill; it rests upon the bodies of the dorsal
vertebræ; it continues to ascend still on the right side of the aorta,
until it reaches the sixth or fifth dorsal vertebra, when changing its
direction, it passes obliquely over to the left side (fig. CLXXVII. 7).
From this point it continues its course upwards, on the left side of
the neck, as high as the sixth cervical vertebra; when suddenly turning
forwards and a little downwards, it terminates its course in the angle
formed by the union of the internal jugular with the subclavian vein
(fig. CLXXVII. 8, 9). At its termination in these great venous trunks
are placed two valves, which prevent alike the return of the chyle, and
the entrance of the blood into the duct (fig. CLXXVIII.).

[Illustration: Fig. CLXXVIII.—_Valve at the termination of the Thoracic
Duct._

 1. The Thoracic Duct. 2. Lymphatics entering the duct. 3. The vein
 laid open, showing the valve at the termination of the duct. 4. The
 left internal jugular vein. 5. The left subclavian vein. 6. The vein
 called innominata. formed by the union of the internal jugular and
 subclavian veins. 7. The right jugular vein. 8. The right subclavian
 vein. 9. The superior cava formed by the union of the veins above. 10.
 The inferior cava formed by the union of the veins below. 11. The two
 venæ cavæ passing to the right auricle of the heart. 12. The heart.
 13. The pulmonary artery dividing into right and left branches. 14.
 The aorta.]

688. This account of the course of the thoracic duct is a description
of the course of the chyle. Performing a double, circuitous, and
slow circulation through the minute convoluted tubes of which the
double series of mesenteric glands are composed, the chyle, in its
receptaculum, is mixed with the contents of the lymphatic vessels,
lymph (fig. CLXXVII. 6, 5), that is, organic matter brought from every
surface and tissue of the body. Both fluids, chyle and lymph, mixed and
mingled, flow together into the thoracic duct, by which in the course
traced (687) they are poured into the blood, just as the venous torrent
is rushing to the heart (fig. CLXXVIII. 6, 9, 11).

689. Thus, the final product of digestion, the chyle; particles of
organized matter, the lymph; and venous blood, that is, blood which has
already circulated through the system commingled, flow together to the
right heart, by which it is transmitted to the lungs, where all these
different fluids are converted into one substance, arterial blood, to
be by the left heart sent out to the system for its support.

690. While these processes are going on, another and a very important
function is performed by the remaining portion of the alimentary canal.
It is the office of this part of the apparatus to carry out of the body
that portion of the aliment which is incapable of being converted into
chyle. The preparation of the excrementitious part of the aliment for
its expulsion constitutes the process of fecation. The organs in which
this process is carried on, and by which the excrementitious matter,
when duly prepared for its removal, is conveyed from the body, are the
large intestines.

691. The large intestines (fig. CLXXIX.) consist of the cæcum, the
colon and the rectum (fig. CLXXIX.). The cæcum varies in length from
two inches to six; the colon is about five feet in length, and the
rectum is about eight inches.

692. The ilium opens into the cæcum (fig. CLXXIX. 8, 10), just as the
esophagus opens into the stomach. At this point the ilium is elongated,
forming two concentric folds which join at their horns, and between
the folds are placed a number of muscular fibres. In this manner is
constructed a valve, which is termed the valve of the colon. It is
placed in a transverse direction across the intestine, and its action
as a valve is very complete. It admits of the free passage of the
contents of the small intestines into the large, but it prevents the
return of any portion of the contents of the latter into the former.

[Illustration: Fig. CLXXIX.—_View of the Abdominal Portion of the
Digestive Organs._

 1. Esophagus. 2. Stomach. 3. Spleen. 4. Liver. 5. Gall-bladder
 with its ducts. 6. Pancreas with its duct. 7. Duodenum. 8. Small
 intestines. 9. Large intestines dividing into—10. Cæcum. 11. Ascending
 colon. 12. Arch of the colon. 13. Descending colon. 14. Sigmoid
 flexure here imperfectly represented. 15. Rectum.]

[Illustration: Fig. CLXXX.

 Portion of the large intestine, showing the arrangement of the
 muscular fibres. 1. The longitudinal fibres collected into bands, and
 forming larger fasciculi. 2. The circular fibres arranged as in the
 other intestines.]

693. The colon is distinguished by its capacious size, its great
length, and its longitudinal bands, which consist of strong muscular
fasciculi (fig. CLXXIX. 11). It is divided into an ascending portion
which occupies the right iliac and hypochondriac regions (fig. CLXXIX.
11); the transverse portion, called its arch, which is placed directly
across the epigastric region (fig. CLXXIX. 12), a descending portion
which occupies the left hypochondriac region (fig. CLXXIX. 13), and a
fourth portion, which being curved somewhat like the italic letter S,
is called the sigmoid flexure, which occupies the left iliac region
(fig. CLXXIX. 14). The sigmoid flexure terminates in the last portion
of the alimentary canal, called the rectum (fig. CLXXIX. 15), which is
placed in the hollow of the sacrum, and which follows the curvature
of that bone (fig. XLV. 5). The circular fibres of the rectum are
accumulated at the termination of the bowel to form the internal
sphincter of the anus. External to this is placed another set of
fibres, which constitute the external sphincter.

694. The mucous membrane of the large intestines is disposed
differently from that of the small intestines, and the mucous membrane
of the colon still differently from that of the rectum. In the colon
the mucous membrane, instead of being disposed in the form of valvulæ
conniventes, is so arranged as to divide its whole surface into
minute apartments or cells by which the descent of the fecal matter
is retarded still more than the descent of the chyle by the valvulæ
conniventes. Some particles of chyle do, however, continue to be
separated from the fecal matter, even in the large intestines; and in
order that nothing may be lost, a few valvulæ conniventes, with their
lacteals, appear here also, while the cells of the colon, by retarding
the descent of the fecal matter, allow time for the more complete
separation and absorption of the chylous particles.

695. In the rectum the mucous membrane is plaited into large transverse
folds, which disappear as the fecal matter descends into the bowel,
accumulates in it, and distends it; an arrangement which gives to this
portion of the intestine its power of distension, so closely connected
with our convenience and comfort.

696. As soon as that portion of the alimentary matter which is
transmitted to the large intestines reaches the colon it ceases to
be alkaline, the distinctive character of the contents of the small
intestines, and becomes acid, just as the whole alimentary mass is
acid at the commencement of digestion in the stomach. It acquires
albumen; its gases are no longer the same, for whereas pure hydrogen
is contained in the small intestines, none is ever found in the large,
but in the place of it, carbureted and sulphureted hydrogen; and now
for the first time it receives its peculiar odour. As it continues
to descend, its fluid parts are progressively absorbed, so that it
becomes more and more solid, until it reaches the rectum, when it is
almost dry. Here the accumulation of it goes on to a considerable
extent, the peristaltic action at first excited by the distension of
the rectum being, it would appear, counteracted by the contraction
of the external sphincter of the anus. When, however, the distension
of the bowel reaches a certain point, it produces a sensation which
leads to the desire to expel its contents. The bowel is now thrown
into action by an effort of the will, and that action is powerfully
assisted by the descent of the diaphragm and the contraction of the
abdominal muscles, actions also induced by an effort of the will. Thus
the action of the first part of the digestive apparatus, that which is
connected with the reception and partly with the deglutition of the
food, is attended with consciousness, and is placed under the control
of the will; the main portion of the digestive apparatus, that in
which the essential part of the digestive process is carried on, is
without consciousness, and is placed beyond the influence of volition;
the last portion of the digestive apparatus, that connected with the
expulsion of the non-nutrient portion of the aliment, again acquires
sensibility and consciousness, and is placed under the control of the
will. The striking differences in the arrangement of the muscular
fibres in these different parts of the apparatus, in accordance with
the widely different function performed by them; the powerful muscles
connected with the prehension, mastication and deglutition of the food;
the delicate and transparent tissue of fibres forming the muscular
coat of the stomach and small intestines; the increase in the number
and strength of the fibres of the large intestines, and the prodigious
accession to them in the rectum, are adjustments not only exquisite and
admirable in their own nature, but so indispensable to our well-being
and comfort, that were the appropriate action of either to be suspended
but for a short period, life would be extinguished, or if it could be
protracted, it would be changed into a state of unbearable torment.

697. From the preceding account of the structure and action of the
apparatus of digestion, on a comparison of all the phenomena, it
appears that the successive stages of the process are marked by the
progressive approximation of the food to the nature of the blood. The
main constituents, of the blood are albumen, fibrin, an oily principle,
and red particles. Even in the chyme there are traces of albumen, with
globules, not indeed to be compared in number with the red particles
of the blood, smaller in size, and without colour, but still of an
analogous nature. In the chyle of the duodenum the quantity of albumen
is larger, there are traces of fibrin, and of an oily matter, and the
number of the globules is increased. In the chyle, after its exit from
the mesenteric glands, the albumen, the fibrin, the oil, the globules,
and more especially the two first and the last, are greatly increased.
But in the chyle when it reaches the thoracic duct, these principles
are so augmented, concentrated, and approximated to the state in which
they exist in the blood, that the chyle is now capable of undergoing
the characteristic process of the blood; for as the blood, when drawn
from a vein, undergoes spontaneous coagulation, so the chyle, when
drawn from the thoracic duct, separates into three parts; a solid
substance or clot, which remains at the bottom of the vessel; a fluid
which surrounds the clot; and a thin layer of matter, which is spread
over the surface of the fluid. The solid substance is analogous to the
fibrin, and the fluid to the serum of the blood; while the layer of
matter which is spread over the fluid is of an oily nature: moreover,
the chyle, when in contact with the air, quickly changes to a red
colour, and abounds with minute particles of various sizes, but the
largest of which is not yet equal to the diameter of the red particles
of the blood.

698. The changes wrought upon the food, by which it is thus
approximated to the chemical composition of the blood, are effected,
as has been shown, partly by the gastric and intestinal juices, and
partly by matters combined with the food highly animalized in their
own nature, and endowed with assimilative properties, as the salivary
secretion mixed with the food during mastication; the pancreatic
and biliary secretions mixed with the food during the conversion of
the chyme into chyle; and the mesenteric secretions mixed with the
elaborated chyle of the mesenteric glands, and lastly, organized
particles which have already formed a part of the living structures of
the body mixed with the chyle under the form of lymph in the thoracic
duct.

699. The lymph, until lately regarded as excrementitious, is really
highly animalized, partly combined with the chyle as its last and
highest assimilative matter; whence the compound formed by the
admixture of chyle and lymph is far more proximate to the blood than
the purest and most concentrated chyle; and partly returning with the
chyle to the lungs, to receive there a second depuration, and thereby a
higher elaboration.

700. There is evidence that there is a series of organs specially
provided for the elaboration of the lymph no less than of the chyle.
There are organs manifestly connected with the digestive apparatus,
to which physiologists have found it extremely difficult to assign a
specific office. These organs have a structure in some essential points
alike; that structure is strikingly analogous to the organization of
glands: like glands, they receive a prodigious quantity of arterial
blood, and are supplied with a proportionate number of organic nerves;
yet they are without an excretory duct. The organs in question are
the bodies called the renal capsules, placed above the kidneys; the
thyroid and thymus glands situated in the neck, and the spleen in close
connexion with the stomach.

701. These organs, however analogous in structure to glands, cannot,
it has been argued, be secreting organs, because they are destitute of
an excretory duct, do not manifestly form from the blood any peculiar
secretion, or, if they do, since there are no means of detecting
where it is conveyed, it is impossible to understand how it is
appropriated. But if these organs collect, concentrate, and elaborate
lymph, preparatory to its admixture with the chyle and to its being
sent a second time into the blood to undergo a second process of
depuration, they perform the function of glands; and their want of an
excretory duct, which has hitherto rendered their office so obscure, is
accounted for; they do not need distinct tubes for the transmission of
any product of secretion; the lymphatic vessels which proceed from them
and which convey the fluid they elaborate into the receptacle of the
chyle, are their excretory ducts. That one of these organs, the spleen,
is specially connected with the elaboration of the lymph, is manifest,
both from its chemical nature and from the remarkable change which
takes place in the chyle the moment the lymph from the spleen is mixed
with it. Tiedemann and Gmelin state, as the uniform result of their
observations and experiments, that the quantity of fibrin contained
in the chyle is greatly increased, and that it actually acquires red
particles as soon as the lymph from the spleen is mixed with it, and
that the lymph from the spleen superabounds both with fibrin and with
red particles. That the organs just enumerated, with the spleen,
perform a similar function, is inferred from their being, like it, of a
glandular structure, and without any excretory duct. If the spleen be
really one of a circle of organs appropriated to a function such as is
here supposed, a purpose is assigned to it adequate to its rank in the
scale of organization; inferior to few, if its importance be estimated
by the quantity of arterial blood with which it is supplied; yet this
is the organ for which Paley could find no better use than that of
serving for package.

702. But in whatever mode the lymph be elaborated, it is certain that
it consists of matter highly animalized, and that its most important
principles, its albumen, its fibrin, its globules, and even its salts,
are in a chemical condition closely resembling that in which they exist
in the blood.

703. It will appear hereafter that all the proximate principles of
which the body is composed are reducible by analysis to three, namely,
sugar, oil, and albumen: of these, sugar and oil are the least, and
albumen the most highly organized. Every alimentary substance must
contain at least one of these proximate principles, and in the various
articles which compose an ordinary meal always two, and often all
three, are afforded in abundance. From the phenomena which have been
stated, it is clear that the digestive organs, in acting on these
principles, exert the following powers.

1. A solvent power. The first action of the stomach on the alimentary
substances presented to it is to reduce them to a fluid state. No
substance is nutritious which is not a fluid, or capable of being
reduced to a fluid. The stomach reduces alimentary substances to
a fluid state by combining them with water. Water enters into the
composition of organized bodies in two states, as an essential and
as an accidental element. A quantity of water is contained in sugar
when reduced to its dryest state; this water cannot be dissipated
without the decomposition of the sugar; it is therefore an essential
constituent of the compound. Water is combined with sugar in its
moist state: of this water much may be removed without destroying the
essential properties of the sugar: this part of the water is therefore
said to be an accidental constituent of the sugar. In most cases
organized bodies contain water in both these forms; and though it is
commonly impossible to discriminate between the water that is essential
and that which is accidental, yet the mode of union among the elements
of bodies in these two states of their combination with water are
essentially different. The stomach has the power of combining water
with alimentary substances in both these forms. Thus fluid albumen,
or white of egg, presented to the stomach is immediately coagulated
or converted into a solid. Soon this solid begins to be softened, and
the softening goes on until it is again reduced to a fluid. What was
fluid albumen in the white of egg is now fluid albumen in chyme; but
the albumen has undergone a remarkable change. Out of the stomach the
albumen of the egg may be converted by heat into a firm solid; but the
albumen of the chyme is capable of being converted only into a loose
and tender solid. In passing from its state in the egg to its state
in the chyme, the albumen has combined with a portion of water which
has entered as an essential ingredient into its composition. By this
combination the compound is reduced from what may be called a strong
to a weak state. This is the first action exerted by the stomach on
most alimentary substances. They are changed from a concentrated to a
diluted, from a strong to a weak state: the power by which the stomach
effects this change is called its reducing power, and the agent by
which it accomplishes it is the gastric juice; the essential ingredient
of which has been shown to be muriatic acid, or chlorine (639, _et
seq._). The muriatic acid obtained from the common salt of the blood
is poured in the form of gastric juice into the stomach, dissolves the
food, combines it with water, reduces it from a concentrated solid to
a dilute fluid; and thus brings it into the condition proper for the
subsequent part of the process.

2. A converting power. Since whatever be the varieties of food, the
chyme invariably forms a homogeneous fluid, the stomach must be endowed
with the power of transforming the simple alimentary principles into
one another; the saccharine into the oily, and the oily into the
albuminous. The transformation of the saccharine into the oleaginous
principle is traceable out of the body in the conversion of sugar into
alcohol, which is essentially an oil. That the same transformation
takes place within the body is indubitable. The oleagenous and the
albuminous principles are already so nearly allied in nature to animal
substance that they do not need to undergo any essential change in
their composition.

3. A completing power. When the alimentary substances have been reduced
and formed into chyme, when the chyme has been converted into chyle,
and when the chyle absorbed by the lacteals is transmitted to the
mesenteric glands, it undergoes during its passage through these organs
a process the direct reverse of that to which it is subjected in the
stomach; for whereas it is the office of the stomach to combine the
alimentary substances with water, it is one office of the mesenteric
glands to remove the superfluous water of the chyle; to abstract
whatever particles of matter may be contained in the compound which are
not indispensable to it, and to concentrate its essential constituents;
and consequently these organs exert on the digested aliment a
completing, in contradistinction to a reducing power.

4. A vitalizing power. When sugar is converted into oil, when oil is
converted into albumen, when albumen, by the successive processes
to which it is subjected is completed, that is, when the alimentary
substances are made to approximate in the closest possible degree to
the nature of animal substance, they must undergo a still further
change, more wonderful than any of the preceding, and far more
inscrutible; they must be endowed with vitality; must be changed from
dead into living matter. Living substance only is capable of forming
a constituent part of living substance. The ultimate action of the
digestive organs is the communication of life to the food, to which
last and crowning process the reducing, converting, and completing
processes are merely subordinate and preparatory. Of the agency by
which this process is effected we are wholly ignorant; we know that
it goes on; but the mode in which it is accomplished is veiled in
inscrutable darkness.

704. Blood is alive; blood is formed from the food; life is
communicated to the food before it is mixed with the blood. The blood
is essentially albumen, which it contains in the form of albumen
properly so called, in that of fibrin, and in that of red particles.
In the thoracic duct the strong albumen of the lymph is mixed with
the weaker albumen of the chyle. At the point where the thoracic duct
terminates in the venous system, lymph and chyle are mixed with venous
blood, and all commingled are borne directly to the lungs. There the
carbon with which the venous blood is loaded is expelled in the form
of carbonic acid gas; the particles of the lymph undergo some, as yet,
unknown change, exalting their organization; and the water hitherto
held in chemical union with the weak albumen of the chyle, is separated
and carried out of the system together with the carbonic acid gas in
the form of aqueous vapour. By this removal of its aqueous particles
the ultimate completion is given to the digested aliment; and the weak
and delicate albumen of the chyle is converted into the strong and firm
albumen of the blood.

705. It has been stated (539), that though gelatin enters abundantly
into the composition of many tissues of the body, and performs most
important uses in the economy, it is never found in the blood; that
it is formed from the albumen of the blood by a reducing process, in
consequence of which carbon is evolved, which unites with the free
oxygen of the blood, forming carbonic acid, thus conducing, among other
purposes, to the production of animal heat. It is equally remarkable,
that though the lymphatics or absorbents arise in countless numbers
from every tissue of the body, and are endowed with the power of taking
up every constituent particle of every organ, solid as well as fluid,
yet gelatin is never found in the lymphatic vessels. The lymphatics
contain only albumen in a form far more proximate to the blood than
that of the chyle; consequently, before the gelatin of the body is
taken up by the lymphatics, it must be reconverted into albumen; that
is, the absorbed gelatin must undergo a process analogous to that which
gelatin and other matters undergo in the stomach and duodenum; it
follows that the digestive process is not confined to the stomach and
duodenum, but is carried on at every point of the body. Hence there are
two processes of digestion, a crude and a refined process. The crude
process is carried on in the stomach and duodenum, in which dead animal
matter is converted into living substance, as yet, however, possessing
only the lowest kind of vitality. The capillary arteries receiving the
substance thus prepared for them, build it up into structure perhaps
the lowest and coarsest, the least organized, and capable of performing
only the inferior functions.

706. Capillary arteries in countless numbers terminate in the tissues
in membraneless canals (304 and 310). Particles of the blood are seen
to quit the arterial stream and to enter into the tissues, becoming a
component part of them: other particles are seen to quit the tissues
and to enter the current of the blood. The latter are probably organic
particles, to which a certain degree of elaboration has been already
given, now transmitted to the capillary veins, to be carried back to
the lungs to undergo there a further depuration, fitting them on their
return to the system for a higher organization.

707. Thus the lymphatic vessels, analogous in so many other respects
to the veins, are probably similar to them in this also—that they take
up from the tissues particles already organized, in order to submit
them to processes which communicate to them a progressively higher
organization. The notion that the contents of the lymphatics consist of
worn-out particles, capable of accomplishing no further purpose in the
economy, is not tenable:—

1. Because it is not analogous to the ordinary operations of nature to
mix wholly excrementitious matter with a substance for the production,
elaboration, and perfection of which, she has constructed such an
expensive apparatus.

2. Because, on the other hand, the admixture of matter already highly
animalized with matter, as yet but imperfectly animalized, exalts the
nature of the latter, and is conducive to its complete animalization.

3. Because the lymph, almost wholly albuminous, is already closely
allied in nature to the blood; it is, therefore, reasonable to infer,
that it is matter passing through an advancing stage of purification
and exaltation.

4. Because this plan of progressive organization is in harmony with the
ordinary operations of nature, in which there is traceable a successive
ascent from the low to the high, the former being preparatory and
necessary to the latter. The tender and delicate organs of animal life,
the brain, the nerves, the apparatus of sense, the muscles, inasmuch as
they perform the highest functions, probably require to be constructed
of a more highly organized material, for the production of which the
matter primarily derived from crude aliment is subjected to different
processes, rising one above the other in delicacy and refinement; by
each of which it is made successively more and more perfect, until it
acquires the highest qualities of living substance, and is capable of
becoming the instrument of performing its most exalted functions.




CHAPTER XI.

OF SECRETION.

 Nature of the function—Why involved in obscurity—Basis of the
 apparatus consists of membrane—Arrangement of membrane into elementary
 secreting bodies—Cryptæ, follicles, cæca and tubuli—Primary
 combinations of elementary bodies to form compound organs—Relation of
 the primary secreting organs to the blood-vessels and nerves—Glands
 simple and compound—Their structure and office—Development of glands
 from their simplest form in the lowest animals to their most complex
 form in the highest animals—Development in the embryo—Number and
 distribution of the secreting organs—How secreting organs act upon
 the blood—Degree in which the products of secretion agree with, and
 differ from, the blood—Modes in which modifications of the secreting
 apparatus influence the products of secretion—Vital agent by which the
 function is controlled—Physical agent by which it is effected.


708. Secretion is the function by which a substance, gaseous, liquid,
or solid, is separated or formed from the nutritive fluid. It is a
function as necessary to the plant as to the animal, and indispensable
alike to the life of both. It is of equal importance to the
preservation of the individual and to the perpetuation of the species.
In all living beings secretions are separated from the nutritive fluid,
and added to the aliment to assist in converting it into nutriment,
and are separated from the nutriment to maintain the composition of
the nutritive mass in a state fit for the continued performance of the
act of nutrition, and to form the germ on the development of which the
continuance of the species depends.

709. The secretions of the plant, varied and abundant, are
indispensable to its nourishment, growth, and fructification. The
secretions of the animal more diversified, and far more constantly
performed, increase in number and elaborateness in proportion to
the range and intensity of the vital endowments and actions. In all
animals high in the scale of organization, and especially in man, the
products of secretion are vast in number, and exceedingly complex in
nature,—membrane, muscle, brain, bone;—the skin, the fat, the nail,
the hair;—water, milk, bile, wax, saliva, gastric juice;—whatever
substances enter as constituents into the corporeal structure;—whatever
substances are specially produced, in order to perform some definite
purpose in the economy;—whatever substances are separated from the
mass, and carried out of the system on account of their useless or
noxious properties:—all are derived from the nutritive fluid, the
blood, and are formed from it by the process of secretion.

710. In this function are included the most secret and subtle processes
of the vital economy,—the ultimate actions of the organic life. Of
the real nature of those actions nothing definite is known; and
they are modified by agencies over which the art and skill of the
experimentalist can exert no adequate control. It is not wonderful
therefore that they should be involved in obscurity: nevertheless,
when all the phenomena are collected and compared, much of the
mysteriousness in which the function appears at first view to be
involved vanishes.

711. The apparatus of secretion is infinity varied in form: when
examined in its complex combinations it appears inextricable in
structure, but the diligence and skill of modern research have unfolded
much of its mechanism, and enabled us to trace the successive steps by
which it passes from its simple to its complex condition.

712. To form an organ of secretion there must be an artery, a vein,
a nerve, an absorbent, and a sufficient quantity of cellular tissue
to allow of the free expansion of these vessels and of their complete
intercommunication. Membrane constitutes such an organ; for membrane
is composed of arteries, veins, nerves, and absorbents sustained and
connected by cellular tissue. Hence membrane constitutes a secreting
organ, in its simplest form. The most important secreting membranes are
the serous (30), the cutaneous (34), and the mucous (33).

713. Serous membrane which lines the great cavities of the body, and
which gives an external covering to the organs contained in them (fig.
LX. a, c), forms an extensive secreting surface. Synovial membrane, or
that which covers the internal surface of joints, and which constitutes
an important portion of the apparatus of locomotion, is essentially the
same in structure and office.

714. Cutaneous membrane, or the skin, which forms the external covering
of the body, is an organ in which manifold secretions are constantly
elaborated; but the skin is only a modification of the membrane
which lines the interior of the body, the mucous. Mucous membrane
forms the basis of the secreting apparatus placed in the mouth,
fauces, esophagus, stomach, and intestines in their whole extent; of
the secreting apparatus auxiliary to that of the alimentary canal,
namely, the pancreas and the liver; probably also of the mesenteric,
or lacteal glands, together with the vast system of lymphatic glands,
and certainly of the glands of the larynx, trachea, bronchi and
air vesicles of the lungs. Hence, while membrane forms the basis
of the secreting apparatus in general, mucous membrane is far more
extensively employed in its construction than any other form of
membrane.

