Produced by R. L. Garnett





POPULAR LECTURES
ON
ZOONOMIA,
OR
THE LAWS OF ANIMAL LIFE,
IN
HEALTH AND DISEASE.

BY THOMAS GARNETT, M.D.

MEMBER OF THE ROYAL COLLEGE OF PHYSICIANS, LONDON; OF THE ROYAL
IRISH ACADEMY; OF THE ROYAL MEDICAL SOCIETY OF EDINBURGH; HONORARY
MEMBER OF THE BOARD OF AGRICULTURE; FELLOW OF THE LINNEAN SOCIETY;
MEMBER OF THE MEDICAL SOCIETY, LONDON; AND OF THE LITERARY AND
PHILOSOPHICAL SOCIETY OF MANCHESTER: &c. &c.
FORMERLY PROFESSOR OF NATURAL PHILOSOPHY AND CHEMISTRY IN THE
ROYAL INSTITUTION OF GREAT BRITAIN.

LONDON:
FROM THE PRESS OF THE ROYAL INSTITUTION OF GREAT BRITAIN:
W. SAVAGE, PRINTER.
PUBLISHED FOR THE BENEFIT OF THE AUTHOR'S CHILDREN BY HIS EXECUTORS.
TO BE HAD OF MR. NICHOLSON, SOHO SQUARE, MR. PRICE,
WESTMINSTER LIBRARY, JERMYN STREET,
AND OF ALL THE BOOKSELLERS.
1804.

[FRONTISPIECE PORTRAIT]

THOMAS GARNETT. M.D.

L. R. Smith, del.
Lenney, sculpt.

Published Jan. 1, 1805, by the Executors, for the benefit of his
orphan children.

ENTERED AT STATIONERS HALL.

TO THE RIGHT HONOURABLE, AND HONOURABLE,
THE MANAGERS OF THE ROYAL INSTITUTION OF GREAT
BRITAIN,
THESE LECTURES,
COMPOSED BY A MAN, WHO, IN HIS LIFE TIME, WAS
HONOURED BY THEIR SELECTION,
AS THEIR FIRST LECTURER;
AND WHOSE INFANT FAMILY HAVE SINCE EXPERIENCED
THEIR BENEVOLENCE AND PROTECTION,
ARE, WITH PERMISSION, DEDICATED,
BY THE TRUSTEES OF THE SUBSCRIPTION,
IN FAVOUR OF THOSE ORPHANS.
CONTENTS.

THE AUTHOR'S LIFE.

His early amusements. His apprenticeship to Mr. Dawson. His studies
at Edinburgh. In London. His establishment at Bradford. At
Knaresborough. At Harrowgate. His marriage. His lectures at
Liverpool. At Manchester. At Warrington. At Lancaster. At Glasgow.
His tour in the Highlands. The death of his wife. His engagement in
the Royal Institution. His resignation. His establishment in
Marlborough Street. His appointment as physician to the Mary-le-bonne
Dispensary. His death.

LECTURE I, INTRODUCTION.

Difficulties and advantages of a popular course of lectures. General
view of the human frame. Bones. Muscles. Joints. Powers of the
muscles. Brain and Nerves. Senses. Hypothesis of sensation.
Galvanism. Distribution of the subjects of the course.

LECTURE II, ON RESPIRATION.

Air. Trachea. Thorax. Animal heat. Its uniformity. Chemical
properties of the air. Combustion. Effects of cold.

LECTURE III, ON THE CIRCULATION OF THE BLOOD.

Respiration partially voluntary. Heart. Circulation. Pulsation.
Hepatic vessels. Action of the arteries. Causes propelling the blood.
Varieties of the pulse. Changes of the blood. Harvey's merits.

LECTURE IV, ON DIGESTION AND NUTRITION.

Necessity of food. Structure of the viscera. Bile. Food of man.
Gastric juice. Absorption. Assimilation. Lymphatics. Diseases
affecting digestion. Advantages of temperance and exercise.

LECTURE V, OF THE SENSES IN GENERAL.

Sensation. Attention. Internal senses. Habit. Touch. Skin. Pain.

LECTURE VI, ON TASTE AND SMELL.

Tongue. Kinds of taste. Diseases of taste. Smell. Mucous membrane.
Odours. Smell in animals. Diseases of smell.

LECTURE VII, ON SOUND AND HEARING.

Production of sound. Medium. Ear. Hearing. Pendulums. Chords. Wind
instruments. Tones. Velocity of sound. Music. Echo. Deafness.

LECTURE VIII, ON VISION.

The eye. Figure. Light. Vision. Accommodation to different distances.
Seat of vision. Erect vision. Single vision. Squinting.

LECTURE IX, ON THE LAWS OF ANIMAL LIFE.

Action of external objects. Excitability. Its laws. Action of light.
Of Heat. Of food. Sound. Odours.

LECTURE X, ON THE LAWS OF ANIMAL LIFE.

General laws. Sleep. Degrees of excitability. Health. Comparison with
a furnace. Oxidation. Electricity. Hydrogen. Theory of muscular
contraction.

LECTURE XI, OF THE NATURE AND CAUSES OF DISEASES.

Brown's theory. Sthenic and asthenic diseases. Debility. Sthenic
depression of spirits. Scale of excitability. Fallacy of symptoms
Effects of cold. Alcohol. Sthenic diseases.

LECTURE XII, ON INFLAMMATION AND ASTHENIC DISEASES.

Nature of inflammation. Distention of the arteries. Cure of
ophthalmias. Asthenic diseases. Cold. Intemperance. Mental exertions.
Classes of diseases. Cure. Oxidation.

LECTURE XIII, ON THE GOUT.

Effects of the gout. Gout not hereditary. Symptoms. Causes.
Affections of the stomach. Cure. Use of electricity. Diet.

LECTURE XIV, ON NERVOUS COMPLAINTS.

Predisposition. Classes. Sthenic kinds. Case of the author. Bad
effects of wine. Asthenic kinds. Passions. Direct debility.
Treatment. Torpor. Remedies. Exercise and temperance. Conclusion.

AN ACCOUNT
OF
THE LIFE OF THE AUTHOR.

DR. GARNETT was born at Casterton, near Kirkby Lonsdale,
Westmoreland, on the 21st of April, 1766. During the first fifteen
years of his life, he remained with his parents, and was instructed
by them in the precepts of the established church of England, from
which he drew that scheme of virtue, by which every action of his
future life was to be governed. The only school education he received
during these early years, was at Barbon, a small village near his
native place, to which his father had removed the year after he was
born. The school was of so little consequence, that its master
changed not less than three times during the space of seven or eight
years, and the whole instruction he received, was comprehended in the
rudiments of the English grammar, a small portion of Latin, and a
little French, together with the general principles of arithmetic.
His bodily constitution was from the beginning weak and susceptible;
he was unequal to joining in the boisterous amusements of his
companions, while from the liveliness of his disposition he could not
remain a moment idle. To these circumstances we are, perhaps, to
attribute the uncommon progress he made in every branch of knowledge
to which he afterwards applied himself.

Whilst a schoolboy, the susceptibility of his mind, and a diffidence
of character connected with it, caused him to associate very little
with his schoolfellows: he dreaded the displeasure of his preceptor,
as the greatest misfortune which could befal him The moment he
arrived at home, he set about preparing his lesson for the next day;
and as soon as this was accomplished, he amused himself by contriving
small pieces of mechanism, which he exhibited with conscious
satisfaction to his friends. His temper was warm and enthusiastic;
whatever came within the narrow circle of his early knowledge he
would attempt to imitate. He saw no difficulties before hand, nor was
he discouraged when he met with them. At the early age of eleven
years, he had somewhere seen a dial and a quadrant, and was able to
imitate these instruments, nay, with the assistance of the latter,
and the small knowledge of arithmetic and trigonometry, which he had
then obtained, he formally marched out with his younger brother, and
rudely attempted to measure the height of a mountain behind his
father's house. When he was nearly fifteen years of age, he was, at
his earnest desire, put apprentice to the celebrated mathematician,
Mr. Dawson, of Sedbergh, who was at that time a surgeon and
apothecary. This situation was peculiarly advantageous to him, on
account of the great mathematical knowledge of his master, by whom he
was instructed in the different branches of this science; and,
notwithstanding his constant employment in necessary business, his
ardent pursuit of professional information, and his extreme youth, in
the course of four years, he became well acquainted with mechanics,
hydrostatics, optics, and astronomy. He afterwards applied himself
with energy to the study of chemistry, and other subjects, with which
it was thought expedient that he should be acquainted, previously to
attending the medical lectures in the University of Edinburgh.
Strongly impressed with a sense of the value of time, he was
indefatigable in the pursuit of knowledge: by a concurrence of
fortunate circumstances, his talents had become so flexible, that he
succeeded almost equally well in every subject to which he applied
himself; but of chemistry he was particularly fond, and from this
time it became his favourite study.

During the four years of his apprenticeship, his conduct was in every
respect highly commendable; he was assiduous, he was virtuous. His
pursuit after general knowledge was restrained to one object only at
a time; he had advanced far in the abstruse sciences; his inclination
for study was increased: when in the year 1785, he went to Edinburgh
with a degree of scientific knowledge, seldom attained by young men
beginning the study of medicine. He became a member of the Medical
and Physical Societies, where he soon made himself conspicuous, and
of the latter of which, he was afterwards president.

Well acquainted with the first principles of natural philosophy, he
had considerable advantages over his contemporaries; and his
superiority was soon acknowledged. He was not, however, on this
account inclined to remit his industry; he attended the lectures of
the ablest professors of the day, and more particularly those of Dr.
Black, with the most scrupulous punctuality, and endeavoured to
elucidate his subject by every collateral information he could
obtain. He avoided almost all society; and it is said, he never
allowed himself, at this time, more than four hours sleep out of the
twenty four. The famous Dr. Brown was then delivering lectures on his
new theory of medicine. Dr. Garnett, fired with the enthusiasm of
this noted teacher, and struck with the conformity of his theory to
the general laws of nature, became one of the most zealous advocates
of his doctrine; and from this period, he took, during the remainder
of his life, every opportunity of supporting it.

During two summers he returned to Mr. Dawson at Sedbergh, passing the
intervening winters in Edinburgh: about this time he wrote the essay,
which, in the year 1797, he published under the title of a Lecture on
Health, which very neatly and perspicuously explains the fundamental
parts of the Brunonian theory of medicine: in September 1788, he
published his inaugural dissertation de Visu, and obtained the degree
of M.D. Very soon afterwards he went to London, to pursue his
professional studies, which he continued to do with the greatest
perseverance: he attended with unceasing diligence the lectures of
the most eminent lecturers, and he sought practical knowledge in the
chief hospitals of the metropolis with the most ardent zeal; so that
whilst he gained information to himself, he set an impressive example
to his contemporary medical students, who in the delusive pursuits of
a great city, are too apt to neglect the objects their parents had in
view in sending them to the capital. Having finished his studies in
London, Dr. Garnett, in 1789, returned to his parents. At the time he
left London, he had lost none of his ardour; still he continued
indefatigable and observant. He had been flattered and respected by
his fellow students, and praised by his seniors; and his previous
success animated him with the strongest expectation of future
advancement. At this time, it is supposed, he wrote the justly
admired Treatise on Optics, which is in the Encyclopaedia Britannica.
Soon after his establishment as a physician, at Bradford, in
Yorkshire, which took place in the year 1790, he began to give
private lectures on philosophy and chemistry. He wrote his treatise
on the Horley Green Spa; and in a short time, gained a deserved
character of ingenuity and skill as a chemist, a physician, and a
benevolent member of society. Bradford did not afford scope for his
practice as a physician, equal to the sanguine expectations he had
formed; and he was induced to change his situation.

In the year 1791, therefore, he removed to Knaresborough, intending
to reside at that place during the winter, and at Harrowgate during
the summer. This plan he put in execution till the year 1794; his
reputation rapidly increased, and his future prospects appeared
cheering and bright. He continued to apply himself very closely to
chemistry, which was now decidedly his most pleasant and interesting
study. He endeavoured to apply his various knowledge to practical
purposes, and in many instances was peculiarly successful. No sooner
had he arrived at Knaresborough, than anxious to investigate every
thing in the neighbourhood, which could at all affect the health of
the inhabitants, he began to analyse the Crescent Water at
Harrowgate; which he did, with all the accuracy a subject so
difficult could admit of; and in 1791, he published his treatise upon
it. The same spirit led him, in 1792, to analyse the other mineral
waters at the same place of fashionable and general resort, the
detail of which he published in the same year. These publications
became generally read, and gained him a very extensive reputation.
The late Dr. Withering, whose knowledge on these subjects could not
be disputed, before he had seen his general analysis of the
Harrowgate Waters, said, that "excepting only the few examples given
us by Bergman, the analysis of the Crescent Waters was one of the
neatest and most satisfactory accounts he had ever read of any
mineral water." But his exertions were not confined to professional
and scientific pursuits; laudably desirous of advancing knowledge
amongst every branch of the community, he formed the plan of a
subscription library, which has, since 1791, been of great
convenience and utility to the inhabitants of Knaresborough. Far from
joining in the opinion which has so much prevailed in modern times,
that it was sufficient to aim at general utility, he lost no
opportunity of doing good to every member of society. He greatly
promoted and encouraged the making of the pleasure grounds and
building on the rock, called Fort Montague; and he instructed and
assisted the poor man, who is called the Governor, to institute a
bank, and to print and issue small bills of the value of a few
halfpence, in imitation of the notes of the country bankers, but
drawn and signed with a reference of humour to the fort, the flag,
the hill, and the cannon. These notes, the nobility and gentry, who
during the Harrowgate season crowd to visit this remarkable place,
take in exchange for their silver, and by these means the governor,
who is a man of gentle and inoffensive manners, has been enabled,
with the assistance of his loom, to support himself and a numerous
family, and to ameliorate their condition, by giving education to his
children.

No station in life escaped his benevolent attentions. In order to
benefit John Metcalf, who is perhaps more generally known by the name
of Blind Jack of Knaresborough, he assisted him to publish an account
of the very singular and remarkable occurrences of his life, during a
long series of years, under the heavy affliction of total blindness;
by the sale of which, this venerable old man derived a considerable
contribution towards his subsistence.

Whilst at Harrowgate, Dr. Garnett obtained the patronage and
protection of the Earl of Rosslyn, then Lord Loughborough, who in the
year 1794 built a house for him, which for the future Dr. Garnett
meant should be his only residence; it was not long however before he
discovered that his situation at Harrowgate was but ill calculated to
forward his liberal and extended views. At this place he had small
opportunities of attaching himself to his favourite sciences; in the
winter months he was without literary society, and it was not for his
ardent spirit to remain inactive. About this time also, he formed the
idea of going to America, where he thought he might live both
honourably and profitably as a teacher of chemistry and natural
philosophy. All these circumstances were floating in his mind, when
in the year 1794, about the end of July, at the instance of a medical
friend, who resided in London, he received as boarders into his
house, which was kept by his sister, Miss Catharine Grace Cleveland,
daughter of the late Mr. Cleveland, of Salisbury Square, Fleet
Street, who was recommended to the use of the Harrowgate waters,
together with her friend Miss Worboys. To all who were acquainted
with the prepossessing exterior of Dr. Garnett, the liveliness of his
conversation, the urbanity of his manners, and his general desire of
communicating knowledge to whomever he saw desirous of gaining
information, it will be no surprise, that a mutual attachment grew up
between him and his inmate, Miss Cleveland, a young lady possessing,
in all respects, a mind similar to his own, and who must have felt a
natural gratification in the zeal with which the company of the
person, on whom she had placed her affections, was sought by all
ranks resorting to this fashionable watering place, where every one
thought himself most fortunate who sat nearest to him at the table,
and where he enlivened the circle around him with his conversation,
which was not only instructive, but playfully gay, and entertaining,
ever striving to amuse, and always successful in his attempts. The
Doctor now began to project plans of happiness, which he had only
before held in idea. Previous to his visitors leaving Harrowgate,
which was towards the latter end of December, he communicated to Miss
Cleveland his intention of going to America. At first she hesitated
about accompanying him; but finding his resolution fixed, she at
length consented. From this time, till the beginning of March 1795,
he continued deliberating upon and maturing his plan. He now departed
from Harrowgate, and followed the object of his affection to her
mother's residence at Hare Hatch, Berks. He was married to her on the
16th of March, and a fortnight afterwards returned to Harrowgate, to
dispose of the lease of his house, and his furniture. Having again
joined his wife, he then went to London, where he purchased apparatus
for his lectures, and after visiting his parents, he proceeded to
Liverpool, in order to obtain a passage to America.

Whilst he was thus waiting for the opportunity of a vessel to
transport him across the Atlantic, he was solicited by the medical
gentlemen at Liverpool, to unpack his apparatus, and give a public
course of lectures on chemistry and experimental philosophy. At all
times desirous of diffusing the knowledge he had acquired, and eager
to fulfil the wishes of his friends, he complied with their request,
and entered upon a plan, which in the end completely overturned the
scheme he had for several months been contemplating with such ardent
hopes of happiness and prosperity. No sooner had he been prevailed
upon, than he set about getting every thing ready for his lectures,
and after a single week's preparation; he commenced his course. The
deep interest he took in his subject, the anxiety he showed to make
himself understood, and the enthusiastic hope he constantly expressed
of the advancement of science, had a remarkable effect upon his
audience; and his lectures were received with the most flattering
marks of attention, and excited the most general applause and
satisfaction. In a short time, he received a pressing invitation from
the most eminent characters at Manchester, to repeat his course in
that town. This invitation he accepted, and, encouraged by the
success he had just experienced, he postponed the idea of leaving his
country. He arrived at Manchester about the middle of January 1796,
and began his lectures on the 22nd of that month. Before his arrival,
not less than sixty subscribers had put down their names, the more
strongly to induce him to comply with their wishes, and many more had
promised to do it, as soon as his proposals were published.
Notwithstanding he was thus led to expect a large audience, and had
procured apartments, which he imagined would be sufficiently spacious
for their reception, he was obliged, for want of room, to change them
not less than three times during one course. With such success did
the career of his philosophical teaching begin, and with such extreme
attention and respect was he every where received, that he used
afterwards to mention this period, as not only the most profitable,
but the most happy of his life. On the 24th of February, his wife was
brought to bed of a daughter, the eldest of the two orphans who have
now to lament the death of so valuable a parent, to deplore the loss
of that independence which his exertions were certain to have raised
them, and to rely on a generous public for protection, in testimony
of the virtues and merit of their father.

After this time Dr. Garnett repeated nearly the same course of
lectures at Warrington and at Lancaster; to both which places he was
followed by the same success.

Whilst he was in this manner exerting himself for the general
diffusion of knowledge, his fame spread with the delight and
instruction he had every where communicated to his audience. The
inhabitants of Birmingham wished to have the advantage of his
lectures; and he also received a most pressing invitation from
Dublin, where a very large subscription had already been formed. It
was his intention to have accepted of the latter invitation, but
previous to his departure for Ireland (from whence he had even yet
some thoughts of emigrating to America) he was informed of the
vacancy of the professorship in Anderson's Institution, at Glasgow,
by his friend the late Dr. Easton of Manchester, who strongly urged
him to become a candidate. As this situation must inevitably destroy
all his future prospects, he for a long time hesitated; but Dr.
Easton having informed the Managers of the Institution, that there
was a possibility of their obtaining a professor, so eminently
qualified as Dr. Garnett, they, after making further inquiry
concerning him, offered it to him in so handsome a manner, that,
although the situation was by no means likely to be productive of so
much emolument as the plan of life he had lately been pursuing, he
yielded to their proposal, strengthened as it was by the earnest
solicitation of Mrs. Garnett, who felt considerable apprehension at
the thoughts of going to America, and consented to accept of the
professorship.

He began his lectures at Glasgow in November 1796, and a short
account of them may be found in his Tour to the Highlands, vol. ii.
p. 196. The peculiar clearness with which he was wont to explain the
most difficult parts of science, together with the simplicity of the
terms he employed, rendered his lectures particularly acceptable to
those who had not been initiated in the technical terms, generally
used on such occasions. Every thing he delivered might easily be
understood by those who had not previously attended to the subject;
and of consequence, all who had been disgusted, or frightened by the
difficulties they had before met with, or imagined, were eager to
receive his instructions; and the audience he obtained, was much more
numerous, than either the trustees, or himself, had deemed probable.

When the session was completed, he repaired to Liverpool for the
purpose of fulfilling a promise he had formerly given to his friends,
to repeat his course of lectures in that town. Mrs. Garnett, in the
mean time, remained at Kirkby Lonsdale, where he joined her as soon
as his lectures were finished. He spent the latter part of the summer
chiefly in botanical pursuits, and returned to Glasgow in the autumn,
when he made known his intention of practising as a physician.
Fortune continued to favour him, his reputation increased, and he
rapidly advanced towards the first professional situation in Glasgow.

In July 1798, he began his Tour to the Highlands, an account of which
he published in 1800, and having returned to his duties in the
Institution, the success of his lectures suffered no interruption,
but whilst he was reaping the benefit due to his industry and his
talents, his happiness received a blow, which was irrecoverable, by
the loss of his wife, who died in child birth, December the 25th
1798: the infant was preserved. The sentiments of Dr. Garnett on this
occasion will be best expressed in his own words, in a letter to Mr.
Ort, of Bury in Lancashire.

"Glasgow, January 1st. 1799.

"Oh my dear cousin, little did I expect that I should begin the new
year with telling you that I am now deprived of all earthly comforts;
yes, the dear companion of my studies, the friend of my heart, the
partner of my bosom, is now a piece of cold clay. The senseless earth
is closed on that form which was so lately animated by every virtue;
and whose only wish was to make me happy.

"Is there any thing, which can now afford me any consolation? Yes,
she is not lost, but gone before: but still it is hard to have all
our schemes of happiness wrecked: when our bark was within sight of
port, when we were promising ourselves more than common felicity, it
struck upon a rock: my only treasure went to the bottom, and I am
cast ashore, friendless, and deprived of every comfort. My poor, dear
love had been as well as usual during the two or three last months,
and even on the dreadful evening (christmas eve) she spoke with
pleasure of the approaching event. My spirits were elevated to so
uncommon a pitch, by the birth of a lovely daughter, that they were
by no means prepared for the succeeding scene; and they have been so
overwhelmed, that I sometimes hope it may be a dream, out of which I
wish to awake. The little infant is well, and I have called it
Catharine, a name which must ever be dear to me, and which I wish to
be able to apply to some object whom I love; for though it caused the
death of my hopes, it is dear to me, as being the last precious relic
of her, whom every body who knew her esteemed, and I loved. I must
now bid adieu to every comfort, and live only for the sweet babes.
Oh! 'tis hard, very hard.
"THOMAS GARNETT."
"To Mr. Ort, Bury,
Lancashire.

The affliction Dr. Garnett experienced on the death of his wife, was
never recovered. On all occasions of anxiety which were multiplied
upon him, by reason of his exquisite sensibility, he longed for the
consolation her society used to afford him; and although his
susceptibility to the action of external causes, would not allow him
to remain in continued and unalterable gloom and melancholy, yet in
solitude, and on the slightest accident, his distress returned, and
he despaired of the possibility of ever retrieving his lost
happiness. Had it not been for his philosophical pursuits, and the
duties of his extensive practice, which kept him almost constantly
engaged, it may be doubted, whether he could at this time have
sustained the load of sorrow with which he was oppressed.

The circumstances which remain to be mentioned are few. From the
death of his wife, Dr. Garnett may be considered as unfortunate; for
although a fair prospect opened before him, a series of occurrences
took place, which neither his state of mind, nor his constitutional
firmness enabled him to support.

At the time of the formation of the Royal Institution of Great
Britain, in London, Count Rumford wrote to Dr. Garnett, to whom he
was then an entire stranger, inquiring into the nature and economy of
Anderson's Institution, Glasgow; the plan of the lectures given, &c.
&c.; and after hinting at the opportunities of acquiring reputation
in London, he finally proposed that Dr. Garnett should become
lecturer of the new Institution. With this proposal, arduous as was
the task, to deliver a course of lectures on almost every branch of
human attainment, Dr. Garnett complied, relying on his acquirements,
and the tried excellence of his nature; and conscious that no
difficulty could resist the indefatigable exertions which on other
occasions he had so successfully applied. Flattered by the honour and
respect he conceived to be paid to his abilities and qualifications;
pleased with the prospect of more rapidly accumulating an
independence for himself and his children; and animated with the hope
of meeting with more frequent opportunities of gratifying his thirst
after knowledge, his spirits were again roused, and he looked forward
to new objects of interest in the advancement of his favourite
pursuits. In the enthusiasm of the moment, he was known to say, that
he considered his connexion with the Royal Institution, from which
the country had a right to expect so much, as one of the most
fortunate occurrences of his life. On the 15th October 1799, he
informed a special meeting of the Managers of Anderson's Institution,
of his appointment to the Professorship of Philosophy, Chemistry, and
Mechanics, in the Royal Institution of Great Britain, and on that
account requested permission to resign his situation. The resignation
of a man, whom all loved and revered, was reluctantly, though, as
tending to his personal advancement, and the promotion of science,
unanimously accepted by the meeting; he was congratulated on his new
appointment, and thanked for the unremitting attention he had paid to
the interests of Anderson's Institution, ever since he had been
connected with it. As an instance of the high esteem in which he was
held by the trustees, it may be observed, that his successor, Dr.
Birkbeck, was elected by a very great majority of votes, principally
on account of his recommendation. In November, he pursued his journey
to London, leaving his children at Kirkby Lonsdale, under the care of
Miss Worboys. This lady, whose friendship for Mrs. Garnett had
induced her to become almost her constant companion, and had even
determined her to go with her friend to America, if the Doctor had
put his intentions in execution; soon after the death of Mrs.
Garnett, had pledged herself, never to desert the children, so long
as she could be of any use to them. How faithfully she observes this
obligation, all who know her must acknowledge; nor can we, without
increased anxiety, reflect upon the situation the poor orphans must
have been in without her protection.

Dr. Garnett was received by the Managers of the Royal Institution
with attention, civility, and respect. During the winter, the lecture
room was crowded with persons of the first distinction and fashion,
as well as by those who had individually contributed much to the
promotion of science; and although the northern accent, which Dr.
Garnett still retained in a slight degree, rendered his voice
somewhat inharmonious to an audience in London, his modest and
unaffected manner of delivering his opinions, his familiar, and at
the same time elegant language, rendered him the object of almost
universal kindness and approbation.

The exertions of the winter had in some measure injured his health,
and a degree of uncertainty that he saw in his prospects, tended
greatly to depress his spirits. He determined, however, to keep his
situation at the Institution, in order that he might at a more
convenient time be justified to himself in resigning it. In the
summer of 1800, he visited his children in Westmoreland; but his
anxiety of mind was not diminished, nor consequently his health
improved, by this relaxation from active employment. He walked over
the same ground, and viewed the same prospects that he had formerly
enjoyed in the company of his wife. He had not resolution to check
the impressions as they arose; and thus, instead of being solaced by
the beauties which surrounded him, he gave the reins to his
melancholy fancy, which, unchecked by any other remembrance, dwelt
only on the affection and the virtues of her, whose loss he had ever
to deplore; the want of whose society he imagined to be the chief
source of his misery. Towards the end of autumn, he returned to the
Institution, and in the winter, recommenced his duties as professor.
The effect produced upon his lecturing by these and other irritating
circumstances was remarkable. Debility of body, as well as uneasiness
of mind, incapacitated him for that ardent and energetic pursuit of
knowledge, by which he had been so eminently distinguished. His
spirited, and at the same time modest method of delivery was changed
into one languid and hesitating, that, during this period, occasioned
an erroneous judgment to be formed of his abilities as a man of
science, and a teacher, by such of his audience as were unacquainted
with the cause, or the intrinsic value and merit of the man. At the
close of the season, his determination of retiring from the
Institution was fixed; and he presented to the Managers his
resignation.