715. 1. In the construction of the secreting apparatus, membrane
disposed in the simplest form, constitutes merely a uniform, smooth,
extended surface. Serous membrane is always disposed in this simple
mode. The costal pleura which lines the internal surface of the walls
of the chest (fig. LX. a); the pulmonary pleura which is continued from
the walls of the chest over the lungs (fig. LX. 5); the peritoneum
which lines the internal surface of the cavity of the abdomen, and
which is reflected over the viscera contained in it (fig. LX. c, and
6, 7, 8, &c.); the synovial membrane which covers all the articular
surfaces; the arachnoid membrane which envelopes the brain, form
simple continuous, serous, secreting surfaces. On the contrary, mucous
membrane is never disposed in this perfectly simple mode; even when it
forms a continuous surface, as in the lining, which it affords to the
alimentary canals, it is more or less plaited into folds or rugæ (fig.
CLXVII. 1).

[Illustration: Fig. CLXXXI.

 A portion of the mucous surface of the intestines, showing some of the
 mucous glands which present the appearance of fovæ or cryptæ.]

716. 2. The second disposition of membrane in the construction of the
secreting apparatus, is the depression of it into a minute pit or fova,
called a crypt (CLXXXI.), which is sometimes inclosed on all sides,
forming a cell or vesicle (fig. CXXXVIII.).


[Illustration: Fig. CLXXXII.

 Portion of the skin and cellular tissue, showing the sebaceous
 follicles, as seen under the microscope very highly magnified. 1. The
 external surface of the follicles with the blood-vessels ramifying
 upon it. 2. Follicles laid open, showing the interior cavity into
 which the secreted fluid is poured.]

717. 3. Next, the vesicle, instead of being rounded, is elongated into
a peduncle or neck, not unlike the neck of a bottle (fig. CLXXXII. 1).
This pedunculated vesicle is called a follicle.

718. 4. Then, the follicle is somewhat elongated, without neck and
without terminal expansion (fig. CLXXXVI. 1); and this is called a
cæcum or pouch.

719. 5. And, lastly, the cæcum itself is elongated; so that instead of
presenting the appearance of a pouch, it rather resembles a tube (fig.
CLXXXV. 1), and is accordingly named tubulum.

720. In the construction of the secreting apparatus, membrane, then,
may be said to be disposed into four elementary forms constituting
cryptæ or vesicles, follicles, cæca and tubuli. Membrane, disposed
into these elementary forms, constitutes the simple bodies by the
accumulation and the varied arrangement of which the compound organs
are composed. There is no other known element which enters into the
composition of the most complex secreting organ.

721. One of these elementary bodies may exist as a simple organ, or
many may be collected into a mass to form a compound organ. When single
they are called solitary: when collected into a mass, aggregated. Each
elementary body has a mode of aggregation peculiar to itself. Vesicles
aggregate by clustering together (fig. CXXXVIII.), and adhering as
if by a common stem (fig. CXXXVIII.); follicles by uniting at their
orifices (fig. CLXXXIII.), and forming masses which are disposed either
in a linear direction (fig. CLXXXIII.) or in fasciculi (fig. CLXXXIV.);
cæca by forming bundles, parallel or branched (fig. CLXXXVI.);
and tubuli by forming masses straight (fig. CLXXXV.), tortuous or
convoluted (figs. CLXXXV. and CLXXXIX.).

[Illustration: Fig. CLXXXIII.

 Aggregated follicles disposed in a linear direction, here represented
 of their natural size, as seen near the mouth in the goose.]

[Illustration: Fig. CLXXXIV.

 Conglomerated follicles.]

722. When a single elementary body, as a vesicle or follicle, forms
a distinct secreting organ, the matter secreted is elaborated at the
inner surface of the organ (fig. CLXXXII. 2), and is contained within
its cavity. When needed it quits this cavity through the walls of
the vesicle, or at the orifice of the follicle, on the application
of the appropriate stimulus. When a number of cryptæ or vesicles are
aggregated into clusters, the individual vesicles sometimes open by
distinct orifices into a common receptacle or sac (fig. CLXXXIV.). When
follicles are aggregated into a mass, and the mass is disposed in a
linear direction (fig. CLXXXIII.), each follicle pours out its secreted
matter by its own orifice (fig. CLXXXIII.); but if conglomerated, into
a common mass by a common orifice (fig. CLXXXIV.).

[Illustration: Fig. CLXXXV.

 1. Parallel tubuli, opening by distinct orifices into—2. A common
 cavity.]

[Illustration: Fig. CLXXXVI.

 Branched cæca, showing—1. The cæca terminating in—2. Excretory ducts
 which unite to form—3. A common trunk.]

723. In like manner, in some very simple arrangements of cæca and
tubuli, each body opens by its own distinct orifice (fig. CLXXXV.
2). But in the more complex arrangements of these bodies, it is
indispensably necessary to modify this mode of parting with their
contents. When the elementary bodies are aggregated into dense,
thick masses (fig. CLXXXIX.), when layer after layer of these masses
containing myriads of myriads of follicles, cæca, or tubuli, are
superimposed one upon another, (fig. CLXXXIX.), it is impossible that
each individual body can have a separate orifice. In this case a
minute tube springs from each body (fig. CLXXXVI. 2); and a complete
connexion is established between all the individuals composing the
mass by the free intercommunication of these tubes (fig. CLXXXVI. 2).
Of these tubes the minutest unite together, and form larger branches
(fig. CLXXXVI. 2); these larger branches again uniting form still
larger branches (fig. CLXXXVI. 2), until, by their successive union,
the branches form at length a single trunk (fig. CLXXXVI. 3), with
which all the individual branches, whether great or small, communicate,
and into which they all pour their contents (fig. CLXXXII. 2, 3).
The bodies from which these tubes take their origin, and the minute
tubes themselves, are called secreting canals (fig. CLXXXII. 1, 2);
the common trunk formed by their union is termed the excretory duct
(fig. CLXXXII. 3). The secreting canals contain the secreted matter;
the excretory duct collects this matter, and conveys it to the part of
the body in which it is appropriated to the specific purpose which it
serves in the economy.

724. The basis of the secreting canals consists, then, of membrane
disposed in one or other of the elementary forms described (712, _et
seq._), These secreting canals constitute a peculiar system of organs
wholly different from all the other organs of the body. The form of
these organs, their structure and their relation to the blood-vessels
and nerves, have formed subjects of laborious investigation and of
keen controversy during several centuries. The honour of discovering
the exact truth on these points is due to very recent researches.

725. Malpighi, an Italian, who flourished at Bologna in the middle of
the 17th century, was the first to establish a special inquiry into
the intimate structure of the secreting apparatus. After many years
of laborious examination he arrived at the conclusion that a minute
sac or follicle is invariably interposed between the termination of
the capillary artery and the commencement of the excretory duct.
According to him, the capillary artery conveys the blood to the
follicle, separates from the blood the substance secreted, and the
excretory duct arising from one extremity of the follicle conveys the
secreted fluid, when duly prepared, to its destined situation. By
injection, by dissection, by the microscope, by experiment on living
animals, and by the phenomena of disease, he conceived that he had
demonstrated that this is the true structure of the secreting apparatus
in its most complex form. This view was generally acquiesced in by his
contemporaries and by succeeding anatomists and physiologists; and in
the time when Ruysh wrote was the received opinion.

726. Ruysh, who flourished at Amsterdam, and was contemporary with
Malpighi, but a younger man, and who published on the glands about
twenty years after Malpighi, according to the account of Haller,
“employed wonderful patience, with the assistance of his daughters,
in rendering all his preparations elegant and beautiful, being
equally skilled in the methods of softening, hardening, filling, and
drying.” Of Ruysh it was said that while others, in their anatomical
preparations, merely exhibited the horrid features of death, he
preserved the human body in all the freshness of life, even to the
expression of the features. The fineness of his injections, the
dexterity with which he unfolded the minute vessels, nerves, and
absorbents, and exhibited their combinations and relations in the most
delicate structures, the skill with which he preserved his preparations
in transparent fluids, and the elegance with which he displayed them
in their natural forms and folds, excited universal admiration; and
philosophers, statesmen, princes, kings, all the learned and noble of
the day, crowded to his museum.

727. By his superior method of injecting, Ruysh conceived that he was
able completely to disprove Malpighi’s doctrine. He maintained that
the bodies which Malpighi mistook for sacs or follicles are in reality
convoluted vessels; that these vessels are capable of being completely
unravelled; that, when unfolded, their continuity with the excretory
duct is perfectly demonstrated; that secretion is performed by the
capillary artery itself, without the intervention of any other organ;
and that when the secreted substance is duly prepared, it is poured by
the capillary directly into the excretory duct.

728. Modern research has demonstrated that the opinion of Malpighi
approaches nearer the truth than that of Ruysh, who appears to have
mistaken the secreting canals for the ultimate division of the
arterial vessels. Malpighi, indeed, did not succeed in discovering
the elementary bodies of which the secreting apparatus is composed;
but he arrived at the very verge of the truth. Profiting by the art
which Ruysh brought to so much perfection, by the facts which Malpighi
disclosed, and, above all, by the improved structure of the microscope,
and the increased skill which has been acquired in the manipulation
of the instrument, the modern physiologist is enabled to see what
was formerly beyond the cognizance of sense, and to demonstrate what
before could only be matter of conjecture. Availing himself of these
advantages with consummate skill, and applying himself to the task with
indefatigable industry, Professor Müller, of Berlin, has investigated
the structure of the secreting apparatus in the whole animal kingdom,
and has traced the progressive development of the several secreting
organs through the entire animal series, from their simplest form in
the lowest animal, to their most complex in the highest.

729. From the researches of this physiologist, and from the labours of
others, his countrymen and contemporaries, who have engaged in the
investigation with an ardour second only to his own, it is demonstrated
that the secreting apparatus of the animal body is disposed in one
or other of the elementary forms which have been described. The
blood-vessels are distributed upon the walls of these elementary
bodies, whether simple cryptæ follicles, cæca, or tubuli, or whether
these bodies are accumulated and combined into the largest and most
complex series of secreting canals, just as the branches of the
pulmonary artery are distributed upon the walls of the air-vesicles
in the rete mirabile of the lungs. The air-vesicles of the lungs are
secreting organs, and afford an excellent example of the mode in which
the blood-vessels are distributed upon the walls of the elementary
secreting bodies. The arteries do not form continuous tubes with the
secreting bodies or their excretory ducts, as was maintained by Ruysh;
neither is the secreting body interposed between the termination of
the artery and the commencement of the excretory duct, as was thought
by Malpighi; but the ultimate divisions of the arteries are spread
out upon the walls of the secreting bodies, where they terminate in
veins by a delicate vascular net-work (fig. CLXXXVII. 2). The minutest
branch of the artery is always smaller than the minutest secreting
body on the walls of which it is distributed. According to Müller, the
arteries, spread out upon the walls of the secreting bodies, form a
distinct and peculiar system of vessels visible under the microscope.
In the more complex secreting organs, immediately before reaching their
distribution upon the walls of the secreting canals, the ultimate
divisions of the arteries form an intricate and delicate net-work
(fig. CLXXXVII. 2). When at length they reach the secreting canals the
arteries no longer divide and subdivide, but are always of the same
uniform size in the same secreting organ, though their magnitude is
different in every different kind of secreting organ. These ultimate
divisions of the arteries are the proper capillary arteries. It is in
these arteries that the changes are wrought upon the blood which it
is the object of the various processes of secretion to effect. In the
walls of these arteries there are visible no pores, no apertures, no
open extremities by which the secreted fluid, when formed from the
blood, is conveyed into the cavity of the secreting canals; it probably
passes through the walls of the vessels into the secreting canals by
the process of endosmose (804).

[Illustration: Fig. CLXXXVII.

 A thin portion of the surface of the kidney taken from the scianus,
 showing—1. The termination of the cæca forming the uriniferous
 duct; and—2. A delicate vascular net-work, consisting of capillary
 blood-vessels about to be distributed on the walls of the cæca.]

730. Secreting organs are very abundantly supplied with nerves,
which are derived for the most part from the organic portion of the
nervous system; although for the reasons assigned (vol. i. p. 77, _et
seq._) sentient nerves are mixed with the organic. The more important
secreting organs have each a distinct net-work or plexus of organic
nerves, which surround the blood-vessels distributed to the organ,
(fig. CLXX. 3), and which envelopes more especially the arterial trunks
and their larger branches (fig. CLXX. 3). From these plexuses nervous
filaments spring in countless numbers (fig. CLXX. 3), which are spread
out upon the walls of the arteries, just as the arteries are spread
out upon the walls of the secreting canals. The nerves never quit
the arteries; are never spent upon the membranous matter which forms
the basis of the secreting organ, but are lost upon the walls of the
capillary arteries. The nerves uniformly increase in number and size as
the arteries diminish in magnitude and as their capillary terminations
become thinner and thinner.

731. When the secreting apparatus consists of simply extended membrane,
a close net-work of capillary arteries with their accompanying nerves
is spread out over the whole extent of the secreting surface. This
simple arrangement is sufficient to separate from the blood the simple
secretion in this case required.

732. When the secreting apparatus consists of simple cryptæ, follicles,
cæca, or tubuli, a similar net-work of capillary arteries and nerves
is spread out on the sides of this more extended surface. The more
elaborate secretion now formed is received into the interior of these
organs, where it remains for some time, and whence it is ultimately
conveyed as it is needed by the actions of the system.

733. But when the secreting apparatus consists of aggregates of cryptæ,
follicles, cæca, and tubuli, with their net-works of arteries and
nerves, a much more complex structure is built up, which is destined to
perform a proportionably elaborate function. An aggregation of these
secreting bodies into a large mass, enveloped in a common membrane,
so as to form a distinct body of a solid consistence, constitutes the
organ termed a gland. Simply extended membrane, with its apparatus
of arteries and nerves does not constitute a gland. Simple cryptæ,
follicles, cæca, and tubuli, with their larger apparatus of arteries
and nerves, do not constitute a gland. The first is simply secreting
surface; the second are simply secreting cryptæ, follicles, cæca or
tubuli; but when these bodies are aggregated into dense and solid
masses with an extended system of excretory ducts, and when the whole
of this apparatus is inclosed in a proper membrane so as to form a
distinct body, such a body is termed a gland.

734. Primary aggregations of these secreting bodies constitute what is
termed a conglobate, that is, a simple gland; such are all the glands
connected with the absorbent or lymphatic system. Secondary aggregates,
or aggregates composed of simple glands, constitute what is termed
a conglomerate, that is, a compound gland; such are all the organs
commonly termed viscera, as the liver, the spleen, the pancreas, the
kidney, and so on.

735. The conglobate, or simple gland, being formed by the aggregation
of cryptæ, follicles, cæca, or tubuli, inclosed in a proper membrane,
presents the appearance of a simple solid body, commonly of a rounded
or oblong form (fig. CLXXVI. 516). On the contrary, the conglomerate
or compound gland, being formed by the aggregation of conglobate or
simple glands, presents the appearance of a compound body composed of
a congeries of masses (fig. CLXV. 1). The larger masses enveloped in
their own proper membrane are termed lobes (fig. CXCI.); the smaller
masses, also enveloped in their own proper membrane, are termed lobules
(fig. CXCI.); the lobules, when carefully examined, are seen to be
composed of still smaller masses, and these of masses yet more minute,
until at length patient, laborious, and skilful dissection brings into
view the ultimate constituent elements, which are invariably found to
consist of simple cryptæ, follicles, cæca, or tubuli.

736. Thus membrane having a specific arrangement of blood-vessels and
nerves, from being simply extended, is folded into a few elementary
forms; the bodies which result constitute simple secreting organs;
these bodies collected together form, by their aggregation, compound
organs; the compound organs, uniting, form aggregates still more
compound, until at length a structure is built up highly elaborate and
complex. But this complexity of combination and arrangement does not
alter the constitution of the organs; their form varies, but their
nature remains essentially the same. All consist alike of membrane
organized in a similar mode. The complex contains no element not
possessed by the simple gland, and the gland contains no element not
possessed by the secreting surface. But there is this difference in
the complex organs. Every kind and degree of change in the form of the
secreting apparatus, from membrane simply extended, to membrane coiled
up into the most complex gland, is attended with an accumulation and
concentration of secreting surface. The crypt contains a larger extent
of secreting surface than the simple membrane; the follicle than the
crypt; the cæcum than the follicle; and the tubulum than the cæcum. A
certain amount of secreting surface is gained by the disposition of the
simple membrane into the form of the crypt. The collection of a number
of crypts into a cluster doubles the extent of the secreting surface by
the extent of every crypt that is added to the cluster. The addition
of every cluster doubles the whole extent of surface acquired by a
single cluster. But when stems spring as if from a common trunk; when
branches spring from a stem; when small branches spring from the large
branches, and yet smaller branches from the small in a series, which
the eye, assisted by the most powerful microscope, is wholly unable to
trace; when all the clusters thus formed are collected, and combined
into a compact mass, the intricacy of which no art can completely
unravel, the extent of surface obtained is altogether immeasurable. How
immense must be the extent of surface thus acquired in such an organ as
the human lungs, in such a gland as the human liver!

737. In such an aggregation the concentration is also equal to the
accumulation; the maximum of surface is comprised in the minimum of
space, and the energy and elaborateness of the function of a secreting
organ is uniformly proportionate to such a concentration of its
secreting substance.

[Illustration: Fig. CLXXXVIII.

 Aggregated and clustered cæca opening into the alimentary canal,
 performing the function of the liver.]

738. Hence the complexity of the compound gland in the higher animals
would appear to arise solely from the intricate arrangement of the
immense mass of secreting matter concentrated in a small compass;
hence also the progressively increased complication indicated in the
successive development of the glandular system in the animal series.
Thus, for example, among the distinct organs formed for the purpose
of elaborating a specific secretion, being intimately connected with
the process of digestion, one of the first is the salivary gland. Low
down in the scale, in the animal in which the first rudiment of a
salivary gland is traceable, it consists of a single follicle, which
appears to serve the office of a gland. In an animal a little higher
in structure, two, three, or four follicles combine to form a somewhat
less simple organ. In an animal still higher in the series, a number of
follicles are clustered together and form a much more complex organ;
and in this manner, as the organization of the animal becomes higher
and higher, the complexity of the gland increases, until at length it
is composed of a countless number of follicles collected into clusters,
the clusters disposed into lobes, the lobes subdivided into lobules,
and the lobules into still smaller particles, the ultimate elements
of the glandular apparatus. In like manner, when the first rudiment
of the liver is discoverable, it consists of a single pouch or cæcum;
somewhat higher in the series, the organ is composed of two or more
cæca distinct and free; and then, as its complexity increases with
the perfection of the organization, cæca are accumulated upon cæca;
the aggregates so formed are closely compacted, disposed into lobes,
divided into lobules, and subdivided into the ultimate particles of the
glandular apparatus. So in a gland composed of tubuli, as the kidney,
the organ in its rudimentary state consists of a few straight tubuli:
as its structure advances more tubuli are added: next, the increasing
tubuli superimposed one upon another become tortuous; then the tubuli
still accumulating, become not merely tortuous, but convoluted; and
last of all, countless numbers of tubuli are closely compacted into
exceedingly convoluted masses. Uniformly, the lower the animal and the
simpler the organ, the larger and the more manifest are the elementary
parts of the gland; but in the higher animals these elementary bodies
are so minute as to be altogether microscopical and their arrangement
is so complex that it can be unravelled only with extreme difficulty.

[Illustration: Fig. CLXXXIX.

 Portions of the kidney taken from the ophidian reptile, as seen
 under the microscope, highly magnified. A one portion of the kidney,
 showing—1. The trunk of the artery passing to be distributed to—2. The
 diverging tubuli, forming the uriniferous ducts which terminate in—3.
 The common excretory duct called ureter.—B another portion of the same
 kidney, showing the extremely convoluted course of—4. The uriniferous
 ducts. 5. The smaller excretory ducts, or secreting canals, converging
 and uniting to form—6. The common excretory duct called the ureter.]

739. It is a striking confirmation of the correctness of this view
of the structure of the glandular apparatus, that whenever in the
ascending series a gland appears for the first time in any class, the
elementary bodies are so large, and are disposed in so simple a mode,
that a slight examination is sufficient to demonstrate their primitive
form, and to render it manifest that they consist either of vesicles,
follicles, cæca, or tubuli, more or less aggregated. This is seen
in the obvious structure presented by the liver, the pancreas, the
salivary glands, and the mammæ, in the simple animals in which these
organs first appear. Thus the liver in animals low down in the scale
is manifestly composed of simple clustering follicles: in the fish the
pancreas is composed of simple branched follicles: in the bird, the
salivary glands are composed of simple parallel tubuli; and in the
cetacea the breasts are composed of simple branched tubuli.

[Illustration: Fig. CXC.

 A lobule of a gland in the progress of development in the ovum of
 the bird, as seen under the microscope, showing the origin of the
 excretory ducts in the semipellucid gelatinous blastema, and the
 branching and foliated arrangement of the follicles in which the
 excretory ducts terminate.]

740. But the microscope, by bringing the successive development of the
compound gland in the embryo of the higher animal under the cognizance
of sense, perfectly discloses the nature of its composition. In
the development of the incubated egg every step of the progressive
formation of the compound gland is rendered visible to the eye.
When this process is carefully watched, it is seen that the part of
the gland first formed is the excretory duct, which springs from
the blastema, the common mass of matter out of which all the organs
are formed. From this duct the elementary parts of the gland bud
just as bunches of grapes bud from the stalk. The buds, at first
at considerable distances from each other, approach nearer as they
increase by new growths, until at length they come into actual
contact. The growth continuing, and the compactness of the substance
of the gland proportionally increasing, the primitive form of the
elementary bodies which compose it is ultimately lost. The substance
of the gland now appears to consist of compact solid matter, which
is commonly termed parenchyma. The component particles of this
parenchymatous and apparently solid substance present a clustered or
grape-like appearance, from which they early obtained the name of
acini, from the Latin word acinus, a berry. This term, originally
employed merely to express the clustered and branching appearance
of the elementary parts of the gland, has since been used in widely
different senses. By some it has been employed to express solid
glandular grains constituting a supposed distinct parenchymatous
substance, differing in every different gland. It is now proved that no
such solid granular particles enter into the composition of any gland
in the animal kingdom. By others the term acini has been employed to
express granular bodies composed of blood-vessels, directly continuous
with the excretory ducts, and from which the excretory ducts derive
their origin. Recent investigation has demonstrated that there is no
continuity of the blood-vessels into the excretory duct either in the
acini or in any other part of the gland. It is established that the
blood-vessels are spread out upon the walls of the secreting canals
and do not form with them continuous tubes. The bodies which have been
mistaken for granular particles, constituting the so called solid
acini, are really the shut extremities of hollow follicles, cæca, or
tubuli, which appear solid only from the closeness with which they are
compacted. When carefully dissected and examined under the microscope,
their real nature becomes apparent, and this is also sometimes capable
of being demonstrated by injection; for some of these elementary bodies
are vesicular, and can be filled with mercury, when they present a
beautiful appearance like clusters of diamonds; or they may be inflated
with air, just as the air vesicles of the lungs.

[Illustration: Fig. CXCI.

 Section of the liver in the lower animal in the progress of
 development, as seen under the microscope, showing the rudimentary
 division into lobes and lobules, and the elongated terminations of
 the biliferous ducts, or cylindrical acini variously disposed in a
 branching and foliated manner.]


741. On watching the formation of the gland in the development of
the embryo, it would appear that at first free streams of blood, or
blood not contained in proper vessels, pass around the acini, the shut
extremities of the excretory ducts, or the secreting canals. “So it
would seem,” says Müller, “when we examine the evolution of the liver
and kidney in the embryo of the lower animal; for the interstices of
the canals appear bloody, without the slightest trace of the walls of
blood-vessels. I conceive that in the beginning new streams arise in an
amorphous mass (a mass without form), not bounded by proper parieties;
but that soon walls are formed, which present definite boundaries
to the streams, the density of the substance around the streams
gradually increasing.” It is in this manner that the connexion is first
established between the system of capillary blood-vessels and that of
the secreting organs.

742. In its embryo state the compound gland of the highest animal
consists of mere excretory ducts, wonderfully similar to the simple
secreting bodies of the lowest classes. But in the higher animal this
simple form of the gland is transient: gradually, with the progressive
evolution of the embryo, it passes into a more complex structure; while
in the lower animal the simple form of the gland remains permanently
the same through the whole term of life.

743. Such are the main points which have been ascertained relative to
the structure of the secreting apparatus, which enters in one or other
of its forms, as a constituent element, into almost every part of
the animal body. Wherever there is nutrition there is secretion, and
wherever there is secretion there is one or other of these secreting
bodies. How immense the number of these organs in the human body! Every
point in the interior of the walls that bound the great cavities is a
secreting surface. Every point of the secreting surface that lines the
alimentary canal, from its commencement to its termination, is studded
with distinct secreting organs. Every point of the skin is still more
thickly studded with distinct secreting organs. By the naked eye, and
still more distinctly with a lens, may be seen the pores through which
the vapour that constitutes the insensible perspiration incessantly
exudes. Next are the open mouths of myriads of sebacious follicles that
pour out upon the skin the oily matter which gives it its suppleness
and softness; and besides all these, are the hairs, each the product
of a secreting organ placed immediately beneath the skin. An attempt
to count the number of pores and hairs visible to the eye within the
compass of an inch, and thence to compute the number on the whole
surface of the skin, may convey some conception of the amount of these
organs; yet these form but a small part of the secreting apparatus.
The great viscera of the body, the brain, the lungs, the liver, the
pancreas, the spleen, are portions of it; all the organs of the senses,
the eyes, the ears, the nose, the tongue; all the organs of locomotion;
every point of the surface of every muscle, and a great part of the
surface and substance of the very bones are crowded with secreting
organs.

744. Since every secreting organ is copiously supplied with blood,
it follows that a great part of the blood of the body is always
circulating in secreting organs; and, indeed, it is to afford materials
for the action of these organs that the blood itself is formed.

745. How do these organs act upon the blood? All that is known of the
course of that portion of the blood which flows through an organ of
secretion is, that it passes into arteries of extreme minuteness, which
are spread out upon the external walls of the elementary secreting
bodies, and which, as far as they can be traced, pass into capillary
veins,—nowhere terminating by open mouths—nowhere presenting visible
outlets or pores; their contents probably transuding through their thin
and tender coats by the process of endosmose.

746. As it is flowing through these capillary arteries, the blood
undergoes the transformations effected by secretion, forming—1. The
fluids, which are added to the aliment, and which accomplish its
solution, and change it into chyme. 2. The fluids, which are added to
the chyme to convert it into chyle, and both to chyle and lymph, to
assist in their assimilation. 3. The fluids which, poured into the
cavities, facilitate automatic or voluntary movements. 4. The fluids,
which serve as the media to the organs of the senses by which external
objects are conveyed to the sentient extremities of the nerves for
their excitement. 5. The fluids which, deposited at different points
of the cellular tissue, when more aliment is received than is needed,
serve as reservoirs of nutriment to be absorbed when more aliment is
required than can be afforded by the digestive organs. 6. The fluids
which are subsequently to be converted into solids. 7. The fluids which
are eliminated from the common mass, whether of fluids or solids, to be
carried out of the system as excrementitious substances. 8. In addition
to all these substances, which are indispensable to the preservation of
the individual, those which are necessary to the perpetuation of the
species.

747. In order to form any conception of the mode in which the secreting
organs act upon the blood, so as to elaborate from it such diversified
substances, it is necessary to consider the chemical composition of the
different products of secretion, and the degrees in which they really
differ from each other, and form the common mass of blood out of which
they are eliminated.

748. By chemical analysis, it is established that all the substances
which are formed from the blood by the process of secretion are either
water, albumen, mucus, jelly, fibrin, oil, resin, or salts; and,
consequently, that all the secretions are either aqueous, albuminous,
mucous, gelatinous, fibrinous, resinous, oleaginous, or saline.

749. 1. AQUEOUS SECRETIONS.—From the entire surface of the skin, and
also from that of the lungs, there is constantly poured a quantity of
water, derived from the blood, mixed with some animal matters, which,
however, are so minute in quantity, that they do not communicate to the
aqueous fluid any specific character.

750. 2. ALBUMINOUS SECRETIONS.—All the close cavities, as the thorax,
the abdomen, the pericardium, the ventricles of the brain, and even
the interstices of the cellular tissue, are constantly moistened by a
fluid which is termed serous, because it is derived from the serum of
the blood. This serous fluid consists of albumen in a fluid form, and
it differs from the serum of the blood chiefly in containing in equal
volumes a smaller proportion of albumen. Membranes of all kinds consist
essentially of coagulated albumen; and the albumen, as constituting
these tissues, differs from albumen as existing in the serum of the
blood only in being unmixed with extraneous matter, and in being in a
solid form.

751. 3. MUCOUS SECRETIONS.—As all the close cavities, or those which
are protected from the external air, are moistened with a serous
fluid, so all the surfaces which are exposed to the external air, as
the mouth, the nostrils, the air-passages, and the whole extent of the
alimentary canal, are moistened with a mucous fluid. Mucus does not
exist already formed in the blood. It is always the product of a gland.
Some of the mucous glands are among the most elaborate of the body;
still the main action of the gland seems to be to coagulate the albumen
of the blood, for the basis of mucous is coagulated albumen. The fluid
that lubricates the mucous surfaces in their whole extent, the saliva,
the gastric juice, the tears, the essential part of the fluid formed
in the testes and in the ovaria, are mucous secretions. Hence the most
complex and elaborate functions of the body, respiration, digestion,
reproduction, are intimately connected with the mucous secretions:
nevertheless, as far as regards their chemical nature, the mucous
differ but slightly from the albuminous secretions; and it is probable
that a slight change in the secreting organ is sufficient to convert
the one into the other. By the irritation of mercury on the salivary
glands, the saliva, properly of a mucous, is sometimes converted into a
substance of an albuminous nature; and irritation in some of the serous
membranes occasionally causes them to secrete a mucous fluid.

752. 4. GELATINOUS SECRETIONS.—The proximate principle termed jelly
abounds plentifully in several of the solids of the body, and more
especially in the skin; but jelly does not exist already formed in
the blood. Yet it is not the product of a gland, neither is there any
known organ by which it is formed. Out of the body albumen is capable
of being converted into jelly by digestion in dilute nitric acid: this
conversion is probably effected by the addition of a portion of oxygen
to the albumen. Albumen contains more carbon and less oxygen than
jelly; the proportions of hydrogen and nitrogen in both being nearly
the same. According to MM. Gay Lussac and Thénard, the elements of
albumen and jelly are,

             Carbon.   Oxygen.  Hydrogen.   Nitrogen.

  Albumen    52.883    23.872    7.54        15.765
  Jelly      47.881    27.207    7.914       16.988

The conversion of albumen into jelly is incessantly going on in the
system; and the process accomplishes most extended and important uses.
In the lungs at the moment of inspiration oxygen enters into the blood
in a state of loose combination; but in the system, at every point
where the conversion of albumen into jelly takes place, oxygen probably
enters into a state of chemical combination with albumen; and the new
proximate principle, jelly, is the result. The agent by which this
conversion is effected appears to be the capillary artery: the primary
object of the action is the production of a material necessary for
the formation of the tissues of which jelly constitutes the basis, as
the skin; but a secondary and most important object is the production
of animal heat; the carbon that furnishes one material of the fire
being given off by the albumen at the moment of its transition into
jelly; and the oxygen that furnishes the other material of the fire
being afforded to the blood at the moment of inspiration. This view
affords a beautiful exposition of the reason why jelly forms so large a
constituent of the skin in all animals. The great combustion of oxygen
and carbon, the main fire that supports the temperature of the body, is
placed where it is most needed, at the external surface.

753. 5. FIBRINOUS SECRETIONS.—The pure muscular fibre, or the basis
of the flesh, is identical with the fibrin of the blood. It contains
a larger proportion of nitrogen, the peculiar animal principle, and
is consequently more highly animalized than the preceding substances.
It appears to be simply discharged from the circulating blood by the
capillary arteries, and deposited in its appropriate situation; no
material change in its constitution being, it would seem, necessary to
fit it for its office.

754. 6. OLEAGENOUS SECRETIONS.—Fat of all kinds, which is found so
extensively connected with the muscles, and with many of the viscera,
and which is more or less diffused through the whole extent of the
cellular tissue, marrow, milk, and nervous and cerebral matter, are
essentially of the same nature. The basis of them all is oil; and oil
exists already formed both in the chyle and in the blood.

755. 7. RESINOUS SECRETIONS.—The peculiar substance forming the basis
of bile, picromel; the peculiar substance forming the basis of urine,
urea; the peculiar substance connected with the muscular fibre, and
forming a component part of almost all the solids and fluids of the
body, osmazome, consists of a common principle—a resin, which exists
already formed in the blood, and more especially in the serosity of the
blood.

756. 8. SALINE SECRETIONS.—The substances termed saline, namely, the
acids, the alkalis, and the neutral and earthy salts, are disposed
over every part of the system: they enter more or less into all the
constituents both of the solids and fluids; they form more especially
the phosphate of lime, the earthy matter of which bones are composed;
and they all exist already formed in the blood.

757. From this account, then, it appears, that by chemical analysis,
the blood is ascertained to contain water, albumen, fibrin, oil, resin,
and various saline and earthy substances: it follows, that, with the
exception of the absence of jelly, the constituents of the body and the
constituents of the blood are nearly identical; and it is probable that
they will be found to be perfectly identical when their analysis shall
have become complete.

758. It is also manifest that in by far the greater number of cases the
various substances of which the body is composed are simply separated
from the nutritive fluid at the parts of the body at which they are
deposited; and that, existing already formed in the blood, they are
merely deposited there, and not generated. Still, however, since it is
certain that gelatin cannot be recognized in the blood, and since it
is doubtful whether some other substances found in different textures
and secretions really exist in the blood, it is necessary, in the
present state of our knowledge, to suppose, that although most of the
constituents of the living tissues are contained in the blood, yet that
in some instances a material change is effected in their nature at
the time and place of their escape from the circulation; and that in
these cases the secreted substances are not simple extracts from, but
products of, the blood.

759. It is by the apparatus of secretion that this separation,
evolution, or re-formation, is effected. Out of a fluid which contains,
blended together, almost all the heterogeneous substances of which the
body is built up, particular substances are selected from the common
mass, and are deposited in certain parts, and only in certain parts.
Although by the most careful examination of the structure of the
apparatus, it is not possible to form a precise conception of the mode
in which this separation is effected, yet we are enabled to perceive a
number of contrivances which we can readily understand must conduce to
the accomplishment of the object.

760. 1. Of these, the most obvious is mechanical arrangement.

761. In its passage to different organs the blood is propelled
through canals of extreme minuteness: in every different case these
canals differ from each other in size; pass off from their respective
trunks at different angles; possess different degrees of density; are
variously contorted, and are of various lengths. In some they are
straight, in others convoluted; at one time branching, at another
pencillated, and at another starry. The veins, too, in some cases, are
almost straight, in others exceedingly tortuous, in others reticulated;
and the freedom of their communication with the arteries varies so
much, that in some cases fine injections pass from the one set of
vessels to the other with the greatest facility, while, in others
they pass with extreme difficulty. The consequence of these divers
arrangements of the capillary blood-vessels is, that the current of
the blood must necessarily flow in them with different degrees of
velocity; its particles must be placed at different distances from each
other, and must be presented to each other in different positions and
in widely different proportions. In no two secreting organs are any
two of these conditions exactly alike. In the lower orders of animals,
in which secretion is seen in its simplest condition, the general
nutritive fluid, elaborated and contained in a single internal cavity,
appears to furnish a variety of products very different from itself, by
a process hardly more complex than mere transudation through a living
membrane. In the higher animals the different secreting organs may be
considered, in part at least, as mechanical contrivances adapted to
carry on analogous transudations—fine sieves or strainers diversly
constructed. A fluid containing such heterogeneous matters as the
blood, held in combination by so slight an affinity, slowly transuding
through series of tubes, the mechanical arrangement of which is so
varied, must yield a different substance in every different case.
Thus by simply filtering the blood a vast variety of products may be
obtained, merely in consequence of a varied disposition of the minute
tubes of which the filters are composed.

762. 2. But in the second place, this diversity of mechanical
arrangement is calculated in a high degree to promote and to modify
chemical action. The contact or proximity of the particles of bodies,
the extent of surface which those particles present to each other, the
space of time in which they continue in contact, the degree of force
with which they impinge against each other, the degree of temperature
to which they are exposed,—these, and circumstances such as these,
are conditions which exert the most powerful influence over chemical
decomposition and re-combination. In the different secreting organs,
as has been shown, the blood must necessarily pass through vessels
having every conceivable diversity of diameter: in those vessels it
must consequently flow with corresponding differences of velocity. Some
of these diameters will admit one constituent of the blood, as one of
the red particles; others may be large enough to admit two or more of
the red particles abreast; others may be so small as to be incapable of
admitting a single red particle, receiving only the more fluid portions
of the blood; in some vessels these different constituents will be in
one degree of proximity, in others in another; in some they will remain
long in contact, in others only for an instant: it is obvious that
from such different conditions the chemical products may be infinitely
varied.

763. Such is the composition of chemical bodies, that a great diversity
of substances is obtainable merely by changing one condition, the
proportions in which the elementary particles combine.

764. Oxygen and nitrogen combined in one proportion form atmospheric
air; in another proportion, nitrous oxide; in another, nitric oxide;
in a fourth, nitrous acid; and in a fifth, nitric acid. Few secretions
formed from the blood differ more widely from each other than the
products thus formed from these two elementary bodies.

765. Urea consists of two prime equivalents of hydrogen, one of
carbon, one of oxygen, and one of nitrogen. Remove one of the atoms
of hydrogen, and take away the atom of nitrogen, urea is converted
into sugar; combine with urea an additional atom of carbon, it is
changed into lithic acid. In like manner add a small quantity of water
to farina, it is converted into sugar; to fibrin, it is changed into
adipocere. From a reservoir containing a quantity of substances in
the state of vinous fermentation, draw off portions of the liquor at
different stages of the process, and cause these to pass through tubes
of various diameters and with various degrees of velocity, there will
be obtained at one time an unfermented syrup, at another, a fermenting
fluid, at another, wine, at another, vinegar. Out of the body place the
blood in a state of rest, it will spontaneously separate into serum and
crassamentum, and the crassamentum will further separate into fibrin
and red particles. Add to the serum a certain portion of acid, it will
be coagulated into solid albumen; add to this solid albumen another
portion of acid, it will be converted into jelly. Add a certain portion
of acid to fibrin, it will be changed into adipose matter; bring the
acid into contact with the red particles, they will be converted into a
substance closely resembling bile. If by the rough chemistry which the
art of man can conduct so great a variety of substances may be obtained
out of a single compound, is it not wonderful that a far greater
variety should be produced by the delicate and subtle chemistry of life.

766. 3. But a third most important agent in the process of secretion is
some influence derived from the nervous system.

1. It is proved, by direct experiment, that the destruction of the
nervous apparatus, or of any considerable portion of it, stops the
process of secretion. By experiments performed by Mr. Brodie, it is
ascertained that the secretion of the urine is suspended by the removal
or destruction of the brain, though the circulation be maintained in
its full vigour by artificial respiration.

2. The section, and still more the removal, of a portion of the
sentient nerves of the stomach (the par vagum, or eighth pair),
according to some experimentalists, deranges and impedes; according to
others, totally arrests the process of digestion.

3. Other classes of phenomena illustrate in a striking manner the
influence of the nervous system over the process of secretion.
The sight, nay, even the thought of agreeable food, increases the
secretions of the mouth. Pleasurable ideas excite, painful ideas
destroy, the appetite for food; probably, in the one case, by
increasing, and, in the other, by suspending the secretion of the
gastric juice: the emotion of grief instantly causes a flow of tears;
that of fear, of urine; the sight or thought of her child fills the
maternal breasts with milk, while the removal of the child from the
mother diminishes and ultimately stops the secretion.

767. Even the imagination is capable of exerting a powerful influence
over the process. A female who had a great aversion to calomel was
taking that medicine in very small doses for some disease under which
she was labouring. Some one told her that she was taking mercury:
immediately she began to complain of soreness in the mouth; salivated
profusely, and even put on the expression of countenance peculiar to a
salivating person. On being persuaded that she had been misinformed,
the discharge instantly began to diminish, and ceased altogether
in a single night. Two days afterwards she was again told, on good
authority, that calomel was contained in her medicines, upon which
the salivation immediately began again, and was profuse. That this
salivation was not produced by the calomel, but was the effect solely
of the influence of imagination on the salivary glands, was proved
by the absence of redness of the gums, which always takes place in
mercurial salivation, and also by the absence of the peculiar fætor,
which is characteristic of the action of this metal on the system.

768. The same influence is apparent even in the lower animals: exhibit
food to a hungry dog, the saliva will pour from its mouth. Rob the nest
of the bird of its eggs as soon as they are laid, the bird may be made
to deposit eggs almost without end, though if the eggs are allowed to
remain undisturbed, it will lay only a certain number. The bird is led
by instinct to continue to deposit eggs in the nest until a certain
number is accumulated; that is, a mental operation acts upon the
ovarium, the secreting organ in which the eggs are formed, maintaining
it in a state of active secretion for an indefinite period; whereas
without that mental operation the secretion would be limited to a
definite number.

769. In all these cases it is probable that the vital agent by which
the effect is produced on the secreting organs is the organic nerve.
Though the sentient part of the nervous system may in many cases be
the part primarily acted on, yet there is reason to believe that
the ultimate effect is invariably produced on the organic part, the
sentient nerves in this case acting on the organic, as in other cases
the organic act on the sentient, in consequence of that intimate
connexion which, for the reason assigned (vol. i. p. 79), is
established between both parts of this system. For,

770. 1. The true object of the sentient part of the nervous system is
to establish a relation between the body and the external world; the
object of the organic part is to preside over the functions by which
the body is sustained and nourished, that is, over the processes of
secretion.

771. 2. The nerves which are distributed to the secreting arteries, and
which increase in number and size as the arteries become capillary,
are, for the most part, derived from the organic portion of the nervous
system (fig. CLXX. 3). This anatomical arrangement clearly points to
some physiological purpose, and indicates the closeness of the relation
between the function of the organic nerve and the ultimate action of
the capillary artery.

772. 3. It is demonstrated that the sentient part of the nervous
system, though occasionally influencing and modifying secretion, is not
indispensable to it. In tracing the normal or regular development of
the human fœtus, it is found that the heart is constructed and is in
full action before the brain and spinal cord, the central masses of the
sentient part of the nervous system, are in existence; and that these
masses are themselves built up by processes to which the action of the
heart is indispensable; consequently, innumerable acts of secretion
must have taken place, those, for example, which have been necessary
to form the different substances which enter into the composition of
the heart, before the brain and spinal cord exist. In like manner in
the anormal or irregular development of the fœtus, as in the production
of monsters, there may be not a vestige of head, neck, brain or spinal
cord, while there may be a perfect heart, perfect lungs, perfect
intestines, and various portions even of the osseous system.

773. However in the perfect animal secretion may be under the influence
of the brain and spinal cord, it is clear that, since the process can
go on without them, it must be independent of them. It is a false
induction from these facts drawn by some physiologists that secretion
is independent of the nervous system. They do prove that it is
independent of one part of the nervous system, the sentient; but it
does not follow that it is independent of the other part, the organic.

774. 4. It is demonstrated that the organic part of the nervous
system is not only independent of the sentient part, but that it
is even pre-existent to it. Researches into the development of the
nervous system, as shown in the progressive growth of the fœtus of
different animals, have proved that the existence of the organic
nerves is manifest long before that of the sentient; that nerves are
discoverable in the tissues, before the brain and the spinal cord are
formed; that as these masses become visible and grow, nerves springing
from the tissues advance towards the central nervous masses, and
at length unite with them; but that this union does not take place
until the development of the nervous system is considerably advanced.
These curious and most instructive facts show that in the fœtus,
though the brain and spinal cord may have been destroyed or have
been non-existent, yet that the organic nerves may have been in full
action. After a communication has been once established between the two
parts of the system, indeed, the destruction of the brain or spinal
cord may stop secretion, not because these organs are indispensable
to secretion; but because the destruction of one part of the system
involves the death of the other, just as the organic life itself
perishes soon after the destruction of the animal.

775. The existence of the organic nerve is probably simultaneous
with that of the secreting artery: from the first to the last moment
of life the nerve regulates the artery; the influence of the one is
indispensable to the operation of the other; and, by their conjoint
action, the sentient nerve itself, as well as every other organ, is
constructed.

776. There is reason to believe that the physical agent by which the
organic nerve influences secretion is electricity. The nerve appears to
be the medium by which electrical fluid is conveyed to the secreting
organs, and the nerve probably influences secretion by influencing
chemical combination, through the intervention of this most powerful
chemical agent. This is rendered probable by the observation of various
phenomena, and by the result of direct experiment.

777. 1. It is proved that galvanic phenomena may be excited by
the contact of the nerve and muscle in an animal recently dead. A
galvanic pile may be constructed of alternate layers of nervous and
muscular substance, or of nervous substance and other animal tissues.
A secreting organ liberally supplied with organic nerve is probably
then in its physical structure nothing but a galvanic apparatus. It
is certain that some animals, as the raia torpedo, possess a special
electrical apparatus composed essentially of nervous matter; that
the nerves which compose this apparatus correspond strictly with the
organic nerves of the human body; that they are distributed principally
to the organs of digestion and secretion, and that they exert a
powerful influence over these processes; for, when the animal is
frequently excited to give shocks, digestion appears to be completely
arrested; so that, after the animal’s death, food swallowed some time
previously is found wholly unchanged.

778. 2. It is universally admitted that the nerves in all animals
possess an extreme sensibility to the stimulus of electricity, and more
especially to that form of it which is termed galvanism.

779. 3. Direct experiment proves that the stimulus of galvanism may
be made to produce in the living-body precisely the same effect as
the nervous influence. It has been stated, that the division of the
par vagum, in the neck of a living animal, suspends the digestion of
the food probably by stopping indirectly the secretion of the gastric
juice. If after the division of the nerves, their lower ends, that
is, that portion of the nerves which is still in communication with
the stomach, but no longer in communication with the brain, be made
to conduct galvanic fluid to the stomach, secretion goes on as fast
as when the nerves are entire and conduct nervous influence. Dr.
Wilson Philip having divided the par vagum in the neck of a living
animal, coated a portion of the lower end of the nerves with tin foil,
placed a silver plate over the stomach of the animal, and connected
respectively the tin and silver with the opposite extremities of a
galvanic apparatus. The result was that the animal remained entirely
free from the distressing symptoms which had always before attended the
division of the nerves, and that the process of digestion, which had
been invariably suspended by this operation, now went on just as in the
natural state of the stomach. On examining the stomach after death, the
food was found perfectly digested, and afforded a striking contrast to
the state of the food contained in the stomach of a similar animal, in
whom the nerves had been divided, but which had not been subjected to
the galvanic influence.

780. 4. On applying a low galvanic power to a saline solution contained
in an organic membrane, Dr. Wollaston found that the galvanic fluid
decomposed the saline solution, and that the component parts of the
solution transuded through the membrane; each constituent being
separately attracted to the corresponding wire of the interrupted
circuit. This experiment, says this acute and philosophical
physiologist, illustrates in a very striking manner the agency of
galvanism on the animal fluids. Thus the quality of the secreted fluid
may probably enable us to judge of the electrical state of the organ
which produces it; as for example, the general redundance of acid
in urine, though secreted from blood that is known to be alkaline,
appears to indicate in the kidney a state of positive electricity; and
since the proportion of alkali in bile seems to be greater than is
contained in the blood of the same animal, it is not improbable that
the secretory vessels in the liver may be comparatively negative.

781. We may imagine, says Dr. Young, that at the division of a minute
artery a nervous filament pierces it on one side, and affords a pole
positively electrical, and another opposite filament a negative pole.
Then the particles of oxygen and nitrogen contained in the blood, being
most attracted by the positive point, tend towards the branch which is
nearest to it; while those of the hydrogen and carbon take the opposite
channel; and that both these portions may be again subdivided, if it
be required; and the fluid thus analysed may be recombined into new
forms by the reunion of a certain number of each of the kinds of minute
ramifications. In some cases the apparatus may be somewhat more simple
than this; in others, perhaps, much more complicated; but we cannot
expect to trace the processes of Nature through every particular step;
we can only inquire into the general direction of the path she follows.

782. Considerations such as these afford us a glimpse into the mode in
which Nature conducts some of her most secret and subtile operations;
or rather into the immediate agency by which she effects them; for,
properly speaking, of the mode in which she works, we do not obtain
the slightest insight, and even of her immediate agency our view, at
least in the present state of our knowledge, is indistinct and vague.
By the study of the apparatus which she builds up, we can trace back
her operations a step or two; but in every case, at a certain point,
the apparatus itself becomes so delicate as to elude our senses, and
then of course we are necessarily at a stand. So, the rough materials
with which she carries on her great work of secretion, by careful
analysis we can separate into divers parts, and ascertain that each
part possesses peculiar properties. The main channels by which she
conveys these varied constituents to the different parts of the system
we can trace; the delicate organs by which she produces on these rude
materials her wonderful transformations we can see; but beyond the
threshold of these organs we cannot go. Why from one common mass of
fluid the same variety of peculiar substances are constantly separated,
and each in its respective place: why the kidney never secretes milk,
nor the liver urine, nor the breast bile: why membrane, and muscle, and
bone, and fat, and brain, are uniformly deposited in the same precise
situation: why these depositions go on with uniformity, constancy and
regularity; and by what laws each process is controlled and modified,
we do not know. But though with whatever diligence we investigate these
operations, the great problem remains, and probably ever will remain
unresolved, still it is both a pleasurable and a profitable labour to
follow Nature in her path, to the extreme point to which it is possible
to trace her footstep; for the phenomena themselves are often in the
highest degree curious and interesting; while their order and relation
can seldom be so considered as to be understood, without the suggestion
of practical applications of great and permanent usefulness.




CHAPTER XII.

OF THE FUNCTION OF ABSORPTION.

 Evidence of the process in the plant, in the animal—Apparatus
 general and special—Experiments which prove the absorbing power of
 blood-vessels and membrane—Decomposing and analysing properties
 of membrane—Endosmose and exosmose—Absorbing surfaces, pulmonary,
 digestive, and cutaneous—Lacteal and lymphatic vessels—Absorbent
 glands—Motion of the fluid in the special absorbent vessels—Discovery
 of the lacteals and lymphatics—Specific office performed by the
 several parts of the apparatus of absorption—Condition of the system
 on which the activity of the process depends—Uses of the function.