It was well known to Dr. Garnett's particular friends, that during
the early part of this session, he determined to withdraw himself
from the Institution; but the success and advancement of the
establishment, which he sanguinely hoped would stand unrivalled in
the universe, was so intimately connected with the affections of his
mind, that he resolved to forego every personal consideration, rather
than risk an inconvenience to the Institution, by ceasing to deliver
his lectures in the middle of a course; liberally considering, that
the Managers, after the business of the season was over, would have
time and opportunity before the ensuing session, to fill the
professor's chair with talents competent to the arduous undertaking;
a circumstance the Managers afterwards so eminently profited by, with
the highest credit to themselves, and advantage to the public, in the
nomination of the gentlemen who now fill the situation held by Dr.
Garnett, and who discharge its important duties with the most
distinguished abilities.

The transactions of the last winter almost completely served to
undermine the small strength of constitution he had left; he was
constantly harrassed by complaints in the organs of digestion; head
ache deprived him of the power of application; his countenance
assumed a sallow complexion; the eye which had beamed with animation,
retired within its socket, deprived of lustre; melancholy conceptions
filled his imagination more habitually, and were excited by slighter
causes; at times, they altogether deprived him of the power of
exertion; and while he lamented their effect, the contemplation of
themselves rendered him the more their prey. At this time, a gloomy
day, or the smallest disappointment, gave him inconceivable distress;
but he was not altogether incapable of temporary relief, and the few
moments of pleasure he seemed to enjoy, would have given reason to
believe, that he might once more have recovered, and have long
continued to be the delight and instructor of his friends. A more
close observation would at the same time have justified the
supposition, that the strong and painful emotions of mind he had
suffered, had already induced disorders of the bodily system, which
were irrecoverable. Before Doctor Garnett had left his situation at
Glasgow, he had determined to practice as a physician in London; but
from this he was restrained, during the time he was at the Royal
Institution. To his former intention he now determined to apply
himself, and in addition to the attempt, by giving private lectures,
to assure himself of that independency, of which his unfortunate
destiny, though with every reasonable expectation before him, had
hitherto deprived him.

With this intention, he purchased the lease of a house in Great
Marlborough Street; and in the summer of 1801, built a lecture room.
He brought his family to town, and once more looked forward with
hope. The flattering success he soon met with, and a short residence
at Harrowgate in the autumn, contributed to afford a temporary
renovation of health and spirits; it was, however, but a short and
delusive gleam of prosperity which now dawned upon him; for,
confiding too much in his newly increased strength, he exerted
himself to a much greater degree than prudence would have suggested.
In the course of the following winter, he delivered not less than
eight courses of lectures, two on chemistry, two on experimental
philosophy, a private course on the same subject, one on mineralogy,
and the course to which this sketch is prefixed, which he also
delivered in an apartment at Tom's Coffee house, for the convenience
of medical students, and others, in the city. Besides these, he
commenced two courses on botany, one at Brompton, and the other at
his own house; but a return of ill health prevented his concluding
them. It was not to be expected, that a constitution so impaired and
debilitated, could long support this continued labour of composition
and recitation; accordingly he became affected with a consequent
disorder, which rapidly exhausted his strength; and, being unable to
employ the only probable means of recovering it, he became more
incapable of exertion. His spirits however were roused, and he ceased
not to use every means of increasing his practice. In the spring of
1802, the office of physician to the St. Mary le Bonne Dispensary
happened to be vacant, and he became a candidate; he was more than
commonly anxious to obtain this situation. It seemed to him, as if
his future good or ill fortune depended altogether upon the event of
his canvass, he spared no effort to ensure his success; and
accordingly was appointed to the situation in May. His life now drew
near a close. Little was he calculated to bear the accumulated
labours, and extreme fatigue, to which he was daily exposed. Any
benefit which might have resulted from constant and well regulated
occupation was frustrated; for whilst he still suffered from the
vividness of his conception, representing to him in mournful colours
the occurrences of his past life, he became liable to other evils,
not less injurious and destructive. The practice of medicine requires
both vigorous health of body and firmness of mind. Dr. Garnett, now
greatly weakened in body, and not exempt from anxiety of mind, became
more and more susceptible to the action of morbific matter. It was
not long before he received the contagion of typhous fever, whilst
attending a patient, belonging to that very dispensary of which he
had been so anxious to become physician. He laboured under the
disorder for two or three weeks, and died the 28th of June, 1802; and
was buried in the new burial ground of the parish of St. James,
Westminster.

Thus was lost to society a man, the ornament of his country, and the
general friend of humanity. In his personal attachments, he was warm
and zealous. In his religion he was sincere, yet liberal to the
professors of contrary doctrines. In his political principles, he saw
no end, but the general good of mankind; and, conscious of the
infirmity of human judgment, he never failed to make allowances for
error. As a philosopher, and a man of science, he was candid,
ingenuous, and open to conviction; he never dealt in mystery, or
pretended to any secret in art; he was always ready in explanation,
and desirous of assisting every person willing to acquire knowledge.
Virtue was the basis of all his actions; science never possessed a
fairer fabric, nor did society ever sustain a greater loss.

LECTURES ON ZOONOMIA.

LECTURE I.
INTRODUCTION.

I AM well aware of the difficulties attending the proper
composition of a popular course of lectures on the animal economy,
which must be essentially different from those generally delivered
in the schools of medicine; because it professes to explain the
structure and functions of the living body, to those who are
supposed to be unacquainted with the usual preliminary and
collateral branches of knowledge. It must be obvious to every one,
that it can be by no means an easy task to give in a few lectures,
a perspicuous view of so extensive a subject; but I trust that the
consideration of this difficulty will readily extend to me your
indulgence.

That such a course, if properly conducted, must be interesting,
needs scarcely to be observed; for the more we examine the
structure and functions of the human body, the more we admire the
excellence of the workmanship, and beauty of contrivance, which
presents itself in every part, and which continually shows the
hand of omniscience. The most ingenious of human inventions, when
compared with the animal frame, indicate a poverty of contrivance
which cannot fail to humble the pretensions of the sons of men.
Surely then there are few who will not feel a desire to become
acquainted with subjects so interesting.

But there is another point of view which will place the utility of
such inquiries in a still stronger light. We shall afterwards see,
that our life is continually supported by the action of a number
of substances, by which the body is surrounded, and which are
taken into the stomach for its nourishment. On the due action of
these depends the pleasant performance of the different functions,
or the state of health; without which, riches, honours, and every
other gratification, become joyless and insipid.

By understanding the manner in which these powers act, or, in
other words, by becoming acquainted with the principles of
physiology, we shall be enabled to regulate them, so as, in a
great measure, to guard against the numerous ills that flesh is
heir to: for it is universally agreed, that by far the greatest
part of the diseases to which mankind are subject, have been
brought on by intemperance, imprudence, and the neglect of
precautions, which often arises from carelessness, but much
oftener from ignorance of those precautions.

Physiological ignorance is, undoubtedly, the most abundant source
of our sufferings; every person accustomed to the sick must have
heard them deplore their ignorance of the necessary consequences
of those practises, by which their health has been destroyed: and
when men shall be deeply convinced, that the eternal laws of
nature have connected pain and decrepitude with one mode of life,
and health and vigour with another, they will avoid the former and
adhere to the latter.

It is strange, however, to observe that the generality of mankind
do not seem to bestow a single thought on the preservation of
their health, till it is too late to reap any benefit from their
conviction: so that we may say of health, as we do of time, we
take no notice of it but by its loss; and feel the value of it
when we can no longer think of it but with retrospect and regret.

When we take a view of the human frame, and see how admirably each
part is contrived for the performance of its different functions,
and even for repairing its own injuries, we might at first sight
imagine, that such a structure, unless destroyed by external
force, should continue for ever in vigour, and in health: and it
is by mournful experience alone that we are convinced of the
contrary. The strongest constitution, which never experienced the
qualms of sickness, or the torture of disease, and which seems to
bid defiance to the enemies of health that surround it, is not
proof against the attacks of age. Even in the midst of life we are
in death; how many of us have contemplated with admiration the
graceful motion of the female form; the eye sparkling with
intelligence; the countenance enlivened by wit, or animated by
feeling: a single instant is sufficient to dispel the charm: often
without apparent cause, sensation and motion cease at once; the
body loses its warmth, the eyes their lustre, and the lips and
cheeks become livid. These, as Cuvier observes, are but preludes
to changes still more hideous. The colour passes successively to a
blue, a green, and a black; the flesh absorbs moisture, and while
one part of it escapes in pestilential exhalations, the remaining
part falls down into a putrid liquid mass. In a short time no part
of the body remains, but a few earthy and saline principles; its
other elements being dispersed through air, or carried off by
water, to form new combinations, and afford food for other
animals.

The human body has been defined to be a machine composed of bones
and muscles, with their proper appendages, for the purpose of
motion, at the instance of its intelligent principle. From this
principle, nerves, or instruments of sensation, are likewise
detached to the various parts of the body, for such information as
may be necessary to determine it to those motions of the body,
which may conduce to the happiness of the former, and the
preservation of both.

It may perhaps be objected to this definition, that the body
consists of other parts besides bones, muscles, and nerves; this
is undoubtedly true; but, if we examine more minutely, we shall
find that all the other parts, as well as functions of the body,
seem only to be subservient to the purposes I have mentioned. For,
in the first place, the muscles which are necessary to the motions
of the body, are, from the nature of their constitution, subject
to continual waste; to repair which waste, some of the other
functions have been contrived.

Secondly, most of the other parts and functions of the body, are
either necessary to the action of the muscles, or to the operation
of the intelligent principle, or both.

Lastly, from the sensibility, and delicate structure, of the
muscles and nerves, they require to be defended from external
injuries: this is done by membranes, and other contrivances,
fitted for the purpose.

To see this more clearly, we shall examine a little more
particularly how each of the functions is subservient to the
muscular and nervous systems. For this purpose it may be observed,
1st. that the stomach and digestive faculties serve to assimilate
the food, or convert it into matter proper to repair the continual
waste of solids and fluids. The circulation of the blood besides
being absolutely necessary, as we shall afterwards see, to the
action of the muscles, distributes the nourishment, thus
assimilated and prepared by the stomach, to all parts of the body.
The different glands separate liquors from the blood, for useful,
but still for subservient purposes. Thus the salivary glands,
stomach, pancreas, and liver, separate juices necessary to the
proper digestion and assimilation of the food. The kidneys serve
to strain off from the blood the useless and superfluous water,
salts, &c. which if allowed to remain in the body would be very
injurious to it.

We shall afterwards see, that the nerves are not only instruments
of sensation, but the origin of motion; it being immediately by
their means that the muscles are moved. A certain degree of heat
is necessary to keep the blood fluid, and also to the action of
the nerves; without either of which, no motion could be performed.
Respiration or breathing is so necessary to life, that it cannot
exist, even a few minutes, without the exercise of that function;
and yet we shall afterwards see, that the ultimate end of
respiration is to keep the body in a proper state, for the
purposes of muscular motion and sensation.

The skin serves like a sheath to defend the body from injuries;
the skull serves the same purpose to the brain, which is the
origin of the nerves. The different membranes separate the fibres,
muscles, nerves, and various organs of the body, from each other.
Hence we see that there is no impropriety, in calling the human
body a machine composed of bones and muscles, with their proper
appendages, for the purpose of motion, at the instance of its
intelligent principle.

In order to show more clearly how each part is subservient to
these ends, I shall give a short account of the structure of the
human body, but I must premise, that the nature of this course
will prevent my entering minutely into anatomical detail. All that
can be done is, to give such a general outline of anatomy and
physiology, as will furnish individuals with so much knowledge of
themselves, as may enable them to guard against habitual sickness.

Among the solid parts of the human frame the bones stand
conspicuous. Their use is, to give firmness and shape to the body.
Some of them likewise serve as armour, or defense, to guard
important parts; thus the skull is admirably contrived to defend
the brain; and the spine or backbone is designed, not only to
strengthen the body, but to shield that continuation of the brain,
called the spinal marrow, from whence originate great numbers of
nerves, which pass through convenient openings of this bone, and
are distributed to various parts of the body. In the structure of
this, as well as every other part, the wisdom of the Creator is
manifest. Had it been a single bone, the loins must have been
inflexible; to avoid which, it consists of a number of small
bones, articulated or joined together with great exactness, which
are strengthened by compact ligaments. Hence it becomes capable of
various inflections, without injuring the nerves, or diminishing
that strength which is so much required.

The whole system of bones, or skeleton, is constructed of several
parts, of different shapes and sizes, joining with one another in
various manners, and so knit together, as best to answer to the
motions which the occasions of the animal may require.

These bones serve as levers for the muscles to act on; which last
serve as mechanical powers, to give the machine various motions,
at the command of the will.

The muscles are fleshy fibres, attached by their extremities to
the bones. When the fibres shorten themselves, the two parts into
which the muscle is inserted are brought nearer; and, by this
simple contrivance, all the motions of animals are performed, and
their bodies carried from one place to another.

Joints are provided with muscles for performing the motions for
which they are adapted; every muscle pulling the bone, to which it
is attached, in its own particular direction. Hence the muscles
may be considered as so many moving forces, as was before hinted;
and their strength, the distance of their insertion from the
centre of motion, the length of the lever to which they are
attached, and the weight connected with it, determine the duration
and velocity of the motions which they produce. Upon these
different circumstances depend the different kinds of motion
performed by various animals, such as the force of their leap, the
extent of their flight, the rapidity of their course, and their
address in catching their prey.

Most of the muscles act upon the bones, so as to produce the
effects of a lever of the third kind, as it is termed by
mechanics, where the power acts between the centre of motion and
the weight; hence it has a mechanical disadvantage; as an instance
of this, the muscle which bends the forearm, is inserted about one
eighth or one tenth of the distance from the centre of motion that
the hand is, where the weight or resistance is applied; hence the
muscle must exert a force eight or ten times greater than the
weight to be raised. But this disadvantage is amply compensated by
making the limbs move with greater velocity; besides, if room had
been given for the muscles to act with greater advantage, the
limbs must have been exceedingly deformed and unwieldy. [1]

The muscles, in general, at least those which serve for voluntary
motion, are balanced by antagonists, by means of which they are
kept beyond their natural stretch. When one of two antagonists is
contracted by the will, the other relaxes in order to give it
play; or at least becomes overpowered by the contraction of the
first. Also when one of such muscles happens to be paralytic, the
other being no longer balanced, or kept on the stretch,
immediately contracts to its natural length, and remains in that
situation. The part to which it is fixed will, of course, be
affected accordingly. If one of the muscles which move the mouth
sideways be destroyed, the other immediately contracting, draws
the mouth awry; and in that situation it remains. The same may be
observed of the leg, the arm, and other parts. Some muscles assist
one another in their action, while others have different actions;
according to their shapes, the course of their fibres, and the
structure of the parts they move.

According to the shape and nature of the bones to be moved, and of
the motions to be performed, the muscles are either long, or
short; slender, or bulky; straight, or round. Where a great motion
is required, as in the leg, or arm, the muscles are long; where a
small motion is necessary, they are short; for a strong motion
they are thick, and for a weak one slender.

Some of the muscles are fastened to, and move bones; others
cartilages, and others again other muscles, as may best suit the
intention to be answered.

With respect to the bones, some are solid and flattened; others
hollow and cylindrical. Every cylindrical bone is hollow, or has a
cavity containing a great number of cells, filled with an oily
marrow. Each of these cells is lined with a fine membrane, which
forms the marrow. On this membrane, the blood vessels are spread,
which enter the bones obliquely, and generally near their middle;
from some branches of these vessels the marrow is secreted; while
others enter the internal substance of the bones for their
nourishment; and the reason why they enter the bones obliquely is,
that they may not weaken them by dividing too many fibres in the
same place.

The bones being made hollow, their strength is greatly increased
without any addition to their weight; for if they had been formed
of the same quantity of matter without any cavities, they would
have been much weaker; their strength to resist breaking
transversely being proportionate to their diameters, as is evident
from mechanics.

All the bones, excepting so much of the teeth as are out of the
sockets, and those parts of other bones which are covered with
cartilages, are surrounded by a fine membrane, which on the skull
is called pericranium, but in other parts periosteum. This
membrane serves for the muscles to slide easily upon, and to
hinder them from being lacerated by the hardness and roughness of
the bones.

But though the apparatus which I have been describing is admirably
contrived for the performance of motion; it would continue for
ever inactive, if not animated by the nervous system.

The brain is the seat of the intelligent principle: from this
organ, white, soft, and medullary threads, called nerves, are sent
off to different parts of the body: some of them proceed
immediately from the brain to their destined places, while the
greater number, united together, perforate the skull, and enter
the cavity of the backbone, forming what we call the spinal
marrow, which may be regarded as a continuation of the brain.
Portions of the spinal marrow pass through different apertures to
all parts of the body.

We are not conscious of the impression of external objects on our
body, unless there be a free communication of nerves, between the
place where the impression is made and the brain. If a nerve be
divided, or have a ligature put round it, sensation is intercepted.

There is perhaps only one sense which is common to all classes of
animals, and which exists over every part of the surface of the
body; I mean the sense of touch. The seat of this sense is in the
extremities of the nerves distributed over the skin; and by means
of it we ascertain the resistance of bodies, their figure, and
their temperature.

The other senses have been thought to be only more refined
modifications of the sense of touch; and the organs of each are
placed near the brain on the external surface of the head. The
sense of sight, for instance, is seated in the eye; the hearing in
the ear; the smell in the internal membrane of the nose; and the
taste in the tongue.

The light; the pulses, or vibrations of the air; the effluvia
floating in the atmosphere; saline particles, or particles which
are soluble in water or saliva, are the substances which act upon
these four senses; and the organs which transmit their action to
the nerves, are admirably adapted to the respective nature of
each. The eye presents to the light a succession of transparent
lenses to refract its rays; the ear opposes to the air membranes,
fluids, and bones, well fitted to transmit its vibrations; the
nostrils, while they afford a passage to the air in its way to the
lungs, intercept any odorous particles which it contains, and the
tongue is provided with spongy papillae to imbibe the sapid
liquors which are the objects of taste.

It is by these organs that we become acquainted with what passes
around us; by these we know that a material world exists. We may
however observe, that the nervous system, besides making us
acquainted with external things, gives us notice of many changes
that take place within our own body. Internal pain warns us of the
presence of disease; and the disagreeable sensations of hunger,
thirst, and fatigue, are signs of the body standing in need of
refreshment or repose.

Concerning the manner in which we become acquainted with external
things, by means of the senses, we know nothing. Many hypotheses
have been offered to explain this: none of them however are the
result of experiment and observation. Many philosophers have
supposed the universe to be filled with an extremely subtile
fluid, which they have termed ethereal; and this hypothesis has
been sanctioned by the illustrious authority of Newton. He however
merely offered it in the modest form of a query, for the attention
of other philosophers; little thinking that it would be made use
of to explain phenomena which they did not understand. His query
about a subtile elastic fluid pervading the universe, and giving
motion and activity to inert masses of matter, and thereby causing
the phenomena of attraction, gravitation, and many other
appearances in nature, was immediately laid hold of by his
followers, as a fact sufficiently supported, because it seemed to
have the sanction of so great an authority.

This hypothesis was made use of to explain a great number of
phenomena, and the physiologists, whose theories were generally
influenced by the prevailing philosophy, eagerly laid hold of it
to explain the phenomena of sensation, and muscular motion. When
an impression was made upon any part of the external surface of
the body, whether it was occasioned by heat, or mechanical
impulse, they supposed, that the ether in the extremities of the
nerves was set in motion. This motion, from the energy of the
ether, is communicated along the nerves to the brain, and there
produces such a change as occasions a consciousness of the
original impression, and a reference in the mind to the place
where it was made. Next they supposed, that the action of the will
caused a motion of the ether to be instantly propagated along the
nerves that terminate in the fibres of the muscles, which
stimulated them to contraction.

Other philosophers imagined, that a tremulous motion was excited
in the nerves themselves, by the action of external impulses, like
the motions excited in the string of a harp. These motions they
supposed to be propagated along the nerves of sense, to the brain,
and from thence along the motory nerves, to the muscles.

Before they attempted this explanation of the phenomena, they
should have proved the existence of such a fluid, or at least
brought forward such circumstances, as rendered its existence
credible. But supposing we grant them the hypothesis, it will, in
my opinion, not avail much; for it is not easy to conceive how the
motion of a subtile fluid, or the vibration of a nerve, can cause
sensation.

Nor are the internal senses, as they are generally called, namely,
memory, and imagination, any better explained on this supposition;
for we cannot conceive how this nervous fluid is stored up and
propelled by the will.

After all, I think we must confess, that this subject is still
enveloped in obscurity. One observation is worth making, namely,
that our sensations have not the smallest resemblance to the
substance or impression, which causes them; thus the sensation
occasioned by the smell of camphor, possesses not the smallest
resemblance to small particles of camphor floating in the
atmosphere; a sensation of pain has no similitude whatever to the
point of a sword which occasions it; nor has the sensation of
sound any resemblance to waves or tremors in the air. In our
present state of knowledge, therefore, all that we can say, is,
that nature has so formed us, that when an impression is made on
any of the organs of sense, it causes a sensation, which forces us
to believe in the existence of an external object, though we
cannot see any connexion between the sensation and the object
which produces it.

But though philosophers were certainly blameable for assuming an
unknown cause, to account for various phenomena, yet later
experiments would seem to prove that even the conjectures of
Newton were not founded on slight grounds. His idea that the
diamond was inflammable, has been confirmed by various
experiments: and that there exists in nature a subtile fluid,
capable of pervading with ease the densest bodies, the phenomena
of electricity would seem to show, and some late experiments
render it by no means improbable, that this fluid or influence,
acts a conspicuous part in the nervous system. That it exists in
great quantity in animal bodies, is evident on gently rubbing the
back of a cat in the dark; and it would seem that, in some
instances, it may be given out or secreted by the nerves. We shall
afterwards see that the seat of vision is a delicate expansion of
a large nerve which comes from the brain, and is spread out on the
bottom of the eye; and flashes of light, or a kind of sparkling,
is often seen to dart from the eyes of persons in high health, and
possessed of much nervous energy. These luminous flashes are very
apparent in the dark in some animals; such as the lion, the lynx,
and the cat; and it is difficult to account for this appearance
unless we suppose it electrical.

The experiments of Galvani and others, have however proved beyond
all doubt, that this fluid, when applied to the nerves and
muscles, is capable of exciting various sensations and motions. To
produce this fluid by the application of two metals, it is
necessary that one of them should be in such a situation, as to be
easily oxidable, while the other is prevented from oxidation. If a
piece of zinc be put into water, no change will take place; but if
a piece of silver be put along with it, the zinc will immediately
oxidate, by decomposing the water, and a current of electricity
will pass through the silver. If the upper and under surfaces of
the tongue be coated with two different metals, one of which is
easily oxidable, and these be brought into contact, a sensation is
produced resembling taste, which takes place suddenly, like a
slight electrical shock. This taste may likewise be produced by
applying one part of the metals to the tongue and the other to any
part of the body deprived of the cuticle, and bringing them in
contact.

The sensation of light may be produced in various ways; such as by
applying one metal between the gum and the upper lip, and the
other under the tongue; or by putting a silver probe up one of the
nostrils, and a piece of zinc upon the tongue; a sensation
resembling a very strong flash of light is perceived in the
corresponding eye, at the instant of contact.

But the experiments which tend most strongly to prove what I have
hinted, are made in the following manner. Lay bare a portion of a
great nerve leading to any muscle or limb of an animal, for
instance, the leg of a frog separated from the body. Touch the
bared nerve with a piece of zinc, and the muscle with a piece of
silver, and strong contractions take place the instant these
metals are brought into contact. The same effect may be produced
by placing a piece of silver on a larger piece of zinc, and
putting a worm or a leech on the silver; in moving about, the
instant it touches the zinc it is thrown into strong convulsions.

These phenomena have been clearly proved to be electrical; for by
a number of pieces of these metals, properly disposed, strong
shocks can be given, the electrometer can be affected, a Leyden
vial charged, the electric spark seen, and combustible bodies
inflamed.

Some animals likewise possess the power of accumulating this
influence in a great degree; for instanc the torpedo, and
electrical eel, which will both give strong shocks; and if the
circuit have a small interruption a spark may be seen, as was
shown by Mr. Walsh. On dissecting these fish, Mr. Hunter found an
organ very similar to the pile of Volta; it consists of numerous
membranaceous columns, filled with plates or pellicles, in the
form of thin disks, separated from each other by small intervals,
which intervals contain a fluid substance; this organ, like the
pile of Volta, is capable of giving repeated shocks, even when
surrounded by water.

It is not absolutely necessary to use two metals to produce the
galvanic phenomena; for if one side of a metal be made to oxidate,
while the other is prevented from oxidation, these appearances
will still be produced. It is not indeed necessary to use any
metal; for a piece of charcoal, oxidated in the same way, produces
galvanism; so does fresh muscular fibre, and perhaps any substance
capable of oxidation. The most striking circumstance in galvanism,
is, that it accompanies oxidation, and is perhaps never produced
without it. But oxidation is always going on in the body by means
of respiration and the circulation of the blood. We shall
afterwards find reason to believe, that the oxygen, received from
the atmosphere by the lungs, is the cause of animal heat, and
probably of animal irritability; and it is perhaps not
unreasonable to suppose, that the nervous influence or electricity
may be separated by the brain, and sent along the nerves, which
seem the most powerful conductors of it, to stimulate the muscles
to action.

What the nature of the electric fluid is, we are ignorant; some
galvanic experiments have led me to suppose that it may be
hydrogen, which when combined with caloric appears in the form of
gas, but when pure, or perhaps in a different state, may be
capable of passing through solid bodies in the form of
electricity.

Having given this short view of the human body, considered as a
machine composed of bones, muscles, and nerves, I shall proceed to
state the different subjects which I shall consider in this
course. It is extremely difficult to begin a course like this; for
we must either enter abruptly into the middle, or the outset must
be in some measure tedious and dry. I have chosen however rather
to hazard the latter appellation, with respect to this lecture,
than to enter more abruptly into the subject, in order to make it
more entertaining. As we proceed, I trust you will feel an
increasing interest in the subject; and, I think I may venture to
promise, that this will be found the least entertaining lecture in
the course. The subjects will be illustrated by experiments, in
order to render the deductions more striking.

I shall next proceed to consider the phenomena of respiration, and
animal heat; after which I shall explain the circulation of the
blood; and the phenomena of digestion and nutrition. I shall then
examine, more minutely than has been done in this lecture, the
connexion of man with the external world, which will lead to a
discussion of the different senses; vision, hearing, smelling,
tasting, and feeling.

When these subjects have been discussed as fully as our time will
allow, I shall examine, at considerable length, the manner in
which the powers that support life, which have been improperly
called by physiologists, the nonnaturals, act upon the body. This
will naturally lead to a fuller explanation of the system which I
have attempted to defend, in my lecture on health. And, after I
have fully explained the laws by which the irritable principle is
regulated, I shall proceed to show, how those variations from the
healthy state, called diseases, are produced; I shall point out
the difference that exists between the debility which is brought
on by the diminished action of the powers which support life, and
that which results from their too powerful action; I shall then
inquire into the nature of diseases of increased excitement; and
after having shown how the undue action of the powers which
support life, operates in producing disease, I shall endeavour to
lay down such rules for the preservation of health, as are the
result of reasoning on these subjects, and are also confirmed by
experience.

[1] [FIGURE] Suppose AC to be a lever, held in equilibrio by the
force B and weight W, then the whole momentum exerted at B must be
equal to that at W, but the forces will be different. For B x AC = W
x AB, and if AC = 10AB, then a force equal to ten times the weight to
be raised must be exerted by the muscle. Hence we see, that in the
actions of muscles there is a loss of power, from their insertions
being nearer the fulcrum than the weight. For example, suppose the
deltoid muscle to act and raise a weight of 55 lb.: the weight of the
arm is 5 lb., and the distance of its insertion is only 1/3 of the
arms length, hence the force exerted must be (55 + 5) x 3 = 180 lb.

[FIGURE] But by this contrivance we gain a greater extent of motion,
and also a greater velocity, and both with less contraction. Let A be
the centre of motion, or articulation; B the insertion of a muscle,
and AC the length of the lever or bone; then, by a contraction only
equal to B_b_, C is carried through C_c_, which is to B_b_ as AC to AB.
It is obvious also, that the velocity is greater, since C moves to _c_
in the same time as B to _b_.

A loss of power is likewise occasioned by the obliquity of the
muscular action, and the oblique direction of the fibres.

For, in this case, there is a compound of two forces, and a
consequent loss of power: for the forces are proportioned to the two
sides of a parallelogram, but the effects produced are proportioned
only to the diagonal.

LECTURE II.
RESPIRATION.

In the last lecture I took a short view of the human body, as a
moving machine, regulated by the will. We shall now proceed to
examine some of its functions more particularly.

I need not tell any of my audience, how necessary air is to the
living body; for every person knows that we cannot live when excluded
from this fluid; but, before we can understand the manner in which it
acts on the body, we must become acquainted with some of its
properties.

That the air is a fluid, consisting of such particles as have little
or no cohesion, and which slide easily among each other, and yield to
the slightest force, is evident from the ease with which animals
breathe it, and move through it. Indeed from its being transparent,
and therefore invisible, as well as from its extreme tenuity, and the
ease with which bodies move through it, people will scarcely believe
that they are living at the bottom of an aerial ocean, like fishes at
the bottom of the sea. We become, however, very sensible of it, when
it flows rapidly in streams or currents, so as to form what is called
a wind, which will sometimes act so violently as to tear up the
strongest trees by the roots, and blow down to the ground the best
and firmest buildings.

Some may still be inclined to ask, what is this air in which we are
said to live? We see nothing; we feel nothing; we find ourselves at
liberty to move about in any direction, without any hindrance. Whence
then comes the assertion, that we are surrounded by a fluid, called
air? When we pour water out of a vessel, it appears to be empty; for
our senses do not inform us that any thing occupies the place of the
water, for instance, when we pour water out of a vial. But this
operation is exactly similar to pouring out mercury from a vial in a
jar of water, the water gets in and supplies the place of the
mercury; so does the air which supplies the place of the water; and
this air will prevent water from rising, or filling a vessel which
contains it.

Hence we see that air possesses similar appearances of
impenetrability with other matter: for it excludes bodies from the
space which itself occupies.

Air being therefore material must have weight; and we shall
accordingly find, that a quart of it weighs about fifteen grains. But
a quart of water weighs about two pounds; this fluid therefore is
nearly a thousand times heavier than air.

But though air is so much lighter than water, yet, because it extends
to a considerable height above the surface of the earth, it is
evident, that it must press strongly on the surfaces of bodies. It is
thought to extend nearly fifty miles above the surface of the earth,
and must therefore press heavily on this surface. This may be evinced
by different experiments, performed by means of the air pump.

Another property of the air, by which it is distinguished from most
other fluids, is its elasticity. It may be compressed into a less
space than it naturally occupies, and when the compressing force is
removed, it expands to its former bulk, by its spring or elasticity.
Indeed it is always compressed into less space than it would
naturally occupy, by the weight of the superincumbent air.

The trachea, or windpipe, commences at the further end of the mouth,
between the root of the tongue, and the passage into the stomach: its
upper part is termed the larynx; it forms the projection in the fore
part of the neck, which is more prominent in the male than the
female: its opening is called the glottis, and is covered with a
small valve, or lid, called the epiglottis, which is open while we
breathe, but shuts when we swallow any thing, to prevent its getting
into the lungs: sometimes, however, particularly when we attempt to
speak at the time we swallow, a small portion of our food or drink
gets into the larynx, and excites violent coughing until it is thrown
back again.

The windpipe is composed of cartilaginous rings, covered with
membrane, which keep it open: after having run downwards for the
space of a few inches, it divides into two great branches, each of
which is subdivided into a vast number of ramifications, ultimately
terminating in little vesicles, which, when distended with air, make
up the greatest part of the bulk of the lungs.

The cavity in which the lungs are contained is called the thorax, or
chest: and is bounded by the ribs, and backbone or spine, and
separated from the abdomen by a muscular membrane, called the
diaphragm. The thorax, by the action of the diaphragm and intercostal
muscles, is alternately enlarged and diminished. Suppose then the
thorax to be in its least state; if it become larger, a vacuum will
be formed, into which the external air will descend by its weight,
filling and distending the vesicles of the lungs.

The thorax, thus dilated, is brought back to its former magnitude,
principally by the relaxation of the muscles, which distended it, and
the natural elasticity of the parts, aided by the contraction of the
abdominal muscles; the thorax being thus diminished, a quantity of
air is expelled from the lungs. The muscles which distend the thorax
beginning again to act, the air reenters; and this alternate dilatation
and contraction, is called respiration. The entrance of the air into
the lungs, is termed inspiration, and its expulsion, expiration.

To form a more accurate idea of the manner in which respiration is
performed, let us suppose this room to be filled with water. On
enlarging the thorax, in the manner before mentioned, the water by
its weight would rush in, and fill the newly formed void; and, upon
the diminution of the capacity of the thorax, a part of this water
would be expelled. Just in the same manner the air will alternately
enter and be expelled from the lungs by this alternate dilatation and
contraction of the thorax.

Respiration is a function of such consequence, that death follows if
it is suspended for a few minutes only. By means of this function the
blood is elaborated, and rendered fit to nourish the body; by means
of it the system is, most probably, supplied with irritability; by
means of it the nervous energy is, most likely, conveyed into the
body, to be expended in sensation, and muscular motion. It appears,
likewise, that in this way, animals are supplied with that heat which
preserves their temperatures nearly the same, whatever may be the
temperatures of surrounding bodies.

If any number of inanimate bodies, possessed of different degrees of
heat, be placed near each other, the heat will begin to pass from the
hotter bodies to the colder, till there be an equilibrium of
temperature. But this is by no means the case with respect to
animated matter; for whatever be the degree of heat peculiar to
individual animals, they preserve it, nearly unchanged, in every
temperature, provided the temperature be not altogether incompatible
with life or health. Thus, we find, from experiments that have been
made, that the human body is not only capable of supporting, in
certain circumstances, without any material change in its
temperature, a degree of heat considerably above that at which water
boils; but it likewise maintains its usual temperature, whilst the
surrounding medium is several degrees below frost.

It is evident, therefore, that animals neither receive their heat
from the bodies which surround them, nor suffer, from the influence
of external circumstances, any material alterations in that heat
which is peculiar to their nature. These general facts are confirmed
and elucidated by many accurate and well authenticated observations,
which show, that the degree of heat in the same genus and species of
the more perfect animals, continues uniformly the same, whether they
be surrounded by mountains of snow, in the neighbourhood of the pole,
or exposed to a vertical sun, in the sultry regions of the torrid
zone.

This stability and uniformity of animal heat, under such a disparity
of external circumstances, and so vast a latitude in the temperature
of the ambient air, prove, beyond doubt, that the living body is
furnished with a peculiar mechanism, or power of generating,
supporting, and regulating its own temperature; and that this is so
wisely adapted to the circumstances of its economy, or so dependent
upon them, that, whatever be the temperature of the atmosphere, it
will have very little influence either in diminishing or increasing
that of the animal.

In order that we may see how this effect is produced, we must examine
the chemical properties of the air. Previously to this, however, it
will be necessary to point out briefly how bodies are affected, with
respect to heat, when they change their form.

When a body passes from a state of solidity to that of fluidity, it
absorbs a quantity of heat, which becomes chemically combined with
it, and insensible to the touch or the thermometer; in the same
manner, when it passes from a fluid state to that of vapour or gas,
it combines with a still larger quantity of heat, which remains
latent in it, so long as it continues in the state of gas, but when
it returns to the liquid or solid state, it gives out the heat which
was combined with it, which, being set at liberty, flows into the
surrounding bodies, and augments their temperature.

This is evinced by the conversion of ice into water, and of water
into steam; and by the return of steam into water. It is evinced
likewise by the evaporation of ether, and by numberless other
experiments.

Modern chemistry has shown that the atmosphere is not a homogeneous
fluid, but consists of two elastic fluids, endowed with opposite and
different properties.

If a combustible body, for instance a candle, be confined in a given
quantity of atmospheric air, it will burn only for a certain time;
after it is extinguished, if another combustible body be lighted and
immersed in the same air, it will not burn, but will immediately be
extinguished.

It has been proved by chemical experiments, that in this instance,
the combustible body absorbs that portion of the air which is fitted
for combustion, but produces no change on that which is unfit: so
that, according to this, the air of the atmosphere consists of two
elastic fluids, one of which is capable of supporting combustion, and
the other not; and that they exist in the proportion of one part of
the former to three of the latter nearly.

These two parts may be separated from each other, and experiments
made with them.

Many metals, and particularly manganese, when exposed to the
atmosphere, attract the combustible air from it, without touching the
other; and it may be procured from these metals by the application of
heat, in very great purity.

Because this air is essential to the formation of acids, it has been
called by chemists the acidifying principle, or oxygen gas.

On plunging a combustible body into the remaining air, it is
instantly extinguished; an animal in the same situation is
immediately deprived of life: from this latter circumstance this air
has been called azote, or azotic gas. If we take three parts of azote
and one of oxygen, and mix them together, we shall form an air in
every respect similar to that of the atmosphere.

If I plunge a piece of iron, previously heated, into oxygen gas, it
will burn with great brilliancy, the gas will be diminished in
quantity, and the iron augmented in weight, and this increase of
weight in the metal will be in proportion to the oxygen which has
disappeared: at the same time a great quantity of heat is given out.
This is the heat which was combined with the oxygen in the state of
gas, and which now becomes free, when the oxygen becomes solid and
joins with the iron.

The same phenomena take place when phosphorus is burned in oxygen
gas; the gas becomes diminished, the phosphorus increased, in weight,
and converted into an acid, and a great quantity of heat is given
out. The same is the case when charcoal is burned in this gas. In
short, in every instance of combustion, the oxygen combines with the
combustible body, and at the same time gives out its heat, which
supported it in the form of gas. This is the case of the combustion
of coal in a common fire, as well as in other cases of combustion;
the heat comes from the air, and not from the coal.

When we examine the phenomena of respiration with attention, we shall
find them very analogous to those of combustion. A candle will not
burn in an exhausted receiver: an animal in the same situation ceases
to live.

When a candle is confined in a given quantity of atmospheric air, it
will burn only for a certain length of time. On examining the air in
which it has been burned, the oxygen is found to be all extracted,
nothing remaining but azotic gas, and a quantity of carbonic acid
gas, produced by the union of the charcoal of the candle with the
oxygen of the atmospheric air.

In the same manner, if an animal be confined in a given quantity of
atmospheric air, it will live only a short time; on examining the air
in which it has ceased to live, it will be found to have lost its
oxygen: what remains being a mixture of azotic and carbonic acid
gases.

When a candle is enclosed in a given quantity of pure oxygen gas, it
will burn four times as long as in the same quantity of atmospheric
air.

In the same manner it has been proved, that an animal will be four
times as long in consuming a given quantity of pure oxygen gas, as in
rendering unfit for respiration the same quantity of atmospheric air.

Here then we observe a striking similarity between combustion and
animal respiration. The ancients seem to have had a more accurate
idea of respiration than most of the philosophers who followed them.
They supposed that the air contained a principle proper for the
support and nourishment of life, which they called pabulum vitae.
This idea, which was unconnected with any hypothesis, was followed by
systems destitute of foundation. Sometimes it was thought that the
air in the lungs incessantly acted as a stimulus or spur to drive on
the circulation; sometimes the lungs were considered in the light of
a pair of bellows, or fan, to cool the body, which was supposed to be
heated by a thousand imaginary causes: and when philosophers were
convinced, by experiments, that the bulk of the air was diminished by
respiration, they explained it by saying, that the air had lost its
spring.

Modern chemistry however enables us to explain the phenomena of
respiration in a satisfactory manner.

In order to see this, we shall proceed to examine the changes
produced by respiration; firstly, on the air, and secondly, on the
blood.

The air which has served for respiration, is found to contain a
mixture of azotic and carbonic acid gas, with a small quantity of
oxygen gas; and a considerable quantity of water is thrown off from
the lungs, in the form of vapour, during respiration.

From a variety of facts, it appears that oxygen gas is decomposed in
the lungs during respiration; a part of it unites, as we shall
afterwards see, with the iron contained in the blood, and converts it
into an oxid; another and greater portion unites with the carbon,
brought by the venous blood from all parts of the body to the lungs,
and thus forms carbonic acid gas; while another portion of the oxygen
unites with the hydrogen, brought in the same manner by the blood,
and forms water. Thus then we are able to account for the different
products of respiration.

Hence we see, that the explanation of animal heat follows as a simple
and beautiful corollary from the theory of combustion; and we may
consider respiration as an operation in which oxygen gas is
continually passing from the gaseous to the concrete state; it will
therefore give out at every instant the heat which it held in
combination, and this heat, being conveyed by the circulation of the
blood to all parts of the body, is a constant source of heat to the
animal.

These facts likewise enable us to explain the reason, why an animal
preserves the same temperature, notwithstanding the various changes
which occur in the temperature of the surrounding atmosphere. In
winter the air is condensed by the cold, the lungs therefore receive
a greater quantity of oxygen in the same bulk, and the heat
extricated will be proportionally increased. In summer, on the
contrary, the air being rarefied by the heat, a less quantity of
oxygen will be received by the lungs during each inspiration, and
consequently the heat which is extricated must be less.

For the same reason, in northern latitudes, the heat extricated by
respiration will be much greater than in the southern. By this simple
and beautiful contrivance, nature has moderated the extremes of
climate, and enabled the human body to bear vicissitudes which would
otherwise destroy it.

Of all the phenomena of the animal body, there is none at first sight
more remarkable, than that which animals possess of resisting the
extremes of temperature.

The heat of the body, as has already been observed, continues at the
same degree, though the temperature of the atmosphere be sometimes
considerably hotter, at other times considerably colder, than the
animal body: so that man is able to live, and to preserve the
temperature of health, on the burning sands of Africa, and on the
frozen plains of Siberia.

The alterations of temperature which the human body has been known to
bear, without any fatal or even bad effects, are not less than 400
degrees or 500 degrees of Fahrenheit. The natural heat of the human
body is 96 degrees or 97 degrees. In the West Indies, the heat of the
atmosphere is often 98 degrees or 99 degrees, and sometimes rises
even to 126 degrees, or 30 degrees above the temperature of the human
body, notwithstanding which, a thermometer put in the mouth points to
96 degrees or 97 degrees. The inhabitants of the hot regions of
Surinam support, without inconvenience, the heat of their climate. We
are assured that in Senegal, about the latitude of 17 degrees, the
thermometer in the shade generally stands at 108 degrees, without any
fatal effects to men or animals. The Russians often live in places
heated by stoves to 108 degrees or 109 degrees, and some philosophers
in this country, by way of experiment, remained a considerable time
in a room heated above the boiling point of water.

On the other hand, an equal excess of cold seems to have no greater
effect in altering the degree of heat proper to animal bodies.
Delisle has observed a cold in Siberia 70 degrees below the zero of
Fahrenheit's scale, notwithstanding which animals lived. Gmelin has
seen the inhabitants of Jeniseisk under the 58th degree of northern
latitude, sustaining a degree of cold, which in January became so
severe, that the spirit in the thermometer was 126 degrees below the
freezing point. Professor Pallas in Siberia, and our countrymen at
Hudson's Bay, have experienced a degree of cold almost equal to this.
All these facts tend to prove, that the heat of animals continues
without alteration in very different temperatures. Hence it is
evident that they must be able to produce a greater degree of heat,
when surrounded by a cold medium; and on the contrary, that they must
effect a diminution of the heat, when the surrounding medium is very
hot.

All these circumstances may be accounted for, by the principles we
have laid down; the decomposition of oxygen in the lungs.

There have not been wanting, however, some very eminent
physiologists, who have contended that animal heat is produced
chiefly by the nerves. They have brought forward in proof of this the
well known fact, that when the spinal marrow is injured, the
temperature of the body generally becomes diminished; and that in a
paralytic limb the heat is less than ordinary, though the strength
and velocity of the pulse remain the same. These facts, and others of
a similar nature, have induced them to believe, that the nervous
system is the chief cause and essential organ of heat; and they have
adduced similar arguments, to prove that nutrition is performed by
the nerves, for a limb which is paralytic from an injury of the
nerves, wastes, though the circulation continues. The truth is, that
the nerves exert their influence upon these, and all other functions
of the body, and modify their action. The liver secretes bile, but if
the nerves leading to it be destroyed, the secretion of bile will
cease; but who will say, that the bile is secreted by the nerves? The
nitric acid will dissolve metals, and this solution will go on more
quickly if heat be applied; but surely the nitric acid is the
solvent, the heat being only an aiding cause.

But though the human body has been so wisely constructed, as to bear,
without inconvenience, a considerable variation of temperature; yet
this latitude has its limits, which depend upon the capability of
extricating heat from the atmosphere. There must be a limit below
which the diminution of heat takes place faster than its production.
If this be continued, or increased, the heat of the animal must
diminish, the functions lose their energy, and an insuperable
inclination to sleep is felt, in which if the sufferer indulge, he
will be sure to wake no more.

This is confirmed by what happened to Sir Joseph Banks and his party
on the heights of Terra del Fuego. Dr. Solander, who had more than
once crossed the mountains which divide Sweden from Norway, well knew
that extreme cold produces an irresistible torpor and sleepiness, he
therefore conjured the company to keep always in motion, whatever
exertion it might require, and however great might be their
inclination to rest. Whoever sits down, says he, will sleep; and
whoever sleeps will wake no more. Thus, at once admonished and
alarmed, they set forward; but, while they were still upon the naked
rocks, the cold was so intense, as to produce the effects which had
been so much dreaded. Dr. Solander himself was the first who found
the inclination against which he had warned others, irresistible; and
insisted on being suffered to lie down. Sir Joseph entreated and
remonstrated in vain; he lay down upon the ground, though it was
covered with snow; and it was with great difficulty that his friend
kept him from sleeping. One of his black servants also began to
linger, having suffered from the cold in the same manner as the
Doctor. Partly by persuasion, and partly by force, they were got
forwards; soon however they both declared that they would go no
further. Sir Joseph had recourse again to entreaty and expostulation,
but these produced no effect: when the black was told, that if he did
not go on, he would shortly be frozen to death; he answered, that he
desired nothing so much as to lie down and die. The Doctor did not so
explicitly renounce his life, but said, he would go on, if they would
first allow him to take some sleep, though he had before told them,
that to sleep was to perish. They both in a few minutes fell into a
profound sleep, and after five minutes Sir Joseph Banks happily
succeeded in waking Dr. Solander, who had almost lost the use of his
limbs; the muscles were so shrunk, that his shoes fell from his feet;
but every attempt to recal the unfortunate black to life proved
unsuccessful.

As the circulation of the blood is the means by which the heat
produced is conveyed to all parts of the body; and as it is a
function of the highest importance, I shall, in the next lecture,
proceed to the consideration of it.

LECTURE III.
CIRCULATION OF THE BLOOD.

Two kinds of motion may be distinguished in the animal economy; the
one voluntary, or under the command of the will, which takes place at
certain intervals, but may be stopped at pleasure. The other kind of
motion is called involuntary, as not depending on the will, but going
on constantly, without interruption, both when we sleep and when we
wake.

Of the first kind is the motion of the limbs, of which I have already
spoken in general terms; the object of which is, to change the
situation of the animal, and carry it where the will directs.

Among the involuntary motions, the most remarkable is the circulation
of the blood, which I shall proceed to consider in this lecture.

There is one motion, however, which claims a middle place between the
voluntary and involuntary; I mean respiration. This action is so far
under the command of the will, that it may be suspended, increased,
or diminished in strength and frequency: but we can only suspend it
for a very short time; and it goes on regularly during sleep, and in
general, even when we are awake, without the intervention of the
will; its continuation being always necessary, as we have already
seen, to support life.

The motion of the fluids in the living body is regulated by very
different laws, from those which govern the motion of ordinary
fluids, that depend upon their gravity and fluidity: these last have
a general centre of gravitation to which they incessantly tend. Their
motion is from above downwards, when not prevented by any obstacles;
and when they meet with obstruction, they either stop till the
obstacle is removed, or escape where they find the least resistance.
When they have reached the lowest situations, they remain at rest,
unless acted upon by some internal impulse, which again puts them in
motion.

But the motion of the fluids in an animal body, is less uniform,
constant, and regular; it takes place upwards as well as downwards,
and overcomes numerous obstacles; it carries the blood from the
interior parts of the body to the surface, and from the surface back
again to the internal parts; it forces it from the left side of the
body to the right, and with such rapidity that not a particle of the
fluid remains an instant in the same place.

The principal organ concerned in the circulation of the blood, is the
heart; which is a hollow muscle, of a conical figure, with two
cavities, called ventricles; this organ is situated in the thorax or
chest; its apex or point is inclined downwards and to the left side,
where it is received in a cavity of the left lobe of the lungs.

At the basis of the heart on each side are situated two cavities,
called auricles, to receive the blood; and these contracting, force
the blood into the ventricles, which are two cavities in the heart,
separated from each other by a strong muscular partition. The cavity
which is situated on the right side of the heart, is called the right
ventricle, and that on the left the left ventricle. From the right
ventricle of the heart issues a large artery, called the pulmonary
artery, which goes to the lungs, and is there divided and subdivided
into a vast number of branches, the extremities of which are too
small to be visible. These ultimate ramifications unite again into
larger branches; these again into branches still larger, and so
continually, till at last they form four tubes, called the pulmonary
veins, which are inserted into the left auricle of the heart,

From the left ventricle of the heart there issues another large
artery, called the aorta, which, in its passage, sends off branches
to the heart, arms, legs, head, and every other part of the body.
These branches, in the course of their progress, are divided and
subdivided into innumerable minute ramifications, the last of which
are invisible. These small ramifications unite again into branches
continually larger and larger, till they form two great tubes, called
the venae cavae; which large veins are inserted into the right
auricle of the heart; where a vein, termed the coronary vein of the
heart, which returns the blood from the heart itself, also
terminates.

From what has been said, it will be evident, that strictly speaking,
there are only two arteries and seven veins in the body; one
pulmonary artery, which carries the blood from the right ventricle of
the heart to the lungs, and four pulmonary veins, which bring it back
again; then the aorta or large artery, which carries the blood from
the left ventricle of the heart to all parts of the body; the two
venae cavae, and the coronary vein of the heart, which bring it back
again.

At the beginning of both arteries, where they leave the heart, are
placed valves, which allow the blood to flow freely from the heart
into the arteries, but which prevent its return to the heart. There
are likewise valves between the auricles and ventricles, which permit
the blood to flow from the former into the latter, but prevent its
return into the auricles. The veins are likewise furnished with
valves, which allow the blood to flow from their minute branches
along the larger toward the heart, but prevent its returning to these
minute branches.

The blood being brought back from all parts of the body into the
right auricle of the heart, distends this cavity, and thus causes it
to contract; this auricle, by contracting, forces the blood into the
right ventricle; this muscular cavity being distended and irritated
by the blood, contracts, and propels the blood through the pulmonary
artery into the lungs: from hence it is brought back by the pulmonary
veins, to the left auricle of the heart, by whose contraction it is
forced into the left ventricle. The contraction of this ventricle
propels the blood, with great force, into the aorta, through the
innumerable ramifications of which, it is carried to every part of
the body, and brought back by veins, which accompany these arterial
ramifications, and form the venae cavae, which conduct the blood into
the right auricle of the heart, from whence it is again sent into the
right ventricle, which sends it through the pulmonary artery, to the
lungs; the pulmonary veins bring it back again to the heart, from
whence it is propelled through the aorta, to all parts of the body:
thus running a perpetual round, called the circulation of the blood.

Thus then we see, that the circulation consists of two circles or
stages, one through the lungs, which may be called the pulmonary, or
lesser circle, and the other through all parts of the body, which may
be termed the aortal, or greater circle.

That the blood circulates in this manner, is evident, from the valves
placed at the origin of the arteries, and in the large branches of
the veins, which prevent the return of the blood to the heart, in any
other manner than that I have described. This is likewise evident, in
the common operation of blood letting: when the arm is tied, the vein
swells below the ligature, instead of above, and we do not make the
opening above the ligature, or on the side next the heart. If the
vein were opened above the ligature, it would not bleed. For it only
swells next the hand, which shows that the blood does not flow into
the vein downwards from the heart, but upwards from the hand.

If the ligature be too tight, the blood will not flow through the
opening in the vein. The reason of this, is, that the artery is
compressed, in this case, as well as the vein; and as the veins
derive their blood from the arteries, it follows that if the blood's
motion be obstructed in the latter, none can flow from them into the
former: when we wish to open an artery, the orifice must be made
above the ligature.

Another proof of the circulation being performed in this manner, is
derived from microscopic observations, on the transparent parts of
animals, in which the blood can be seen to move towards the
extremities, along the arteries, and return by the veins.

The blood, however, does not flow out of the heart into the arteries
in a continued stream, but by jets, or pulses; when the ventricles
are filled with blood from the auricles, this blood stimulates them,
and thereby causes them to contract; by such contraction, they force
the blood, which they contain, into the arteries; this contraction is
called the systole of the heart. As soon as they have finished their
contraction, they relax, till they are again filled with blood from
the auricles, and this state of relaxation of the heart, is called
the diastole.

This causes the pulsation or beating of the heart. The arteries must,
of course, have a similar pulsation, the blood being driven into them
only by starts; and accordingly we find it in the artery of the
wrist; this beating we call the pulse; the like may also be observed
in the arteries of the temples, and other parts of the body. The
veins, however, have no pulsation, for the blood flowing on, in an
uninterrupted course, from smaller tubes to wider, its pulse becomes
entirely destroyed.

The different cavities of the heart do not contract at the same time;
but the two auricles contract together, the ventricles being at that
time in a state of relaxation; these ventricles then contract
together, while the auricles become relaxed.

Both the arteries and veins may be compared to a tree, whose trunk is
divided into large branches; these are subdivided into smaller, the
smaller again into others still smaller; and we may observe,
likewise, that the sum of the capacities of the branches, which arise
from any trunk, is always greater than the capacity of the trunk.