783. Absorption is the function by which external substances are
received into the body, and the component particles of the body are
taken up from one part of the system, and deposited in some other
part. So universal and constant is the operation, that there is not a
fluid nor a solid, not a surface nor a tissue, not an external nor an
internal organ, which is not, in its turn, the seat and the subject of
the process. By its action the component particles of the living body
are kept in a state of perpetual mutation.

784. The plant in a humid atmosphere increases in weight. The nutritive
matter of the plant diffused in the soil is taken up by its capillary
rootlets, or by the spongolæ which are attached to them, and conveyed
into the system. The fall of dew or rain upon leaves promotes the
growth of the plant. Leaves placed on water are capable of preserving
not only their own vitality, but that of the branches and twigs to
which they are attached. These phenomena show that the process of
absorption is carried on by the plant.

785. The evidence of the absorbing power possessed by the animal is
still more striking.

786. 1. If an animal be immersed in water the amount of which is
ascertained by measure, its head being kept out of the water, so that
none can enter the mouth, the body increases in weight and the water
diminishes in quantity. If certain animals, as snails, are plunged in
water impregnated with colouring matter, the fluids in the interior
of their body soon acquire the colour of the water by which they are
surrounded. Frogs, previously kept for some time in dry air, when
placed in water, absorb a quantity equal in weight to their whole body.

787. 2. In a humid atmosphere the animal increases in weight still more
than the plant.

788. 3. If a quantity of water be injected into any of the great
cavities of the body, as into that of the peritoneum, the whole of the
fluid after a certain time disappears; it is spontaneously removed.

789. 4. If in the progress of disease a fluid be poured into any cavity
of the body, as often happens in dropsy, the whole of the fluid is
removed, sometimes spontaneously and quite suddenly; but more often
slowly, under the influence of medicinal agents.

790. 5. Certain substances, whether applied to an external or an
internal surface, produce specific effects on the system, just as when
they are received into the stomach or injected into the blood-vessels.
Mercury in mere contact with the skin, but more rapidly when the
application is aided by friction, produces the same specific action
upon the salivary glands, and the same general action upon the system
as when the preparation of the metal is received into the stomach.
By the like external and local application arsenic, opium, tobacco,
and other narcotics produce their distinct and peculiar effects on
the nervous system, and their remote and general effects on the other
systems.

791. 6. If an organ or tissue be deprived of nourishment, it gradually
diminishes in bulk, and at length wholly disappears from the system.
By long-continued pressure, such as that occasioned by the pulsation
of a diseased artery, as in aneurism, or by the growth of a fleshy
tumor, portions of the firmest and strongest muscle, nay, even of the
most dense and compact bone, wholly disappear. At one time the fluids
diminish in quantity, the flesh wastes, and the weight of the body is
reduced one half or more. Under other circumstances, while the state of
the general system remains stationary, some particular part diminishes
in size, or altogether disappears.

792. 7. Healthy and strong men, engaged in hard labour and exposed to
intense heat, sometimes lose, in the space of a single hour, upwards
of five pounds of their weight. Though daily engaged for months
together in this occupation at two different periods of the day, for
the space of an hour each time, and though consequently these men lose
five pounds twice every day, yet when weighed at intervals of three,
six, or nine months, it is found that the weight of the body remains
stationary, not varying, perhaps, more than a pound or two. It follows
that the bodies of these men must absorb, twice every day, a quantity
equal in weight to that which they lose.

793. These phenomena depend on a power inherent in the body, that of
taking up and carrying into the system certain substances in contact
with its surfaces, and of transporting from one part of its system to
another its own component particles.

794. The apparatus by which these operations are carried on is general
and special.

795. The general apparatus consists of blood-vessels and membrane. The
special apparatus consists of a peculiar system of vessels, namely,
the lacteals and lymphatics, together with the system of glands termed
conglobate.

796. It is proved by direct experiment that the walls of blood-vessels
exert a power by which substances in contact with their external
surface penetrate their tissue, reach their internal surface, and mix
with the mass of the circulating fluids, and that this property is
possessed by all blood-vessels, arteries and veins, great and small,
dead and living.

797. If a portion of a vein or artery taken from the body be attached
by either extremity to two glass tubes in order to establish a current
of warm water in its interior, if the vein be then placed in a fluid
slightly acidulated, and the fluid which flows through the vessel be
collected in a flask, this latter fluid becomes, in the space of a few
minutes, sensibly acid. In this experiment there is no possibility
of communication between the current of warm water and the external
acidulated fluid, consequently the latter must penetrate the parietes
of the vessel, that is, absorption must take place through its
membranous walls.

798. A striking experiment demonstrates the absorbing power of the
living blood-vessels. If the trunk of a vein or artery be exposed in a
living animal, and a poisonous substance in solution be dropped on the
external surface of either, the animal is killed in a few minutes,
just as when the poison is injected into the blood-vessel itself.
Analogous experiments on the minute blood-vessels not only show that
they are endowed with the like absorbing power, but that their number,
tenuity and extent, are conditions which greatly favour the activity of
the process.

799. Membrane is an organised substance abounding with blood-vessels.
Whether the absorbing power possessed by this tissue be due only to
these vessels, or whether it be assisted in the operation by other
agents not yet fully ascertained, it is certain that the absorbing
power it exerts is highly curious and wonderful.

800. An animal membrane placed in contact with water becomes saturated
with fluid: placed in contact with a compound fluid, as with water or
spirit holding colouring matter in solution, the membrane actually
decomposes the compound and resolves it into its elementary parts,
just as accurately as can be done by the chemist. If one extremity
of a piece of membrane be placed in a vessel containing the tincture
of iodine, for example, and the other extremity be kept out of the
fluid, that portion of the membrane which is in immediate contact with
the tincture acquires a perfectly dark colour, because the iodine
completely penetrates the substance of the membrane. This dark-coloured
portion is bounded by a definite line, above which the membrane
is penetrated by a different part of the solution, by a pearly,
colourless fluid, the alcohol in which the iodine was suspended. Above
this again there are traces of a still lighter coloured fluid, which
is probably water. In like manner, if strips of membrane are placed in
glasses containing port wine, the same analytical process is effected
by the membrane. The colouring matter of the wine is imbibed by the
lower portion of the membrane; above this is the alcohol, and above
this the water.

801. These and many analogous experiments demonstrate that the
process of absorption is accompanied with the further phenomena of
decomposition and analysis; and that membrane, at the very moment
it imbibes certain compound substances, resolves them into their
constituent elements.

802. It is further established by numerous experiments that different
compound substances are decomposed and absorbed by membrane with
different degrees of facility. If strips of membrane are placed in
phials containing different kinds of fluids, one fluid rises only
a line or two; others rise to the height of many inches. There is
indubitable evidence that analogous properties are possessed by living
membrane; that the mucous membrane of the stomach at the moment
it imbibes, decomposes and analyses the alimentary and medicinal
substances in contact with its surface; and consequently that in all
animals membrane becomes a most important agent in carrying on the
digestive process.

803. But perhaps the most remarkable property possessed by membrane is
that of establishing in fluids in contact with its surfaces currents
through its parietes, which proceed in opposite directions, according
to the different natures of the fluids, and more especially according
to their different densities. If small bladders composed of membrane
are filled with a fluid of greater density than water, and securely
fastened, and then thrown into water, they acquire weight and become
swollen and tense. If the experiment be reversed; if the bladders be
filled with water and immersed in a denser fluid, the denser fluid
flows inwards to the water, and the water passes from the interior
outwards. M. Dutrochet, who was led by accident to the observation
of these phenomena, and who saw at once the possible importance of
this agency in some organic processes hitherto involved in great
obscurity, commenced an extended series of experiments with a view
to ascertain the exact facts. He took the cæca of fowls, membranous
bags already made to his hand, into which he introduced a quantity
of fluid consisting of milk, thin syrup, or gum-arabic dissolved in
water. Having securely tied the membranes, he placed the bags thus
filled in water, and found that two opposite currents are established
through the walls of the cæca. The first and strongest current, that
from without inwards, is formed by the flow of the external water
towards the thicker fluid contained in the cæca; the second and weaker
current, that from within outwards, is formed by the flow of the
thicker interior fluid towards the external water. The first or the
in-going current is termed _endosmose_, from ενδον, intus, and
ωσμος, impulsus, and the second or out-going current is termed
_exosmose_, from a similar combination of Greek words signifying an
impulse outwards.

804. The velocity and strength of these currents are capable of exact
admeasurement. The amount of endosmose is measured by an apparatus
termed an endosmometer, which consists of a small bottle, the bottom
of which is taken out and the aperture closed by a piece of bladder.
Into this bottle is poured some dense fluid; the neck of the bottle is
closed with a cork, through which a glass tube, fixed upon a graduated
scale, is passed. The bottle is then placed in pure water. The water
by endosmose penetrates the bottle in various quantities according
to the density of the fluid contained in its interior through the
membrane closing its bottom. The dense fluid in the bottle, increased
in quantity by the addition of the water, rises in the tube fitted to
its neck, and the velocity of its ascent is the measure of the velocity
of the endosmose.

805. The strength of endosmose is measured by a similar apparatus,
in which a tube is twice bent upon itself, and the ascending branch
containing a column of mercury which is raised by the fluid in the
interior of the endosmometer, as the volume of this fluid is increased
by the endosmose. By means of these two instruments it is found that
the velocity and strength of endosmose follow the same law, and that
both are proportionate to the excess of the density of the fluid
contained in the endosmometer above the density of water. By numerous
experiments it is ascertained that by employing syrup of ordinary
density (I. 33) an endosmose is obtained, the strength of which is
capable of raising water more than 150 feet.

806. But though difference of density is necessary to the production
of endosmose, yet numerous and decisive experiments show that the
different natures of fluids, irrespective of their proportionate
densities, materially influence the activity and energy of the process.
Thus, if sugar-water and gum-water of the same density be placed in
the same endosmometer, the former produces endosmose with a velocity
as seventeen and the latter only as eight. The endosmose produced
by a solution of the sulphate of soda is double that produced by a
solution of the hydro-chlorate of soda of the same density. A solution
of albumen exerts an endosmose four times greater than a solution of
gelatin of the same density.

807. With organic fluids endosmose goes on without ceasing until the
chemical nature of the fluids becomes altered by putrefaction; but
with alkalies, soluble salts, acids, and chemical agents in general,
the endosmose excited is capable only of short continuance, because
such agents enter into chemical combination with the organic tissue of
the endosmometer, and thus destroy endosmose.

808. It is remarkable that the direction of the endosmotic currents
produced by vegetable membrane is the exact reverse of that produced
by animal membrane under precisely the same circumstances. Thus oxalic
acid, when separated from water by an animal membrane, invariably
exhibits endosmose from the acid towards the water; when separated
by a vegetable membrane, from the water towards the acid: and the
same is the case with the tartaric and citric acids, and with the
sulphuric, the hydro-sulphuric, and the sulphurous acids. I filled,
says Dutrochet, a pod of the _colutea arborescens_, which being opened
at one end only, and forming a little bag, was readily attached by
means of a ligature to a glass tube, with a solution of oxalic acid,
and having plunged it into rainwater, endosmose was manifested by the
ascent of the contained acid fluid in the tube, that is to say, the
current flowed from the water towards the acid. The lower part of the
leek (_allium porrum_) is enveloped or sheathed by the tubular petioles
of the leaves. By slitting these cylindrical tubes down one side,
vegetable membranous webs of sufficient breadth and strength to be
tied upon the reservoir of an endosmometer are readily obtained. An
endosmometer, fitted with one of these vegetable membranes, having been
filled with a solution of oxalic acid and then plunged into rainwater,
the included fluid rose gradually in the tube of the endosmometer, so
that the endosmose was from the water towards the acid, the reverse
of that which takes place when the endosmometer is furnished with
an animal membrane. Vegetable membrane, then, at least with fluids
containing a preponderance of acid, produces a current, the direction
of which is the exact reverse of that produced by animal membrane.

809. The bodies of organised beings are composed in great part of
various fluids of different density, separated from each other by thin
septa, precisely the conditions which are necessary to the production
of endosmose. But such conditions never concur in inorganic bodies,
whence inorganic bodies never exhibit endosmotic phenomena. Vegetable
tissue of every kind consists of vast multitudes of aggregated cells
intermingled with tubes. The parietes of these hollow organs are
exceedingly delicate and thin; the organs themselves are at all times
filled with fluids, the densities of which are infinitely various;
consequently, by endosmose and exosmose, mutual interchanges of their
contents incessantly go on; those contents brought into contact by
currents moving now in one direction and now in another, now rapidly
and now slowly intermingle, and in consequence of their admixture
changes in their chemical composition take place. It is by these
powers that water holding in solution nutrient matter diffused through
the soil penetrates the spongeolæ of the capillary rootlets, always
filled with a denser fluid than the water contained in the soil,—that
the energetic motion by which the sap ascends is generated,—that the
ascending sap is attracted into fruits, always of greater density
than the crude sap,—that buds are capable of emptying the tissue
that surrounds them when they begin to grow, and that almost all the
phenomena connected with the motions of fluids in plants, and the
chemical changes which those fluids undergo in consequence of this
admixture, is effected. And there cannot be a question that analogous
phenomena take place in the various cells, cavities, and minute
capillary vessels of the animal body.

810. It is then established on indubitable evidence that all animal
tissues, without exception, possess an inherent property by which they
are capable of transmitting through their substance certain fluids, and
even solids, convertible into fluids; and that the great agent by which
this transmission is effected is membranous tissue, whether in the form
of blood-vessels or of proper membrane. By virtue of this property
fluids and solids are absorbed, by the animal body, with whatever
surface or organ they are in contact, whether with an external or an
internal surface, or with the eye, the mouth, the tongue, the stomach,
the lungs, the liver, or the heart.

811. But membrane is so disposed and modified, in different parts of
the body, as to admit of the introduction of fluids and solids from the
exterior to the interior of the system with widely different degrees
of facility. There may be said to be in the human body three great
absorbing surfaces, the pulmonary, the digestive, and the cutaneous,
each highly important, but each endowed with exceedingly different
degrees of absorbing power.

812. The pulmonary surface, for reasons which will be readily
understood from what has been already stated relative to the structure
of the air vesicles of the lungs, is by far the most active absorbing
surface of the body. The mode in which the air vesicles are formed and
disposed has been shown to be such as to give to the lungs an almost
incredible extent of membranous surface, while the membrane of which
the cells are composed is exceedingly fine and delicate. Moreover,
there is the freest possible communication between all the branches of
the pulmonary vascular system, whether arteries or veins; the distance
between the lungs and the heart is short; the course of the blood
from the pulmonary capillaries to the central engine that works the
circulation is rapid, and the lungs are at the same time close to the
central masses of the nervous system, with which indeed they are placed
in direct communication by nerves of great magnitude and of most
extensive distribution. These circumstances account for the wonderful
rapidity with which substances are absorbed, when placed in contact
with the pulmonary surface, and for the instantaneousness and intensity
of the impression produced upon the system, when the substance thus
introduced is of a deleterious nature.

813. They also afford an explanation of a phenomenon not to have been
credited without experience of the fact, that innoxious substances,
introduced into the air cells of the lungs in moderate quantities
produce no more inconvenience there than when taken into the stomach.
A single drop of pure water, when in contact near the glottis with the
same membrane that forms the air vesicles of the lungs, excites the
most violent and spasmodic cough, and the smallest particle of a solid
substance permanently remaining there occasions so much irritation
that inevitable suffocation and death result. Yet so different is the
sensibility of this membrane in different parts of its course, that
while at the upper portion of the trachea it will not bear a drop
of water without exciting violent disturbance, in the air vesicles
it tolerates with only slight inconvenience a considerable quantity
even of solid matter. An accident of a nature sufficiently alarming,
which occurred to Dessault, affords a striking illustration of this
curious fact. This celebrated surgeon had to treat a case in which the
trachea and esophagus were cut through. It was necessary to introduce
a tube through the divided esophagus into the stomach, and to sustain
the patient by food introduced in this manner. On one occasion the
tube, instead of being passed through the esophagus to the stomach,
was introduced into the trachea down to the division of the bronchi.
Several injections of soup were actually thrown into the lungs before
the mistake was discovered; yet no fatal, and even no dangerous
consequences ensued. Since that period, in various experiments on
animals, several substances of an innoxious nature have been thrown
into the lungs without producing any inconvenience beyond slight
disturbance of the respiration and cough. The reason is, that after a
short time the substances are absorbed by the membrane composing the
air vesicles, and are thus removed from the lungs and borne into the
general circulating mass. At every point of the pulmonary tissue there
is a vascular tube ready to receive any substance imbibed by it, and to
carry it at once into the general current of the circulation.

814. Hence the instantaneousness and the dreadful energy with which
poisons and other noxious substances act upon the system when brought
into contact with the pulmonary tissue. A solution of nux vomica
injected into the trachea produces death in a few seconds. A single
inspiration of the concentrated prussic acid kills with the rapidity
of a stroke of lightning. This acid in its concentrated form is so
potent a poison, that it requires the most extreme care in the use of
it, and more than one physiologist has been poisoned by it through
the want of proper precaution while employing it for the purpose of
experiment. If the nose of an animal be slowly passed over a bottle
containing this poison, and the animal happen to inspire during the
moment of the passage, it drops down dead instantaneously, just as
when the poison is applied in the form of liquid to the tongue or the
stomach. The vapour of chlorine possesses the property of arresting
the poisonous effects of prussic acid, unless the latter be introduced
into the system in a dose sufficiently strong to kill instantly; and,
hence, when an animal is all but dead from the effects of prussic acid,
it is sometimes suddenly restored to life by holding its mouth over the
vapour of chlorine.

815. Examples of the transmission of gaseous bodies through the
pulmonary membrane have been already fully described in the account of
the passage of atmospheric air to the lungs, and of carbonic acid gas
from the lungs, in natural respiration. But foreign substances may be
mixed with or suspended in the atmospheric air, which it is the proper
office of the pulmonary membrane to transmit to the lungs, and may be
immediately carried with it into the circulating mass. Thus, merely
passing through a recently-painted chamber gives to the urine the odour
of turpentine. The vapour of turpentine diffused through the chamber is
transmitted to the lungs with the inspired air, and passing into the
circulation through the pulmonary membrane, exhibits its effects in the
system more rapidly than if it had been taken into the stomach, and
thence absorbed.

816. Vegetable and animal matter in a state of decomposition generates
a poison, which when diffused in the atmosphere, and transmitted
to the lungs in the inspired air, produces various diseases of the
most destructive kind. The exhalations arising from marshes, bogs,
and other uncultivated and undrained places, constitute a poison of
a vegetable nature, which produces principally intermittent fever
or ague. Exhalations accumulating in close, ill-ventilated, and
crowded apartments in the confined situations of densely-populated
cities, where no attention is paid to the removal of putrefying and
excrementitious matters, constitute a poison chiefly of an animal
nature, which produces continued fever of the typhoid character. It is
proved by fatal experience that there are situations in which these
putrefying matters, aided by heat and other peculiarities of climate,
generate a poison so intense and deadly that a single inspiration
of the air in which they are diffused is capable of producing
instantaneous death; and that there are other situations in which a
less highly concentrated poison accumulates, the inspiration of which
for a few minutes produces a fever capable of destroying life in from
two to twelve hours. In dirty and neglected ships, in which especially
the bilge-water is allowed to remain uncleansed; in damp, crowded, and
filthy gaols; in the crowded wards of ill-ventilated hospitals filled
with persons labouring under malignant surgical diseases, or some forms
of typhus fever, an atmosphere is generated which cannot be breathed
long, even by the most healthy and robust, without producing highly
dangerous fever.

817. The true nature of these poisonous exhalations is demonstrated by
direct experiment. If a quantity of the air in which they are diffused
be collected, the vapour may be condensed by cold and other agents, and
a residuum of vegetable or animal matter obtained, which is found to
be highly putrescent, constituting a deadly poison. A minute quantity
of this concentrated poison applied to an animal previously in sound
health, destroys life with the most intense symptoms of malignant
fever. If, for example, ten or twelve drops of a fluid containing
this highly putrid matter be injected into the jugular vein of a
dog, the animal is seized with acute fever; the action of the heart
is inordinately excited, the respiration is accelerated, the heat
increased, the prostration of strength extreme, the muscular power so
exhausted, that the animal lies on the ground wholly unable to stir or
to make the slightest effort; and, after a short time, it is actually
seized with the black vomit, identical, in the nature of the matter
evacuated with that which is thrown up by an individual labouring
under yellow fever. It is possible, by varying the intensity and the
dose of the poison thus obtained, to produce fever of almost any type,
endowed with almost any degree of mortal power. These facts, of which
practical applications of the highest utility are hereafter to be made,
may suffice to show the importance of the pulmonary membrane as an
absorbing surface. By the extent and energy of its absorbing power, it
is one of the great portals of life and health, or of disease and death.

818. The digestive surface is of much less extent than the pulmonary;
it is less vascular; it is further removed from the centre of the
circulating system, and it is covered with a thick mucus, which is
closely adherent to it; hence its absorbing power is neither so great
as that of the pulmonary membrane, nor do noxious substances in contact
with it affect the system so rapidly. An appreciable interval commonly
elapses between the introduction of a poison into the stomach and
its action upon the system. An emetic is commonly a quarter of an
hour before it begins to operate: arsenic itself is generally half an
hour, and sometimes three quarters of an hour, before it produces
any decided effect on the system: but at length a noxious substance,
applied to any part of the digestive membrane is introduced into the
circulating mass and produces its appropriate effects on the system,
just as when it is in contact with the pulmonary tissue.

819. Over the external surface of the body or the skin, there is spread
a thin layer of solid, inorganic, insensible matter, like a varnish of
Indian rubber. The obvious effect of such a barrier placed between the
external surface of the body and external objects, is to moderate the
entrance of substances from without, and the transmission of substances
from within, that is, to regulate both the absorbing and the exhaling
power of the skin. Hence the comparative slowness with which substances
enter the system by the cutaneous surface; the impunity with which the
most deadly poisons may remain for a time in contact with the skin,
with which prussic acid, arsenic, corrosive sublimate, may be touched
and even handled. The internal surface of the body is protected from
the action of acrid substances introduced into the alimentary canal by
a layer of mucus through which an irritant must penetrate before it can
pain the sentient nerve or irritate the capillary vessel; but were not
a still denser shield thrown over the external surface, pain, disease,
and death must inevitably result from the mere contact of innumerable
bodies, which now are not only perfectly innoxious, but capable of
ministering in a high degree to human comfort and improvement.

820. Immediately beneath the cuticle is a surface as vascular as it is
sensitive, from which absorption takes place with extreme rapidity.
Poison in very minute quantity introduced beneath the cuticle kills
in a few minutes. Arsenic applied to surfaces from which the cuticle
has been removed by ulceration produces its poisonous effects upon
the system just as surely as when introduced into the stomach.
The poisonous matter of small-pox and of cow-pox placed in almost
inappreciable quantity by the lancet beneath the cuticle produces in a
given time its specific action upon the system. When, in certain states
of disease, with the view of bringing the system rapidly under the
influence of a medicinal agent, the cuticle is removed by a blister,
and the exposed surface is moistened with a solution of the substance
whose action is required, the constitutional effects are developed with
such intensity, that if extreme care be not taken in the employment of
any deleterious substance in this mode the result is fatal in a few
minutes.

821. The phenomena which have been stated may suffice to illustrate the
absorbing power of the general tissues and surfaces of the body; but
superadded to this, there is carried on in particular parts of the
system a specific absorption for which a special apparatus is provided.

[Illustration: Fig. CXCII.

 An enlarged view of an absorbent vessel.—1. External surface, with the
 jointed appearance produced by the valves.—2. The same vessel laid
 open, showing the arrangement of the valves.]

822. The special apparatus of absorption, commonly termed the proper
absorbent system, consists of the lacteal and lymphatic vessels and of
the conglobate glands. The lacteals arise only from the intestines; the
lymphatics, it is presumed, from every organ, tissue, and surface of
the body. Both sets of vessels possess a structure strikingly analogous
to that of veins, the common agents of absorption. The coats of the
lacteals and lymphatics are somewhat thinner and a good deal more
transparent than those of veins; yet thin and delicate as they are,
they possess considerable strength, for they are capable of bearing,
without rupture, injections which distend them far beyond their natural
magnitude.

823. When fully distended, these vessels present a jointed appearance
somewhat resembling a string of beads (fig. CXCII. 1). Each joint
indicates the situation of a pair of valves (fig. CXCII. 2). These
valves are of a semilunar form, and are composed of a fold of the inner
coat of the vessel (fig. CXCII. 2). The convex side of the valve, in
the lacteals, is towards the intestines; in the lymphatics towards the
surfaces; in both towards the origins of the vessels. The valves allow
the contents of the vessels to pass freely towards the main trunk of
the system, but prevent any retrograde motion towards the origins of
the vessels.

824. By continued pressure the resistance of the valves may be
overcome, so that mercury may be made to pass from the trunk into the
branches. When this is done in an absorbent trunk proceeding from
certain organs, such as the liver, it is seen that the absorbents are
distributed, arborescently, in such vast numbers that the surface of
the viscus appears as if it were covered with a reticular sheet of
quicksilver.

825. The internal coat of the small intestines has been shown to
present a fleecy surface, crowded with minute elevations called villi,
which give this surface an appearance closely resembling the pile of
velvet. Each villus consists of an artery, a vein, a nerve, and a
lacteal, united and sustained by delicate cellular tissue. After a meal
the lacteals become so turgid with chyle that they completely conceal
the blood-vessels and nerves, so that the surface of the intestine
presents to the eye only a white mass, or a surface thickly crowded
with white spots (fig. CXCIII.)

[Illustration: Fig. CXCIII.

 Appearance of the lacteals turgid with chyle, as seen in the jejunum
 some time after a meal.]

[Illustration: Fig. CXCIV.

 Magnified view of two ampullulæ turgid with chyle, terminating the
 lacteal vessels.]

826. When a portion of the intestine in this condition of the lacteal
vessels is examined under the microscope, there is said to be visible
on the villus an oval vesicle, termed an ampullula (fig. CXCIV.). This
vesicle is described as having an aperture at its apex, which it is
conceived constitutes the open mouth of the lacteal, and through which
the chyle is supposed to be taken up.

[Illustration: Fig. CXCV.

 View of villi, with the lacteals arising from their surface by open
 mouths and forming radiated branches. The surface of one of these
 villi is represented as entirely white, from the lacteals being so
 turgid with chyle as completely to obscure their orifices and their
 radiating branches.]

827. Mr. Cruikshank, who particularly devoted himself to the study of
this part of the absorbent system, states that he had an opportunity
of examining these vessels in a person who died suddenly some hours
after having taken a hearty meal, and who had been previously in sound
health. “In some hundred villi,” he says, “I saw the trunk of the
lacteal beginning by radiated branches (fig. CXCV.). The orifices of
these radii were very distinct on the surface of the villus as well
as the radii themselves (fig. CXCV.). There was but one trunk in each
villus. The orifices on the villi of the jejunum, as Dr. Hunter said
(when I asked him as he viewed them in the microscope how many he
thought there might be) were about fifteen or twenty in each villus,
and in some I saw them still more numerous” (fig. CXCV.).

828. The course of the lacteals, from their origin in the villi to
their termination in the thoracic duct, has been traced (687). It is
conjectured that the lymphatics take their origin from every point of
the body, but it is admitted that they have not been actually seen
even in every organ; still they have been found in so many that it is
inferred that they really exist in all, and that in those in which they
have not been hitherto detected they have eluded observation on account
of their extreme delicacy and transparency and our imperfect means of
examining them.

829. Though, like veins, lymphatics anastomose freely with each other,
yet they do not proceed from smaller to larger branches and from larger
branches to trunks, but continue of nearly the same magnitude from
their origin to their termination. They are disposed in two sets, one
of which always keeps near the external surface of the body, and the
other is deeply seated, accompanying more especially the great trunks
of the blood-vessels.

[Illustration: Fig. CXCVI.

Fig. CXCVII.

Fig. CXCVIII.

 CXCVI.—1. Trunks of absorbent vessels entering a gland. 2. Gland laid
 open. 3. Highly magnified views of the cells or follicles of which
 the gland is supposed to consist. CXCVII.—1. Absorbent vessels called
 vasa inferentia, entering (2) the gland. 3. Absorbent vessels emerging
 from the gland, called vasa efferentia, and forming (4) a common
 trunk.
 CXCVIII.—1. Trunk of absorbent vessel entering a gland. 2. Gland
 apparently composed entirely of convoluted vessels. 3. Vessels
 emerging from the gland and forming (4) a common trunk.]

830. In the human body every vessel that can be distinctly recognised
either as a lacteal or a lymphatic, passes, in some part of its course,
through a conglobate or lymphatic gland (figs. CXCVII., CXCVIII.).
These glands, small, flattened, circular or oval bodies, resembling
beans in shape, are enclosed in a distinct membranous envelope. Their
intimate structure has been already fully described (chap. xi.). They
are of various sizes, ranging from three to ten lines in diameter: they
are placed in determinate parts of the body, and are grouped together
in various ways, being sometimes single, but more often collected
in masses of considerable magnitude. Numerous absorbent vessels,
termed vasa inferentia, enter the gland on the side remote from the
heart (figs. CXCVII. 1 and CXCVIII. 1); a smaller number, called vasa
efferentia, leave it on the side proximate to the heart (fig. CXCVII.
3). If mercury be injected into the vasa inferentia (fig. CXCVI.), it
is seen to pass into a series of cells of the corresponding gland (fig.
CXCVI. 3), and then to escape by the vasa efferentia; but if the gland
be more minutely injected, as by wax, all appearance of cells vanishes;
the whole substance of the gland seems then to consist of convoluted
absorbents (fig. CXCVIII. 2), irregularly dilated, and communicating
with each other so intimately that every branch that leaves the gland
appears to have been put in communication with every branch that
entered it (fig. CXCVIII. 1, 2, 3).

831. The motion of the fluid within the absorbent vessels, though not
rapid, is energetic. If a ligature be placed around the thoracic duct
in a living animal, the tube will swell and ultimately burst, from the
rupture of its coat, in consequence of the force of the distension
that takes place below the ligature. If the thoracic duct in the neck
of a dog be opened some hours after the animal has taken a full meal,
the chyle flows from the vessel in a full stream, and in the space
of five minutes half an ounce of the fluid may be obtained. Yet this
system of vessels is beyond the influence of the circulating blood: it
has no heart to propel it; no current behind always in rapid motion
to urge it onwards; it is therefore inferred that it is moved by a
vital contractile power inherent in the vessels, analogous to, if not
identical with, muscular contractility. The flow of blood through the
arterial tubes is universally believed to be effected, in part at
least, by such a contractile power, for this, among other reasons, that
if in a living animal the trunk of an artery be laid bare, the mere
exposure of it to the atmospheric air causes it to contract to such
a degree that its size becomes obviously and strikingly diminished
(298.1). The same phenomenon has been observed in the main trunk of
the absorbent system. Tiedemann and Gmelin state that in the course of
their experiments they saw the thoracic duct contract from exposure to
the air.


832. The delicacy and transparency of the lacteals and lymphatics
long concealed them from the view of the anatomist. The lacteals had
indeed been occasionally seen in ancient times, but their office
was altogether unknown. In the year 1563 Eustachius discovered the
thoracic duct, but did not perceive its use. About half a century
afterwards, in the year 1622, the lacteals were again one day by
chance seen by Asellius, in Italy, while investigating the function of
certain nerves. Mistaking the lacteals for nerves, he at first paid no
attention to them; but soon observing that they did not pursue the same
course as the nerves, and “astonished at the novelty of the thing,” he
hesitated for some time in silence. Resolving in his mind the doubts
and controversies of anatomists, of which it chanced that he had been
reading the very day before, in order to examine the matter further,
“I took,” he says, “a sharp scalpel to cut one of these chords, but
scarcely had I struck it when I found a liquor white as milk, or rather
like cream, to leap out. At this sight I could not contain myself for
joy; but turning to the by-standers, Alexander Tadinus and the senator
Septalius, I cried out Εὕρηκα! with Archimedes; and at the
same time invited them to look at so rare and pleasing a spectacle;
with the novelty of which they were much moved. But I was not long
permitted to enjoy it, for the dog now expired, and, wonderful to
tell, at the same instant the whole of that astonishing series and
congeries of vessels, losing its brilliant whiteness, that fluid being
gone, in our very hands, and almost before our eyes, so evanished and
disappeared that hardly a vestige was left to my most diligent search.”
The next day he procured another dog, but could not discover the
smallest white vessel. “And now,” he continues, “I began to be downcast
in my mind, thinking to myself that what had been observed in the first
dog must be ranked among those rare things which, according to Galen,
are sometimes seen in anatomy.” But at length recollecting that the dog
had been opened “athirst and unfed,” he opened a third “after feeding
him to satiety; and now everything was more manifest and brilliant
than in the first case.” The zeal with which he followed out the clue
he had obtained is indicated by the number of dogs, cats, iambs, hogs,
and cows which he dissected, and by the statement that he even bought
a horse and opened it alive; but, he adds, “a living man, however,
which Erasistratus and Herophilus of old did not fear to anatomize, I
_confess_ I did not open.”

833. Nearly thirty years elapsed before the lacteals, which were long
thought to terminate in the liver, were traced to the thoracic duct;
and it was not until the year 1651, about eighty years after the
discovery of Asellius, that the lymphatics were discovered, and that
the whole of this portion of the absorbent system was brought to light.

834. Taking together the whole of the apparatus of absorption, the
specific office performed by its several parts seems to be as follows:—

835. 1. It is established that the lacteals absorb chyle, and that they
refuse to take up almost every other substance which can be presented
to them. Experimentalists are uniform in stating that however various
the substances introduced into the stomach, it is exceedingly rare to
find in the lacteals anything but chyle. These vessels appear to be
endowed with a peculiar sensibility, derived from the nervous system,
by which they are rendered capable of exerting an elective power,
readily absorbing some substances and absolutely rejecting others.

836. 2. The lymphatics absorb a far greater variety of substances
than the lacteals, but not all substances indiscriminately; chiefly
organized matter in a certain stage of purification; particles passing
through successive processes of refinement (707).

837. 3. The blood-vessels, and more especially the capillary veins,
appear to absorb indiscriminately all substances, however heterogeneous
their nature, which are dissolved or dissolvable in the fluids
presented to them.

838. 4. The absorbent glands appear by various modes, either by
removing superfluous and noxious matters, or by the addition of
secreted substances possessing assimilative properties, to approximate
the fluid which flows through them more and more closely to the nature
of the blood. Fatal effects result from the artificial infusion of
minute portions even of mild substances into the blood. Hence the
extended and winding course which Nature causes the new matter formed
from the food to undergo, even after its elaboration in the digestive
apparatus, in order that, before it is allowed to mingle with the
blood, its perfect purification and assimilation may be secured.

839. The activity or inactivity of the process of absorption is mainly
dependant on the emptiness or the plethora of the system. There is
a point of saturation beyond which the absorbent vessels, though in
immediate and continued contact with absorbable matters, will take
up no more. The nearer the system to this point the less active the
process; the further the system from this point the more active the
process. Thus, when an animal whose vessels are full to saturation is
immersed in water, or exposed to humid air, its body does not increase
in weight, and there is no sensible diminution of the water; but the
longer an animal is kept without fluid, and the more it is exposed
to the action of a dry air, the further its system is removed from
the point of saturation, and exactly in that proportion, when it is
brought in contact with water, is the diminution of the quantity of the
fluid and the increase in the weight of the body. This law explains
many circumstances of the animal economy,—why it is impossible to
dilute the blood or any other animal fluid beyond a certain point,
by any quantity of liquid which may be in contact with the external
surface, or which may be taken into the stomach; why it is impossible
to introduce nutrient matter into the system, beyond a certain point,
by any quantity of food, which the digestive organs may convert into
chyle; why, consequently, the bulk and weight of the body are incapable
of indefinite increase; why that bulk and weight are so rapidly
regained after long abstinence; and why the appetite is so keen, and
the ordinary fulness and plumpness of the body are so soon restored,
after recovery from fever and other acute diseases, when the digestive
organs have been uninjured.

840. Different portions of the absorbent apparatus accomplish specific
uses. With the absorbent action of the capillary blood-vessels and of
membranous surfaces every organic function, but more especially the
processes of digestion and respiration, are intimately connected.

841. The specific absorption carried on by the lacteals has for its
object the introduction of new materials into the system, for the
reparation of the losses which it is constantly sustaining by the
unceasing actions of life.

842. The specific absorption carried on by the lymphatics has a
two-fold object. First, the introduction of particles, which have
already formed an integrant part of the system, a second time into the
blood, in order to subject them anew to the process of respiration,
thereby affording them a second purification, and giving them new and
higher properties; and, secondly, the regulation of the growth of the
body, and the communication and preservation of its proper form.

843. It is the office of the lacteals to replenish the blood by
constantly pouring into it new matter, duly prepared for its conversion
into the nutritive fluid. It is the office of the lymphatics to preside
over the distribution of the blood as it is deposited in the system in
the act of nutrition. The lymphatics are the architects which mould and
fashion the body. They not only regulate the extension of the frame,
but they retain each individual part in its exact position, and give to
it its exact size and shape. Growth is not mere accretion, not simple
distension; it consists of a specific addition to every individual
part, while all the parts retain the same exact relation to each other
and to the whole. When a bone grows it does not increase in bulk by
the mere accumulation of bony matter; but every osseous particle is so
increased in length and breadth that the relative size of every part,
and the general configuration of the whole organ, remain precisely
the same. When a muscle grows, while the entire organ enlarges in
bulk by the augmentation of every individual part, each part retains
exactly its former proportions and its relative connexions. When the
brain grows a certain quantity of cerebral matter is added to every
individual part, but at the same time the proportionate size and
original form of each part, and the primitive configuration of the
entire organ, are retained exactly the same. How is this effected? By
a totally new disposition of every integrant particle of every part of
every organ. New matter is not deposited before the removal of the old:
the lymphatic, in the very act of removing the old, fashions a mould
for the reception of the new, and then the capillary artery brings
the new particle and deposits it with unerring exactness in the bed
prepared for it. Thus, by removing the old materials of the body in a
determinate manner, and thereby fashioning a mould for the reception of
the new, the lymphatics may be said, in the strictest sense, to be the
architects of the frame.




CHAPTER XIII.

OF THE FUNCTION OF EXCRETION.

 In what excretion differs from secretion—Excretion in the
 plant—Quantity excreted by the plant compared with that excreted
 by the animal—Organs of excretion in the human body—Organization
 of the skin—Excretory processes performed by it—Excretory
 processes of the lungs—Analogous processes of the liver—Use of the
 deposition of fat—Function of the kidneys—Function of the large
 intestines—Compensating and vicarious actions—Reasons why excretory
 processes are necessary—Adjustments.


844. The various matters contained in organized bodies, and even
those which enter as constituent elements into their composition, are
constantly removed from the system, and thrown off into the external
world. The matters thus rejected are called excretions; and the various
processes by which their elimination is effected constitute a common
function termed excretion.

845. Excretion is the necessary consequence of the deterioration which
all organized matter undergoes by the actions of life. The matters
removed by the process consist of the waste particles of the body, or
the particles expended in the vital actions, as the aliment contains
the particles which replenish the waste, and compensate the expenditure.

846. The excretions are separated from the common organized mass by
processes perfectly analogous to those comprehended in the great
function of secretion. Excretion is only a particular form of
secretion: the difference between the two functions is, that, in the
former, the matter eliminated being either noxious or useless, is
separated for the sole purpose of being rejected; while, in the latter,
the matter eliminated is destined to perform some useful purpose
in the economy. Accordingly, the products of excretion are termed
excrementitious; and those of secretion, recrementitious.

847. The chief matters excreted by the plant are oxygen, carbonic acid,
air; water, in some few cases, under peculiar circumstances, ammonia
and chlorine; and in still rarer cases, during the night, poisonous
substances, as carburetted hydrogen, together with acrid, and even
narcotic principles.

848. The forms under which these excretions are eliminated are
exceedingly various. Sometimes the matter excreted is in the shape of
gas, at other times it is in that of vapour, and at others in that of
liquid. The chief gaseous exhalations are oxygen and carbonic acid;
the vaporous exhalations consist principally of water, in the state
of vapour; and the liquid exhalations are either pure water, or water
holding in combination sugar, mucilage, and other proximate vegetable
principles. Even the peculiar products formed by the vital actions
of the plant, as the volatile oils, the fixed oils, the balsams, the
resins, and perhaps, with the exception of gum, sugar, starch, and
lignine, all the substances formed out of the proper juices of the
plant, are true excretions; for these substances are fixed immovably
in the cells, sacs, or tubes which secrete and contain them: they are
not consumed in the growth of the plant; they do not appear to be
applied to any useful purpose in the economy; they are injurious, and
even poisonous to the very plant in which they are formed when taken up
by the roots and combined with the sap: as long as they remain in the
plant they are isolated in the individual parts in which they are first
deposited, until with the advancing age of the plant they lose their
aqueous particles, and are finally dried up; they, therefore, possess
all the essential characters of excrementitious substances.

849. The organs by which these matters are excreted are the leaves, the
flowers, the fruits, the roots, and certain bodies called glands.

850. The gaseous and vaporous exhalations are effected chiefly by the
leaves, which it has been shown (320 and 465), under the influence of
the solar ray, are always pouring out a large quantity of oxygen, and
still larger quantities of fluid in the state of vapour.

851. Similar matters are exhaled by the flowers either in the form
of vapour or of liquid; and this exhalation commonly bears with it
a peculiar odour, which proceeds from an essential oil, sometimes
evaporated with the pollen, and at other times secreted by glandular
bodies which have their seat in the petals.

852. Fruits, and especially green fruits, as raspberries, pears,
apples, plums, apricots, figs, cherries, gooseberries, and grapes, pour
out oxygen during the day, and carbonic acid gas during the night, and
thus co-operate with leaves in carrying on the function of excretion.

853. The more elaborate excretions contained in special receptacles,
and formed by diverse organs from the proper juices of the plant,
descend chiefly by the bark, and are poured by the roots into the soil.
These excretions, if re-absorbed by the roots, and re-introduced into
the system of the plant that has rejected them, poison that plant.
Consequently, two processes of deterioration are always going on in the
soil; first, the absorption of the nutrient matter contained in it;
and, secondly, the accumulation of excrementitious matter constantly
poured into it by the growing plant. By the addition of manure, the
soil is replenished with fresh nutritive materials; by a rotation of
crops, it is purified from noxious excretions. It is a remarkable
and beautiful adjustment, that excrementitious substances which are
destructive to plants of one natural family, actually promote the
growth of plants of a different species. Thus, if wheat be sown upon
a tract of land proper for that grain, it may produce a good crop the
first, the second, and perhaps even the third year, as long as the
ground is what the farmers call in good heart. But, after a time, it
will yield no more of that particular kind of corn. Barley it may still
bear, and, after this, oats, and perhaps after these, pease, or some
other species belonging to a different family. The excrementitious
matter deposited in the soil by a preceding is absorbed by a succeeding
crop; the matter excreted by the former serving as nutriment or
stimulus to the latter. But though in this mode all noxious matter is
removed from the soil, yet the ground at last becomes quite barren, in
consequence of having parted with all its nutrient particles, and then
it will yield no more produce until it is supplied with a new fund of
matter. This new matter is afforded by vegetable or animal substances,
in which, the principle of life having become extinct, the peculiar
bond that held their particles together is dissolved. Leaves, flowers,
fruits, bark, roots; hair, skin, horns, hoofs, fat, muscle, bone, the
blood itself, whatever has formed a part of the organized body, now
dead, and repassing through the process of decomposition, back to the
simple physical elements, all its forms of beauty gone, and exhaling
only matters highly deleterious to animal life, mixed with the soil,
are recombined into new products, spring up into new plants, and thus
re-appear under new forms of beauty, and afford fresh nutriment to
myriads of animals. The very refuse of the matters which have served as
food and clothing to the inhabitants of the crowded city, and which,
allowed to accumulate there, taint the air, and render it pestilential,
promptly removed, and spread out on the surface of the surrounding
country, give it healthfulness, clothe it with verdure, and endow it
with inexhaustible fertility.

854. The quantity of matter excreted by the plant is proportionate to
the energy of its vital actions. Hence it is always greatest in spring,
when the tender leaves are beginning to shoot; gradually diminishes as
autumn approaches; and, at last, as the leaves turn yellow, and the
vessels which connect the leaves with the stalk dry up and are closed,
it almost wholly ceases.

855. It is copious in proportion to the number of the leaves, and to
the extent of the surface they present. From experiments performed as
long ago as the year 1699, by Woodward, it appears that, of the whole
quantity of water absorbed by the plant, the least proportion exhaled
to that retained is as 46 or 50 to 1; in many cases it is as 100 or 200
to 1, and in some above 700 to 1. In one experiment, a plant which
imbibed 2501 grains of water, increased in weight only three grains
and a half: hence the dampness and humidity of the air in all places
in which trees and the larger vegetables abound; more especially when
the leaves are young, and most numerous and active; and hence also the
magnitude of the rivers in all extensive countries which are covered
with forests.

856. Exhalation, scarcely appreciable in the night, is most abundant
during the day under the influence of the solar light. If two plants
of the same size are covered with two glass bells, and one be exposed
to the sun’s light, while the other is left in the shade, the inner
surface of the former bell becomes covered with drops of water, while
that of the second remains perfectly dry.

857. The absolute quantity of matter excreted by the plant is widely
different in different species. According to Hales, in a sun-flower
three feet and a half high, the leaves of which presented a surface
of 5616 square inches, or 39 square feet, the greatest quantity
exhaled in twelve hours, during the day, was one pound fourteen
ounces avoirdupois; the medium quantity one pound four ounces. In a
middle-sized cabbage, the greatest quantity exhaled was one pound
nine ounces; the medium quantity one pound three ounces. In a vine,
the greatest quantity exhaled was six ounces; the medium quantity
five ounces. In a young apple tree having 163 leaves, the surface
of which was equal to 1589 square inches, or 11 square feet, the
greatest quantity exhaled was eleven ounces; the medium quantity nine
ounces. Martino calculated the quantity exhaled by a cabbage, in the
twenty-four hours, at twenty-three ounces; by a young mulberry-tree,
eighteen ounces; and, by a maize plant, seven drachms.

858. Supposing the weight of the human body to be 160 pounds, and
the weight of a sun-flower 3 pounds, the relative weights of the two
bodies will be as 160 to 3, or as 53 to 1. The surface of such a human
body is equal to 15 square feet, or 2160 square inches; the surface
of the sun-flower is 5616 square inches, or as 26 to 10. The quantity
perspired in the twenty-four hours by an ordinary-sized man, according
to the estimate of Keill, is about thirty-one ounces. Allowing two
ounces for the exhalation during the beginning and the ending of
the night, the quantity exhaled by the plant, in the same time, is
twenty-two ounces; so that the perspiration of a man to that of a
sun-flower is nearly as 141 to 100, though the weight of the man to
that of the sun-flower is as 53 to 1. Taking bulk for bulk, the plant
imbibes seventeen times more fresh fluid than the man, partly, no
doubt, for the reason assigned by Hales—because, “the fluid which is
filtered through the roots of the plant is not near so full freighted
with nutrient particles as the chyle which enters the lacteals of the
animal; the plant, therefore, requires a much larger supply of fluid.”

859. As soon in the animal series as organs are formed distinct from
the homogeneous mass of which the minute and simple beings placed
at the bottom of the scale appear to consist, these organs are
appropriated, at least in part, to the function of excretion. In the
human being, six organs take a part, and are chiefly appropriated to
this function—namely, the skin, the lungs, the liver, the adipose
tissue, the kidneys, and the intestinal canal. All these organs serve
other purposes in the economy; but still the removal, in some specific
form, of excrementitious matter from the system, is a most important
part of the office of each.

860. The skin (34), to which are assigned numerous and highly important
offices, seems to be specially constructed for performing the function
of excretion. It is composed of three layers, of which the internal
is called the cutis, or true skin; the external the cuticle, or scarf
skin; and the middle, by which the other two are united, the rete
mucosum. The latter is indistinct, excepting in the negro, in whom it
is the seat of colour.

861. The cutis, or true skin, is a dense membrane, composed of firm
and strong fibres, interwoven like a felt. Its internal surface is
marked by numerous depressions, which receive processes of the adipose
tissue beneath. Over its external surface is spread a delicate and
complex net-work of vessels, termed the vascular plexus, of such
extent and capacity that, in the natural state of the circulation, a
very large proportion of the whole blood of the body is constantly
flowing in these blood-vessels of the cutis. A prodigious number of
nerves accompany the cutaneous blood-vessels, some derived from the
organic, and others from the sentient portion of the nervous system.
The organic nerves endow the arteries with the power of performing
the organic processes proper to the cutis, which are principally of
an excrementitious nature. The sentient nerves communicate to every
point of the external surface of the cutis the exquisite degree of
sensibility possessed by the skin. Innumerable absorbent vessels
terminate at the same points, with the capillary arteries and the
sentient nerves.

862. The extreme smoothness and softness natural to the skin is
communicated to it by a number of follicles which are placed in the
cutis, and are termed sebaceous, from the oily substance they secrete.
It is the matter secreted by these organs which communicates to the
animal body the odour peculiar to it, on which the scent depends.

863. In many parts the cutis is perforated obliquely by hairs, which
spring from little bulbs beneath it, to which the growth of the hairs
is confined. The human hair, which is hollow, consists of fine tubes
filled with an oily matter. This matter is either of a black, red,
yellow, or pale colour, as the hair is black, red, yellow, or white.

864. The nails are products formed by the cutis, and are essentially
the same as the cuticle.

865. By long-continued boiling the cutis is resolvable into gelatin,
which by evaporation becomes glue, and by combining with tannin and the
extractive of oak bark is converted into leather.

866. The third portion of the skin, the cuticle, is a thin, elastic
membrane spread over the external surface of the cutis, from which it
is easily detached, by the action of a blister in the living, and by
the process of putrefaction in the dead body. It is without vessels and
nerves, and consequently it is insensible and inorganic. It is formed
as a secretion by the cutis, and is composed almost entirely of solid
albumen. When any portion of it is removed, it is renewed with great
rapidity. Since it is subject to constant waste from friction, and is
much increased by pressure, as is manifest in the palms of the hands
and the soles of the feet, its formation must be continual; yet even in
the fœtus it is thicker in the parts where pressure is ultimately to be
made than in the other parts of the body.

867. The cuticle is a sheath in which the body is enclosed for the
purpose of restraining the organic actions which take place at its
surface, and for tempering the sentient impressions received there. For
restraining the organic actions it is fitted by the cohesion of its
parts, which is such as to receive and transmit any fluid very slowly,
as is manifest from the dryness of its surface when it is raised in
a blister, and from the extreme rapidity with which the cutis dries,
until it becomes as hard as parchment, when the cuticle is removed from
it in the dead body.