The minutest branches of the arteries, being reflected, become veins,
or else they enter veins that are already formed, by anastomosis, as
it is called; the small veins continually receiving others, become,
like a river, gradually larger, till they form the venae cavae, which
conduct the blood to the heart.

Anatomical injections prove, that the last branches of the arteries
terminate in the beginning of veins; but it is the opinion of many
celebrated physiologists, that the arteries carry the blood to the
different parts of the body to nourish them, and that the veins
commence by open mouths, which absorb or suck up what is superfluous,
and return it back to the heart.

From what has been said, it must be evident that there is a
considerable resemblance between the circulation of the blood in the
animal body, and the circulation of the aqueous fluid on the surface
of the globe. In the latter case the water is raised from the ocean,
by the heat of the sun, and poured down upon the dry land, in minute
drops, for the nourishment and economy of its different parts. What
is superfluous is collected into little rills; these meeting with
others, form brooks; the union of which produce rivers, that conduct
the water to its original source, from which it is again circulated.

In the same manner, the blood is sent by the heart to different parts
of the body, for the nourishment and economy of its different parts;
what is superfluous is brought back by veins, which, continually
uniting, form those large trunks, which convey the vital fluid to the
heart.

The blood does not circulate, however, in the manner which I have
mentioned, in all parts of the body; for that which is carried by
arteries to the viscera, serving for digestion, such as the stomach,
bowels, mesentery, omentum, and spleen, is collected by small veins
which unite into a large trunk called the vena portarum; this vein
enters the liver, and is subdivided in it like an artery,
distributing through the liver a great quantity of blood, from which
the bile is secreted: and, having served this purpose, the blood is
collected by small veins; these unite and form the hepatic vein,
which pours the blood into the vena cava, to be conducted to the
heart.

The reason of this deviation, is probably, to diminish the velocity
of the blood in the liver, for the secretion of the bile; which could
not have been effected by means of an artery.

The force which impels the blood, is, first, the contraction of the
heart, which propels the blood into the arteries with great velocity;
but this is not the only force concerned in keeping up the
circulation; this is evident, from the diminished heat, and weakened
pulse, in a paralytic limb, which ought not to take place, if the
blood were propelled merely by the action of the heart.

The arteries are possessed of an elastic and muscular power, by means
of which they contract when they are distended or stimulated. It is
however by the muscular power alone, that they assist in propelling
the blood; for the elasticity of their coats can serve no other
purpose than preserving the mean diameter of the vessel. If we
suppose the arteries to be dilated by the blood, poured into them by
the heart, they will, by their contraction, as elastic tubes,
undoubtedly propel the blood: but supposing them to be perfectly
elastic, the force of the heart will be just as much diminished in
dilating them as the force of the blood is increased by their
contraction. We are not however acquainted with any substance
perfectly elastic, or which restores itself with a force equal to
that with which it was distended: hence the elastic power of the
arteries will subtract from, instead of adding to, the power of the
heart. It is evident, therefore, that it must be by the muscular
power of the arteries, which causes them to contract like the heart,
that they propel the blood.

That such is the case, appears from the muscular structure of the
arteries observed by anatomists; as also from the effects of
mechanical irritation of their coats, which causes them to contract;
this is likewise evident from the inflammation produced by the
application of stimulating substances to particular parts; for
instance, cantharides and mustard. It appears likewise, from the
secretion in some parts being preternaturally increased, while the
motion of the general mass of the blood continues unaltered.

The contraction of the arteries always propels the blood towards the
extreme parts of the body: this must necessarily happen, because the
valves at the origin of the arteries prevent its return to the heart,
it must therefore move in the direction in which it finds least
resistance.

If it were not for this muscular power of the arteries, the force of
the heart would not alone be able to propel the blood to the extreme
parts of the body, and overcome the different kinds of resistance it
has to encounter. Among the causes that lessen the velocity of the
blood, may be mentioned the increasing area of the artery; for it was
before observed, that the sum of the cavities of the branches from
any trunk exceeded the cavity of the trunk: and from the principles
of hydrostatics, the velocities of fluids, propelled by the same
force, in tubes of different diameters, are inversely as the squares
of the diameters, so that in a tube of double the diameter, the
velocity will only be one fourth; in one of the triple, only one
ninth: and since the arteries may be looked upon as conical, it is
evident that the velocity of the blood must be diminished from this
cause.

The curvilinear course of the arteries likewise gives considerable
resistance; for at every bending the blood loses part of its momentum
against the sides; and this loss is evidently proportioned to the
magnitude of the angle, at which the branch goes off. Convolutions
are frequently made, in order to diminish the force of the blood in
particular organs; this is especially the case with the carotid
artery before it enters the brain.

The angles which the ramifications of the arteries make, are greater
or more obtuse nearer the heart, and more acute as the distance
increases; by which means the velocity of the blood is rendered more
equal in different parts.

The anastomosing or union of different branches of arteries, likewise
retards the velocity of the blood, the particles of which, from
different vessels, impinging, disturb each other's motion, and
produce a compound force, in which there is always a loss of
velocity: and it is evident, from the composition of forces, that
this loss must be proportioned to the obliquity of the angle at which
the vessels unite.

The adhesion of the blood to the sides of the vessels, likewise
causes a loss of velocity in the minuter branches, which may be owing
to a chemical affinity: the viscidity or imperfect fluidity of the
blood is another retarding cause. All these causes united, would
render it impossible for the heart to propel the blood with the
velocity with which it moves in the very minute branches of the
arteries, if these arteries were not endowed with a living muscular
power like the heart, by which they contract and propel their
contents.

In the veins, the motion of the blood is occasioned partly by the vis
a tergo, and partly by the contraction of the neighbouring muscles,
which press upon the veins; and these veins being furnished with
valves, the return of the blood towards the arteries is prevented; it
must therefore move towards the heart.

That the contraction of the muscles of the body tends very much to
promote the circulation of the blood, is evident, from the increase
of the circulation from exercise, and likewise from the languid
motion of the blood in sedentary persons, and those given to
indolence. Hence we may account for the different diseases to which
such persons are subject, and know how to apply the proper remedies.
Hence likewise, we see the reason why rest is so absolutely necessary
in acute and inflammatory diseases, where the momentum of the blood
is already too great.

It has been doubted by anatomists, whether the veins were possessed
with muscular power; but this seems now to be confirmed. Haller found
the vena cava near the heart to contract on the application of
stimulants, though he could see no muscular fibres; these, however,
have been discovered by succeeding anatomists.

The magnitude of the veins is always greater than that of the
corresponding arteries; hence the velocity of the blood must be less
in the veins; and hence likewise we may account for their want of
pulsation; for the action of the heart upon the arteries is at first
very great; but as we recede from the heart, this effect becomes less
perceptible; the arterial tube increases both in size and
muscularity, in proportion to its distance from the source of
circulation. The powers of the heart are spent in overcoming the
different resistances which I have noticed, before the blood enters
the veins; hence the blood will flow uniformly in these last.

The blood is subject in the veins to retarding causes, similar to
those which operate in the arteries, but perhaps not in an equal
degree; for the flexures are less frequent in the veins than in the
arteries. As the capacity of the arterial tube increases with its
distance from the heart, the velocity, from this cause, as has
already been observed, is continually diminished; but a contrary
effect takes place in the veins; for the different branches uniting,
form trunks, whose capacities are smaller than the sums of the
capacities of the branches, hence the velocity of the blood in the
veins will increase as it approaches the heart.

Another retarding cause may be mentioned, namely, gravity, which acts
more on the venous than the arterial system. The effects of gravity
on the veins may be exemplified, by a ring being pulled off the
finger with ease when the hand is elevated; also by the swellings of
the feet that occur in relaxed habits, which swellings increase
towards night, and subside in the morning, after the body has been in
a horizontal posture for some hours.

In weak persons, the frequency of the pulse is increased by an erect
posture, which may probably depend on gravity; as we know, from the
observations of Macdonald and others, that an erect posture will make
a difference of 15 or 20 beats in a minute. The experiments alluded
to, were made by gently raising a person fastened to a board, where
there being no muscular exertion, respiration would not be increased;
so that the whole effect was probably owing to gravity accelerating
the column of arterial blood.

The inverted posture produces a still more remarkable effect in
accelerating the pulse, than the erect, for it sometimes causes it to
beat 10 or 12 times more in the former case than in the latter.

While we are on this subject, it may not be improper to take notice
of the effects of swinging on the circulation, which have been found
by Dr. Carmichael Smyth, and others, to diminish the strength and
velocity to such a degree, as to bring on fainting. These effects
have never been satisfactorily accounted for; but they would seem to
admit of an easy explanation on mechanical principles: they are
undoubtedly owing, at least in a great measure, to the centrifugal
force acquired by the blood.

By a centrifugal force, I mean, the tendency which revolving bodies
have to fly off from the centre, which arises from their tendency to
move in a straight line, agreeably to the laws of motion. Hence a
tumbler of water may be whirled in a circle vertically without
spilling it; the centrifugal force pushing the water against the
bottom of the tumbler. In the same manner when the human body is made
to revolve vertically in the arch of a circle, this centrifugal force
will propel the blood from the head and heart towards he extremities;
hence the circulation of the blood will be weakened, and the energy of
the brain diminished. The contrary, however, will take place on a
horizontal swing, as I have frequently observed, both on myself and
others; for the centrifugal force in this case will propel the blood
from the extremities towards the head.

It has been already observed, that the pulsations of the artery which
we feel at the wrist, are occasioned by its alternate dilatations and
contractions, which vary according to the strength and regularity of
the circulation, which is liable to be affected by the smallest
changes in the state of health. Hence physicians make use of the
pulse as a criterion whereby to judge of the health of the body. And we
may observe that there are few more certain characteristics of the
state of the body than the pulse; yet the conclusions that have been
drawn from it have often been erroneous; and this has arisen from
trusting to observation without the aid of reason.

That we may better understand the phenomena of the pulse, I shall lay
down the following postulata. 1st. It is now generally believed, that
every part of the arterial system is endowed with irritability, or a
power of contracting on the application of a stimulus, and that the
blood acting on this contractibility, if the term may be allowed,
causes contraction; and that the alternate relaxation and contraction
gives the phenomenon pulsation. 2d. The greater the action of the
stimulus of the blood, the greater will be the contraction, that is,
the nearer will the sides of the artery approach towards the axis.
3d. That the velocity with which a muscular fibre, in a state of
debility, contracts, is at least equal to that with which a fibre in
a state of strength contracts, is a fact generally allowed by
physiologists.

We shall afterwards see, that a deficient action of stimulus on the
vessels may arise, either directly from diminishing the quantity of
blood contained in them, or indirectly, from the application of too
great a stimulant power, which has diminished the capability of
contracting inherent in the vessels.

From these postulata, it will be evident, that the greater the action
of the arteries, that is, the more powerful their contraction, the
longer will be the intervals between the pulsations.

For the velocity being at least equal in debility and in strength,
the times between the pulsations will be proportioned to the approach
of the sides of the artery towards its axis: but the approach of the
sides towards the axis is greater when the arteries are in a state of
vigour than when debilitated; consequently the intervals between the
pulsations will be greater when the arteries are in a state of vigour
than when debilitated.

Hence it is evident, that a frequency of pulse must generally
indicate a diminished action or debility; while a moderate slowness
indicates a vigorous or just action.

Hence likewise the opinion of increased action, which has been
supposed to take place in fevers, because a frequent pulse was
observed, must be false, because the frequency arises from a directly
opposite state, and indicates a diminished action of the vascular
system.

In a sound and adult man the frequency of the pulse is about seventy
beats in a minute; and in an infant, within the first five or six
months, the pulse is seldom less than one hundred and twenty, and
diminishes in frequency as the child grows older. But though seventy
beats in the minute may be taken as a general standard; yet in
persons of irritable constitutions the frequency is greater than
this, and many, who are in the prime of life, have the pulse only
between fifty and sixty.

It is generally observed, that the pulse is slower in the morning,
that it increases in frequency till noon, after dinner it again
becomes slow, and in the evening its frequency returns, which
increases till midnight.

These phenomena may be rationally explained on the principles just
laid down. When we rise in the morning, the contractibility being
abundant, the stimulus of the blood produces a greater effect, the
pulse becomes slow, and the contractions strong; it becomes more
frequent, however, till dinner time, from a diminished
contractibility; after dinner, from the addition of the stimulus of
food and chyle, it again decreases in frequency, and becomes slow
till the evening, when its frequency returns, because the
contractibility becomes exhausted: and this frequency continues till
the vital power have been recruited by sleep.

By the same principles it is easy to explain the quickness of the
pulse in infancy, its gradual decrease till maturity, its slowness
and strength during the meridian of life, and the return of its
frequency during the decline.

Having now described the phenomena of the circulation, it will be
proper to examine the changes produced by this function on the blood;
and, in the first place, it may be observed, that the blood which
returns by the vena cava to the heart, is of a dark colour inclining
to purple; while that which passes from the left ventricle into the
arteries, is of a bright vermilion hue. The blood which is found in
the pulmonary artery has the same dark purple colour with that in the
vena cava, while that in the pulmonary vein resembles the aortal
blood in its brightness. Hence it would appear, that the blood,
during its passage through the lungs, has its colour changed from a
dark purple to a bright vermilion, in which state it is brought by
the pulmonary vein to the left auricle of the heart; this auricle,
contracting, expels the blood into the corresponding ventricle, by
whose action, and that of the arteries, it is distributed to all
parts of the body. When it returns, however, by the veins, it is
found to have lost its fine bright colour. It would appear,
therefore, that the blood obtains its red colour during its passage
through the lungs, and becomes deprived of it during its circulation
through the rest of the body.

That the blood contains iron, may be proved by various experiments:
if a quantity of blood be exposed to a red heat in a crucible, the
greatest part will be volatilised and burnt; but a quantity of brown
ashes will be left behind, which will be attracted by the magnet. If
diluted sulphuric acid be poured on these ashes, a considerable
portion of them will dissolve; if into this solution we drop tincture
of galls, a black precipitate will take place, or if we use prussiate
of potash, a precipitate of prussian blue will be formed. These facts
prove, beyond doubt, that a quantity of iron exists in the blood.

I shall not now particularly inquire how it comes there; it may
partly be taken into the blood along with the vegetable and animal
food, which is received into the stomach; for the greatest part of
the animal and vegetable substances, which we receive as food,
contain a greater or less quantity of iron. Or it may be partly
formed by the animal powers, as would appear from the following
circumstance. The analysis of an egg, before incubation, affords not
the least vestige of iron, but as soon as the chick exists, though it
has been perfectly shut up from all external communication, if the
egg be burnt, the ashes will be attracted by the magnet.

But, however we may suppose the blood to obtain its iron, it
certainly does contain it; if the coagulable lymph and serum of the
blood be carefully freed from the red particles, by repeated washing,
the strictest analysis will not discover in either of them a particle
of iron, while the red globules thus separated will be found to
contain a considerable quantity of this metal.

That the red colour of the blood depends upon iron, appears likewise
from the experiments of Menghini, which show, that the blood of
persons who have been taking chalybeate medicines for some time, is
much more florid that it is naturally; the same is agreeable to my
own observation. A late analysis, by Fourcroy, has likewise proved,
that the red colour of the blood resides in the iron; but, though the
red colour of the blood may reside in the iron which it contains, we
shall find that this colour is likewise connected with oxidation.

If the dark coloured blood, drawn from the veins, be put under a
vessel containing oxygen gas, its surface will immediately become
florid, while the bulk of the gas will be diminished. Mr. Hewson
enclosed a portion of a vein between two ligatures, and injected into
it a quantity of oxygen gas; the blood, which was before dark
coloured, instantly assumed the hue of arterial blood. Thuvenal put a
quantity of arterial blood under the receiver of an air pump; on
exhausting the air it became of the dark colour of venous blood; on
readmitting the air, it became again florid. He put it under a
receiver filled with oxygen gas, and found the florid colour much
increased.

Dr. Priestly exposed the blood of a sheep successively to oxygen gas,
atmospheric air, and carbonic acid gas; and found, that in oxygen gas
its colour became very florid, less so in atmospheric air, and in
carbonic acid gas it became quite black. He filled a bladder with
venous blood, and exposed it to oxygen gas; the surface in contact
with the bladder immediately became florid, while the interior parts
remained dark coloured.

All these facts prove, that the red colour which the blood acquires
in the lungs, is owing to the oxygen, which probably combines with
it, and the last mentioned fact shows, that oxygen will act on the
blood, even though a membrane similar to the bladder, be interposed
between them.

The same effect, probably, takes place in the lungs; the blood is
circulated through that organ by a number of fine capillary arteries;
and it is probable that the oxygen acts upon the blood through the
membranes of these arteries, in the same manner that it does through
the bladder.

In short, it seems likely, that the blood, during its circulation
through the lungs, becomes combined with oxygen; that this oxidated
blood, on its return to the heart, is circulated by the arteries to
all parts of the body; and that, during this circulation, its oxygen
combines with the hydrogen and carbon of the blood, and perhaps with
those parts of the body with which it comes into contact; it is
therefore brought back to the heart, by the veins, of a dark colour,
and deprived of the greatest part of its oxygen.

This is the most probable theory, in the present state of our
knowledge; it was proposed by Lavoisier, who imagines the focus of
heat, or fireplace to warm the body, to be in the lungs: others,
however, have thought it more consonant to facts, to suppose, that,
instead of the oxygen uniting with carbon and hydrogen in the lungs,
and there giving out its heat, the oxygen is absorbed by the blood,
and unites with these substances during the circulation, so that heat
is produced in every part of the body; and this doctrine seems
certainly supported by several facts and experiments.

The circulation of the blood, though so simple and beautiful a
function, was unknown to the ancient physicians, and was first
demonstrated by our countryman, Harvey; when he first published his
account of this discovery, he met with the treatment which is
generally experienced by those who enlighten and improve the comfort
of their fellow creatures, by valuable discoveries. The novelty and
merit of this discovery drew upon him the envy of most of his
contemporaries in Europe, who accordingly opposed him with all their
power; and some universities even went so far, as to refuse the
honours of medicine to those students, who had the audacity to defend
this doctrine; but afterwards, when they could not argue against
truth and conviction, they attempted to rob him of the discovery, and
asserted that many of the ancient physicians, and particularly
Hippocrates, were acquainted with it. Posterity, however, who can
alone review subjects of controversy without prejudice, have done
ample justice to his memory.

LECTURE IV.
DIGESTION, NUTRITION, &c.

The human body, by the various actions to which it is subject, and
the various functions which it performs, becomes, in a short time,
exhausted; the fluids become dissipated, the solids wasted, while
both are continually tending towards putrefaction. Notwithstanding
which, the body still continues to perform its proper functions,
often for a considerable length of time; some contrivance, therefore,
was necessary to guard against these accelerators of its destruction.
There are two ways in which the living body may be preserved; the one
by assimilating nutritious substances, to repair the loss of
different parts; the other to collect, in secretory organs, the
humours secreted from these substances.

We are admonished of the necessity of receiving substances into the
body, to repair the continual waste, by the appetites of hunger and
thirst. For the stomach being gradually emptied of its contents, and
the body, in some degree, exhausted by exercise, we experience a
disagreeable sensation in the region of the stomach, accompanied by a
desire to eat, at first slight, but gradually increasing, and at last
growing intolerable, unless it be satisfied.

When the fluid parts have been much dissipated, or when we have
taken, by the mouth, any dry food, or acrid substance, we experience
a sensation of heat in the fauces, and at the same time a great
desire of swallowing liquids. The former sensation is called hunger,
and the latter thirst.

From the back part of the mouth passes a tube, called the oesophagus
or gullet, its upper end is wide and open, spread behind the tongue
to receive the masticated aliment: the lower part of this pipe, after
it has passed through the thorax, and pierced the diaphragm, enters
the stomach, which is a membranous bag, situated under the left side
of the diaphragm: its figure nearly resembles the pouch of a bagpipe,
the left end being most capacious; the upper side is concave, and the
lower convex: it has two orifices, both on its upper part; the left,
which is a continuation of the oesophagus, and through which the food
passes into the stomach, is named cardia; and the right, through
which the food is conveyed out of the stomach, is called pylorus:
within this last orifice is a circular valve, which, in some degree,
prevents the return of the aliment into the stomach.

From the pylorus, or right orifice of the stomach, arise the
intestines, or bowels, which consist of a long and large tube, making
several circumvolutions, in the cavity of the abdomen; this tube is
about five or six times as long as the body to which it belongs.
Though it is one continued pipe, it has been divided, by anatomists,
into six parts, three small, three large. The three small intestines
are the duodenum, the jejunum, and the ileum; the duodenum commences
at the pylorus, and is continued into the jejunum, which is so called
from its being generally found empty: the ileum is only a
prolongation of the jejunum, and terminates in the first of the great
intestines, called the caecum. The other great guts are the colon and
the rectum.

The whole of what has been described is only a production of the same
tube, beginning at the oesophagus. It is called by anatomists the
intestinal canal, or prima via, because it is the first passage of
the food. It has circular muscular fibres, which give it a power of
contracting when irritated by distension; and this urges forward the
food which is contained in it. This occasions a worm like motion of
the whole intestines, which is called their peristaltic motion.

The mesentery is a membrane beginning loosely on the loins, and
thence extending to all the intestines; which it preserves from
twisting by their peristaltic motion. It serves also to sustain all
the vessels going to and from the intestines, namely the arteries,
veins, lacteals, and nerves; it also contains several glands, called,
from their situation, mesenteric glands.

The lacteal vessels consist of a vast number of fine pellucid tubes,
which arise by open mouths from the intestines, and proceeding thence
through the mesentery, they frequently unite, and form fewer and
larger vessels, which pass through the mesenteric glands, into a
common receptacle or bag, called the receptacle of the chyle. The use
of these vessels is to absorb the fluid part of the digested aliment,
called chyle, and convey it into the receptacle of the chyle, that it
may be thence carried through the thoracic duct into the blood.

The receptacle of the chyle is a membranous bag, about two thirds of
an inch long, and one third of an inch wide, at its superior part it
is contracted into a slender membranous pipe, called the thoracic
duct, because its course is principally through the thorax; it passes
between the aorta and the vena azygos, then obliquely over the
oesophagus, and great curvature of the aorta, and continuing its
course towards the internal jugular vein, it enters the left
subclavian vein on its superior part.

There are several other viscera besides those I have described, which
are subservient to digestion; among these may be mentioned the liver,
gall bladder, and pancreas. The liver is the largest gland in the
body, and is situated immediately under the diaphragm, principally on
the right side. Its blood vessels that compose it as a gland, are the
branches of the vena portarum, which, as I mentioned in the last
lecture, enters the liver and distributes its blood like an artery.
From this blood the liver secretes the bile, which is conveyed by the
hepatic duct, towards the intestines: before this duct reaches the
intestines, it is joined by another, coming from the gall bladder:
these two ducts uniting, form a common duct, which enters the
duodenum obliquely, about four inches below the pylorus of the
stomach.

The gall bladder, which is a receptacle of bile, is situated between
the stomach and the liver; and the bile which comes from the liver,
along the hepatic duct, partly passes into the duodenum, and partly
along the cystic duct into the gall bladder. When the stomach is
full, it presses on the gall bladder, which will squeeze out the bile
into the duodenum at the time when it is most wanted.

The bile is a thick bitter fluid, of a yellowish green colour,
composed chiefly of soda and animal oil, forming a soap; and it is
most probably in consequence of this saponaceous property that it
assists digestion, by causing the different parts of the food to
unite together by intermediate affinity. When the bile is prevented
from flowing into the intestines, by any obstruction in the ducts,
digestion is badly performed, costiveness takes place, and the
excrements are of a white colour, from being deprived of the bile.
This fluid, stagnating in the gall bladder, is absorbed by the
lymphatics, and carried into the blood, communicating to the whole
surface of the body a yellow tinge, and other symptoms of jaundice.

The jaundice therefore is occasioned by an obstruction to the passage
of the bile into the intestines, and its subsequent absorption into
the blood: this obstruction may be caused either by concretions of
the bile, called gall stones, or by a greater viscidity of the fluid,
or by a spasm, or paralysis of the biliary ducts.

The pancreas, or sweet bread, is a large gland lying across the upper
and back part of the abdomen, near the duodenum. It has a short
excretory duct, about half as wide as a crow quill, which enters the
duodenum at the same place where the bile duct enters it.

The food being received into the mouth, is there masticated or broken
down, by the teeth, and impregnated with saliva, which is pressed out
of the salivary glands, by the motions of the jaw and the muscles of
the mouth. It then descends, through the oesophagus, into the
stomach, where it becomes digested, and, in a great measure,
dissolved, by the gastric juice, which is secreted by the arteries of
the stomach. It is then pushed through the pylorus, or right orifice
of the stomach into the duodenum, where it becomes mixed with the
bile from the gall bladder and liver, and the pancreatic juice from
the pancreas. These fluids seem to complete the digestion: after
this, the food is continually moved forwards by the peristaltic
motion of the intestines.

The chyle, or thin and milky part of the aliment, being absorbed by
the lacteals, which rise, by open mouths, from the intestines, is
carried into the receptacle of the chyle, and from thence the
thoracic duct carries it to the subclavian vein, where it mixes with
the blood, and passes with it to the heart.

The food of animals is derived from the animal or vegetable kingdoms.
There are some animals which eat only vegetables, while others live
only on animal substances. The number and form of the teeth, and the
structure of the stomach, and bowels, determine whether an animal be
herbivorous, or carnivorous. The first class have a considerable
number of grinders, or dentes molares; and the intestines are much
more long and bulky; in the second class, the cutting teeth are
predominant, and the intestines are much shorter.

Man seems to form an intermediate link between these two classes: his
teeth, and the structure of the intestines, show, that he may subsist
both on vegetable and animal food; and, in fact, he is best nourished
by a proper mixture of both. This appears from those people who live
solely on vegetables, as the Gentoo tribes, and those who subsist
solely on animals, as the fish eaters of the northern latitudes,
being a feebler generation than those of this country, who exist on a
proper mixture of both. A due proportion, therefore, of the two kinds
of nourishment, seems undoubtedly the best.

Having taken a general view of the course of the aliment into the
blood, I shall now examine more particularly, how each part of the
organs concerned in digestion, or connected with that function,
contributes to that end.

The food being received into the mouth, undergoes various
preparations, which fit it for those changes it is afterwards to
undergo. By the teeth the parts of it are divided and ground,
softened and liquified by the saliva, and properly compressed by the
action of the tongue and mastication.

The mouth, in most animals, is armed with very hard substances,
which, by the motion of the lower jaw, are brought strongly into
contact. Those parts of the teeth which are above the sockets, are
not simply bony, they are much harder than the bones, and possess the
property of resisting putrefaction, as long as this hard crust
continues to cover them. The teeth are divided into three classes:
1st. The cutting teeth, which are sharp and thin, and which serve to
cut or divide the food: 2nd. The canine teeth, which serve to tear it
into pieces still smaller: 3rd. The grinders, which present large and
uneven surfaces, and actually grind the substance already broken down
by the other teeth. Birds, whom nature has deprived of teeth, have a
strong muscular stomach, called the gizzard, which serves the
purposes of teeth, and they even take into the stomach small pieces
of grit, to assist in grinding to a powder the grain that they have
swallowed.

Among those parts of the mouth which contribute to the preparation of
the food, we must reckon the numerous glands which secrete saliva,
and which have therefore been called salivary glands. The saliva is a
saponaceous liquor, destitute of taste or smell, which is squeezed
out from these glands, and mixed with the food during mastication. In
the mouth, therefore, the food becomes first broken down by the
teeth, impregnated with saliva, and reduced to a soft pasty
substance, capable of passing with these, through the oesophagus,
into the stomach. It is here that it undergoes the change, which is
particularly termed digestion.

Digestion comprehends two classes of phenomena, distinct from each
other: 1st. Physical and chemical: 2nd. Organic and vital. The object
of the first, is to bring the alimentary substances to such a state
as is necessary, that they may be capable of the new combinations
into which they are to enter, to obtain the animal character. The
object of the second is, to produce those combinations which some
have thought to be very different from those produced by simple
chemical attractions.

The physical and chemical phenomena of digestion, relate chiefly,
1st. To the action of heat; 2ndly. To the dissolution of the
alimentary substances. The heat of the animal is such, as is well
fitted to promote solution.

That digestion is performed by solution, is evident, from several
experiments, particularly those made by Dr. Stevens, who enclosed
different alimentary substances in hollow spheres of silver, pierced
with small holes. These were swallowed, and after remaining some time
in the stomach, the contents were found dissolved. The great agent of
solution is the gastric juice, which possesses a very strong solvent
power. This juice is secreted by the arteries of the stomach; it may
be collected in considerable quantity, by causing an animal that has
been fasting for some time, to swallow small hollow spheres, or tubes
of metal filled with sponge.

This liquid does not act indiscriminately upon all substances; for if
grains of corn be put into a perforated tube, and a granivorous bird
be made to swallow it, the corn will remain the usual time in the
stomach without alteration; whereas if the husk of the grain be
previously taken off, the whole of it will be dissolved. There are
many substances likewise which pass unaltered through the intestines
of animals, and consequently are not acted upon by the gastric juice.
This is the case frequently with grains of oats, when they have been
swallowed by horses entire, with their husks on. This is the case
likewise with the seeds of apples and other fruits, when swallowed
entire by man; yet if these substances have been previously ground by
the teeth, they will be digested. It would appear therefore, that it
is chiefly the husk or outside of these substances which resists the
action of the gastric juice.

This juice is not the same in all animals; for many animals cannot
digest the food on which others live. Thus sheep live wholly on
vegetables, and if they are made to feed on animals, their stomachs
will not digest them: others again, as the eagle, feed wholly on
animal substances, and cannot digest vegetables.

The accounts of the experiments made on gastric juice are very
various: sometimes it has been found of an acid nature, at other
times not. The experiments of Spallanzani show, however, that this
acidity is not owing to the gastric juice, but to the food. The
result of his experiments, which have been very numerous, prove, that
the gastric juice is naturally neither acid nor alkaline. No
conclusion, however, can be drawn from these experiments made out of
the stomach, with respect to the nature of the gastric juice; nor do
the analyses which have been made of it throw any light on its mode
of action. But, from the experiments which have been made on
digestion, in the stomach, particularly by Spallanzani, the following
facts have been established.

The gastric juice attacks the surfaces of bodies, and combines
chemically with their particles. It operates with more energy and
rapidity, the more the food is divided, and its action increased by a
warm temperature. By the action of digestion, the food is not merely
reduced to very minute parts, but its chemical properties become
changed; its sensible properties are destroyed, and it acquires new
and very different ones. This juice does not act as a ferment; so far
from it, it is a powerful antiseptic, and even restores flesh which
is already putrid.

When the alimentary substances have continued a sufficient time in
the stomach, they are pushed into the intestines, where they become
mixed with the bile and pancreatic juice, as was before observed.
What changes are caused by these substances, we have yet to learn;
but there is no doubt, that they serve some important purposes. By
the peristaltic motion of the bowels, the alimentary matters thus
changed are carried along, and applied to the mouths of the lacteal
vessels, which open into the intestines, like a sponge, and by some
power, not well understood, absorb that part which is fitted for
assimilation, while the remainder is rejected as an excrement.

The lacteal vessels are furnished with valves, which allow a free
passage to the chyle from the intestines, but prevent its return. The
most inexplicable thing in this operation, is the power which these
vessels possess of selecting from the intestinal mass, those
substances which are proper for nutrition, and rejecting those which
are not.

These lacteal vessels, as was before observed, pass through the
mesentery, and their contents seem to undergo some important change
in the mesenteric glands. The chyle which passes through vessels,
appears to be an oily liquor, less animalised than milk, and its
particles seem to be held in solution by the intermedium of a
mucilaginous principle. It is conveyed along the thoracic duct in the
manner already described, and enters the blood slowly, and, as it
were, drop by drop, by the subclavian vein; in this way it becomes
intimately mixed with the blood, and combining with oxygen in the
lungs, it acquires a fibrous character, and becomes fit to nourish
the body.

We have now seen how the process of digestion is performed, at least,
so far as we are acquainted with it, and how its products are
conveyed into the blood. But to what purposes the blood is employed,
which is formed with so much care, we have yet to discover. It seems
to answer two purposes. The parts of which the body is composed,
namely, bones, muscles, ligaments, membranes, &c. are continually
changing: in youth they are increasing in size and strength, and in
mature age they are continually acting, and, consequently,
continually liable to waste and decay. They are often exposed to
accidents, which render them unfit for performing their various
functions; and even when no such accidents happen, it seems necessary
for the health of the system that they should be perpetually renewed.
Materials must therefore be provided for repairing, increasing, or
renewing all the various organs of the body. The bones require
phosphate of lime, and gelatine, the muscles fibrine, and the
cartilages and membranes albumen; and accordingly we find all these
substances contained in the blood, from whence they are drawn, as
from a storehouse, whenever they are wanted. The process by which
these different parts of the blood become various parts of the body
is called assimilation.

Over the nature of assimilation the thickest darkness still hangs;
all that we know for certain is, that there are some conditions
necessary to its action, without which it cannot take place. These
are, 1. A sound and uninterrupted state of the nerves. 2. A sound
state of the blood vessels. 3. A certain degree of tone or vigour in
the vessels of the part.

There remains yet to be noticed another set of vessels, connected
with the circulating and nutritive systems, called lymphatics. These
vessels are very minute, and filled with a transparent fluid: they
rise by open mouths in every cavity of the body, as well as from
every part of the surface, and the course of those from the lower
extremities, and indeed from most of the lower parts of the body, is
towards the thoracic duct, which they enter at the same time with the
lacteal vessels already described. They are furnished, like the
lacteals, with numerous valves, which prevent their contents from
returning towards their extremities.

The minute arteries in every part of the body exhale a colourless
fluid, for lubricating the different parts, and other important
purposes: and the lymphatic vessels absorb the superfluous quantity
of this fluid, and convey it back to the blood.

It must be evident therefore, that, if the lymphatics in any cavity
become debilitated, or by any other means be prevented from absorbing
this exhaled fluid, an accumulation of it will take place: the same
will happen, if the exhaling arteries be debilitated, so as to allow
a greater quantity of fluid to escape than the absorbents can take
up. When the balance between exhalation and absorption is destroyed,
by either or both of these means, a dropsy will be the consequence.

Before we finish the subject of digestion, I shall take a short view
of some of the morbid affections, attending this important function
of the animal economy.

A deficiency of appetite may arise, either from an affection of the
stomach, or a morbid state of the body: for there is such a sympathy
between the stomach and the rest of the system, that the first is
very seldom disordered, without communicating more or less disorder
to the system: nor can the system become deranged and the stomach
remain sound.

A want of appetite may arise from overloading the stomach, whereby
its digestive powers will be weakened. And this may be occasioned in
two ways. First, by taking food of the common quality in too great
quantity, which will certainly weaken the powers of the stomach. An
excellent rule, and one which if more attended to, would prevent the
dreadful consequences of indigestion, is always to rise from the
table with some remains of appetite. This is a rule applicable to
every constitution, but particularly to the sedentary and
debilitated.

The second way in which the stomach may be debilitated, is by taking
food too highly stimulating or seasoned; and this even produces much
worse effects than an over dose with respect to quantity. The tone of
the stomach is destroyed, and a crude unassimilated chyle is absorbed
by the lacteals, and carried into the blood, contaminating its whole
mass. Made dishes, enriched with hot sauces, stimulate infinitely
more than plain food, and bring on diseases of the worst kind: such
as gout, apoplexy, and paralysis. "For my part," says an elegant
writer, "when I behold a fashionable table set out in all its
magnificence, I fancy I see gouts, and dropsies, fevers, and
lethargies, with other innumerable distempers, lying in ambuscade
among the dishes."

All times of the day are not equally fitted for the reception of
nourishment. That digestion may be well performed, the functions of
the stomach and of the body must be in full vigour. The early part of
the day therefore is the proper time for taking nutriment; and, in my
opinion, the principal meal should not be taken after two or three
o'clock, and there should always be a sufficient time between each
meal to enable the stomach to digest its contents. I need not remark
how very different this is from the common practice of jumbling two
or three meals together, and at a time of the day likewise when the
system is overloaded. The breakfast at sunrise, the noontide repast
and the twilight pillow, which distinguished the days of Elizabeth,
are now changed for the evening breakfast, and the midnight dinner.
The evening is by no means the proper time to take much nourishment:
for the powers of the system, and particularly of the stomach, are
then almost exhausted, and the food will be but half digested.
Besides, the addition of fresh chyle to the blood, together with the
stimulus of food acting on the stomach, always prevents sleep, or
renders it confused and disturbed, and instead of having our worn out
spirits recruited, by what is emphatically called by Shakespeare,
"the chief nourisher in life's feast," and rising in the morning
fresh and vigorous, we become heavy and stupid, and feel the whole
system relaxed.

It is by no means uncommon, for a physician to have patients, chiefly
among people of fashion and fortune, who complain of being hot and
restless all night, and having a bad taste in the mouth in the
morning. On examination, I have found that, at least in nineteen
cases out of twenty, this has arisen from their having overloaded
their stomachs, and particularly from eating hot suppers; nor do I
recollect a single instance of a complaint of this kind in any person
not in the habit of eating such suppers.

The immoderate use of spirituous and fermented liquors, is still more
destructive of the digestive powers of the stomach; but this will be
better understood, when we have examined the laws by which external
powers act upon the body. The remarks I have made could not, however,
I think, have come in better, than immediately after our examination
of the structure of the digestive organs, as the impropriety of
intemperance, with respect to food, is thus rendered more evident.

The appetite becomes deficient from want of exercise, independently
of the other causes that have been mentioned. Of all the various
modes of preserving health, and preventing diseases, there is none
more efficacious than exercise; it quickens the motion of the fluids,
strengthens the solids, causes a more perfect sanguification in the
lungs, and, in short, gives strength and vigour to every function of
the body. Hence it is, that the Author of nature has made exercise
absolutely necessary to the greater part of mankind for obtaining
means of existence. Had not exercise been absolutely necessary for
our well being, says the elegant Addison, nature would not have made
the body so proper for it, by giving such an activity to the limbs,
and such a pliancy to every part, as necessarily produce those
compressions, extensions, contortions, dilatations, and all other
kinds of motion, as are necessary for the preservation of such a
system of tubes and glands.

We may, indeed, observe, that nature has never given limbs to any
animal, without intending that they should be used. To fish she has
given fins, and to the fowls of the air wings, which are incessantly
used in swimming and flying; and if she had destined mankind to be
eternally dragged about by horses, her provident economy would surely
have denied them legs.

The appetite becomes deficient on the commencement of many diseases,
but this is to be looked upon here rather as a salutary than as a
morbid symptom, and as a proof that nature refuses the load, which
she can neither digest nor bear with impunity.

In healthy people the appetite is various, some requiring more food
than others; but it sometimes becomes praeternaturally great, and
then may be regarded as a morbid symptom. The appetite may be
praeternaturally increased, either by an unusual secretion of the
gastric juice, which acts upon the coats of the stomach, or by any
acrimony, either generated in, or received into the stomach, or,
lastly, by habit, for people undoubtedly may gradually accustom
themselves to take more food than is necessary.

The appetite sometimes becomes depraved, and a person thus affected,
feels a desire to eat substances that are by no means nutritious, or
even esculent: this often depends on a debilitated state of the whole
system. There are some instances, however, in which this depravity of
the appetite is salutary; for example, the great desire which some
persons, whose stomachs abound with acid, have for eating chalk, and
other absorbent earths: likewise, the desire which scorbutic patients
have for grass, and other fresh vegetables. Appetites of this kind,
if moderately indulged in, are salutary, rather than hurtful.

The appetite for liquids as well as solids is sometimes observed to
be deficient, and sometimes too great. The former can scarcely be
considered as a morbid symptom, provided the digestion and health be
otherwise good. But when along with diminished thirst, the fauces and
tongue are dry, this deficiency may be regarded as a morbid and
dangerous symptom.

A more common morbid symptom, however, is too great thirst, which may
arise from a deficiency of fluids in the body, produced by violent
exercise, perspiration, too great a flow of urine, or too great an
evacuation of the intestines. A praeternatural thirst may likewise
arise from any acrid substance received into the stomach, which our
provident mother, nature, teaches us to correct by dilution; this is
the case with respect to salted meats, or those highly seasoned with
pepper. It may arise also from the stomach being overloaded with
unconcocted aliment, or from a suppressed or diminished secretion of
the salivary liquors in the mouth, which may arise from fever, spasm,
or affections of the mind; an increased thirst may likewise take
place, from a derivation or determination of the fluids to other
parts of the body; of this, dropsy affords an example. Indeed,
various causes may concur to increase the thirst; this is the case in
most fevers, where great thirst is occasioned by the dissipation of
the fluids of the body by heat, as well as by the diminished
secretion of the salivary humours which should moisten the mouth; to
which may be added, the heat and diminished concoctive powers of the
stomach.

From what has been said, we can easily understand, why praeternatural
thirst may sometimes be a necessary instinct of nature, at other
times, an unnecessary craving; why acids, acescent fruits, and weak
fermented liquors quench thirst more powerfully than pure water; and
lastly, why thirst, in some instances, may be relieved by emetics,
when it has resisted other remedies.

There is no organ of the body whose functions are so easily deranged
as those of the stomach; and these derangements prove a very fertile
source of disease; they ought, therefore, carefully to be guarded
against; and it is fortunate for us that we have this generally in
our power, if we would but avail ourselves of it: for most of the
derangements proceed from the improper use of food and drink, and a
neglect of exercise. Indeed, when we examine, we shall find but a
short list in the long catalogue of human diseases, which it is not
in our power to guard against and prevent: and which surely will be
guarded against, when their causes are known, and consequences
understood.

Among the diseases arising from a disordered state of the stomach and
indigestion, may be enumerated the following: great oppression and
anxiety, pain in the region of the stomach, with acid eructations,
nausea, vomiting, the bowels sometimes costive, sometimes too loose,
but seldom regular, depression of spirits, and all the long list,
commonly, but very improperly, termed nervous complaints, deficient
nutrition, and consequently general weakness, a relaxed state of the
solids, too great a tenuity of the fluids, headach, vertigo, and
many other complaints, too numerous to mention here.

The greatest misfortune, and which indeed arises from a want of
physiological knowledge, is, that people labouring under these
disorders, imagine they may be cured by the reception of drugs into
the stomach, and thus they are induced to receive into that organ,
half the contents of an apothecary's shop. There is no doubt that
these complaints may oftentimes be alleviated, and the cure assisted,
by medicines: thus, when the stomach is overloaded, this may be
removed by an emetic; the same complaint of the bowels may be removed
by a cathartic; and when the stomach is debilitated, we are
acquainted with some substances which will give it vigour, such as
iron, the Peruvian bark, and several kinds of bitters. These however,
when used alone, afford but temporary relief; and unless the cause
which induced the disease be removed, it will afterwards return with
redoubled violence. When the stomach, for instance, is debilitated by
want of exercise, I would ask, is there an article in the whole
materia medica, that can cure the complaints of sedentary people,
unless proper exercise at the same time be taken? With exercise tonic
remedies will undoubtedly accelerate the cure, but without it, they
will only make bad worse.

Again, when the stomach is debilitated by the use of improper food,
or the abuse of fermented or spirituous liquors, I would say to any
one who pretended to cure me of these complaints, without my making a
total change in the manner of living, that he either was ignorant of
the matter, or intended to deceive me.

In many cases the change of food must be strictly observed and
persevered in for a long time before a cure can be effected. In some
instances where the powers of the stomach were too weak to prevent
the food from undergoing perhaps both a vinous and acetous
fermentation, and where, in consequence of the disengagement of gas
and the formation of acid, the most excruciating pains were felt, the
most dreadful sickness experienced, and all the symptoms of
indigestion present in the most aggravated state; after almost every
article in the materia medica, generally employed, had been tried
without success, I have cured the patient merely by prohibiting food
subject to fermentation, such as vegetables, and enjoining a strict
use of animal food alone.

In short, wherever the cause of a disease can be ascertained, the
grand and simple secret in the cure, is the careful removal of that
cause.

LECTURE V.
OF THE SENSES IN GENERAL.

In this lecture, I propose to take a view of the connexion of man
with the external world, and shall endeavour to point out the manner
in which he becomes acquainted with external objects, by means of the
faculties called senses.

A human creature is an animal endowed with understanding, and reason;
a being composed of an organized body, and a rational mind.

With respect to his body, he is pretty similar to other animals,
having similar organs, powers, and wants. All animals have a body
composed of several parts, and, though these may differ from the
structure of the human body in some circumstances, to accommodate it
to peculiar habits and wants of the animal, still there is a great
similarity in the general structure.

The human body is feeble at its commencement, increases gradually in
its progress by the help of nourishment and exercise, till it arrives
at a certain period, when it appears in full vigour; from this time
it insensibly declines to old age, which conducts it at length to
dissolution. This is the ordinary course of human life, unless it
happens to be abridged either by disease or accident.

With regard to his reasoning faculties, or mind, man is eminently
distinguished from other animals. It is by this noble part that he
thinks, and is capable of forming just ideas of the different objects
that surround him: of comparing them together; of inferring from
known principles unknown truths; of passing a solid judgment on the
mutual agreement of things, as well as on the relations they bear to
him; of deliberating on what is proper or improper to be done; and of
determining how to act. The mind recollects what is past, joins it
with the present, and extends its views to futurity. It is capable of
penetrating into the causes of events, and discovering the connexion
that exists between them.

Governed by invariable laws, which connect him with all the beings,
whether animate or inanimate, among which he exists, man has certain
relations of convenience, and inconvenience, arising from the
particular constitution of the surrounding objects, as well as of his
own body. These external objects possess qualities which may be
useful or prejudicial to him; and his interest requires, that he
should be capable of ascertaining and appreciating these properties.

It is by sensation, or feeling, that the knowledge of external
objects is obtained. The faculty of feeling, modified in every organ,
perceives those qualities for which the peculiar structure of the
organ is fitted; and all the various sensations of sound, colour,
taste, smell, resistance, and temperature, find appropriate organs by
which they are perceived, without mixing with, or confounding each
other. External objects, therefore, act upon the parts of the body
endowed with feeling, and their action is diversified in such a
manner, as to give us a great number of sensations, which appear to
have no resemblance to each other, and which make us acquainted with
the various properties of surrounding objects.

It would not, however, have been sufficient for man, merely to have
possessed this power of perceiving the different properties of the
objects which surround him: it was necessary likewise, that he should
be possessed of motion, that he might be able to approach or avoid
them, to seize or repulse them, as it suited his convenience or
advantage. By the extreme mobility of his limbs, he is able to move
his body, and transport it from place to place; to bring external
objects nearer to him, to remove them to a greater distance, and to
place them in such situations and such circumstances, as may enable
them to act on each other, and produce the changes which he wishes.

The human body, therefore, may be regarded as a machine composed
(besides the moving parts which have formerly been noticed) of divers
organs upon which external objects act, and produce those impressions
which convince us of their presence, and make us acquainted with
their properties. These impressions are transmitted to the sentient
principle, or mind; and the faculty we possess of perceiving these
impressions has been called by physiologists, sensibility.

Sensation has generally been defined by metaphysicians to be a change
in the mind, of which we are conscious, caused by a correspondent
change in the state of the body. This definition, however, leaves the
matter where they found it, and throws no light whatever on the
nature of sensation; nor can we say any thing more concerning it,
than that, when the organs are in a sound state, certain sensations
are perceived, which force us to believe in the existence of external
objects, though there is no similarity whatever, nor any necessary
connexion, that we can perceive, between the sensation and the object
which caused it.

All the different degrees of sensation may be reduced to two kinds:
pleasant and painful. The nature of these two primitive modes of
sensation, is as little known to us as their different species: all
that can be said, is, that the general laws by which the body is
governed, are such, that pleasure is generally connected with those
impressions which tend to its preservation, and pain with those which
cause its destruction.

In a general point of view, sensibility may be regarded as an
essential property of every part of the living body, disposing each
part to perform those functions, the object of which is to preserve
the life of the animal. Sensibility presides over the most necessary
functions, and watches carefully over the health of the body: she
directs the choice of the air proper for respiration, and also of
alimentary substances; the mechanism of the secretions is likewise
placed under her power; and in the same way that the eye perceives
colours, and the ear sounds, so every animated and living part is
fitted to receive impressions from the objects appropriated to it.

That every part of the animal is endowed with sensibility, is evident
from a variety of facts, particularly from the action which follows
when a muscle taken out of the animal body is irritated by any
stimulus: this is evident, by a variety of facts mentioned by Whytt,
Boerhaave, and others, which show, that parts recently taken from the
animal body retain a portion of sensibility, which continues to
animate them, and render them capable of action for a considerable
time.

The primary organ of sensation appears to be the brain, its
continuation in the form of medulla oblongata and spinal marrow, and
the various nerves proceeding from these; and it seems now generally
agreed, that unless there be a free communication of nerves between
the part where the impression is made, and the brain, no sensation
will take place; for instance, if the nerves be cut or compressed.

In a sound body, sensation is caused, whenever a change takes place
in the state of the nervous power, whether that change be produced by
an external, or an internal cause. The former kind of sensation is
said to arise from impression or impulse, the latter from
consciousness.

Every impression or impulse is not, however, equally calculated to
produce sensation; for this purpose, a middle degree of impulse
appears the best. An impulse considerably less produces no sensation,
and one more violent may cause pain, but no proper sensation denoting
the presence or properties of external objects. Thus too small a
degree of light makes no impression on the optic nerve; and if the
object be too strongly illuminated, the eye is pained, but has no
proper idea of the figure or colour of the object. In the same way,
if the vibrations which give us an idea of sound, be either too quick
or too slow, we shall not obtain this idea. When the vibration is too
quick, a very disagreeable and irritating sensation is perceived, as
for instance, in the whetting of a saw: and on the other hand, when
the vibrations are too slow, they will not produce a tone or sound.
This might be proved of all the senses, and shows, that a certain
degree of impression is necessary to produce perfect sensation.

There is another circumstance likewise requisite to produce
sensation: it is not enough, that the impression should be of the
proper strength; it is necessary likewise, that it should remain for
some time, otherwise no sensation will be produced. There are many
bodies whose magnitude is amply sufficient to be perceived by the
eye; yet, by reason of their great velocity, the impulse they make on
any part of the retina is so short, that they are not visible. This
is proved by our not perceiving the motions of cannon and musket
balls, and many other kinds of motion. On this principle depends the
art of conjuration, or legerdemain; the fundamental maxim of those
who practise them, is, that the motion is too quick for sight.

If the impulse be of a proper degree, and be continued for a
sufficient length of time, the impression made by it will not
immediately vanish with the impulse which caused it, but will remain
for a time proportioned to the strength of the impulse. This, with
respect to sight, is proved by whirling a firebrand in a circular
manner, by which the impression of a circle is caused, instead of a
moving point: and, with respect to hearing, it may be observed, that
when children run with a stick quickly along railing, or when a drum
is beaten quickly, the idea of a continued sound is produced, because
the impression remains some time: for it is evident, that the sounds
produced in succession are perfectly distinct and insulated.

Sensation likewise depends, in a great measure, on the state of the
mind, and on the degree of attention which it gives. For if we are
engaged in attention to any object, we are insensible of the
impressions made upon us by others, though they are sufficiently
strong to affect us at other times. Thus, when our attention is fixed
strongly upon any particular object, we become insensible of the
various noises that surround us, though these may be sometimes very
loud. On the contrary, if our attention be upon the watch, we can
perceive slight, and almost neglected impressions, while those of
greater magnitude become insensible. The ticking of a clock becomes
insensible to us from repetition, but if we attend to it, we become
easily sensible of it, though at the same time we become insensible
of much stronger impressions, such as the rattling of coaches in the
streets.

The attention depends in some degree on the will, but is generally
given to those impressions which are particularly strong, new,
pleasant, or disagreeable; in short, to those which particularly
affect the mind. Hence it is, that things which are new, produce the
most vivid impressions, which gradually grow fainter, and at last
become imperceptible.

There is one circumstance respecting sensation, which will probably
account for our only perceiving those impressions to which the mind
attends: and this is, that the mind is incapable of perceiving more
than one impression at a time: the more accurately we examine this,
the greater reason we shall have to think it true; but the mind can
turn its attention so quickly, from one object to another, that at
first sight, we are led to believe, that we are able to attend to
several at the same time.

But though the mind cannot perceive or attend to various sensations
at the same time, yet if two or more of these are capable of uniting
in such a manner as to produce a compound sensation, this may be
perceived by the mind.

This compound sensation may be produced either by impressions made at
the same instant, or succeeding each other so quickly, that the
second takes place before the first has vanished.

As an instance of the first, we may mention musical chords, or the
sounds produced by the union of two or more tones at the same time.
We have another instance likewise in odours or smells; if two or more
perfumes be mixed together, a compound odour will be perceived,
different from any of them.

As an instance of the latter, if a paper painted of various colours
be made to revolve rapidly in a circle, a compound colour, different
from any of them, will be perceived. These observations apply
particularly to the senses we have mentioned, and likewise to taste:
but the sensations afforded us by touch do not seem capable of being
compounded in this manner.

There are many things necessary to perfect sensation, besides those
that have been mentioned. The degree and perfection of sensation will
depend much on the mind, and will be continually altered by delirium,
torpor, sleep, and other circumstances; much likewise depends on the
state of the organs with respect to preceding impressions; for if any
organ of sense have been subjected to a strong impression, it will
become nearly insensible of those which are weaker.

Of this innumerable instances may be given: an eye which has been
subjected to a strong light, becomes insensible of a weaker: and on
the contrary, if the organs of sense have been deprived of their
accustomed impressions for some time, they are affected by very
slight ones. Hence it is, that when a person goes from daylight into
a darkened room, he can at first see nothing; by degrees however he
begins to have an imperfect perception of the different objects, and
if he remain long enough, he will see them with tolerable
distinctness, though the quantity of light be the same as when he
entered the room, when they were invisible to him.

Sensation often arises from internal causes, without any external
impulse. To this source may be referred consciousness, memory,
imagination, volition, and other affections of the mind. These are
called the internal senses. The senses, whether internal or external,
have never been accurately reduced to classes, orders, or genera; the
external indeed are generally referred to five orders; namely,
seeing, hearing, smelling, tasting, and feeling, or touch. With
respect to the four first, the few qualities of external bodies which
each perceives may be easily reduced to classes, each of which may be
referred to its peculiar organ of sensation, because each organ is so
constituted, that it can only be affected by one class of properties;
thus the eye can only be affected by light; the ear by the vibrations
of the air, and so of the rest.