868. Diffused over every part and particle of the cutis is the seat
of common sensation, that cognizance may be taken of the presence of
external objects. Restricted to particular points, the tips of the
fingers, is the seat of one of the special senses, that of touch.
Had the nerves which communicate to this extended surface its acute
sensibility been placed in direct contact with external bodies,
intolerable pain would have been the result; but by covering this
surface with an inorganic and insensible substance, yet so thin that it
is a pellicle rather than a membrane, the organ of sense is shielded,
while the delicacy of the sensation is not impaired. But the control
of the organic process and the protection of the sentient nerve are
not the only offices performed by the cuticle; it serves further to
hide what it is undesirable to have constantly in view. All that is
beautiful in the blood as an object of sense is rendered visible
through the cuticle, in the bright and rosy hue of health, at the same
time that every process, the sight of which would excite anxiety or
terror, is effectually concealed.

869. The skin, an organ of secretion, an organ of absorption, an organ
of excretion, and an organ of sense, is thus the immediate seat of
three organic processes and of one animal process.

870. The chief excretion performed by the skin, in the human body, is
commonly known under the name of perspiration. The perspiration is
either sensible or insensible. Sensible perspiration is the liquid
commonly called the sweat. Insensible perspiration consists of a vapour
which, under the ordinary circumstances in which the body is placed,
is invisible. The invisible vapour is constantly exhaling; the visible
liquid is only occasionally formed. The quantity of matter carried out
of the system under the form of invisible vapour is much greater than
that lost by the visible liquid.

871. That a quantity of matter is incessantly passing off from the
surface of the skin, under the form of an invisible vapour, is proved
by the following facts:—

1. If the hand and arm are enclosed in a glass jar, the inner surface
of the glass soon becomes covered with moisture.

2. If the tip of the finger be held at about the twelfth of an inch
from a mirror, or any other highly polished surface, the surface
rapidly becomes dimmed by the vapour which condenses upon it in small
drops, and which disappear on the removal of the finger.

3. If the body be weighed at different periods, an accurate account
being taken of the ingesta and the egesta, it is found to undergo a
loss of weight sensibly greater than can be attributed to any of the
visible discharges: this loss must be owing to the transmission of a
quantity of matter out of the body, under the form of invisible vapour.

872. The matters excreted under the form of perspiration are separated
from the blood by a true and proper secretion, like the other
secretions of the body. The process by which this is effected is called
transudation. The matter of transudation deposited on the surface of
the skin by a vital function is removed from the body by evaporation,
a physical process which consists of the conversion of a liquid
into a vapour by the addition of heat. Consequently the process of
perspiration is a cooling process, and it is chiefly by the increase of
the perspiration that the body is enabled to bear the intense degrees
of heat which it has been shown (491, _et seq._) to be capable of
sustaining. Sitting one day in repose in the shade during the intense
heat of an American summer’s day, the skin freely perspiring at every
pore, Dr. Franklin happened to examine the temperature of his body with
a thermometer. He found that the temperature of his body was several
degrees lower than that of the surrounding air. The physiologists who
exposed themselves in heated chambers, for the sake of ascertaining the
greatest degree of heat which the human body is capable of enduring,
perspired profusely during the experiment (495). The artisans who
carry on their daily occupations in elevated temperatures perspire
most profusely (884, _et seq._). Under such circumstances, caloric is
communicated to the human body just as freely as to inorganic matter
yet it does not injure the body, because it does not accumulate in the
system, but is immediately expended in supplying the heat necessary
to convert the water, which is poured out upon the skin, into vapour.
In this manner that surface of the body at which, under ordinary
circumstances, a large portion of its animal heat is generated, is
the very surface at which, under extraordinary circumstances, cold is
generated, and the heat of the system positively reduced.

873. The physical process of evaporation would go on to a certain
extent, though the vital function of transudation did not exist, and
does go on in the dead body when the vital function is at an end. An
organic tissue enclosing a liquid may not be porous enough to give
passage to a single drop of liquid, and yet sufficiently porous to
admit air. In this case the air in contact with the tissue dissolves
the liquid in its interior, and carries it off in the form of invisible
vapour; hence liquids contained in organic bodies in contact with the
air diminish in quantity by evaporation. But if an animal be placed
in air saturated with moisture, and of the same temperature as its
own, the air can no longer deprive that animal of a single particle of
its moisture: evaporation from the body, in such a condition of the
air, is suppressed. On the other hand, when an animal is placed in
air saturated with moisture, and of the same temperature as its own,
so far is transudation from being suppressed, that the sweat streams
from every part of the external surface of the body. By modifying
the condition of the air, in regard to its hygrometrical state and
its temperature, the result of the physical process and of the vital
function may thus be separated from each other, and the amount of each
may be ascertained with perfect exactness. Now, by numerous experiments
on the cold-blooded vertebrata, placed under such conditions of the
air, it is found that, in these animals, perspiration by evaporation is
to that by transudation as 6 to 1. But since the human body presents to
the air an immense extent of surface over which is constantly flowing
a large proportion of the whole quantity of blood contained in the
system, the loss by the physical process compared with that by the
vital function must be still greater in man than in the cold-blooded
animal.

874. Taking together the average quantity of matter removed from
the human body by both processes, or the whole loss of weight
sustained from perspiration, on the comparison of the results of
many observations, it is estimated to vary from twenty ounces in the
twenty-four hours of the colder, to forty ounces in the warmer climates
of Europe. Keill estimated it at thirty-one ounces. In the climate of
Paris it is stated to be thirty ounces.

875. By the delicate tests of modern chemistry, various substances are
found to be contained in the aqueous fluid which constitutes the great
proportion of the matter of perspiration, namely, an acid, probably the
lactic, a small proportion of animal matter, some alkaline and earthy
salts, an oily or fatty substance, probably derived from the sebaceous
follicles. All these matters are so analogous to the constituents of
the serum of the blood as to leave little ground for doubt that they
are merely separated from this part of the blood as it is flowing
through the complex net-work of vessels spread over the surface of the
cutis (861).

876. The skin, when in contact with the air, also separates a portion
of carbon from the blood, and to the extent in which it does this it
is auxiliary to the lungs; but the quantity of carbonic acid excreted
by the skin is small and variable in amount. The primary office of
the skin as an organ of excretion is to relieve the blood of its
superabundant watery particles, that is, to remove from the system its
superfluous hydrogen.

877. A full account has been given (359, _et seq._) of the primary
office of the lungs, which, it has been shown, is to decarbonize the
blood. The details of the calculations have been stated (457), from
which it is estimated that 10 ounces and 116 grains of carbon are daily
exhaled by the lungs under the form of carbonic acid; and the reasons
have been assigned which favour the conclusion that the carbonic acid
expired is not formed immediately in the lungs by the combination of
the oxygen of the atmospheric air with the carbon of the blood; but in
the system, where the oxygen taken into the blood at the lungs unites
with carbon, the carbonic acid resulting from the combination passing
as soon as formed into the capillary veins. The blood contained in
these vessels, thus become venous, returns to the lungs, where it gives
off the carbonic acid accumulated in it, and by that depuration again
assumes its arterial character.

878. Some interesting experiments performed by Dr. Stevens appear
to show that there exists a powerful attraction between oxygen and
carbonic acid, and that the venous blood, as it is flowing through the
lungs, is freed from its carbonic acid by virtue of that attraction.
Chemists were so universally agreed that the carbon in carbonic acid is
united with its maximum dose of oxygen, that the idea of an attraction
between carbonic acid and oxygen appeared highly improbable. The
evidence of the fact, however, is decisive. If a receiver, filled with
carbonic acid, and closed by a piece of bladder, firmly tied over it,
be exposed to the atmospheric air, the carbonic acid, notwithstanding
its superior specific gravity, rapidly escapes, and does so without
the exchange of an equivalent portion of atmospheric air; the bladder
is consequently forcibly depressed into the receiver. If the converse
of this experiment be tried, and the receiver, containing atmospheric
air, be tied over with a piece of bladder or thin leather, and then be
immersed in carbonic acid, this gas will so abundantly penetrate the
membrane and enter the receiver as to endanger its bursting.

879. Dr. Stevens had repeated opportunities of verifying these facts,
during a stay which he made at Saratoga, in the United States, the
springs at which place liberate a large quantity of carbonic acid.
In the high rocks it often collects in considerable quantity and
purity, and experiments on dogs and rabbits are often made for the
entertainment of strangers, as at the Grotto del Cano, near Naples.
This rock stands by itself in a low valley, through which there run two
currents of water, the one fresh and superficial, the other beneath
and charged with salts and carbonic acid. A current of this water rises
to some height in a cavity of the high rock, which appears to have been
formed by a deposition of earthy salts from the water. It has a conical
figure, the base of which is below the surface of the ground, and is
about nine feet in diameter. It rises about five feet from the ground,
where it is truncated, and presents an aperture a foot in diameter. The
water rises in general only about two feet above the ground, and in the
three feet above the surface of the water the liberated carbonic acid
collects. By luting a large funnel over the aperture, carbonic acid may
be collected at the mouth of the funnel in indefinite quantities, of
which Dr. Stevens availed himself to multiply and vary his experiments,
the result of which appears to be the complete establishment of the
fact that there exists a powerful attraction between carbonic acid and
oxygen.

880. The application of this fact to the explanation of the phenomena
of respiration is highly interesting. By virtue of this mutual
attraction, two currents are established, which flow in opposite
directions, through the membranous matter of the air-vesicles of the
lungs and the pulmonary blood-vessels spread out upon their surface;
the oxygen of the air flows to the blood attracted by its carbonic
acid, and the carbonic acid of the blood flows to the air attracted by
its oxygen. According to Dr. Stevens, the moment the blood parts with
its carbonic acid it loses its dark colour, and becomes of a bright
vermilion colour, for the following reason: all acids impart a dark
colour to the blood. With respect to most acids, this colour remains,
although the added acid be afterwards saturated. Carbonic acid forms an
exception, for on the removal of this aërial acid the blood resumes its
bright and arterial colour. Alkalies, like acids, darken the colour of
the blood, but salts produce a bright and vermilion colour when added
to the colouring matter of the blood. When the blood loses its carbonic
acid, the salts contained in the blood produce upon its colouring
matter the vermilion tint natural to the combination when the influence
of the salts is not counteracted by the presence of a redundant acid.
At the moment the venous blood gives up its carbonic acid it receives
in exchange a portion of the inspired air, which is chiefly at the
expense of the oxygen. It retains somewhat more oxygen than it yields
back in the shape of carbonic acid. The reddened and oxygenated blood,
having returned to the heart, is diffused over the system, where
it parts with its oxygen and combines with carbon, forming by the
union carbonic acid; the necessary result of this combination is the
generation of animal heat in the exact proportion to the quantity of
the carbonic acid which is produced. The venous blood, which receives
the carbonic acid as it is formed in the system, is darkened by its
presence, which counteracts the effects of the salts of the blood upon
its colouring matter.

881. An account has been given (439) of the experiments, which prove
that the lungs also constantly exhale a quantity of azote.

882. It has been further shown (469) that, together with the carbonic
acid, which passes off in the inspired air, there is always present a
quantity of aqueous vapour. This aqueous vapour is not visible at the
ordinary temperature of the air in its ordinary hygrometric state,
because the water is then dissolved in the air, and is carried off
in the form of invisible vapour; but it becomes abundantly manifest
at a low temperature, or when the air is loaded with moisture. By
the removal of this aqueous vapour, the lungs assist the skin in the
depuration of the blood. The water transpired by the lungs, like that
perspired through the skin, is separated from the blood by a true
and proper secretion constituting the pulmonary transudation. It is
commonly estimated that the lungs exhale about one-third as much as
the skin, or fifteen ounces daily. Dalton estimates it at twenty-four
ounces.

883. These estimates of the quantity of fluid lost by cutaneous
and pulmonary transpiration relate to the quantities lost at the
ordinary external temperatures in which the human body is placed. The
quantity lost when the body is exposed to an elevated temperature is
prodigiously increased. It did not occur to the physiologists, whose
experiments have been detailed (492, _et seq._), to ascertain this
by causing themselves to be accurately weighed immediately before
they entered their heated chamber and immediately after they left it.
Having heard that the loss daily sustained by the workmen employed in
gas-works is very extraordinary, I endeavoured to ascertain the amount
of it with exactness. This I have been enabled to accomplish by the
assistance of Mr. Monro, the manager of the Phœnix Gas Works, and of
Mr. Cooper. The following are the experiments by which this has been
ascertained.


EXPERIMENT I.—November 18, 1836, at the Phœnix Gas Works, Bankside,
London.

884. Eight of the workmen regularly employed at this establishment in
drawing and charging the retorts and in making up the fires, which
labour they perform twice every day, commonly for the space of one
hour, were accurately weighed in their clothes immediately before they
began and after they had finished their work. On this occasion they
continued at their work exactly three-quarters of an hour. In the
interval between the first and second weighing, the men were allowed
to partake of no solid or liquid, nor to part with either. The day was
bright and clear, with much wind. The men worked in the open air, the
temperature of which was 60° Farh. The barometer 29° 25´ to 29° 4´.

                      Weight of the Men   Weight of the Men      Loss.
                      before they began   after they had
                      their work.         finished their work.

                      cwt. qr. lbs. oz.    cwt. qr. lbs. oz.    lbs. oz.
  Michael Griffiths     1   1   14  10       1   1   12   2       2   8
  John Kenny            1   0   26  10       1   0   24   1       2   9
  John Ives             1   0   14   2       1   0   11   8       2  10
  James Finnigan        1   1   10   6       1   1    7   0       3   6
  William Hummerson     1   0   24   4       1   0   20   8       3  12
  Timothy Frawley       1   1    8  10       1   1    4  12       3  14
  Patrick Nearey        1   1   14  10       1   1   10   8       4   2
  Bryan Glynon          1   1    0   4       1   0   24   1       4   3


EXPERIMENT II.—Nov. 25, 1836.

885. Day foggy, with scarcely any wind. Temperature of the air 39°
Farh., barometer 29° 8´. On this occasion the men continued at their
labour one hour and a quarter.

                           Before.             After.          Loss.

                      cwt. qr. lbs. oz.   cwt. qr. lbs. oz.   lbs. oz.
  Patrick Murphy        1   1    0   0      1   0   27   2      0  14
  John Broderick        1   0    9   4      1   0    8   0      1   4
  Michael Macarthy      1   0   11   9      1   0   10   3      1   6
  Michael Griffiths     1   1   15   8      1   1   13   2      2   6
  James Finnigan        1   1   12   4      1   1    9  12      2   8
  Bryan Duffy           1   1   11  12      1   1    9   0      2  12
  John Didderick        1   1   11   5      1   1    8   8      2  13
  Charles Cahell        1   1    4   5      1   1    1   6      2  15

886. Charles Cahell, the man who on this occasion lost the most, was
weighed previously to the commencement of his work, with all his
clothes off, excepting his shirt, which was kept dry and put on him
again when weighed a second time at the end of his work. He was then
immediately put into a warm bath at 95° Farh., and kept there half an
hour: he complained of being weak and faint, and when reweighed had
gained half a pound.


EXPERIMENT III.—June 4, 1837.

887. Day clear, with some wind. Temperature 60° 5´.

                           Before.            After.         Loss.

                      cwt. qr. lbs. oz.  cwt. qr. lbs. oz.  lbs. oz.
  Robert Bowers         1   1   19   0     1   1   17   0     2   0
  William Mullins       1   1    3   0     1   1    1   0     2   0
  Charles Cahell        1   1    2   0     1   1    0   0     2   0
  John Kenny            1   0   22   2     1   0   19   8     2  10
  Bryan Glynon          1   0   27   0     1   0   24   4     2  12
  John Haley            1   1    4   0     1   1    1   4     2  12
  Benjamin Faulkner     1   1   15  14     1   1   13   0     2  14
  Michael Griffiths     1   1    8   8     1   1    5   8     3   0
  John Broderick        1   0    4   6     0   3   27   8     4  14
  John Didderick        1   1    6  12     1   1    1  10     5   2

888. The two last men worked in a very hot place for one hour and ten
minutes; all the rest worked about one hour. Michael Griffiths, as
soon as he had finished his work, was put into a bath at 98°, where he
remained half an hour. He was reweighed on coming out of the bath, and
had lost 8 oz.

889. From these observations it appears that, towards the end of
November, when the temperature of the external air was 39°, and the day
was foggy and without wind, the greatest loss did not amount to 3 lbs.
(2 lbs. 15 oz.), the least loss was 14 oz., and the average loss was 2
lbs. 3 oz.

890. In the middle of the same month, when the temperature of the air
was 60°, and the day was clear with much wind, the greatest loss was 4
lbs. 3 oz., the least loss was 2 lbs. 8 oz., and the average loss was 3
lbs. 6 oz.

891. In June, when the temperature of the external air was 60°, and the
day exceedingly bright and clear, without much wind, the greatest loss
was 5 lbs. 2 oz., the next greatest loss was 4 lbs. 14 oz., the least
loss was 2 lbs., and the average loss was 2 lbs. 8 oz.

892. The same individuals lose very different quantities at different
times. Thus, James Finnigan in the first experiment lost 3 lbs. 6 oz.,
in the second 2 lbs. 8oz. Michael Griffiths in the first experiment
lost 2 lbs. 8oz., in the second 2 lbs. 6 oz., and in the third 3 lbs.;
while John Kenny in the first experiment lost 2 lbs. 9 oz., and in the
third experiment, which was the second to which he was subjected, he
lost very nearly the same, namely, 2 lbs. 10 oz. On the other hand,
Bryan Glynon in the first experiment lost 4 lbs. 3 oz., and in the
third experiment, which was the second to which he was subjected, he
lost no more than 2 lbs. 12 oz.

893. In one case, when a man who had lost 2 lbs. 15 oz., the greatest
quantity lost by any of the men examined during that day, was put into
a hot bath at 95°, and reweighed on coming out of the bath, where he
had remained exactly half an hour, it was found that he had gained half
a pound. On the other hand, when a man who had lost 3 lbs. was put
into a hot bath at 98°, and kept there for half an hour and reweighed,
it was found that he had lost exactly half a pound.

894. It was our intention to have pursued these experiments, with the
view of ascertaining the influence of the hygrometrical state of the
air on transpiration, as well as the absorbing power of the skin, under
circumstances so favourable to the activity of that power, but the
investigation has been unavoidably postponed.

895. The results of these observations are as interesting in relation
to absorption as to transpiration. Thus, James Finnigan, on the 18th of
November, weighed,

                               cwt.   qr.   lbs.   oz.

  before the experiment          1     1     10     6
  after the experiment           1     1      7     0
  having lost                    0     0      3     6

On the 25th of November he weighed 1 cwt. 1 qr. 12 lbs. 4 oz., having
gained in the interval 1 lb. 14 oz.

Michael Griffiths, on the 18th of November,

                                   cwt.   qr.   lbs.   oz.

  before the experiment, weighed     1     1     14    10
  after the experiment               1     1     12     2
  having lost                        0     0      2     8

On the 25th of November, before the experiment, he weighed 1 cwt. 1 qr.
15 lbs. 8oz., having gained 14 oz.; but on the 3rd of June he weighed
1 cwt. 1 qr. 8 lbs. 8 oz., having lost between the 18th of November and
the 3rd of June, 6 lbs. 2 oz.

896. John Kenny, on the 18th of November,

                                 cwt. lbs.  oz.
  before the experiment, weighed  1    26   10
  after the experiment            1    24    1
  having lost                     0     2    9

On June the 3rd he weighed 1 cwt. 22 lbs. 2oz., having gained in the
interval 4 lbs. 8 oz.

897. Bryan Glynon, November 18th,

                                 cwt. qr. lbs. oz.
  before the experiment, weighed  1    1    0   4
  after the experiment            1    0   24   1
  having lost                     0    0    4   3

On the 3rd of June he weighed 1 cwt. 27 lbs., having lost 1 lb. 4 oz.

898. Thus, in the course of their ordinary occupation, these men are in
the habit of losing from 2 lbs. to 5 lbs. and upwards twice a-day; yet,
when weighed at distant intervals, it is found that some have actually
gained in weight and others have lost only a few pounds; it follows
that the activity of the daily absorption must be proportionate to that
of the daily transpiration.

899. According to the prevalent opinion, the liver is the cause of
a large proportion of the maladies which afflict and destroy human
life. It certainly exercises an important influence over health and
disease, the true reason of which is but little understood by those who
attribute most to its agency.

900. The liver is an organ of digestion and an organ of excretion.

It is an organ of digestion in a two-fold mode:

1. By the secretion of a peculiar fluid, through the direct action of
which chyme is converted into chyle. The several phenomena attending
this operation have been fully described (668 _et seq._).

2. By subjecting alimentary matters which have been partly acted on by
the stomach and intestines to a second digestion.

901. It has been shown (666) that the veins which return the blood from
the digestive organs, the stomach, the intestines, and the mesentery,
together with the veins of the spleen, the omentum and the pancreas,
instead of pursuing a direct course to the right side of the heart in
order to transmit their contents by the shortest route to the lungs, as
is the case with all the other veins of the body, unite together and
form a large trunk termed the vena portæ, which enters the liver and
ramifies through it in the manner of an artery. It has been further
shown (666) that the bile is secreted from the venous blood contained
in this vessel by its capillary branches spread out on the walls of
the biliary ducts, the only known instance in the human body in which
a secretion is formed from venous blood by venous capillaries; that
the trunk of this vein, unlike that of any other, is encompassed with
organic nerves, which accompany its subdivisions, and are spread out
upon its capillary branches just as an organic nerve is spent upon
an artery, and that thus, as this vessel performs the function of an
artery, it has the structure and distribution of an artery.

902. The veins which unite to form the vena portæ take up, by their
capillary branches, certain portions of the contents of their
respective organs, and bear those contents directly into the venous
current. The capillary veins of the stomach take up certain parts of
the contents of the stomach, it would appear the fluid substances
received with the aliment more especially; the capillary veins of the
duodenum take up certain portions of the contents of the duodenum, and
so on of the capillary veins of the spleen, intestines, and all the
organs whose veins combine to form the vena portæ. Further, branches
of the absorbent vessels of these organs have been distinctly traced
opening directly into the veins in their immediate neighbourhood.
Certain products of digestion must, then, be constantly poured, both
by the capillary veins and by the absorbent vessels of the digestive
organs, into the blood of the vena portæ.

903. Accordingly, on the examination of animals soon after a meal,
streaks of a substance like chyle are often observed in the blood of
the vena portæ. It is further established by numerous experiments,
that if alcohol, gamboge, indigo, and other odoriferous and colouring
matters, are mixed with the food, their presence is manifest in the
blood of the digestive organs, and more especially in the blood of the
mesenteric veins and in that of the vena portæ, while no trace of these
substances is ever found in the lacteals.

904. The lacteals, it has been shown (835. 1.), are special organs
appropriated to the performance of a specific function, that of
absorbing chyle. To fit them for this office, they are endowed with an
elective power, by virtue of which they select, from the alimentary
mass, that portion of it only which is converted into chyle; in
a natural and healthy state they would appear to be incapable of
absorbing any other substance excepting pure chyle. But in the
digestive organs there is always present much nutritive matter not
yet converted into proper chyle, and with this matter there are mixed
foreign substances not strictly alimentary. These unassimilated matters
and foreign substances, absorbed by the capillary veins or by the
absorbent vessels, or by both, are conveyed directly into the vena
portæ, by which vessel they are transmitted to the liver, where they
undergo a true and proper digestion. After undergoing this digestion
in the liver, they are sent by a short course to the heart, and thence
to the lungs, where they are assimilated into, or at least commingled
with, arterial blood, and, with arterial blood, are transmitted to the
system. The substances subjected to this hepatic digestion, which is
as real as that effected in the stomach and duodenum, do not appear
to enter the lacteals at all; they have therefore a shorter course
to traverse, and probably a proportionately less elaborate process
to undergo, before their transmission to the lungs and their final
entrance into the arterial system.

905. What the particular substances are for which this slighter
digestive process suffices is not known with certainty. There is,
however, reason to suppose that they consist chiefly of liquids, while
there is direct evidence that vinous and spirituous liquids enter
the system through this shorter course; since these fluids are often
abundantly manifest in the blood of the vena portæ, when not the
slightest trace of them can be detected in the lacteal vessels.

906. According to this view, the liver is a second digestive
apparatus, completing what the first commences, or effecting what
that is incapable of accomplishing; and this view assigns the reason
why certain fluids taken into the stomach sometimes appear in the
secretions and excretions with such astonishing rapidity; why the liver
so constantly becomes diseased when highly stimulating substances, not
properly alimentary, are mixed with the food, and more especially when
ardent spirits or the stronger wines are largely and habitually taken;
why the sympathy is so intimate and intense between the stomach and
the liver and the liver and the stomach, both in health and disease;
why in the ascending animal series the liver so soon appears after
the stomach, and why the magnitude of the organ and the elaborateness
of its structure progressively increase with the extension of the
digestive apparatus and the corresponding complexity of the general
organization.

907. The second function performed by the liver is that of excretion.
The excrementitious matter eliminated from the blood by the liver is
contained in its peculiar secretion, the bile. The bile consists of
two portions, an assimilative part which combines chemically with the
chyle, purifying and exalting its nature; and an excrementitious part
which combines with the residue of the aliment.