The same organ, whatever be its state, or whatever be the degree of
impulse, always gives to the mind a similar sensation; nor is it
possible, by any means we are acquainted with, to communicate the
sensation peculiar to one organ by means of another. Thus we are
incapable, for instance, of hearing with our eyes, and seeing with
our ears: nor have we any reason to believe that similar impressions
produce dissimilar sensations in different people. The pleasure,
however, as well as the pain and disgust, accompanying different
sensations, differ very greatly in different persons, and even in the
same person at different times; for the sensations which sometimes
afford us pleasure, at other times produce disgust.

Habit has a powerful influence in modifying the pleasures of
sensation, without producing any change in the sensation itself, or
in the external qualities suggested by it. Habit, for instance, will
never cause a person to mistake gentian or quassia for sugar, but it
may induce an appetite or liking for what is bitter, and a disgust
for what is sweet. No person perhaps was originally delighted with
the taste of opium or tobacco, they must at first be highly
disgusting to most people; but custom not only reconciles the taste
to them, but they become grateful, and even necessary.

Almost every species of sensation becomes grateful or otherwise,
according to the force of the impression; for there is no sensation
so pleasant, but, that, by increasing its intensity, it will become
ungrateful, and at length intolerable. And, on the contrary, there
are many which on account of their force are naturally unpleasant,
but become, when diminished, highly pleasant. The softest and
sweetest sounds may be increased to such a degree as to be extremely
unpleasant: and when we are in the steeple of a church, the noise of
a peal of bells stuns and confounds our senses, while at a distance
their effect is very pleasant. The smell of musk likewise at a
distance, and in small quantity, is pleasant; but when brought near,
or in large quantity, it becomes highly disagreable. The same may be
observed with respect to the objects of the other senses.

For a similar reason, many sensations which are at first pleasing,
cease to delight by frequent repetition; though the impression
remains the same. This is so well known that illustrations are
unnecessary. Those who are economical of their pleasures, or who wish
them to be permanent, must not repeat them too frequently. In music,
a constant repetition of the sweetest and fullest chords, cloys the
ear; while a judicious mixture of them with tones less harmonious
will be long relished. Those who are best acquainted with the human
heart need not be told, that this observation is not confined to
music.

On the same principle likewise we can account for the pleasure
afforded by objects that are new; and why variety is the source of so
many pleasures; why we gradually wish for an increase in the force of
the impression in proportion to its continuance.

The pleasures of the senses are confined within narrow limits, and
can neither be much increased nor too often repeated, without being
destructive of themselves; thus we are admonished by nature, that our
constitutions were not formed to bear the continual pleasures of
sense; for the too free use of any of them, is not only destructive
of itself, but induces those painful and languid sensations so often
complained of by the voluptuary, and which not unfrequently produce a
state of mind that prompts to suicide.

As the transition from pleasure to pain is natural, so the remission
of pain, particularly if it is great, becomes a source of pleasure.
There is much truth, therefore, in the beautiful allegory of
Socrates, who tells us, that Pleasure and Pain were sisters, who,
however, met with a very different reception by mankind on their visit
to the earth; the former being universally courted, while the latter
was carefully avoided: on this account, Pain petitioned Jupiter, who
decreed that they should not be parted; and that whoever embraced the
one, obtained also the other.

There is a great diversity with respect to the duration of the
pleasures of the different senses: some of the senses become soon
fatigued, and lose the power of distinguishing accurately their
different objects: others, on the contrary, remain perfect a long
time. Thus smell and taste are soon satiated; hearing more slowly;
while, of all the external senses, the objects of sight please us the
longest. We may, however, prolong the pleasures of sense by varying
them properly, and by a proper mixture of objects or circumstances
which are indifferent, and afford less delight. But the very
constitution of our nature limits our enjoyments, and points out the
impossibility of perpetual pleasures in this state of our existence.
To a person who is thirsty, water is delicious nectar; to one who is
hungry, every kind of food is agreeable, and even its smell pleasant;
to a person who is hot and feverish, the cool air is highly
refreshing. But to the same persons in different circumstances, the
same things are not only indifferent, but even disgusting; for
instance, a person cannot bear the sight or smell of food, after
having satiated himself with it, and perpetual feasting will cloy the
appetite of the keenest epicure.

I shall conclude this account of the general laws of sensation, by a
short recapitulation of those laws.

And, in the first place, it may be observed, that the energy or force
of any sensation, is proportioned to the degree of attention given by
the mind to the external object which causes it.

Secondly, A repetition of sensations diminishes their energy, and at
last nearly destroys it; but this energy is restored by rest, or the
absence of these sensations.

Thirdly, The mind cannot attend to two impressions at the same time:
so that two sensations never act with the same force at the same
instant; the stronger generally overcoming the weaker. The mind,
however, can attend to the weaker sensation, in such a manner, as to
overpower the stronger, or to render it insensible.

Having fully considered the general laws of sensation, I shall now
proceed to examine those proper to each sense; and in this
examination, two objects will engage our attention. 1. The structure
of the organ which receives and transmits the impulse to the mind. 2.
The qualities or properties of external bodies, particularly those by
which they are fitted to excite sensation.

The first sense that we shall examine is touch, which, of all the
external senses, is the most simple, as well as the most generally
diffused. By means of this sense, we are capable of perceiving
various qualities and properties of bodies, such as hardness,
softness, roughness, smoothness, temperature, magnitude, figure,
distance, pressure, and weight; this sense is seldom depraved;
because the bodies, whose properties are examined by it, are applied
immediately to the extremities of the nerves, without the
intervention of any medium liable to be deranged, as is the case with
the eye, and ear.

The organ of touch is seated chiefly in the skin, but different parts
of this covering possess different degrees of sensibility. The skin
consists of three parts. 1. The cutis vera, or true skin, which
covers the greatest part of the surface of the body. When the skin is
examined by a microscope, we find it composed of an infinite number
of papillae, or small eminencies, which seem to be the extremities of
nerves, each of which is accompanied by an artery and a vein, so that
when the vessels of the skin are injected, the whole appears red. 2.
Immediately over the true skin, and filling up its various
inequalities, lies a mucous reticulated substance, which has been
called by Malpighi, who first described it, rete mucosum. The real
skin is white in the inhabitants of every climate; but the rete
mucosum is of various colours, being white in Europeans, olive in
Asiatics, black in Africans, and copper coloured in Americans. This
variety depends chiefly on the degree of light and heat; for, if we
were to take a globe, and paint a portion of it with the colour of
the inhabitants of corresponding latitudes, we should have an uniform
gradation of shade, deepening from the pole to the equator.

The diversity of colour depends upon the bleaching power of the
oxygen, which, in temperate climates, combines more completely with
the carbonaceous matter deposited in the rete mucosum; while, in
hotter climates, the oxygen is kept in a gaseous state by the heat
and light, and has less tendency to unite with the carbonaceous
matter. In proof of this, the skins of Africans may be rendered white
by exposure to the oxygneated muriatic acid.

Over the rete mucosum is spread a fine transparent membrane, called
the cuticle, or scarf skin, which defends the organ of feeling from
the action of the air, and other things which would irritate it too
powerfully. In some parts of the body this membrane is very thick, as
in the soles of the feet, and palms of the hands; and this thickness
is much increased by use and pressure.

In general, the thinner the cuticle is, the more acute is the sense
of touch. This sense is very acute and delicate about the ends of the
fingers, where we have the most need of it; but in the lips, mouth,
and tongue, it is still more delicate; a galvanic or electrical shock
being felt by the tongue, when it is impossible for us to perceive it
by the fingers.

This sense, like the others, becomes more exquisite when its organ is
defended from the action of external bodies; it is on this account
that the cuticle becomes so sensible under the end of the nail, which
defends it from the action of external objects; and when part of the
nail is taken away, we can scarcely bear to touch any thing with this
newly exposed part of the skin.

When we place our fingers upon the surface of any body, the first
sensation we experience is that of resistance, after which the other
properties are perceived in a natural order; such as heat or cold,
moisture or dryness, motion or rest, distance, and figure or shape.

With respect to the diseases of this sense, it is very seldom that it
becomes too acute over the whole body; though it frequently does so
in particular parts, which may arise from the cuticle being too thin
or abraded, or from an inflamed state of the part.

It however becomes sometimes obtuse, and indeed almost abolished over
the whole body; and this takes place from compression of the brain,
and various affections of the nervous power. This diminution is
called anaesthesia. The touch becomes deficient, and indeed almost
abolished, when the cuticle is injured by the frequent application of
hot bodies, or acrid substances: thus the cuticle of the hands of
blacksmiths and glassblowers is generally so hard and horny, that
they can take up and grasp in their hand pieces of redhot iron with
impunity.

We generally refer pain to this sense, though it may arise from too
violent an impression made upon any of the organs of sense.

Pain is an unpleasant sensation, which the mind refers to some part
of the body, and very accurately, if any part of the surface is
affected, but less so, if it arises from the affection of an internal
part. The sensation of pain may arise from any thing which tends to
injure the structure of the body, whether that be internal or
external; so that it serves as a monitor to put us on our guard, and
to induce us to remove any thing which might be injurious to us. This
sensation is produced by any thing which punctures, cuts, tears,
distends, compresses, bruises, corrodes, burns, or violently
stimulates any part of the body.

A moderate degree of pain in any part excites the action of the whole
body; a greater quantity of blood and nervous energy is determined to
the part. A still greater degree of pain brings on inflammation and
its consequences, and if it be intense, it will bring on fever,
convulsions, delirium, fainting, and even death.

The endurance of pain depends much on the strength of mind possessed
by the patient, which, in some instances, is such, that the most
violent pains are patiently endured; while in other instances, the
slightest can scarcely be born.

It is a curious circumstance, that a moderate degree of pain, when
unaccompanied by fever, often tends to render the understanding more
clear, lively, and active. This is confirmed by the experience of
people labouring under gout. We have an account of a man who
possessed very ordinary powers of understanding, but who exhibited
the strongest marks of intelligence and genius in consequence of a
severe blow on the head; but that he lost these powers when he
recovered from the effects of the blow. Pechlin mentions a young man,
who during a complaint originating from worms, possessed an
astonishing memory and lively imagination, both of which he nearly
lost by being cured. Haller mentions a man who was able to see in the
night, while his eyes were inflamed, but lost this power as he got
well. All these facts show, that a certain action or energy is
necessary for the performance of any of the functions of the body or
mind; and whatever increases this action will, within certain limits,
increase those functions.

Feeling is by far the most useful, extensive, and important of the
senses, and may be said indeed to be the basis of them all. Vision
would be of very little use to us, if it were not aided by the sense
of feeling; we shall afterwards see that the same observation may be
applied to the other senses. In short, it is to this sense that we
are indebted, either immediately or indirectly, for by far the
greatest part of our knowledge; for without it we should not be able
to procure any idea with respect to the magnitude, distance, shape,
heat, hardness or softness, asperity or smoothness of bodies; indeed,
if we were deprived of this sense, it is difficult to say whether we
should have any idea of the existence of any external bodies; on the
contrary, it seems probable that we should not.

LECTURE VI.
TASTE AND SMELL.

From the sense of touch we proceed naturally to that of taste, for
there seems to be less difference between these two senses than
between any of the others. The sense of taste appears to be seated
chiefly in the tongue; for any sweet substance, such as sugar,
applied to any other part of the mouth, scarcely excites the least
sensation of taste. The same may be observed with respect to any
other sapid body, which, unless it is strongly acrid or irritating,
produces no effect on any other part than the tongue; but if it is
possessed of much acrimony, it then not only affects the palate, and
uvula, but even the oesophagus.

The tongue is a muscular substance, placed in the mouth, connected by
one end with the adjacent bones and cartilages, while the other end
remains free, and easily moveable. The tongue is furnished,
particularly on its upper surface, with innumerable nervous papillae,
which are much larger than those I described as belonging to the
skin. These papillae are of a conical figure, and extremely sensible,
forming, without doubt, the true organ of taste; other papillae are
found between them, which are partly conical, and partly cylindrical.

Over the papillae of the tongue is spread a single mucous, and
semipellucid covering, which adheres firmly to them, and serves the
purpose of a cuticle.

Under these papillae are spread the muscles which make up the fleshy
part of the tongue: these are extremely numerous, and by their means
the tongue possesses the power of performing a great variety of
motions with surprising velocity.

The arteries leading to the tongue are extremely numerous; and, when
injected with a red fluid, the whole substance appears of a beautiful
red.

The tongue is likewise furnished with a large supply of nerves, some
of which undoubtedly serve to supply its muscles with nervous energy,
while others terminate in the papillae, and form the proper organ of
taste: this office seems to be performed by the third branch of the
fifth pair of nerves. The papillae, before described, are formed or
composed of a number of small nerves, arteries, and veins, firmly
united together by cellular substance. These papillae are excited to
action by the application of any sapid body; in consequence of which
they receive a greater supply of blood, become enlarged, and vastly
more sensible.

The structure of the tongue differs in different animals, which
likewise possess corresponding differences with respect to taste. In
those quadrupeds, in which it is armed with sharp points, the sense
of taste is by no means acute. The same is the case with birds and
reptiles, whose tongues are very dry and rough.

In a former lecture I took notice of a liquor which is secreted by
the glands of the mouth and neighbouring parts, which is called
saliva. This liquor acts an important part in the production of
taste; it does not differ much from water, excepting by containing a
quantity of mucilage; and nothing is sapid, or capable of affecting
the sense of taste, unless it is in some degree soluble in this
liquor. Hence earthy substances, which are nearly insoluble, have
little or no taste.

It is not, however, sufficient that the substance be possessed of
solubility alone; it is necessary likewise that it should be
possessed of saline properties, or, at least, of a kind of acrimony,
which renders it capable of stimulating the nervous papillae. Hence
it is that those substances which are less saline, and less acrid
than the saliva, have no taste.

We are capable of distinguishing various kinds of taste, but some of
them with less accuracy than others. Among the different kinds of
taste, the following have been considered by Haller, and some other
physiologists, as primitive: sweet, sour, bitter, and saline. The
others have been thought to be compounded of these; for the sense of
taste, as well as sight and hearing, is capable of perceiving
compound impressions. To these primitive tastes, Boerhaave added
alkaline, spirituous, aromatic, and some others. Of these, in
different proportions, all the varieties of tastes, which are
extremely numerous, are composed.

Some tastes are pleasant and agreeable, others disagreeable, and
scarcely tolerable: there is, however, a great diversity in this
respect experienced by different persons; for the same taste, which
is highly grateful to some, is extremely unpleasant to others.

But the most pleasant tastes, agreeably to the general laws of
sensation, which I described in the last lecture, become gradually
less pleasant, and at last disgusting; while, on the contrary, the
most disagreeable savours, such as tobacco, opium, and assafoetida,
become, by custom, not only tolerable, but highly agreeable.

Nature designed this difference of tastes that we might know and
distinguish such foods as are salutary; for we may in general
observe, that no kind of food which is healthy, and affords proper
nutriment to the body, is disagreeable to the taste; nor are any that
are ill tasted proper for our nourishment. Those substances,
therefore, which possess strong or disagreeable savours, and which,
in general, possess a power of producing great changes on our
constitution, are to be ranked as medicines, and only to be used when
the constitution is deranged; whereas, in general, those which are
pleasant, or mild tasted, are proper for nourishing the body. We are
therefore excited or prompted to receive nourishment by the pleasant
smell or taste of the food; but the avidity with which we take it
depends much on the state of the stomach, and likewise on a certain
inanition or emptiness; for the coarsest food is grateful to those
who are hungry, and whose digestion is good; whereas, to those who
have lately eaten, or whose digestive powers are impaired, the most
delicate food affords little pleasure. While we are eating, the
saliva flows into the mouth more copiously, which excites a more
acute sensation of taste. This flow of saliva is likewise frequently
excited by the smell or sight of substances agreeable to the taste,
which causes an appetite, or desire of eating, similar to that caused
by an accumulation of gastric juice in the stomach.

In brute animals, who have not, like ourselves, the advantage of
learning from each other by instruction, the faculty of taste is much
more acute, by which they are admonished to abstain from noxious or
unhealthy food. This sense, for the same reason, is more acute in
savages than in those who live in civiilsed society, which, whatever
perfection it gives to the reasoning faculties of man, certainly
diminishes the acuteness of all our senses, partly by affording fewer
inducements to exercise them, and partly by our manner of living, and
by the application of substances to the organs of sense, which tend
to vitiate them, and render them depraved.

Taste is modified by age, temperament, habit, and disease; and in
this it obeys the general laws of sensation. Children are pleased
with the taste of what is sweet, and little stimulating; as we
advance in years the taste of more stimulating substances becomes
agreeable to us; so that we are admonished by this sense to take into
the stomach the kind of nourishment fitted to each period of life. We
often, however, counteract this salutary monitor by depraving our
sense of taste, by the too free use of vinous or spirituous liquors,
which so far deadens the sense of taste, that sweet substances become
unpleasant, and nothing but acrid and stimulating things can make an
impression on our diminished and vitiated sense of taste.

This sense, as well as others, is liable to be diseased. In order
that the sense may be perfect, it is necessary that the membrane
which envelopes the nervous papillae of the tongue, and serves as a
cuticle, should neither be too thick nor too thin, too dry nor too
moist. It is necessary likewise that the qualities of the saliva be
natural; for alterations in the nature of this liquor affect very
much the sense of taste; if it is bitter, which sometimes happens in
bilious complaints, all kinds of food have a bitter taste; if it is
sweet, the food has a faint and unpleasant flavour; and if it is
acid, the food too tastes sour.

This sense is seldom observed to be too acute, unless from a vitiated
state of the cuticle, or membrane, which covers the tongue: if this
has been abraded or ulcerated, then the substances applied to the
tongue are more sensibly tasted; in many instances, however, they do
not produce an increased sensation of taste, but only of pain.

The sense of taste, as well as of touch, may become deficient, from
various affections of the brain and nerves; this, however, is not
often the case. Some persons have naturally a diminished sense of
taste, and this generally accompanies a diminished sense of smell.
This sense is frequently diminished in sensibility from a deficiency
of saliva, as well as of the proper moisture of the tongue. Hence, in
many diseases, it becomes defective, such as fevers, colds, and the
like; both from a want of the proper degree of moisture, and from
defect of appetite, which, as was before observed, is necessary to
the sense of taste.

The sense of taste is often diminished by a thickened mucous covering
of the tongue, which prevents the application of substances to its
nervous papillae. This mucous covering arises from a disordered state
of the stomach, as well as from several other affections of the body:
hence physicians inspect the tongue, that they may be able to judge
of the general state of the body; and next to the pulse, it is
undoubtedly the best criterion that we have, as it not only points
out the nature and degree of several fevers, but likewise, in many
instances, the degree of danger to be apprehended.

Having examined the sense of taste, I shall now proceed to consider
that of smell; the use of which, like taste, is to enable us to
distinguish unwholesome from salutary food; indeed, by this sense, we
are taught to avoid what is prejudicial before it reaches the sense
of taste, to which it might be very injurious; and thus we are
enabled to avoid any thing which has a putrid tendency, which, if
received into the stomach, would taint the whole mass of fluids, and
bring on speedy dissolution.

The seat of this sense is a soft pulpy membrane, full of pores, and
small vessels, which lines the whole internal cavity of the nose. On
this membrane are distributed abundance of soft nerves, which arise
chiefly from an expansion of the first pair of nerves coming from the
brain. This membrane is likewise plentifully supplied with arteries;
so that by means of this nervous and arterial apparatus, this
membrane is possessed of very great sensibility; but the nerves of
the nose being almost naked, require a defence from the air, which is
continually drawn through the nostrils into the lungs, and forced out
again by respiration. Nature has therefore supplied this part with a
thick insipid mucus, very fluid at its first separation, but
gradually thickening, as it combines with oxygen, into a dry crust,
approaching often to a membranous matter. This mucus is poured out,
or exhaled, by the numerous minute arteries of the nostrils, and
serves to keep the nervous apparatus moist, and in a proper state for
receiving impressions, as well as to prevent the violent effects
which might arise from the stimulus of the air and other bodies. The
sense of smell is the most acute about the middle of the septum of
the nose, where the nervous membrane which I have described is
thicker and softer, than in the cavities more deeply situated, where
it is less nervous and vascular. These parts are not however
destitute of the sense.

As taste proceeds from the action of the soluble parts of bodies on
the nervous papillae of the tongue, so smell is occasioned by minute
and volatile particles flying off from bodies, which become mixed
with the air, and drawn up with it into the nostrils, where these
small particles stimulate or act upon the nerves before described,
and produce the sensation which we call smelling.

The air therefore, being loaded with the subtile and invisible
effluvia of bodies, is, by the powers of respiration, drawn through
the nose, so as to apply these particles to the almost naked
olfactory nerves, which, as was before observed, excites the sense of
smelling. When we wish to smell accurately, we shut the mouth, open
the nostrils as wide as possible, and making a strong inhalation,
draw up a greater number of these volatile particles, than could be
drawn up by the common action of respiration, by which means the
olfactory nerves are more stimulated, and produce a stronger
sensation.

In order that this sense may be enjoyed in perfection, it is
necessary that the organ of smell be in a proper state or condition
to receive impressions, and that the odorous bodies be likewise in a
proper state. With respect to the first, it is necessary that the
state of the nerves be sound, and particularly that they be kept in a
proper state with respect to moisture.

With regard to the odorous bodies, it is necessary, first, that their
minute particles should be disengaged, either by heat, friction,
fermentation, or other means capable of decomposing those bodies
which are the subjects of smell: secondly, that they may be capable
of assuming the vaporous or gaseous state, by combining with caloric,
or at any rate, that they should remain for a certain time dissolved
or suspended in the air: thirdly, that they should not meet with any
substance in their way to the nostrils, which is capable of
neutralising them, or altering their properties by its chemical
action.

Notwithstanding all the pains which physiologists have taken to
detect the nature of odorous bodies, they have met with but little
success. They are so extremely minute as to escape the other senses,
and we can only say that they must be composed of particles in an
extreme state of division and subtilty, because very small quantities
of odorous matter exhale a sufficient quantity of particles to fill a
large space. A grain of camphor, musk, or amber exhales an odour
which penetrates every part of a large apartment, and which remains
for a long time.

There is perhaps no substance in nature which is absolutely incapable
of being changed from a solid state into that of a fluid or gas, by
combining with caloric; though different substances require very
different quantities of heat to produce those effects. Those which
are with difficulty converted into fluids or gases, are termed fixed,
while those which are easily changed are called volatile; though
these are only terms of comparison, for there is probably no body
which is absolutely fixed, or incapable of being reduced to vapour by
the application of a sufficient degree of heat.

The odorous property is probably as general as that of being
convertible into gas. There is perhaps no body so hard, compact, and
apparently inodorous, as to be absolutely incapable of exciting smell
by proper methods: two pieces of flint rubbed together, produce a
very perceptible smell. Metals which appear nearly inodorous, excite
a sensation of smell by friction, particularly lead, tin, iron, and
copper. Even gold, antimony, bismuth, and arsenic, under particular
circumstances, give out peculiar and powerful odours. The odour of
arsenic in its metallic state, and in a state of vapour, resembles
that of garlic. The chief means of developing the odorous principles
are friction, heat, electricity, fermentation, solution, and mixture.
The effect of mixture is very remarkable in the case of lime and
muriate of ammoniac, neither of which, before mixture, has any
perceptible odour.

There is perhaps then no body which is perfectly inodorous, or
entirely destitute of smell: for those which have been generally
accounted such, may be rendered odorous by some of the methods I have
mentioned.

Several naturalists and physiologists, such as Haller, Linneus, and
Lorri, have attempted to reduce the different kinds of odours to
classes, but without any great success; for we are by no means so
well acquainted with the physical nature of the odorous particles, as
we are with that of light, sound, and the objects of touch; and till
we do obtain a knowledge of these circumstances, which perhaps we
never shall, it will be in vain to attempt any accurate
classification. The division of them into odours peculiar to the
different kingdoms, is very inaccurate; for the odour of musk, which
is thought to be peculiarly an animal odour, is developed in the
solution of gold by some mineral solvents; it is perceptible in the
leaves of the geranium moschatum, and some other vegetables. The
smell of garlic is possessed by many vegetables, by arsenic, and by
toads. The violet smell is perceived in some salts, and in the urine
of persons who have taken turpentine. The same may be observed with
respect to several other odours.

As taste keeps guard, or watches over the passage by which food
enters the body, so smell is placed as a sentinel at the entrance of
the air passage, and prevents any thing noxious from being received
into the lungs by this passage, which is always open. Besides, by
this sense, we are invited or induced, to eat salutary food, and to
avoid such as is corrupted, putrid, or rancid. The influence of the
sense of smell on the animal machine is still more extensive: when a
substance which powerfully affects the olfactory nerves is applied to
the nostrils, it excites, in a wonderful manner, the whole nervous
system, and produces greater effects in an instant, than the most
powerful cordials or stimulants received by the mouth would produce
in a considerable space of time. Hence in syncope or fainting, in
order to restore the action of the body, we apply volatile alkali, or
other strong odorous substances, to the nostrils, and with the
greatest effect. It may indeed for some time supply the place, and
produce the effects, of solid nutriment usually received into the
stomach We are told that Democritus supported his expiring life,
and retarded, for three days, the hour of death, by inhaling the
smell of hot bread, when he could not take nutriment by the stomach.
Bacon likewise gives us an account of a man who lived a considerable
time without meat or drink, and who appeared to be nourished by the
odour of different plants, among which were garlic, onions, and
others which had a powerful smell. In short, the stimulus which
active and pleasant odours give to the nerves, seems to animate the
whole frame; and to increase all the senses, internal and external.

The perfection of the organ of smell is different in different
animals; some possessing it very acutely; others on the contrary
having scarcely any sense of smell. We may in general observe that
this sense is much more acute in many quadrupeds than in man: an in
them the organ is much more extensive: in man, from the shape of the
head, little opportunity is given for extending this organ, without
greatly disfiguring the face. In the dog, the horse, and many other
quadrupeds, the upper jaw being large, and full of cavities, much
more extension is given to the membrane which is the organ of smell,
which in some animals is beautifully plaited, in order to give it
more surface. Hence a dog is capable of following game, or of tracing
his master in a crowd, or in a road where it could not be done by the
mere track. Nay, we are told of a pickpocket being discovered in a
crowd, by a dog who was seeking its master, and who was directed to
the man by the pocket handkerchief of his master, which the
pickpocket had stolen. In dogs the sense of smell must be uncommonly
delicate, to enable them to distinguish the way their master has gone
in a crowded city.

The habit of living in society, however, deadens this sense in man as
well as taste; for we have the advantage of learning the properties
of bodies from each other by instruction, and have therefore less
occasion to exercise this sense; and the less any sense is exercised,
the less acute will it become; hence it is, that those whom necessity
does not oblige to to exercise their senses and mental faculties, and
who have nothing to do but lounge about, and consume the fruits of
the earth, become half blind, half deaf, and, in general, have great
deficiency in the sense of smell. The use of spirituous liquors, and
particularly of tobacco in the form of snuff, serves likewise in a
remarkable manner to deaden this sense.

Savages, however, who are continually obliged to exercise all their
senses, have this, as well as others, in very great perfection. Their
smell is so delicate and perfect, that it approaches to that of dogs.
Soemmering and Blumenbach indeed assert, that in Africans and
Americans the nostrils are more extended, and the cavities in the
bones lined with the olfactory membrane much larger than in
Europeans.

I have already observed the powerful effects which some odours have
upon the nervous system. There are some which agreeably excite it,
and produce a pleasant and active state of the mind, while others, on
the contrary, produce the most terrible convulsions, and even
fainting. Those particular antipathies with respect to smells, arise
sometimes from something in the original constitution of the body,
with which we are unacquainted, but generally from the senses having
been powerfully and unpleasantly affected by certain odours at an
early period of life. The latter may often be cured by resolution and
perseverance, but the former cannot.

The sense of smell sometimes becomes too acute, either from a
vitiated state of the organ itself, which is not often the case; or
from an increased sensibility or irritability of the whole nervous
system, which is observed in hysteria, phrenitis, and some fevers.

This sense is however more often found deficient; and this may arise
from a fault in the brain or nerves, which may either proceed from
external violence, or from internal causes. A defect of smell often
arises from a vitiated state of the organ itself; for instance, if
the nervous membrane is too dry, or covered with a thick mucus; of
both of which we have an example in catarrh or common cold, where, at
the beginning, the nostrils feel unusually dry, but as the disease
advances, the pituitary membrane becomes covered with a thick mucus:
in both states, the sense of smell is in general deficient, and
sometimes nearly abolished.

This sense is sometimes depraved, and smells are perceived when no
odorous substance is present; or odours are perceived to arise from
substances, which are very different from those which we perceive in
a sound state.

There are many diseases likewise of the nose, and neighbouring parts,
which cause a depraved sensation; such as ulcers, cancer, caries; a
diseased state of the mouth, teeth, throat, or lungs; or a vitiated
state of the stomach, which sometimes exhales a vapour similar to
that of sulphureted hydrogen. This sense likewise sometimes becomes
depraved from a diseased state of the brain and nerves.

LECTURE VII.
SOUND AND HEARING.

Having in the last lecture examined the senses of taste and smell, I
now proceed to that of hearing. As the sense of smell enables us to
distinguish the small particles of matter which fly off from the
surfaces of bodies, and float in the air, so that of hearing makes us
acquainted with the elastic tremors or impulses of the air itself.

The sense of hearing opens to us a wide field of pleasure, and though
it is less extensive in its range than that of sight, yet it
frequently surmounts obstacles that are impervious to the eye, and
communicates information of the utmost importance, which would
otherwise escape from and be lost to the mind.

Sound arises from a vibratory or tremulous motion produced by a
stroke on a sounding body, which motion that body communicates to the
surrounding medium, which carries the impression forwards to the ear,
and there produces its sensation. In other words, sound is the
sensation arising from the impression made by a sonorous body upon
the air or some other medium, and carried along by either fluid to
the ear.

Three things are necessary to the production of sound; first, a
sonorous body to give the impression; secondly, a medium or vehicle
to convey this impression; thirdly, an organ of sense or ear to
perceive it. Each of these I shall separately examine.

Strictly speaking, sonorous bodies are those whose sounds are
distinct, of some duration, and which may be compared with each
other, such as those of a bell or a musical string, and not such as
give a confused noise, like that made by a stone falling on the
pavement. To be sonorous, a body must be elastic, so that the tremors
exerted by it in the air may be continued for some time: it must be a
body whose parts are capable of a vibratory motion when forcibly
struck.

All hard bodies, when struck return more or less of a sound; but
those which are destitute of elasticity, give no repetition of the
sound; the noise is at once produced and dies; while other bodies,
which are more elastic and capable of vibration, repeat the sounds
produced several times successively. These last are said to have a
tone; the others are not allowed to have any. If we wish to give
nonelastic bodies a tone, it will be necessary to make them continue
their sound, by repeating our blows quickly upon them. This will
effectually give them a tone; and an unmusical instrument has often
by this means a fine effect in concerts. The effects of a drum depend
upon this principle. Gold, silver, copper, and iron, which are
elastic metals, are sonorous; but lead, which possesses scarcely any
elasticity, produces little or no tone. Tin, which in itself has very
little more sound than lead, highly improves the tone of copper when
mixed with it. Bell metal is formed of ten parts of copper, and one
of tin. Each of these is ductile when separate, though tin is only so
in a small degree, yet they form when united a substance almost as
brittle as glass, and highly elastic. So curious is the power of tin
in this respect, that even the vapour of it, when in fusion, will
give brittleness to gold and silver, the most ductile of all metals.
Sonorous bodies may be divided into three classes; first, bells of
various figures and magnitudes: of these such as are formed of glass
have the most pure and elegant tones, glass being very elastic, and
its sound very powerful; secondly, pipes of wood or metal; thirdly,
strings formed either of metallic or animal substances. The sounds
given by strings are more grave or more acute according to the
thickness, length, and tension of the strings.

Air is universally allowed to be the ordinary medium of sound, or the
medium by which sounds are propagated from sonorous bodies, and
communicated to the ear. This may be shown by an experiment with the
air pump; also with the condenser.

But though air is the general vehicle of sound, yet sound will go
where no air can convey it; thus the scratching of a pin at the end
of a long piece of timber may be heard by an ear applied at the other
end, though it could not be heard at the same distance through the
air. On this account it is that sentinels are accustomed to lay their
ears to the ground, by which means they can often discover the
approach of cavalry, at a much greater distance than they can see
them.

For the same reason two stones being struck together under water, may
be heard at a much greater distance by an ear placed under water
likewise, than it can be heard through the air. Dr. Franklin, who
several times made this experiment, thinks that he has heard it at a
greater distance than a mile. This shows that water is better adapted
to convey sound than air.

When an elastic body is struck, that body, or some part of it, is
made to vibrate. This is evident to sense in the string of a violin
or harpsichord, for we may perceive by the eye, or feel by the hand,
the trembling of the strings, when by striking they are made to
sound. If a bell be struck by a clapper on the inside, the bell is
made to vibrate. The base, of the bell, is a circle, but it has been
found that by striking any part of this circle on the inside, that
part flies out, so that the diameter which passes through this part
of the base will be longer than the other diameter. The base, by the
stroke, is changed into an ellipse or oval, whose longer axis passes
through the part against which the clapper is struck. The elasticity
of the bell restores the figure of the base, and makes that part
which was forced out of its place, return back to its former
situation, from which the same principle throws it out again; so that
the circular figure of the bell will be again changed to an ellipse,
only now the shorter axis will pass through the part which was first
struck.

The same stroke, which makes the bell vibrate, occasions the sound,
and as the vibrations decay, the sound grows weaker. We may be
convinced by our senses that the parts of the bell are in a vibratory
motion while it sounds. If we lay the hand gently on it, we shall
easily feel this tremulous motion, and even be able to stop it, or if
small pieces of paper be put upon the bell, its vibrations will put
them in motion.

These vibrations in the sounding body will cause undulations or waves
in the air; and, as the motions of one fluid may often be illustrated
by those of another, the invisible motions of the air have been
properly enough compared to the visible waves of water produced by
throwing a stone therein. These waves spread themselves in all
directions in concentric circles, whose common centre is the spot
where the stone fell, and when they strike against a bank or other
obstacle, they return in the contrary direction to the place from
whence they proceeded. Sound in like manner expands in every
direction, and the extent of its progress is in proportion to the
impulse on the vibrating chord or bell.

Such is the yielding nature of fluids, that when other waves are
generated near the first waves, and others again near these, they
will perform their vibrations among each other without interruption;
those that are coming back will pass by those that are going
forwards, or even through them, without interruption: for instance,
if we throw a stone into a pond, and immediately after that, another,
and then a third, we shall perceive that their respective circles
will proceed without interruption, and strike the banks in regular
succession.

The atmosphere in the same manner possesses the faculty of conveying
sounds in the most rapid succession or combination, as distinctly as
they were produced. It possesses the power not only of receiving and
propagating simple and compound vibrations in direct lines from the
voice, or an instrument, but of retaining and repeating sounds with
equal fidelity after repeated reflection and reverberation, as is
evident from the sound of a French horn among hills.

Newton was the first who attempted to demonstrate that the waves or
pulses of the air are propagated in all directions round a sounding
body, and that during their progress and regress they are twice
accelerated and twice retarded, according to the law of a pendulum
vibrating in a cycloid. These propositions are the foundation of
almost all our reasoning concerning sound. When sonorous bodies are
struck, they, by their vibration, excite waves in the air, similar to
those caused by a stone thrown into water; some parts of these waves
entering the ear, produce in us that sensation which we call sound.
How these pulsations act upon the auditory nerve, to produce sound,
we know not, as we see no necessary connexion between the pulses and
the sensation, nor the least resemblance between them; but we can
trace their progress to a certain extent, which I shall now endeavour
to do.

The external part of the ear is called the auricle, or outward ear,
which is a cartilaginous funnel, connected to the bones of the
temple, by means of cellular substance, and likewise by its own
proper ligaments and muscles. This cartilage is of a very compound
figure, being a kind of oval, marked with spirals standing up, and
hollows interposed, to which other hollows and ridges correspond on
the opposite side. The outer eminence is called helix. Within the
body of the cartilage arises a forked eminence called antihelix,
which terminates in a small and short tongue called antitragus. The
remaining part of the ear, called the concha or shell, is anteriorly
hollow, but posteriorly convex, growing gradually deeper; with a
crooked line or ridge running along its middle, which is immediately
joined to the meatus auditorius, or entrance into the ear; before
which stands a round moveable appendix, which serves as a defense,
called tragus.

Against this funnel of the ear the sonorous waves strike, and its
different parts are most admirably contrived to reflect them all into
the meatus auditorius: if it would not occupy too much time, it might
be shown, that all these curves and spirals are contrived in the best
manner possible, and with a most perfect knowledge of the geometry of
sounds, to reflect the sonorous pulses accurately, and in the
greatest possible quantity, into the ear.

This external part of the ear is differently formed in different
animals; and admirably suited to their various situations and habits.
In man it is close to the head, but so formed as to collect the
various pulses with great accuracy; in other animals it is more
simple, where less accuracy is required, but it is, in general, much
larger, having the appearance of an oblong funnel; and this gives
them a greater delicacy of hearing, which was necessary for them.

In animals which are defenceless and timid, and which are constantly
obliged to seek their safety in flight, the opening of this funnel is
placed behind, that they may better hear the noises behind them. This
is particularly instanced in the hare. Beasts of prey have this
opening before, that they may more easily discover their prey; as the
lion and tiger. Those that feed on birds have the opening directed
upwards, as the fox; and it is inclined downwards in animals, such as
the weasel, which seek their prey on the earth.

To this external part of the ear, which I have described, is
connected the meatus auditorius, or passage to the internal ear,
which is somewhat of a compressed cylindrical figure, lessening as it
bends inwards: a considerable part of it is bony, and it is bent
towards the middle. Across this passage, at its inner extremity, is
stretched a thin membrane, called membrana tympani. Upon the surface
of this membrane, the sonorous waves, which have been directed
inwards by the external ear, strike, and cause it to vibrate like the
membrane of a drum. This membrane is stretched over a cavity in the
bone, called the os petrosum, which cavity is called the tympanum, or
drum of the ear, which is of a rounded figure, divided in its middle
by a promontory, and continued backwards to the cells of the mastoid
bone. Besides this continuation of the tympanum into the mastoid
cells, it has a free communication with the mouth, by means of a tube
I shall soon describe.

Within this cavity of the tympanum are placed four small bones, which
facilitate the hearing: the first is the malleus or hammer, so called
from its shape: the upper part of its round head rests upon the
concavity of the tympanum, from whence the handle is extended down,
along the membrane of the tympanum; this bone has several muscles,
which move it in different directions, and cause it to stretch or
brace the membrana tympani, when we wish to hear with accuracy.

Connected with the malleus is another small bone, called the incus,
or anvil, which is connected with another, called the stapes, or
stirrup, from its shape. These two bones are connected by a small
oval shaped bone, called os orbiculare, placed between them: the
whole forming a little chain of bones.

The stapes, or stirrup, has its end of an oval shape, which fits a
small hole called fenestra ovalis, in that part of the ear called the
labyrinth, or innermost chamber of the ear.

The labyrinth consists of three parts; first, the vestibule, which is
a round cavity in a hard part of the os petrosum; secondly, the
semicircular canals, so called from their shape, which however is not
exactly semicircular; thirdly, the cochlea, which is a beautifully
convoluted canal, like the shell of a snail. This part has a round
cavity called fenestra rotunda, which is covered with a thin elastic
membrane, and looks into the tympanum.

The vestibule, semicircular canals, and cochlea, the whole of which
is called the labyrinth, form one cavity, which is filled with a very
limpid fluid resembling water, and the whole lined with a fine
delicate membrane, upon which the auditory nerve is expanded, like
the retina upon the vitreous humor of the eye. This beautiful
apparatus was only lately discovered by an Italian physician, Scarpa.
The auditory nerve is a portion of the seventh pair, which is called
the portio mollis or soft portion.

There is one part of the ear still to be described, namely, the
Eustachian tube, so called from Eustachius, the anatomist, who first
described it. This tube opens by a wide elliptical aperture into the
tympanum behind the membrane; the other end, which gradually grows
wider, opens into the cavity of the mouth. By this canal the inspired
air enters the tympanum to be changed and renewed, it likewise serves
some important purpose in hearing, with the nature of which we are
yet unacquainted. It is certain that we can hear through this
passage, for if a watch be put into the mouth, and the ears stopped,
its ticking may be distinctly heard; and in several instances of
deafness, this tube has been found completely blocked up.

The waves, which have been described as propagated in the air, in all
directions from the sounding body, enter the external cartilaginous
part of the ear, which, as has before been observed, is admirably
fitted for collecting and condensing them. As soon as these pulses
excite tremors in the membrane of the tympanum, its muscles stretch
and brace it, whence it becomes more powerfully affected by these
impulses. It is on this account that we hear sounds more distinctly
when we attend to them, the membrane being then stretched.

A tremulous motion, being excited in this membrane, is communicated
to the malleus annexed to it, which communicates it to the incus, by
which it is propagated through the os orbiculare to the stapes, which
imparts this tremulous motion through the foramen ovale to the fluid
contained in the labyrinth. This tremor is impressed by the waves
excited in this fluid, on every part of the auditory nerve in the
labyrinth. The use of the foramen rotundum, or round hole, before
described, is probably the same as that of the hole in the side of a
drum; it allows the fluid in the labyrinth to be compressed,
otherwise it could not vibrate.

If the organization is sound, and tremors are communicated to the
auditory nerve, they are in some way or other conveyed to the mind,
but in what manner we cannot tell. Nature has hid the machinery by
which she connects material and immaterial things entirely from our
view, and if we try to investigate them, we are soon bewildered in
the regions of hypothesis.

Tremors may however be communicated to the auditory nerve in a
different manner from what I have described. If a watch be put
between the teeth, and the ear stopped, tremors will be communicated
to the teeth, by them to the bones of the upper jaw, and by these to
the auditory nerve. In this way a person born deaf, and having no
power of hearing through the medium of the air, may become sensible
of the pleasures of music.

That sound may be propagated by vibrations, independent of pulses of
the air, is evident from the experiment with the string and poker.

There is, strictly speaking, no such thing existing as sound; it
being only a sensation of the mind, caused by tremors of the air, or
vibrations of the sounding body.

In order to understand more clearly how pulses, or waves are caused
by the vibration of bodies, and the manner in which vibrating bodies
are affected, I shall just enumerate some of the properties of
pendulums, which however I shall not stop to demonstrate here, as
that would consume much time.

When two pendulums vibrate which are exactly of the same length,
their vibrations are performed in equal times; if they set out
together to describe equal arcs, they will agree together in their
motions, and the vibrations will be performed in equal times.

But if one of these pendulums be four times as long as the other, the
vibrations of the longer will be twice as slow as those of the
shorter; the number of vibrations being as the square roots of their
lengths.

A pendulum is fixed to one point, but a musical string is extended
between two points, and in its vibrations may be compared to a double
pendulum vibrating in a very small arc, hence we see how strings of
different lengths may agree in their motions after the manner of
pendulums; but we must observe that it is not necessary to quadruple
the length of a musical string, in order to make the time of
vibration twice as long; it will be sufficient merely to double it.
We know that from whatever height a pendulum falls on one side, to
the same height will it rise on the other. In the same manner will an
elastic string continue to vibrate from one side to the other for
some time, till its motion be destroyed by the resistance of the air,
and friction about its fixed points, and each of its small
vibrations, like those of a pendulum, will, for the same reason, be
performed in times exactly equal to each other.

Thus we gain from the analogy between a pendulum and a musical
string, a more adequate conception of a subject which was never
understood till this analogy was discovered. It explains to us why
every musical string preserves the same pitch from the beginning to
the end of its vibration; or as long as it can be distinguished by
the ear; and why the pitch remains still unvaried whether the sound
is loud or soft, and all this because the vibrations of the same
pendulum whether they are longer or shorter, when compared among
themselves, are found to be all performed in equal times till the
pendulum be at rest, the difference of the space, which is moved
over, compensating for the slowness of the motion till its decay.

To illustrate this subject still further, suppose we have a piece of
catgut stretched between two pins; I lay hold of it in the middle and
pull it sideways; I let it go, and you will observe that it first
straightens itself or returns to its original position. This depends
on the elasticity of its particles, which tend to reunite when they
have been separated by an external force, just in the same way that
the particles of a piece of caoutchouc or Indian rubber attract each
other when pulled asunder; and this force not only enables the string
to restore itself to its former situation, but will carry it nearly
to an equal distance on the other side, just in the same manner as a
ball falling down an inclined plane will rise nearly to the same
height up another, or a pendulum will rise nearly to the height from
which it fell.

In this way will a string move backwards and forwards, till friction
and the resistance of the air have destroyed the velocity which it
acquired by the force of elasticity.

It is obvious that when a string is thus let fly from the finger,
whatever be its own motion, such will also be the motion of the
particles of the air which fly before it: the air will be driven
forwards, and by that means condensed. When this condensed air
expands itself, it will expand not only towards the string, but as
its elasticity acts in all directions, it will also expand itself
forwards and condense the air that is beyond it, this last condensed
air, by its expansion, will produce the same effect on the air that
lies still further forwards, and thus the motion produced in the air,
by the vibration of the elastic string, is constantly carried
forwards and conveyed to the ear.

It will be proper however to observe, that these pulses are sometimes
produced without any such vibration of the sounding body, as we find
it in musical strings and bells. In these cases we have to discover
by what cause these condensations or pulses may be produced without
any apparent vibrations in what is considered as the sounding body.
We have two or three instances of this kind; one in wind instruments,
such as the flute or organ pipe; another in the discharge of a gun.

In an organ, or flute, the air, which is driven through the pipe,
strikes against the edge of the lips of the instrument in its
passage, and by being accumulated there, is condensed, and this
condensation produces waves or pulses in the air.

When a gun is discharged, a great quantity of air is produced, by the
firing of the gunpowder, which being violently propelled from the
piece, condenses the air that encompasses the space where the
expansion happens; for whatever is driven out from the space where
the expansion is made will be forcibly driven into the space all
around it. This condensation forms the first pulse, and as this, by
its elasticity, expands again, pulses of the same sort will be
produced and propagated forwards.

There is likewise another curious instance of the production of
sound, when a tube is held over a stream of inflamed hydrogen gas
issuing out of a capillary tube in a bottle.

Sounding bodies propagate their motions on all sides, directly
forwards, by successive condensations and rarefactions, so that sound
is driven in all directions, backwards and forwards, upwards and
downwards, and on every side; the pulses go on succeeding each other
like circles in disturbed water.

Sounds differ from each other both with respect to their tone and
intensity: in respect to their tone, they are distinguished into
grave and acute: in respect to their intensity, they are
distinguished into loud and low, or strong and weak. The tone of a
sound depends on the velocity with which the vibrations are
performed, for the greater the number of vibrations in a given time,
the more acute will be the tone, and on the contrary, the smaller the
number, the more grave it will be. The tone of a sound is not altered
by the distance of the ear from the sounding body; but the intensity
or strength of any sound depends on the force with which the waves of
the air strike the ear; and this force is different at different
distances; so that a sound which is very loud when we are near the
body that produces it, will be weaker if we are further from it,
though its tone will suffer no alteration; and the distance may be so
great that we cannot hear it at all. It has been demonstrated, that
the intensity of sound at different distances from the sounding body
is inversely as the square of the distance.

Sound moves with the same velocity at all distances from the sounding
body, otherwise it would not produce the same tone at all distances.
Sounds of different tones likewise move with the same velocity. This
is evident from a peal of bells being heard in the same order in
which they are rung, whether we are near, or at a distance.

It is likewise found that sounds of the same tone but of different
intensities are propagated with the same velocity. A low sound cannot
indeed be heard so far as a loud one; but sounds, whether low or
loud, will be conveyed in an equal time to any equal distance at
which they can both be heard. The report of a cannon does not move
faster, or pass over a given space sooner, than the sound of a
musical string.

The principal cause of the decay of sound is the want of perfect
elasticity in the air: whence it happens that every subsequent
particle has not the entire motion of the preceding particle
communicated to it, as is the case with equal and perfectly elastic
bodies; consequently the further the motion is propagated, the more
will the velocity with which the particles move be diminished; the
condensation of the air will be diminished also, and consequently its
effect on the ear. That the want of perfect elasticity in the air is
the principal cause of the decay of sound, appears from this, that
sounds are perceived more distinctly when the north and easterly
winds prevail, at which time the air is dry and dense, as appears
from the hygrometer and barometer; and, of course, the air in this
state must be more elastic, for the vapours diffused through the
atmosphere, unless dilated by intense heat, diminish the spring of
the air.

That sound is not propagated to all distances instantaneously, but
requires a sensible time for its passage from one place to another,
is evident from the discharge of a gun at a distance; for the report
is not heard till some time after the flash is seen. Light moves much
more swiftly than sound; it comes from the sun in eight minutes,
which is at the rate of 74,420 leagues in a second; so that the
velocity of light may be considered as instantaneous, at any distance
on the earth; and, as sound takes up a considerable time in its
passage, the interval between the flash and the report of the gun
shows the space it passes over in a given time, which is found to be
1142 feet in a second; so that if three seconds elapse between the
time when we see the flash and hear the report of the gun, it must be
distant 1142 yards.

From experiments that have been made at different times, by various
philosophers, we may collect the following results. First, That the
mean velocity of sound is a mile in about 4 3/4 seconds, or 1142 feet
in a second of time. Secondly, That all sounds, whether they be weak
or strong, have the same velocity. Thirdly, That sound moves over
equal spaces in equal times, from the beginning to the end.

The tone of a musical string, or a bell, appears continuous. This
depends upon a law of sensation, formerly mentioned, namely, that
impressions made upon any of the organs of sense do not immediately
vanish, but remain some time; and we hear sound continuous from these
vibrations, for the same reason that we hear it continuous when we
draw a stick quickly along a rail, or a quill along the teeth of a
comb; the vibrations succeed each other so quickly that we hear the
succeeding before the effect of the preceding is worn off; though it
must be evident that the impression produced by each pulse or wave of
the air is perfectly distinct and insulated.

The act of combining sounds in such a manner as to be agreeable to
the ear, is called music. This art is usually divided into melody and
harmony. An agreeable succession of sounds is called melody; but when
two or more sounds are produced together, and afford an agreeable
sensation, the effect is called harmony. When two sounds are produced
together, and afford pleasure to the sense of hearing, the effect is
called a concord; but when the sensation produced is harsh or
disagreeable, it is called a discord. These different effects seem to
depend upon the coincidence of the vibrations of the two strings, and
consequently on the coincidence of the pulses which they excite in
the air. When the strings are equally stretched, and of the same
length and thickness, their vibrations will always coincide, and they
produce a sound so similar to each other, that it is called unison,
which is the most perfect concord. When one string is only half the
length of the other, the vibrations coincide at every second
vibration of the shorter string: this produces a compound sound,
which is more agreeable to the ear than any other, except the unison;
this note, when compared with the tone produced by the longer string,
is called the octave to it, because the interval between the two
notes is so divided by musicians that from one to the other they
reckon eight different tones.

If the strings be of the length, two and three, the coincidence of
the pulses will happen less frequently, viz. at every third vibration
of the shorter string, and the concord will be less perfect. This
forms what is called a fifth. The less frequent the coincidence of
the vibrations, the less perfect will be the concord, or the less
pleasure will it afford to the mind; till the vibrations coincide so
seldom, that the sound produced by both strings at once is harsh and
disagreeable, and is called a discord.

The effects of music upon the mind, the power by which it moves the
heart, touches the passions, and excites sometimes the highest
pleasure, and sometimes the deepest melancholy, depend upon melody.
By a simple melody the ignorant are affected as well as those skilled
in music. The pleasures arising from harmony or a combination of
sounds are acquired rather than natural. Its pleasures are the result
of experience and knowledge in music; music affords a source of
innocent and inexhaustible pleasure, but its effects are different on
different persons: some are enthusiastically fond of it, while others
hear the sweetest airs with a listlessness bordering upon
indifference. This has been supposed to depend on a musical ear,
which is not given by nature to all. The cause of this difference is
by no means evident. It does not depend on the delicacy of the sense
of hearing, for there are some persons half deaf, who have the
greatest relish for music; while others who have a very acute sense
of hearing have no relish for music. In some instances I think a
musical ear has been acquired where it did not seem originally to
exist.

The force of sound is increased by the reflection of many bodies,
particularly such as are hard or elastic, which receive the waves or
pulses of the air and reflect them back again; these reflected
pulses, striking the ear along with the original, strengthen the
original sound. Hence it is, that the voice of a speaker is louder,
and more distinctly heard, in a room than in the open air. I said
that these reflected sounds entered the ear at the same time with the
original: this however is not strictly the case, for they must enter
the ear after the original, because the sound has a greater space to
move over: but they enter the ear so quickly after the original that
our sense cannot distinguish the difference. If however the
reflecting body should be placed at such a distance, that the
reflected sound should enter the ear some considerable or sensible
time after the original, an echo or distinct sound would be heard.

It appears from experiment that the ear of an experienced musician
can only distinguish such sounds as follow each other at the rate of
nine or ten in a second, or any lower rate; and therefore that we may
have a distinct perception of the direct and reflected sound, there
should at least be an interval of 1/9 of a second; but in this time
sound passes over one hundred and twenty seven feet, and
consequently, unless the space between the sounding body and the
reflecting surface, added to that between the reflecting surface and
the ear, be greater than one hundred and twenty seven feet, no echo
will be heard, because the reflected sound will enter the ear so soon
after the original, that the difference cannot be distinguished; and
therefore it will only serve to augment the original sound.

From what has been said, it is evident, in order that a person may
hear the echo of his own voice, that he should stand at least sixty
three, or sixty four feet from the reflecting obstacle, so that the
sound may have time to move over at least one hundred and twenty
seven feet before it come to his ear, otherwise he could not
distinguish it from the original sound.

But though the first reflected pulses may produce no echo, both on
account of their being too few in number, and too rapid in their
return to the ear; yet it must be evident that the reflecting surface
may be so formed, that the pulses, which come to the ear after two or
more reflections, may, after having passed over one hundred and
twenty seven feet or more, arrive at the ear in sufficient numbers to
produce an echo, though the distance of the reflecting surface from
the ear be less than the limit of echoes. This is instanced by the
echoes that we hear in several caves or caverns.

The sense of hearing is more apt to be vitiated or diseased than any
of the other senses, which indeed is not surprising, when we consider
that its organ is complex, consisting of many minute parts, which are
apt to be deranged.