908. The excrementitious part of the bile contains a large proportion
of carbon and hydrogen. Carbon and hydrogen abound in venous blood;
venous blood in large quantity is sent to the liver to afford the
materials for the secretion of bile; consequently, the more copious
the secretion of bile the greater the quantity of carbon and hydrogen
abstracted from venous blood. It follows that, by this elimination of
carbon and hydrogen from the blood, the liver is auxiliary, as an
organ of excretion, to the skin and the lungs.

909. But it is well worthy of remark, that although the liver at all
times assists the skin and the lungs in carrying on the process of
excretion, it does this most especially under circumstances which
necessarily enfeeble the action of the cutaneous and pulmonary organs.

910. Less carbon is expelled from the lungs in summer than in winter;
at a high than at a low temperature; consequently by a long-continued
exposure to intense heat, as in the hot months of summer, and still
more by a continual residence in a warm climate, an accumulation of
carbon in the blood is favoured. A part of this excess is removed by
the increased exhalation from the skin. The skin, however, is the
chief outlet, not for carbon, but for hydrogen; and accordingly by the
increased perspiration hydrogen is largely removed. Hydrogen and carbon
compose fat. The deposition of fat, could it go on to the requisite
extent, would afford an adequate consumption for the superabundant
carbon; but the formation of fat is prevented by the dissipation of the
hydrogen. Under such circumstances, when the lungs cannot carry off the
requisite quantity of carbon, nor the adipose tissue compensate for its
diminished activity by the deposition of fat, the liver, taking on an
increased action, secretes an extraordinary quantity of bile. In this
manner the superfluous carbon, instead of being removed in the ordinary
mode, by the pulmonary artery through the lungs, under the form of
carbonic acid gas, is excreted by the vena portæ, through the liver,
under the form of bile, while the superabundant hydrogen is removed by
the increased quantity of perspiration; and thus the accumulation of
these inflammable matters in the system is effectually prevented.

911. By the deposition of fat in the adipose tissue material assistance
is afforded to the excretory action of the skin, the lungs, and
the liver. Fat is composed essentially of carbon and hydrogen; it
contains no nitrogen and very little oxygen. It is deposited whenever
an excessive quantity of nutritive matter is poured into the blood,
and especially when at the same time the different secretions and
excretions ordinarily formed from the blood are diminished. The primary
object of this deposition is to relieve the circulation of a load which
would embarrass and ultimately stop the actions of life. It serves,
however, a secondary purpose, that of forming a storehouse of nutritive
matter, duly prepared for supplying the wants of the system, in case
the body should be placed under circumstances in which the digestive
organs can no longer receive food or no longer convert it into chyle.

912. Thus hybernating animals, which pass many months without taking
food, accumulate a store of fat before they fall into the state of
torpor. Marmots and dormice subsist on this store during the winter,
and hence, when spring awakens them from their torpor, they are always
in a state of extreme emaciation. Birds and other animals which live on
food procured with difficulty in the winter, become unusually fat in
the autumn.

913. During fever and other acute diseases, when little food is
received, and still less converted into chyle, the extreme emaciation
which the body undergoes is owing partly to the disappearance of the
fat, which is taken up by the absorbents and carried into the blood, in
order to compensate for the deficiency of nutrient matter supplied by
the digestive organs.

914. The chief depositories of the fat are those intersticial spaces of
the body in which a certain quantity of soft but tenaceous substance is
required to obviate pressure or to preserve symmetry. A large quantity
is also placed immediately beneath the skin; in the interstices of
muscles; along the course of blood-vessels and nerves; in the omentum,
where it is spread like a covering over the viscera of the abdomen
(fig. CLXX. 7); in the mesentery and around the kidneys.

915. Fat is a bad conductor of heat; consequently the layer which is
spread over the external surface immediately beneath the skin, and
that which is collected in the interior of the omentum, must be useful
in preserving the heat of the body. Fat persons bear cold better than
lean persons. Animals which inhabit the northern climates, and the
fishes of the frozen seas, are enveloped in prodigious quantities of
fat. Where the accumulation of this substance would produce deformity
or interfere with function, as about the joints, in the eyelids, within
the skull, not a particle is ever deposited. About the joints it would
impede motion; in the eyelids it would render the face hideous and
obstruct vision; and within the skull, a cavity completely filled
with the brain, an organ impatient of the slightest pressure, had a
substance been placed, the quantity of which is liable to be suddenly
trebled or quadrupled, changes in the system which now produce no
inconvenience would have been fatal. Thus, while provision is made at
once to exonerate the system from too great a load of nourishment, and
to lay up the superfluous matter, as in a magazine, to be ready for
future use, the most extreme care is taken to deposit the store in safe
and convenient situations.

916. The excretory organs and processes, hitherto considered, have for
their object the removal from the blood of its superfluous carbon and
hydrogen; the element peculiar to the animal body, azote, is eliminated
by the kidneys, glandular organs which possess a highly complex
structure.

917. But besides the removal of the superfluous azote, the fluid
secreted by the kidneys would appear to be a general outlet for
whatever is not required in the system, and for the removal of which
no specific apparatus is provided. Chemical analysis shows that, in
different states of the system, the following substances are contained
in this fluid:—water, free phosphoric acid, phosphate of lime,
phosphate of magnesia, floric acid, uric acid, benzoic acid, lactic
acid, urea, gelatin, albumen, lactate of ammonia, sulphate of potash,
sulphate of soda, fluate of lime, muriate of soda, phosphate of soda,
phosphate of ammonia, sulphur, and silex.

918. This catalogue itself suggests the idea that when any matter
employed in carrying on the functions is in excess, or when it has
become decayed, or is decomposed and is not eliminated by any other
excretory process, it is taken up by the absorbents, poured into the
veins, and so conveyed in the course of the circulation to the kidneys,
by which organs it is separated from the blood, and thence by an
appropriate apparatus carried out of the system.

919. The specific matter secreted by the kidneys is that termed urea;
a substance of a resinous nature, highly animalized. One character by
which the animal is distinguished from the plant is its locomotion.
The organ by which the animal is rendered capable of performing the
function of locomotion is muscle or flesh. The basis of muscle is
fibrin, and the basis of fibrin azote. There must be in the animal
body an abundant supply of fibrin, and consequently a proportionate
abundance of azote. Azote is introduced into the system partly by the
food and partly by the lungs. That there may be a sufficiency for all
occasions, more is introduced than is necessary on ordinary occasions,
and a special outlet is established for the excess through the kidneys.

920. Organs appropriated to the removal of substances from the blood,
capable of becoming deleterious by their accumulation, generally in
a state of health perform their office so perfectly that the matters
which it is their part to excrete are eliminated almost as quickly
as they enter the blood, so that they are seldom present in the
circulating fluid in sufficient quantity to be detected by the most
delicate chemical tests. But by the removal of the excretory organ, or
by the suppression of its function, the excretory matter accumulates
in the blood, and is then readily detected. A decisive experiment
disclosed that this is the case with regard to urea. The kidneys were
removed from a living animal. The operation did not appear to be
productive of material injury for some time; but at length symptoms
denoting the presence of a poison in the blood arose, and the animal
died. The blood was carefully examined after death. It was found to
contain a much larger quantity than ordinary of the peculiar animal
substance which enters into the composition of the serosity of the
blood (225). On subjecting this substance to the action of various
re-agents, and also on reducing it to its ultimate elements, it was
found to resemble urea; to be, in fact, nearly identical with urea as
contained in the urine. From this experiment it became manifest that
the source of the urea is the serosity of the blood. It is probable
that the chief office of the kidney is to separate the urea from the
other ingredients of the blood, and to convey it to the organs which
are destined to carry it out of the body.

921. It is estimated that about a thousand ounces of blood pass through
the kidneys in the space of an hour; itself a sufficient indication
of the importance of the excretion performed by this organ, and an
adequate source of the matter actually excreted, although, under
ordinary circumstances, distributed through the circulating mass in
quantities so minute as to be almost inappreciable.

922. From the power of absorption possessed by the veins of the stomach
and intestines, from the connexion proved to be established between
the venous and absorbent systems, and from the discovery of Lippi, that
several absorbent branches in the abdomen terminate directly in the
pelvis of the kidney, that is now an established fact which was long
a conjecture, that there exists a short route from the stomach to the
kidneys, so that the extreme rapidity with which certain substances
mixed with the aliment appear in the fluid secreted by the kidneys is
no longer a matter of wonder.

923. Out of the body urea putrifies with great rapidity. When retained
in the system by the extirpation of the kidney, or by placing a
ligature around the ureter, such is the septic tendency communicated to
the blood that signs of putrescency become manifest even during life,
and after death all the soft parts of the body are reduced to a state
of putrefaction with extreme rapidity. The suppression of the secretion
in the human body, or the undue retention of the matter secreted,
induces fever of a malignant kind, in which the symptoms that denote a
highly putrid taint in the system are rapidly developed. But for the
labour of the kidney, then, a substance would accumulate in the blood,
which would quickly lead to the decomposition of the body.

924. It has been shown that the mucous membrane which lines the
alimentary canal is studded in its whole extent with glands, which
secrete from the blood a large quantity of fluid, These secretions go
on without interruption, whether food be taken or not, so that there
may be copious alvine evacuations though not a particle of food enter
the stomach; and the separation of the matter eliminated from the blood
by this extended membrane can no more be dispensed with than that by
the skin or the lungs. There is, too, a most intimate sympathy between
the secretion of the membrane that lines the internal surface of the
body and that carried on by its external covering; any disorder of
the one immediately and powerfully disturbs the natural course of the
other: hence the diarrhœa, so often produced by the application of
cold to the external skin, and the diseases of the skin, so constantly
connected with a disordered state of the mucous membrane of the
intestines.

925. It is the special office of the large intestines to prepare for
its removal, and to carry out of the system the residue of the aliment,
together with the excrementitious portion of the bile.

926. It was calculated by Haller, that the different excretory organs
remove from the system every twenty-four hours twenty pounds of matter.
Of this total loss sustained daily by the human body, it was estimated
that four pounds are removed by the skin, four pounds by the lungs,
four pounds by the kidneys, and eight pounds by the intestinal canal.
In this estimate, which is considered too large, especially that
by the intestinal canal, the quantity stated must be understood as
denoting the maximum of each secretion.

927. Supposing the ingesta in twenty-four hours to be of food 6 pounds,
or 96 ounces, and of oxygen retained in the system 4 ounces, in all 100
ounces, it is estimated that the egesta will be, in twenty-four hours,
by the skin, 34 ounces, by the lungs 17 ounces, by the intestines 6
ounces, by the kidneys 40 ounces, and by various other excretions
3 ounces, in all, 100 ounces. These calculations must of course be
taken only as approximations to the truth, and as ascribing rather the
relative than the positive quantities of matter excreted.

928. Whatever be the absolute quantity or the form of the excretions,
it is clear, from the preceding account, that there is constantly
removed from the system by the skin a large portion of hydrogen and
some carbon; by the lungs a large portion of carbon and some hydrogen;
by the liver a large portion of hydrogen and some carbon; by the
kidneys a large portion of azote; by the large intestines the residue
of the aliment; while, by the deposition of fat, the superabundant
nutriment withdrawn from the current of the circulation is laid up in
store in some safe part of the body.

929. Most of the processes which have been described are mutually
compensating and vicarious. Besides the office which each habitually
performs, it is capable of having its action occasionally increased,
for the purpose of supplying the deficiency of one or more of its
fellows. If perspiration by the skin languish, transudation by the
lungs increases; if neither the skin nor the lungs be able to remove
the superfluous hydrogen and carbon, these inflammable substances are
carried out of the system by the liver in an augmented secretion of
bile. If the action of the liver be diminished, that of the kidney is
increased; and if the secretion of urine be suppressed, the secretion
of bile is augmented. When the absorbents are oppressed by the quantity
of fluid poured into the stomach, or when the system is at the point
of saturation, and no absorption can go on, the veins take up the
superfluous liquids, pour them into the circulating current, and bear
them to the kidneys, by which organs they are rapidly separated from
the blood, and carried out of the body. The weakness of one organ is
compensated by the strength of another; the diminished activity of one
process is equalized by the increased energy of some other to which it
is allied in nature and linked by sympathy; and thus the evils which
would result from the partial and temporary failure of an important
function are obviated by some vicarious labour, until the enfeebled
organ has recovered its tone, and the natural balance of the functions
is restored.

930. The condition acquired by the elementary particles of organized
bodies, from their long continuance in the system, which induces the
necessity for their excretion, is not known. The chemical elements
of the excretions are the very same as those which constitute the
organized textures and the nourishment by which they are sustained.
Carbon is the basis of the organized body; yet all living bodies,
without exception, excrete carbon. Oxygen, hydrogen, and azote, also,
without which life cannot be maintained, if retained in the system
beyond a given time, are incompatible with the continuance of life.
During the chemical changes which these elementary particles undergo,
in the course of the vital processes, they appear to enter into some
combination, which is no longer compatible with the peculiar mode in
which they are disposed in organized and living structures. And one
such change, of a very remarkable nature, has been observed, which,
it is conceived, has a considerable share in rendering their constant
expulsion and renovation indispensable.

931. Out of the condition of life the component elements of organized
bodies readily combine so as to form crystals; the peculiar
combinations by which they form the constituent textures of organic
structures are never crystalline. No crystal is ever seen in the seat
of a living and growing vegetable cellule; no crystal is ever found
as a constituent part of animal membrane. Whenever a crystal occurs
in an organized body it is always the result either of disease or of
some artificial process, or else it is an excretion separated from the
nourishing fluid and the useful textures. Every one of these textures
contains, even in its minutest parts, saline and earthy, as well
as vegetable or animal, matter. Why do not these saline and earthy
particles as readily combine to form crystals in the organic as they do
in the inorganic body? They never do. In the organic body these saline
and earthy particles are always so arranged that they are diffused
through the membranous fibres or cells, never concentrated in crystals.

932. On the other hand, the elements containing the peculiar matters of
excretion are generally in such a state of combination as readily to
assume the crystalline form, either alone or in the simplest further
combinations of which they are susceptible. It seems probable that this
circumstance may be, at least in part, the cause which necessitates
their expulsion, and it is certain that some such general principle
must determine the incompatibility of the matters of excretion with the
life of the structures

933. The ultimate object of the processes included in the function of
excretion is to maintain the nutritive fluid in a certain chemical
condition. Into the combination of the blood there must enter certain
constituents, and these must be in certain relative proportions, and
in no others. If the salts be diminished or in excess, if the fibrin,
or the red particles, or the serum be abundant or defective beyond a
certain degree, either the necessary chemical elements are not present,
or not present in the form necessary to their entering into the
requisite combinations; the result is, that a proper nutritive fluid
is not formed, and consequently due nourishment is not afforded to the
textures nor due stimulus to the moving powers; there is either too
much nutriment and stimulus or too little; in the one case the machine
is exhausted and worn out, and in the other it is clogged and stopped.

934. The capillary arteries of the skin, and of all the other tissues
into the composition of which gelatin enters as a constituent,
necessarily pour carbon into the capillary veins at the moment they
convert albumen into gelatin (539). The veins, receiving in their
course more and more carbon from the arteries, at length become loaded
with this element, and in order to get rid of the excess they bear
it to the lungs, where it is expelled by the act of expiration under
the form of carbonic acid gas. On the other hand the chyle, gradually
becoming firmer and more condensed by the series of changes which it
undergoes from its first formation in the duodenum to its admixture
with the lymph in the receptacle of the chyle, and with the blood
in the subclavian vein, is hurried to the heart and thence to the
lungs, where it gives off a large portion of its watery particles,
also by the act of expiration, under the form of aqueous vapour. This
excretion of its watery particles is a necessary part of the process of
completion by which the weak albumen of the chyle is converted into the
strong albumen of the blood (703. 3). How completely analogous then is
this excretory process in the plant and in the animal! How precisely
the same is the action of the leaf and of the lung! The leaf dissipates
the superfluous water of the crude sap, concentrates its organic
principles, and brings it into the chemical condition which constitutes
the proper juice of the plant; the lung removes the superfluous water
of the chyle, concentrates its organic principles, and completely
assimilates its chemical nature into that of the blood.

935. It is the same with every other process of excretion; its uniform
result is to alter the chemical composition of the nutritive fluid, to
restore it to a state of concentration and purity. Excretion then is
appropriately termed a depurating process.

936. The effect of the suppression of excretion, when the suppression
is complete, is appalling. Stop the respiration, that is, suspend
the depurating action of the lungs, carbon accumulates in the venous
blood; carbon mixes with the arterial blood; in half a minute the
blood flowing in the arteries is evidently darkened; in three-quarters
of a minute it is of a dusky hue; in a minute and a half it is quite
black; every particle of arterial blood has now disappeared, and the
whole mass is become venous. With the first appearance of the dusky hue
great disturbance is produced in the system; the instant it becomes
dark sensibility is abolished; in a few minutes after it is black the
power of the heart is so enfeebled that it can no longer carry on the
circulation, and in a few minutes more its action wholly ceases, and
can never again be excited. The brain feels the poison first, and is
first killed; but the heart cannot long resist the fatal influence.

937. Stop the excretion of the kidney by the extirpation of the organ,
or the suppression of its secretion, urea accumulates in the blood; the
poison, after a short time, begins to work; fever is excited, and then,
with fearful rapidity, fever is followed by coma, and coma by death.

938. Stop the secretion of bile, a poison accumulates in the blood as
potent, producing insensibility and death as rapidly, as that generated
by the suppression of the depurating action of the kidneys.

939. Only obstruct the secretion of bile, merely prevent its due
elimination from the blood, just in proportion to its suppression does
the system suffer from languor, lassitude, and inaptitude for every
muscular and mental exertion.

940. How do the internal organs suffer when the excretion of the skin
is deficient, and how numberless and hideous are the diseases of the
skin when the depurating process of the alimentary canal is suspended!

941. When, on the contrary, all these excretions are well and duly
performed, how regular and tranquil, yet how full and strong the flow
of the circulating current; how rich the stream poured by it into every
organ; how healthfully exciting its influence on them all; how gentle,
how efficient, every organic action; how complete the absence of all
note or sensible intimation that any such action is going on, yet how
delicious the consciousness produced by its soundness and vigour; how
acute the sense, how bounding the motion, how quick the percipience;
how the pure blood mantles in the cheek and diffuses its sparkling
colour over all the transparent complexion; how the jocund spirits
laugh from the eyes; how the intellectual and sympathizing mind beams
forth from them with a higher and holier happiness! How wonderfully
beautiful is such a human body, and how magnificently endowed in its
capacity to give and to receive enjoyment!

942. There are two adjustments, with regard to the excretions, carried
on by organized bodies, which can never be contemplated with sufficient
admiration. It has been fully shown (464 _et seq._) that the relation
established between the two great classes of organized beings is such
that the excrementitious matter of the plant is nutritious to the
animal, and the excrementitious matter of the animal is nutritious to
the plant; and, consequently, that the two orders of living beings
maintain the world, which is given them as their inheritance, in a
state of perpetual adaptation for the life and health of each other;
the animal receiving healthy stimulation from that which is poisonous
to the plant, and the plant being nourished by particles which the
animal throws off as exhausted and useless. And this relation naturally
suggests that so beautifully described by Milton:—

                  Flow’rs and their fruit,
    Man’s nourishment, by gradual scale sublimed
    To vital spirits aspire, to animal,
    To intellectual; give both life and sense,
    Fancy and understanding; whence the soul
    Reason receives.

943. Secondly, the particles thrown off by organized bodies are
rendered, in the very act of their dissipation, subservient to purposes
of utility and pleasure. How these poisonous elements are converted
into the pabulum of life and health has been shown. To a being with the
senses and faculties of man, how loathsome might these particles have
been rendered during the period of their transition from one organized
kingdom to the other! And if disagreeable at all, how constantly forced
upon his sense, wherever he might be, during every moment of his
waking hours, must these objects of disgust have been! But how does
the matter actually stand? The excretions of the plant are the very
particles that, poured

    “Into the blissful field through groves of myrrh,
    And flow’ring odours, cassia, nard, and balm,”

create “a wilderness of sweets.” It is as these exhalations are passing
off from the economy to which, if retained, they would be noxious
(851), that they become

        “Exhalations of all sweets
    That float o’er vale and upland;”

and which refresh and delight even more than the forms and colours of
the “aery leaf” or “the bright consummate flower.”

944. And the human body, when the functions of its economy are sound
and vigorous, is fresh and fragrant as the flower (862); and by that
intellectual faculty by which man is capable of associating his
conception of beauty and delight with whatever object has been the
source of exquisite gratification, the fragrance of the flower is but
suggestive of what, to him, is inexpressibly sweeter and dearer.

            “As new waked from soundest sleep,
    Soft on the flow’ry herb I found me laid
    In balmy sweat, which with his beams the sun
    Soon dry’d——
    By quick instinctive motion up I sprung,
                    ——— And upright
      Stood on my feet.——
                    ——— All things smiled
    With fragrance, and with joy my heart o’erflow’d.
    Myself I then perused, and limb by limb
    Survey’d, and sometimes went, and sometimes ran.
    With supple joints, as lively vigour led.” MILTON.

                      ——Fresh lily,
    ’Tis her breathing that
    Perfumes her chamber thus. SHAKSPEARE.

                        —— The very air
    With her sweet presence is impregnate richly,
    As in a mead that’s fresh with youngest green
    Some fragrant shrub exhales——
    Ambrosial odours——
                    Charming present sense,
    And sure of memory;—so her person bears
    A natural balm—distilling incense.
    “Death of Marlowe,” by R. H. HORNE.




CHAPTER XIV.

OF NUTRITION.

 Composition of the blood—Liquor sanguinis—Recent account of the
 structure of the red particles—Formation of the red particles in
 the incubated egg—Primary motion of the blood—Vivifying influence
 of the red particles—Influence of arterial and venous blood on
 animal and organic life—Formation of human blood—Course of the new
 constituents of the blood to the lungs—Space of time required for the
 complete conversion of chyle into blood after its first transmission
 through the lungs—Distribution of blood to the capillaries when
 duly concentrated and purified—Changes wrought upon the blood while
 it is traversing the capillaries—Evidence of an interchange of
 particles between the blood and the tissues—Phenomena attending the
 interchange—Nutrition, what, and how distinguished from digestion—How
 the constituents of the blood escape from the circulation—Designation
 of the general power to which vital phenomena are referrible—Conjoint
 influence of the capillaries and absorbents in building up
 structure—Influence of the organic nerves on the process—Physical
 agent by which the organic nerves operate—Conclusion.


945. The object of the greater part of the processes hitherto described
is to form the nutritive fluid, and to bring it to the requisite state
of purity and strength. Recent researches into the composition of the
nutritive fluid confirm the general correctness of the account already
given of it, (211 _et seq._).

946. When examined as it is flowing in the finest vessels of a
transparent part of the body, or immediately after it is abstracted
from the trunk of a vein or artery, before coagulation (218) takes
place, the blood is seen to consist of a colourless fluid, through
which is diffused a countless number of minute solid particles of a red
colour. The colourless fluid is called the liquor sanguinis, and the
solid particles the blood corpuscles or the red particles.

947. By the process of coagulation, the phenomena of which have been
fully described (219 _et seq._), the blood spontaneously separates into
a clear fluid of a yellow colour called serum or blood-water, and into
a solid mass termed the clot or the crassamentum. The serum, which must
be carefully distinguished from the liquor sanguinis, is the fluid
formed from the blood by coagulation; the liquor sanguinis is the fluid
part of the blood which exists before coagulation.

948. The liquor sanguinis contains in solution a large quantity of
animal matter, fibrin (228), which separates spontaneously in a solid
form on coagulation; the serum also contains a quantity of animal
matter in solution, albumen (224), which does not separate in a
solid form spontaneously, but only on the application of heat, acids,
alcohol, &c. (224). The animal matter, the fibrin, which separates
spontaneously from the liquor sanguinis in a solid form, constitutes
one part of the clot, and the other part of it consists of the red
particles which floated in the liquor sanguinis.

949. Thus, by coagulation, the liquor sanguinis separates into a
portion which remains fluid, the serum; and into a portion which
becomes solid, the fibrin; while the fibrin, as it is passing from
the fluid to the solid state, entangles the red particles, and both
together form the clot; consequently the liquor sanguinis contains in
solution two kinds of solid matter, fibrin and albumen; while the serum
contains in solution only one kind of solid matter, albumen.

950. The solution of fibrin in the liquor sanguinis, and its
spontaneous solidification during the process of coagulation, has been
shown by Professor Müller in the following mode. Having carefully
collected blood from the femoral artery of the frog, and also from the
heart laid bare and incised, and having brought a drop of this pure
blood under the microscope, and diluted it with serum, so that the
red particles were separated from each other by distant intervals,
he observed that there formed in those intervals a coagulation of
previously dissolved matter, by which the separated red particles were
connected together. By raising, with a needle, the coagulum occupying
the intervening spaces, this solid matter was obtained free from red
particles. The blood corpuscles of the frog are rendered, by a powerful
microscope, so large, that this operation may be performed with the
greatest distinctness. In consequence of the minuteness of the red
particles of human blood they pass, with the liquor sanguinis, through
filtering-paper; but those of the frog, being four times larger, are
kept back by the filter, while the liquor sanguinis percolates through
as a clear fluid, and then coagulates. This colourless coagulum is so
transparent that it is not even detected, after its formation, until
it is raised out of the fluid with a needle. It gradually thickens and
becomes white. It is the fibrin of the blood in its purest state.

951. Professor Müller’s account of the structure of the red particles
differs in a material point from that given (231 _et seq._). He
agrees that they are rounded bodies (fig. CXII. 1), generally of
the same size, though some are seen larger than common, but never
double the mean diameter; that they are always quite flat (232); that
in a certain light they look as if they were hollowed out from the
edges to the centre (fig. CXII. 1); but, he adds, “that this spot
is a real depression, as some think, appears to me in the highest
degree improbable; for I have at last convinced myself that the blood
corpuscles of man and the mammalia contain a very small nucleus of
the diameter of the flat corpuscle. My observations prove beyond
doubt that the blood corpuscles of frogs and salamanders (fig. CXII.
4) contain a nucleus entirely different in its chemical relations
from the outer layer. With one of Frauenhofer’s microscopes I have
seen very distinctly, in the blood corpuscles of man an exceedingly
small, round, well-defined nucleus, yellower and brighter than the
transparent circumference. When the blood corpuscles are mixed,
under the microscope, with acetic acid, the shell is almost entirely
dissolved, and these small nuclei, which are seen with great difficulty
in human blood, remain, while those of the frog appear, very evidently
the nuclei observed earlier in the blood corpuscles. In man, the nuclei
within the corpuscles are so small, that the diameter does not exceed
the thickness of the flat corpuscles.”

952. The enveloping capsule is stated to be soluble in water, while the
internal nucleus is insoluble; but the capsule is not soluble in serum;
the albumen and the salts contained in the serum probably rendering it
insoluble. The colouring matter of the capsule, which gives the red
colour to the blood, is called hæmatosin. Lecanu considers the capsular
substance as a combination of a specific colouring matter, which he
calls globulin, and of albumen; but Müller regards it as fibrin,
containing a quantity of iron. The latter physiologist states that
the opinion of Brande, that the amount of iron in hæmatosin is not
greater than in serum and other animal substances, has been refuted
by Berzelius and Engelhart. The iron is not an accidental ingredient
obtained from the food; for iron has been found in the blood of a
new-born animal that has never even sucked. According to Berzelius the
colouring matter of the blood contains a quantity of iron corresponding
to somewhat more than a half per cent. its weight of metallic iron, and
he thinks it most probable that the iron exists in the blood in the
metallic state, and not as an oxide.

953. By carefully watching the development of the chick in the
incubated egg, the first formation of the red particles can be
distinctly seen. The blood in the new being, which is elaborated before
the existence of the vessels that are to contain it, is formed from
the substance of the germ or from that of the germinal membrane, and
is augmented by the blood of the egg, which is the substance of the
yolk. First, a number of granules are produced from the substance
of the yolk. These subsequently lose their granular appearance, and
become translucent. On the translucent ring is produced the nucleus
of the blood corpuscles. When completely formed, the blood corpuscles
of the bird, as of all the animals below the bird in the scale of
organization, are of an elliptical figure, and quite flat (fig. CXII.
4, 5); but when first produced they are rounded globules, not flat, and
they gradually assume their proper and permanent form; it is only on
the sixth day of incubation that they begin to be elliptical, by the
ninth day they are all elliptical (fig. CXII. 4, 5).

954. The substance of the fluid yolk is thus changed into blood without
the action of any special organ; for, as yet, no organs such as liver,
spleen, or lungs, exist. When the formation of the blood has arrived
at a certain point, it begins to be in motion. The blood is seen to
be in motion before the heart can be observed to beat. The germinal
membrane arising out of the enlarged germinal disk soon exhibits a
thin upper layer (serous membrane) and a thicker under layer (mucous
membrane). There is also formed in the middle of the germinal membrane
around the appearing trace of the embryo a translucent space, the _area
pellucida_. The exterior of the germinal membrane remains opaque, and
this opaque portion becomes divided by a definite boundary into an
external and internal annular space in from sixteen to twenty hours.
This separation encloses one part of the opaque portion of the germinal
membrane, which surrounds the interior or translucent space of the
germinal membrane, and is termed _area vasculosa_, because the blood
and vessels form the inner half of this space.

955. As far as the area vasculosa extends, a granular layer is
presented between the two layers of the germinal membrane, which soon
divides into numerous granular isolated particles with translucent
intervals, in which the blood collects, first in the form of a
yellowish, and then of a reddish fluid; first distinctly in the
periphery of the area vasculosa, from which it is seen to flow towards
the heart before the heart beats.

956. The blood exerts its vivifying influence chiefly by the red
particles. If an animal be bled to fainting, and pure serum be injected
into its vessels, re-animation does not take place; but if the blood of
another animal of the same species be injected, the animal which was
apparently dead acquires new life at every stroke.

957. The fibrin may be removed from the blood without injuring the
red particles. If the fibrin be abstracted, and a mixture of the
red particles and the serum be brought to a proper temperature, and
injected into the veins of an animal bled to fainting, re-animation is
effected.

958. If the blood of an animal of another species be injected whose red
particles are of the same form, but of a different size, re-animation
is indeed effected, but the restoration is imperfect; the organic
functions are oppressed, and languish, and death takes place generally
within the sixth day. The same effects follow, if a mixture of serum
and red particles of the blood of a different species be injected.

959. If blood with circular particles be injected into the vessels
of an animal whose blood corpuscles are elliptical, the most violent
effects are instantly produced; such blood acts upon the nervous system
like the strongest poisons; and death usually follows with extreme
rapidity after the injection of a very small quantity. Thus, if a few
drops of the blood of the sheep be injected into the vessels of the
bird, the bird is killed instantaneously. It is very remarkable, that
the blood of the mammalia should be thus fatal to the bird. The effect
cannot be dependent on any mechanical principle. The injection of a
fluid with particles, the diameter of which is greater than that of the
capillary blood-vessels would of course destroy life by stopping the
circulation; but the blood corpuscles of the mammalia are much smaller
than those of the bird; yet the pigeon is killed by a few drops of
mammiferous blood; and the blood of the fish is rapidly fatal to all
the mammalia as well as to birds.

960. It is manifest, both from observation and experiment, that
arterial blood is far more necessary to the support of the animal than
of the organic life. When in asphyxia the communication of atmospheric
air with the lungs is suspended, the functions of the brain are
abolished; sensibility and voluntary motion are lost the moment venous
blood circulates in the arteries of the brain. It has been shown (476),
that if this state continue, the animal life is destroyed in a minute
and a half; but that the organic life is not extinguished for many
minutes, and sometimes not even for several hours.

961. It sometimes happens that the communication between the pulmonary
artery and the aorta, and between the right and left auricle, which
naturally exist in the fœtus, is continued after birth. In persons
having this state of the circulation, called ceruleans, some portion
of venous blood is always mixed with arterial blood. In this case the
various processes of secretion and nutrition, the entire circle of
organic functions, are but little disturbed; while the animal functions
are deranged in a remarkable degree. The mind is weak and inactive,
and the muscular power is so feeble, that the least exertion produces
a sense of suffocation; and, if the muscular effort be continued,
occasions fainting, and even suspended animation.

962. But while venous blood is in no case capable of supporting
sensation and voluntary motion, there are decided cases in which
secretion is effected, at least in part, from venous blood, as the bile
from the venous blood that circulates through the liver in man and all
the mammalia, and the urine which is formed from venous blood in some
of the lower orders of animals.

963. The proper nutritive fluid of the human body is directly formed
from chyle, lymph, and venous blood; that is, partly from new matter
introduced into the system from the external world, and partly from
matter which has already formed a constituent part of the body.
The new matter, the white chyle, is prepared partly by the action
of the digestive fluids upon the food, and partly by the addition
to the digested food of highly animalized substances, endowed with
assimilative properties, by which the product is progressively
approximated to the chemical composition of the blood. The old matter
consists partly of the clear lymph, contained in the lymph vessels,
and derived from the interior of the organized parts, particles which
have already formed an integrant portion of the tissues and organs; and
partly of the dark venous blood, the residue of the proper nutritive
fluid, after the latter has yielded to the system the new matter
required by it, and has given off from the system its superfluous and
noxious particles.

964. In the duodenum and jejunum the new matter, the chyle, contains
albumen; but it is without coagulable fibrin: it acquires fibrin in the
lymph vessels on its way to the veins.

965. In the chyle globules appear; but the chyle corpuscles are white,
are without an external envelop, are comparatively few in number, are
somewhat more than half the size of the blood corpuscles, and, like the
nuclei of the latter, are insoluble in water.

966. The fatty or oleaginous matter contained in the chyle is in a free
state, not intimately combined.

967. The chyle is alkaline, but is much less alkaline than the blood;
and the iron contained in the chyle is much less intimately combined
than it is in the blood.

968. Lymph contains in solution more animal matter than chyle, and the
white globules are more abundant in lymph. But though lymph contain
in solution more albumen and fibrin than chyle, it is not so richly
loaded with these substances as blood. Still, however, the solution of
albumen and fibrin in lymph approximates lymph so closely to the blood,
that the lymph very much resembles the clear liquor sanguinis of which
the blood consists when the red particles are abstracted from it. The
colourless liquor sanguinis is the lymph of the blood. Lymph is blood
without red particles; and blood, lymph with red particles.

969. The chyle is transmitted into the lymph-vessels to mingle with the
lymph before it flows into the veins to mingle with the blood.

970. The commingled fluids, chyle and lymph, pass into the blood very
slowly, drop by drop. The regulation of the rapidity of the admixture
seems to be the chief office of the valve placed at the termination of
the thoracic duct. When the operation is observed in a living animal,
it is seen that this valve prevents the new matter from flowing into
the blood in a full stream. If in a dog of ordinary size that has
recently eaten as much animal food as it chose, the thoracic duct be
opened in the neck, the dog being alive, there will flow from the duct
about half an ounce of fluid in five minutes (831); yet when this fluid
reaches the termination of the duct only a few inches further on, it
flows into the vein only drop by drop, at considerable intervals. One
great object of pouring the chyle and lymph into the venous system so
close to the heart (fig. CLXXVIII.), and of causing the commingled
fluid to pass under the action of that powerful engine before it is
transmitted to the lungs, seems to be, by the agitation to which it
is subjected in the right auricle and ventricle to accomplish the
most perfect admixture possible between the particles of the chyle
and lymph and the red particles of the venous blood; an object which
would be counteracted by the too rapid entrance into the current of the
circulation of the new and as yet imperfectly assimilated matter.

971. After their due admixture by the powerful action of the engine
that works the circulation, the commingled fluids are transmitted by
the right heart to the lungs. There the watery portion of the chyle
and lymph is removed; the composition of the albumen and fibrin is
completed, these substances being changed from a weak and loose into
a strong and concentrated state; the solid particles are increased in
number, augmented in size, and changed from a white into a red colour;
carbon is given off; oxygen is absorbed; azote is alternately inhaled
and exhaled; and the ultimate result is, that the three fluids—chyle,
lymph, and venous blood—are converted into one homogeneous fluid,
arterial blood, the proper nutrient fluid.

972. The particles of the chyle and lymph, on mingling with the blood,
are scattered through the mass, and become invisible, being apparently
lost among the innumerable red corpuscles; but it is not probable that
the chyle is immediately converted into blood. If the coagulation
of the blood be retarded by the addition of a small portion of the
carbonate of potass, the red particles gradually sink some lines
below the level of the fluid; and the supernatent liquid is whitish,
evidently from the chylous globules mingled with the blood. In ordinary
coagulation, the chyle globules are included among the immense number
of the red particles of the coagulum, and are thus indistinguishable;
but there is reason to believe that the chyle is not converted into
blood under at least from ten to twelve hours; it is certain, that in
that space of time after the completion of digestion, the serum of
the blood is frequently seen to be milk-white, from the quantity of
unassimilated chyle still contained in it.

973. How the red colour of the blood is obtained, and whence the
capsules of the red particles are derived, if these bodies really
possess an external envelop, is wholly unknown. But it has been shown
(953 and 955) that in incubation the blood is formed from the substance
of the fluid yolk, without the action of any special organ; that at the
period when the blood is first generated, no such organs as appear to
influence the production of the blood in the adult are in existence;
it is, therefore, reasonable to infer that the formation of blood in
the adult may not be so dependent on the action of special organs as is
commonly supposed; and that the formation of blood from chyle, of blood
corpuscles from chyle corpuscles, may take place at all periods of life
under the influence of the same general vital conditions as it does in
the incubated egg.

974. What change the matter of the blood undergoes by respiration,
whether it acquire something without which it is incapable of
maintaining life, or part with something the presence of which is
incompatible with life, is equally unknown. We only know that the
blood, during respiration, changes its colour; but of the nature of
the change produced upon its substance we are wholly ignorant. In the
present state of our knowledge, the ultimate fact is, that without the
change wrought upon the blood by respiration, the blood is incapable
of maintaining life; in fact, no proper nutrient fluid is formed.

975. Once formed, the conservation of the proper proportions of the
composition of the blood is effected by the excretory processes
already described; by the removal of its superfluous water by the
lungs, skin, and kidneys; by the removal of its superfluous carbon,
azote, and oxygen by the lungs, liver, and kidneys; by the removal of
saline and mineral matters chiefly by the kidneys; and finally by the
instantaneous removal of products of decomposition formed in the course
of the organic actions, chiefly, it would appear, by the kidneys.

976. Once formed, and duly concentrated and purified, the blood is sent
out by the left heart to the system. Driven by the heart through the
main trunks and branches of the aorta, the blood ultimately reaches the
capillary arteries, which do not divide and subdivide indefinitely,
but ultimately reach a point beyond which they no longer diminish in
size. Not all of the same magnitude, some are large enough to admit of
three or four of the red particles of the blood abreast; the diameter
of others is only sufficient to admit of two or even of one; others
are capable of transmitting only the clear and transparent liquor
sanguinis; while in many cases the membranous tunics of the capillaries
wholly disappear; the blood no longer flows in actual vessels, but is
contained in the substance of the tissues in channels which it forms
in them for itself (304).

977. Under the microscope, says Müller, the blood corpuscles are
seen distinctly pouring from the smallest ramifying arteries into
vessels which grow no smaller. After leaving these, they again
assemble in the origins of veins formed in collected branches. The
blood corpuscles flow in the finest capillaries, one after another,
and often interruptedly. They are colourless when they flow singly;
accumulated more thickly, they appear yellow, and in still greater
quantity, yellowish red or red. In animals that have lost their
strength, the globules flow without stoppage: when the animal is weak
and the motion is retarded, the globules move by starts; they move
on, but go more rapidly by fits. In a still weaker animal they only
advance during the impulse of the heart, and then fall back a little.
When several arterial currents unite in an anastomosis, one current
always predominates and traverses the anastomosis alone, to mingle its
blood in the other currents. Thus the currents meet and divide in the
reticulate capillaries till all are collected again in veins. Sometimes
the direction of the current changes, when another current becomes
stronger, and the previous leader weaker, according to the pressure
exerted on the part.

978. While the blood is thus traversing the capillaries, its colour
changes from a bright scarlet to a dark red. This change in the colour
of the blood is the certain sign that particles have been abstracted
from the circulating mass, and have been applied to the formation
and support of the fluid and solid parts through which the stream is
flowing. Some physiologists have satisfied themselves that they have
seen the actual escape of particles from the circulating current; that
they have witnessed the immediate combination of those particles with
the substance of the tissues, and even that they have beheld other
particles quitting the tissues and mingling with the flowing blood.
Other physiologists doubt whether the most patient observation, aided
by the most skilful management of the best glasses, can ever have
rendered such phenomena matters of sense. “I imagined,” says Müller,
“at an early period, that I had seen something like this in the setting
circulation; but by prolonging the observation I saw the globules move
on if the current continued.”

979. But whether the human eye have ever actually seen or not an
interchange of particles between the blood and the tissues, it is
absolutely certain that such an interchange does take place. For,—

1. Indubitable evidence has been stated (786, _et seq._) of continual
absorption from all parts of the body, yet there is no loss of
substance; there must therefore of necessity be a proportionate
deposition.

2. Equal evidence has been adduced (688) that constant additions are
made to the blood through the organs of digestion, yet the quantity
of the blood in the body does not progressively and permanently
increase; it follows that a quantity must be abstracted from the blood
proportionate to the quantity added to it.

3. The human germ, from a scarcely visible point, by the successive
additions of new matter progressively acquires the bulk of the adult
man.

4. Organs whose special office it is to abstract particles from the
blood for the elaboration of specific secretions consist almost
entirely of congeries of blood-vessels. The agents are multiplied in
proportion to the extent of the labour assigned them.

5. Growth, which is merely excess of deposition above absorption, is
active in proportion to the quantity of blood which circulates through
the growing part in a given time. The blood-vessels of a growing part
increase in number and augment in size is proportion to the rapidity of
the growth. In morbid growth, it is sometimes sufficient to stop the
process merely to tie the main trunks of the arteries distributed to
the part.

980. By every organ and every tissue; by the membrane, the muscle, the
bone; by the brain, the heart, the liver, the lungs, particles are
abstracted from the countless streams that bathe them, or that flow
through them. In every case in which particles are thus abstracted by a
tissue the following phenomena take place:—

1. Only those constituents of the blood are abstracted by the tissue
which are of the same chemical nature as its own.

2. The constituents of the blood abstracted by a tissue, identical in
chemical composition with its own, are immediately incorporated into
its substance.

3. The constituents of the blood abstracted by a tissue, as they are
incorporated into its substance, are not disposed fortuitously, but are
arranged according to the specific organization of the tissue, and thus
receive its own peculiar structure.

4. The constituents of the blood which thus receive the peculiar
organization and structure of the tissue by which they are
appropriated, acquire all its peculiar vital endowments.

981. It is manifest, then, that the tissues assimilate the blood just
as the digestive fluids assimilate the aliment. And this is nutrition,
the assimilation of the blood by the tissues and organs. Digestion is
the conversion of the food into blood; nutrition is the conversion of
blood into living fluids and solids.

982. For the reasons assigned (757 and 758), it is probable that
the living fluids and solids, formed from the blood by the act of
nutrition, are not generated at the parts of the body where they
appear, but that, pre-existing in the blood, they are merely evolved
at those parts. Hence the variety and complexity of the processes for
the elaboration of the blood which have been described, and all of
which appear to be indispensable to bring the blood to a proper state
of purity and strength. The great effort of the system is put forth
in effecting the constitution of the blood. When the blood is once
formed, all the rest of the work appears to be easy; because, before it
reaches any part of the organization which it is destined to support,
the blood is already adapted, mechanically, chemically, and vitally, to
afford that support. Still since there are cases, as in the production
of gelatin, in which the substance does not appear to be pre-existent
in the blood, we are under the necessity of supposing that a material
change is effected in the constituents of the vital fluid at the time
and place of their escape from the circulation.

983. How the constituents of the blood escape from the circulation and
incorporate themselves with the substance of the tissues there can
be no difficulty in conceiving, wherever the capillaries terminate
in membraneless canals, channels worked out for the reception of the
nutrient stream by the force of the current itself; and in every case
in which the capillaries, retaining their membranous tunics, remain
true and proper vessels, their contents escape through their delicate
walls by the process of endosmose (803), for which their structure
appears to be admirably adapted.

984. But in the capillary vessels there exists only blood. Universally
and invariably before the blood passes from under the influence of the
capillary vessels it has ceased to be blood. Arterial blood is conveyed
by the carotid artery to the brain; but the cerebral arteries do not
deposit blood, but brain. Arterial blood is conveyed by the capillary
arteries to bone; but the osseous capillaries do not deposit blood, but
bone. Arterial blood is conveyed by the muscular arteries to muscle,
but the muscular capillaries do not deposit blood but muscle. The blood
conveyed by the capillaries of brain, bone, and muscle is the same;
all comes alike from the systemic heart, and is alike conveyed to all
tissues; yet in the one it becomes brain, in the other bone, and in the
third muscle. Out of one and the same fluid are manufactured cuticle,
and membrane, and muscle, and brain, and bone; the tears, the wax, the
fat, the saliva, the gastric juice, the milk, the bile, all the fluids,
and all the solids of the body (310).

985. These phenomena are wholly inexplicable on any known mechanical
principles. It is equally impossible to refer them to mere chemical
agency, or to any properties of dead matter. We are therefore under
the necessity of referring them to a principle which, for the sake of
distinguishing it from anything mechanical or chemical, we term vital.
As the actions which take place between the integrant particles of
bodies, giving rise to chemical phenomena, are referred to one general
principle, termed chemical affinity, so the actions which take place in
living bodies, giving rise to vital phenomena, may be referred to one
general principal, termed vital affinity. The term explains nothing,
it is true, it merely expresses the general fact; but still it is
convenient to have a term for the expression of the fact. The property
itself will ever remain an ultimate fact in physiology, however exactly
the limits of its agency, and the laws according to which it modifies
the mechanical and chemical relations of the substances subjected to
its influence, may hereafter be ascertained; just as chemical affinity
will ever be an ultimate fact in physics, whatever discoveries may yet
be made of the extent of its agency and of the conditions on which its
action depends.

986. It is then an ascertained fact, that there exists between the
blood and the tissues a mutual reaction, not of a physical, but of a
vital nature, in which the blood takes as active a part as the tissue,
and the tissue as the blood; the blood exerting a vital attraction on
the tissue, and the tissue on the blood. We only express this ultimate
fact when we say (and this is all we can do) that in every part of the
body, by virtue of a vital affinity, the tissue attracts from the blood
the molecules of matter appropriate to its chemical composition, and
the blood attracts from the tissue the particles which, having served
their purpose there, are destined to other uses in the economy; or, if
wholly useless, are absorbed into the current of the circulation to be
expelled from the system.

987. We can see how the particles of matter which are attracted by the
tissue from the blood are so deposited and disposed that the tissue
always preserves its own shape, bulk, and relation to the surrounding
tissues. This definite arrangement is the result of an action which has
been already stated to be proper to the absorbent vessels. Previously
to the deposition of a new particle of matter by a capillary, an old
particle is removed by an absorbent, either a lymphatic or a vein. In
removing the old matter, the absorbent forms a mould into which the
capillary deposits the new molecules; and the form of every tissue and
organ depends on the kind of mould formed for the reception of its
nutrient matter by the absorbent vessel. The absorbents are thus the
architects of the system; and the capillaries are both chemists which
form the rough material employed in the structure, and masons which
deposit and arrange it. The conjoint action of both sets of vessels is
necessary to the formation of the simplest tissue; and it is by their
united labour that the compound organs are built up out of the simple
tissues.

988. It is conjectured that the immediate living agents by which this
vital attraction is exerted between the blood and the tissues are
the organic nerves. These nerves consist of two sets, those which
enter as constituents into the tissues and those which accompany the
capillaries. It has been shown (304), that while the membranous tunics
of the capillaries diminish, the nervous filaments distributed to them
increase; that the smaller and thinner the capillaries the greater
the proportionate quantity of their nervous matter; and that this is
most remarkably the case in organs of the greatest irritability. It is
conceived that the capillaries, in consequence of the nervous structure
which thus envelops them, exert upon the fluid which is flowing through
them an influence perfectly analogous to that of the secreting organ,
in consequence of which similar particles are abstracted from the blood
as those which compose the tissue in which the operation takes place.

989. It is further conjectured that the physical agent by which this
action upon the blood is effected is the galvanic fluid. Dutrochet
believes that he has actually formed muscular fibre from albumen by
galvanism. He considers the red particles of the blood as pairs of
electrical plates, and thinks that the nucleus is electronegative,
and the capsule electropositive. Müller has repeated and critically
examined the interesting experiments of Dutrochet; and while he arrives
in many essential points at different results, expresses the highest
admiration of the ingenious manner in which this philosopher has sought
to solve a great problem. “If,” says Müller, “a drop of an aqueous
solution of the yolk of egg (in which very small microscopic globules
are suspended) be galvanised, the currents discovered by Dutrochet will
be observed. The wave, proceeding from the copper or negative pole, in
which the alkali of the decomposed salt accumulates, is transparent,
from the solution of albumen by the alkali. The wave, proceeding from
the positive or zinc pole, particularly in its circumference, is
opaque, and white from the acid it contains. Both waves encounter,
and exactly in the line of contact a linear coagulum is immediately
produced, which assumes the form of the line of contact, and is curled
at times as the edges of the waves are meeting. The meeting of both
waves takes place with a lively motion, in the line of contact, when
the deposition of coagulum takes place; but as soon as the deposition
of coagulum has occurred, all is tranquil, and not the least trace of
motion is observed. It is therefore inconceivable how an observer of
the first rank, like Dutrochet, can pronounce this coagulated albumen
contractile muscular fibre, generated by galvanism; it is nothing but
coagulated albumen. This coagulum, besides, like the albumen which
is deposited by galvanism round the zinc pole, has no consistence,
but is composed of globules easily separated by stirring, and only
precipitated in the line where the two waves meet without cohesion.”

990. But though science has not yet succeeded in ascertaining with
certainty the physical agency to which the ultimate changes that
take place in organized matter are to be referred, there cannot be
a question that they are dependent on physical agents; and the
legitimate object of scientific inquiry is to discover what those
agents are, and to ascertain the modifications they undergo by those
vital affinities to the influence of which they are subjected.

991. The discoveries which science has already made relative to the
influence of certain physical agents on particular organs, and to the
influence of the whole circle of physical agents on the whole living
economy, have added not a little to human power over human health
and disease. But these agents also exert an influence scarcely less
momentous on the entire apparatus and action of the animal life, so
inseparably linked with the organic. An account will therefore be
next given of the structure and function of the nervous and muscular
systems. The exposition of these systems, which will be as brief as
possible, will be followed by a full account of the action of physical
agents on the whole of this complex and wonderful organization. The
detail of the ascertained phenomena will have a strict reference to
the development of the physical and mental powers of the human being,
and thereby a close and practical application will be attempted of
physiology to the production and preservation of health.


THE END.