It sometimes becomes too acute, and this may arise either from too
great an irritability of the whole nervous system, which often occurs
in hysteria, also in phrenitis, and some fevers; or from an inflamed
state of the ear itself.

The sense of hearing becomes diminished, and often entirely
abolished; and this may arise from various causes, such as an
original defect in the external ear, or the meatus auditorius, or
both; the meatus auditorius is often blocked up with wax or other
substances, which being removed, the hearing becomes perfect.
Deafness may likewise arise from a rigidity of the membrane of the
tympanum, from its being erodedor ruptured, or from an obstruction
of the Eustachian tube. It may likewise arise from a paralysis or
torpor of the auditory nerve, or from some diseased state of the
labyrinth, or from a vitiated state of the brain and nerves. There
is a kind of nervous deafness which comes on suddenly, and often
leaves the patient as suddenly.

There are various instances, however, in which the membane of the
tympanum has been lacerated or destroyed, without a total loss of
the sense of hearing, or indeed any great diminution of it. A
consideration of these circumstances induced Mr. Astley Cooper to
think of perforating it, in cases of deafness arising from a
permanent obstruction of the Eustachian tube, and he has often
performed this operation with great success. Of this he has given an
account in the last part of the Philosophical Transactions. This
operation ought however only to be performed in case of the closure
of the Eustachian tube. Cases of this kind may be distinguished by
the followingcriteria. If a person on blowing the nose violently,
feel a swelling in the ear, from the membrane of the tympanum being
forced outwards, the tube is open; and though the tube be closed,
if the beating of a watch placed between the teeth, or pressed against
the side of the head, cannot be heard, the operation cannot relieve,
as the sensibility of the auditory nerve must have been destroyed. In
a closed Eustachian tube, there is no noise in the head, like that
accompanying nervous deafness.

There is one species of deafness, which occurs very frequently, and
happens generally to old persons, though sometimes to the delicate
and irritable in the earlier periods of life. Anxiety and distress of
mind have been known to produce it. Its approach is generally
gradual, the patient hears better at one time than at another; a
cloudy day, a warm room, agitated spirits, or the operation of fear,
will produce a considerable diminution in the powers of the organ. In
the open air the hearing is better than in a confined situation; in a
noisy, than in a quiet society; in a coach when it is in motion, than
when it is still. A pulsation is often felt in the ear; a noise
resembling sometimes the roaring of the sea, and at others the
ringing of distant bells is heard. This deafness generally begins
with a diminished secretion of the wax of the ear, which the patient
attributes to cold. It may be cured, particularly at its
commencement, by the application of such stimulants as are capable of
exciting a discharge from the ceruminous glands; for which purpose
thev should be introduced into the meatus auditorius.

In some cases of this kind, where the auditory nerve has been in some
degree torpid, or rather perhaps where there has been a kind of
paralysis, or want of action, in the muscles which brace the membrane
of the tympanum, and keep the chain of bones in their proper state; a
person has not been able to hear, except during a considerable noise.
Willis mentions the case of a person who could only hear when a drum
was beaten near her; and we are told of a woman who could not hear a
word except when the sound of a drum was near, in which case she
could hear perfectly well. When she married, her husband hired a
drummer for his servant. In instances of this kind the noise probably
excites the action of the torpid muscles, which then put the
apparatus in a proper condition to hear.

LECTURE VIII.
VISION.

In order to understand properly the theory of vision, it will be
necessary to premise an anatomical description of the eye: but I
shall content myself with as short a one as will suffice to explain
the effects it produces on the rays of light, so as to produce the
distinct vision of an object.

The shape of the eye is nearly spherical; it is composed of several
coats or tunics, one within another; and is filled with transparent
humours of different densities.

The proper coats of the eye are reckoned five in number; viz. the
sclerotica, cornea, choroides, iris or uvea, and the retina.

After the tunica conjunctiva, or adnata, (a membrane, which, having
lined the eyelids in the manner of a cuticle, surrounds the anterior
part of the globe) is removed, we perceive a white, firm, membrane,
called the sclerotica, which takes its rise from that part of the
globe where the optic nerve enters, and surrounds the whole eye,
except a little in the fore part; which fore part has a membrane,
immediately to be described, called the cornea. The tunica
sclerotica, viewed through the conjunctiva, forms what is called the
white of the eye. Some anatomists have supposed that this coat is a
continuation of the dura mater, which surrounds the optic nerve; but
later observations have shown this opinion to be ill founded. The
tunica sclerotica consists of two layers, which are with difficulty
separated.

The next coat is the cornea, so called from its resemblance to
transparent horn; it arises where the sclerotic coat ends, and forms
the fore part of the eye. The cornea is a segment of a lesser sphere
than the rest of the eye, and consequently makes it more prominent on
the fore part: it is transparent, and firmly connected by its edges
to the sclerotica.

Immediately adherent to the sclerotica, within, is the choroides,
which takes its rise from that part of the eye where the optic nerve
enters, and accompanies the sclerotica to the place where it is
joined to the cornea; here it is very closely connected to the
sclerotica, where it forms that annulus, called ligamentum ciliare;
then leaving the sclerotic coat, it is turned inwards, and surrounds
the crystalline lens; but as this circle, where it embraces the
crystalline, is much narrower than where the membrane leaves the
sclerotic coat, it becomes beautifully corrugated, which folds or
corrugations have been, by the more ancient anatomists, improperly
called ciliary processes.

To the same part of the choroid coat, where the ciliary ligament
begins, is fixed a moveable and curious membrane, called the iris;
this membrane has a perforation in the middle, called the pupil, for
the admission of the rays of light. The iris is composed of two kinds
of fibres: those of the one sort tend, like the radii of a circle,
towards its centre, and the others form a number of concentric
circles round the same centre. The pupil is of no constant magnitude,
for when a very luminous object is viewed, the circular fibres of the
iris contract, and diminish its orifice; and, on the contrary, when
objects are dark and obscure, those fibres relax, and suffer the
pupil to enlarge, in order to admit a greater quantity of light into
the eye: it is thought that the radial fibres also assist in
enlarging the pupil. The iris is variously coloured in different
persons, but according to no certain rule; though in general, they
who have light hair, and a fair complexion, have the iris blue or
grey; and, on the contrary, they whose hair and complexion are dark,
have the iris of a deep brown; but whether this difference in colour
occasions any difference in the sense, is not yet discovered. In the
human eye the whole choroid coat, and even the interior surface of
the iris or uvea, is lined with a black mucus; this mucus, or as it
is called, pigmentum, is darkest in young persons, and becomes more
light coloured as we advance in years. In many animals, but more
particularly those which catch their prey in the night, this
pigmentum is of a bright colour: its use will appear afterwards.

The last, and innermost coat of the eye, is the retina, it differs
much from the above mentioned coats, being very delicate and tender.
It is nothing but an expansion of the medullary part of the optic
nerve, which is inserted into each eye, nearer the nose, and a little
higher, than the axis. This coat has been thought by many to end
where the choroides, going inwards, towards the axis of the eye,
forms the ciliary ligament; Dr. Monro thinks that it is not continued
so far, and we cannot see with what advantage it could have been
continued to the ciliary ligament, since none of the rays of light,
passing through the pupil, could fall upon that part of it. In the
middle of the optic nerve is found the branch of an artery, from the
internal carotid, which is diffused and ramified in a beautiful
manner along the retina. From this artery, a small branch goes
through the middle of the vitreous humour, and giving off branches on
every side, expands itself upon the capsule of the crystalline lens.

We shall now consider the humours of the eye, which are three in
number, the aqueous, the crystalline, and the vitreous; all
transparent, and in general colourless; but of different densities.

The aqueous humour, so called from its resemblance to water, fills up
all the space between the cornea and the crystalline humour. It is
partly before and partly behind the uvea, and is divided by that
membrane into two parts, which are called the chambers of the aqueous
humour; which chambers communicate with each other by means of the
pupil.

The next humour is the crystalline; it is situated between the
aqueous and vitreous humours, and is connected to the choroid coat by
the ciliary ligament: it is not the least of all the humours, as has
been generally supposed, the aqueous and it being of equal weights;
but its substance is more firm and solid than that of the other
humours: its figure is that of a double convex lens; but the fore
part next the pupil is not so convex as its other side, which is
contiguous to the vitreous humour; the diameter of the sphere, of
which its anterior segment is a part, being in general about seven or
eight lines, whereas the diameter of the sphere, of which its
posterior segment forms a portion, is commonly only about five or six
lines. It is covered with a fine transparent capsule, which is called
arachnoides. This humour is situated exactly behind the pupil, but
not in the centre of the eye, as was supposed by Vesalius, being a
good deal nearer its forepart. The convexity of its posterior surface
is received into an equal concavity of the vitreous humour. It is not
of an equal density throughout, but is much more hard and dense
towards its centre than externally, the reason of which will appear
hereafter. Till we arrive at about our thirtieth year, this humour
continues perfectly transparent, and colourless; about that time it
generally has a little tinge of yellow, and this colour increases
with age.

The third humour of the eye, is the vitreous; it is the largest of
all the humours, filling up the whole of that part of the eye which
lies behind the crystalline humour. It is thicker than the aqueous,
but thinner than the crystalline humour; on its back part is spread
the retina, and in the middle of its fore part is a small cavity, in
which the whole posterior surface of the crystalline lens lies; this
humour is also enclosed in a very fine capsule, called tunica vitrea;
this capsule at the edge of the crystalline humour is divided into
two membranes, of which the one is continued over the whole anterior
surface of the vitreous humour, and lines that cavity into which the
back part of the crystalline is received; the other passes over the
crystalline humour, and covers all its fore part, by which means
these two humours are closely connected together. The weights of the
aqueous, crystalline, and vitreous humours in a human eye, are,
according to the accurate Petit, at a medium, to each other, as 1, 1,
and 25.

It was thought necessary to premise this general description of the
structure of the eye, in order that what we are going to add in the
remaining part of this Lecture may be the more easily comprehended. A
more distinct idea will perhaps be had from a contemplation of the
following figure, which represents the section of an eye by a
vertical plane passing through its centre.

[FIGURE]

EXPLANATION.

NOO represents the optic nerve.
The outmost line ALLB represents the sclerotic coat, and the
   part ACB the transparent cornea.
The line ALLB, immediately within the former, represents the
   choroides; the part APB is the iris or uvea, in which the hole at P
   is the pupil.
The line FOOG is the retina.
The cavity ACBEMDA is the aqueous humour.
DE is the crystalline lens or humour.
The space DFOOGE, lying behind the crystalline, represents the
   vitreous humour.
BE and AD is the ligamentum ciliare.

_Nature and Properties of Light._

After this short description of the human eye, I shall next proceed
to take notice of some of the properties of light; but shall confine
myself to such as are absolutely necessary for explaining the
phenomena of vision, as far as that can be done from optical
principles.

1. It is, I believe, generally at present agreed, that light consists
of exceedingly small particles of matter, projected with great
velocity in all directions from the luminous or radiant body. This
hypothesis, to which no solid objection has yet been made, appears to
be more simple than any other; and is so consistent with all the
phenomena yet observed, that we have great reason to think it true:
however, as it is not absolutely and directly demonstrated, it may
have been wrong in optical writers to have given this hypothesis (for
it can only be called a hypothesis) as a definition of light.

2. The space through which light passes is, by opticians, called a
medium, and it is observed, that, when light passes through a medium,
either absolutely void, or containing matter of an uniform density,
and of the same kind, it always proceeds in straight lines.

3. Those rays of light which come directly from a luminous body to
the eye, only give us a perception of light; but when they fall upon
other bodies, and are from them reflected to the eye, they give us an
idea or perception of those bodies.

4. When a ray of light passes out of one medium into another of
different density, it is bent out of its course, and is said to be
refracted. We must, however, except those rays which fall in a
direction perpendicular to the surface of the refracting medium; as
the refractive force acts in the same direction in which those rays
move, they will not be turned out of their course, but proceed in the
same direction they had before they entered the refracting medium.
When a ray passes out of a rarer into a denser medium, it will be
refracted, or bent towards a line which is perpendicular to the
surface which separates the media at the point where it falls; but
when it passes out of a denser into a rarer medium, it will be bent
from the perpendicular.

5. Whenever the rays, which come from all the points of any object,
meet again in so many points, after they have been made to converge
by refraction, there they will form the picture of the object,
distinct, and of the same colours, but inverted. This is beautifully
demonstrated by a common optical instrument, the camera obscura. If a
double convex lens, be placed in the hole of a window shutter in a
dark room, and a sheet of white paper be placed at a certain distance
behind the lens; a beautiful, but inverted picture of the external
objects will be formed: but if the paper be held nearer, or more
remote than this distance, so that the rays from each point shall not
meet at the paper, but betwixt it and the lens, or beyond the paper,
the picture will be indistinct and confused.

_Of the Manner in which Vision is performed._

From the just mentioned properties of light, and the description we
have given of the eye, it will not be difficult to explain the theory
of vision, so far as it depends upon optical principles. For the eye
may, with great propriety, be compared to a camera obscura; the rays
which flow from external objects, and enter the eye, painting an
inverted picture of those objects on the retina: if you carefully
dissect from the bottom of an eye, newly taken out of the head of an
animal, a small portion of the tunica sclerotica and choroides, and
place this eye in a hole made in the window shutter of a dark
chamber, so that the bottom of the eye may be towards you; the
pictures or images of external objects will be painted on the retina
in lively colours, but inverted.

In order to see how the several parts of the eye contribute to
produce this effect, let us follow the rays proceeding from a
luminous point, and see what will happen to them from the
beforementioned properties of light.

Since the rays of light flow from every visible point of a body in
every direction, some of them, issuing from this point, will fall
upon the cornea, and, entering a medium of greater density, will be
refracted towards the perpendicular, and as they fall upon a convex
spherical surface, nearly in a parallel state, the pupil being so
extremely small, it is evident, from the principles of optics, that
they will be made to converge: those which fall very obliquely will
either be reflected, or falling upon the uvea, or pigmentum nigrum,
which covers the ciliary ligaments, will be suffocated, and prevented
from entering the internal parts of the eye: those which fall more
directly, as was before said, become converging, in which state they
fall upon the anterior surface of the crystalline humour, which,
having a greater refracting power than the aqueous humour, and its
surface being convex, will cause them to converge still more, in
which state they will fall upon the posterior surface of the
crystalline, or anterior surface of the vitreous humour; which having
a less refractive power than the crystalline, they will be refracted
from the perpendicular; but, as they fall upon a concave surface, it
is evident, from the principles of optics, that they will be made to
converge still more: in which state they will go on to the retina,
and if the eye is well formed, the refraction of these several
humours will be just sufficient to bring them to a point or focus on
the retina.

The same thing will happen to rays flowing from every other visible
point of the object: the rays which flow from every point will be
collected into a corresponding point on the retina, and,
consequently, will paint the image of that object inverted; the rays
coming from the superior part of any object, being collected on the
inferior part of the retina, and vice versa, as is manifest from the
principles of optics.

If the rays are accurately, or very nearly, collected into a focus on
the retina, distinct vision will be produced; but if they be made to
converge to a point before or beyond the retina, the object will be
seen indistinctly; this is proved by holding a convex or concave
glass before the eye of a good sighted person: in the former case,
the rays will be made to converge to a point before they arrive at
the retina, and in the latter, to a point beyond it. In these cases,
it is plain that the rays which flow from a point in the object, will
not form a point, but a circular spot, upon the retina, and these
various circles intermixing with other, will render the image very
indistinct. This is well illustrated by the camera obscura, where if
you hold the paper nearer or more remote than the focal distance of
the lens, the picture will be indistinct.

So far then, in the theory of vision, are we led by the principles of
optics, and we can with certainty, by their assistance, affirm, that
if the eye is sound, and the image of an object distinctly painted
upon the retina, it will be seen distinctly, erect, and of its proper
colours: so far we can proceed on safe and sure grounds, but if we
venture further, we shall find ourselves bewildered in the regions of
hypothesis and fancy. The machinery by which nature connects the
material and immaterial world is hidden from our view; in most cases
we must be satisfied with knowing that there are such connexions, and
that these connexions invariably follow each other, without our being
able to discover the chain that goes between them. It is to such
connexions that we give the name of laws of nature; and when we say
that one thing produces another by a law of nature, this signifies no
more, than that one thing, which is called the cause, is constantly
and invariably followed by another, which we call the effect, and
that we know not how they are connected. But there seems a natural
propensity in the mind of man, to endeavour to account for every
phenomenon that falls under his view, which has given rise to a
number of absurd and romantic conjectures in almost every branch of
science. From this source has risen the vibration of the fibres of
the optic nerve, or the undulation of a subtile ether, or animal
spirits, by which attempts have been made to explain the theory of
vision; but all of them are absurd and hypothetical.

Kepler was the first who had any distinct notion of the formation of
the pictures of objects on the bottom of the eye; this discovery he
published about the year 1600. Joannes Baptista Porta had indeed got
some rude notion of it prior to the time of Kepler, but as he knew
nothing of the refraction made by the humours of the eye, his
doctrine was lame and defective, for he imagines that the images are
painted on the surface of the crystalline humour.

The disputes concerning the theory of vision had very much divided
the ancient philosophers; some of them imagining that vision was
caused by the reception of rays into the eye; while a great many
others thought it more agreeable to nature, that certain emanations,
which they called visual rays, should flow from the eye to the
object.

We shall now inquire more particularly how each part of the eye is
peculiarly fitted to produce distinct vision. Though the eye is
composed of different humours, yet one might have been sufficient to
collect the rays into a focus, and form the picture of an object upon
the retina. By the experiments of the accurate Dr. Robertson, it
appears that there is less difference in the density, as well as in
the refracting power of the humours, than has been generally thought:
by weighing them in a hydrostatic balance, he found that the specific
gravities of the aqueous and vitreous humours were very nearly equal,
each being nearly equal to that of water: and that the specific
gravity of the crystalline did not exceed the specific gravity of the
other humours in a greater proportion than that of about 11 to 10.
Hence it would seem to follow, that the crystalline is not of such
great use in bringing the rays together, and thereby forming the
pictures of objects on the retina, as has been commonly thought by
optical writers; for though in shape it resembles a double convex
lens, and is, on that account, fitted to make the rays converge; yet,
be cause it is situated between two humours nearly of the same
refractive power with itself, it will alter the direction of the
light but a little. From this, the reason is evident why the sight
continues after the operation for the cataract, in which the
crystalline is depressed, or extracted, and why a glass of small
convexity is sufficient to supply the little refraction wanting,
occasioned by the loss of this humour. But without doubt, several
important purposes are effected by this construction of the eye;
which could not have been attained if it had been composed of one
humour only. Some of those purposes seem sufficiently evident to us;
for instance, by placing the aqueous humor before the crystalline,
and partly before the pupil, and making the cornea convex, a greater
quantity of light is made to enter the eye than could otherwise have
done without enlarging the size of the pupil; the light will also
enter in a less diverging state than it could have done if the pupil
had been enlarged, and consequently be more accurately collected to a
focus on the retina; for a perfect eye can only collect such rays to
a focus on that membrane, as pass through the pupil nearly in a state
of parallelism.

Another, and perhaps a principal advantage derived from the different
humours in the eye, is, probably, to prevent that confusion arising
from colour, which is the consequence of the different degrees of
refrangibility of the rays of light. From the experiments of Mr.
Dollond, it appears, though contrary to the opinion of Newton, and
most other optical writers, that different kinds of matter differ
extremely with respect to the divergency of colour produced by equal
refractions; so that a lens may be contrived, composed of media of
different dispersing powers, which will form the image of any object
free of colour; this discovery Mr. Dollond has applied to the
improvement of telescopes, with great success. It is by no means
improbable, that nature has, for the same purpose, placed the
crystalline lens betwen two media of different densities, and,
probably, different dispersing powers, so that an achromatic image,
free from the prismatic colours, will be formed on the retina. Indeed
we find a conjecture of this kind, so long since as Dr. David
Gregory's time, he says, in speaking of the imperfection of
telescopes, "Quod si ob difficultates physicas, in speculis idoneis
torno elaborandis, et poliendis, etiamnum lentibus uti oporteat,
fortassis media diversae densitatis ad lentem objectivam componendam
adhibere utile foret, ut a natura factum observamus in oculo, ubi
crystallinus humor (fere ejusdem cum vitro virtutis ad radios lucis
refringendos) aqueo et vitreo (aquae quoad refractionem hand
absimilibus) conjungitur, ad imaginem quam distincte fieri poterit, a
natura nihil frustra moliente, in oculi fundo depingendam."

In describing the eye, I observed, that the crystalline humour was
not every where of the same consistence, being much more hard and
dense towards its centre, than externally: in the human eye, it is
soft on the edges, and gradually increases in density as it
approaches the centre: the reason of this construction is evident, at
least we know of one use which it will serve; for, from the
principles of optics, it is plain that the rays which fall at a
distance from the axis of the crystalline, by reason of their greater
obliquity, if the humour were of the same density in all its parts,
would be more refracted than those which fall near its axis, so that
they would meet at different distances behind the crystalline humour;
those which pass towards its extremity, nearer, and those near its
axis, at a greater distance, and could not be united at the same
point on the retina, which would render vision indistinct; though the
indistinctness arising from this cause, is only about the 1/5449 part
of that which arises from the different refrangibility of the rays of
light, as Sir Isaac Newton has demonstrated. Nature has, however,
contrived a remedy for this also, by making the crystalline humour
more dense and solid near its centre, that the rays of light which
fall near its axis, may have their refraction increased, so as to
meet at the same point with those which fall at a distance from its
axis.

_Of the manner in which the Eye conforms itself in order to see
distinctly at different Distances._

It has been much disputed in what manner the eye conforms itself to
see distinctly at different distances; for it is evident, that,
without some change, the rays which flow from objects at different
distances, could not be collected into a focus at the same point,
and, consequently, though the eye might see distinctly at one
distance, it could not at another.

This subject has given rise to a variety of opinions, but few of them
are satisfactory; and though several of them might explain the
phenomena of vision, at different distances, yet it is by no means
proved that those supposed changes do take place in the eye. I shall
content myself with just mentioning the principal opinions on this
subject, without engaging in a controversy, which has for a long time
employed the ingenuity of philosophers to little purpose.

Some are of opinion, that the whole globe of the eye changes its
figure; becoming more oblong when objects are near, and more flat
when they are removed to a greater distance; and this change in the
figure of the eye is differently explained by different authors; some
maintain that it is rendered oblong by the joint contraction of the
two oblique muscles: others think that the four straight muscles
acting together, compress the sides of the globe, and by this
compression, reduce it to an oblong figure, when objects are near;
and that, by its natural elasticity, it recovers its former figure
when these muscles cease to act. Others again think that when these
four straight muscles act together, they render the eye flat by
pulling it inwards, and pressing the bottom of it against the fat;
and that it is reduced to its former figure, either by the joint
contraction of the two oblique muscles, or by the inherent elasticity
of its parts, which exerts itself when the muscles cease to act.

That, if such a change should take place in the eye, it would produce
distinct vision, will be readily granted; but that such a one does
not take place, at least in any of these ways, is, in my opinion,
very certain. Dr. Porterfield thinks that the crystalline lens has a
motion by means of the ligamentum ciliare, by which the distance
between it and the retina is increased or diminished, according to
the different distances of objects. The ligamentum ciliare, he says,
is an organ, the structure and disposition of which excellently
qualify it for changing the situation of the crystalline, and
removing it to a greater distance from the retina, when objects are
too near for us; for that, when it contracts, it will not only draw
the crystalline forwards, but will also compress the vitreous humour,
lying behind it, so that it must press upon the crystalline, and push
it from the retina. Although this hypothesis will, in a great
measure, account for distinct vision at different distances, yet it
could only be of use where the rays enter the eye with a certain
degree of divergency, while, however we are sure, that in looking at
very distant objects which are at different distances from us, the
eye undergoes a change. But a sufficient objection to Dr.
Porterfield's hypothesis is, that it is by no means proved that the
crystalline lens can be moved in the manner he supposes, or that the
ligamentum ciliare is possessed of muscular fibres; on the contrary
some eminent anatomists deny that they are.

We shall now take a view of the opinion of M. de la Hire, who
considered this subject, as well as almost every other relating to
vision, with the closest attention; he maintains, that, in order to
view objects distinctly at different distances, there is no
alteration but in the size of the pupil, which is well known to
contract and dilate itself according to the quantity of light flowing
from the object we look at, being most contracted in the strongest
light, and most dilated when the light is weakest; and consequently
will contract when an object is held near the eye, and dilate as it
is removed, because in the first case the quantity of light entering
the eye is much greater than in the last. That this contraction of
the pupil will have the effect of rendering vision distinct,
especially when objects are within the furthest limits of distinct
vision, will plainly appear, if we consider the cause of indistinct
vision. Dr. Jurin has shown, that objects may be seen with sufficient
distinctness, though the pencils of rays issuing from the points of
them do not unite precisely in another point on the retina, but
instead thereof, if they form a circle which does not exceed a
certain magnitude, distinct vision will be produced; the circle
formed by these rays on the retina he calls the circle of
dissipation. The pupil will, by contracting, not only diminish the
circles of dissipation, and thereby help to produce distinct vision,
but will also prevent so great a quantity of light from falling near
the circumferences of those circles; and Dr. Jurin has shown, that,
if the light on the outer side of the circles of dissipation is
diminished, the remainder will scarce affect the sense. In both these
ways, the contraction of the pupil has a tendency to diminish the
circles of dissipation, and, consequently, to produce distinct
vision. This is likewise confirmed by experiment, for when an object
is placed so near, that the pupil cannot be so much contracted as is
necessary for distinct vision, the same end may be obtained by means
of an artificial pupil: for, if a small hole is made in a card, a
very near object may be viewed through it with the greatest ease and
distinctness. Also, if a person have his back turned towards a
window, and hold a book so near his eyes as not to be able to read,
if he turn his face to the light, he will find, that he will be able
to read it very distinctly; which is owing to the contraction of the
pupil by means of the light.

M. Le Roi, a member of the Royal Academy of Montpelier, has attempted
to defend the opinion of M. de la Hire, and, indeed, it seems, of all
others, the best supported by facts; but perhaps it may not account
so well for vision at great distances. It is likewise rendered more
probable by viewing the pictures of external objects, formed in a
dark chamber, by rays coming through a hole in the window shutter;
those pictures will be rendered distinct, by dilating, or contracting
the aperture, without the assistance of a lens, accordingly as the
object is more or less distant; those who have had the crystalline
lens depressed, or extracted, by means of one glass can see objects
pretty distinctly at different distances. These, and several other
arguments that might be brought, tend to prove that the eye
accommodates itself to view objects distinctly at different
distances, chiefly by means of the motion of the pupil; and though
this does not explain the phenomenon so satisfactorily as we could
wish, yet it is certain, that it has a share in it; we are however
certain, that, in whatever manner it may be produced, the eye has a
power of accommodating itself to view objects distinctly enough at
several different distances.

_Concerning the Seat of Vision._

No subject has been more canvassed than that concerning what is
improperly called the seat of vision. In early times, the crystalline
lens was thought to be best qualified for this office; but this
substance, though situated in the middle of the eye, which Baptista
Porta thought to be the proper centre of observation, had universally
given place to the better founded pretensions of the retina: and,
from the time of Kepler, few ventured to dispute its claim to that
office, till M. Mariotte was led, from some curious circumstances, to
think that vision was not performed by the retina, but by the choroid
coat. Having often observed in the dissections of men, as well as of
brutes, that the optic nerve is not inserted exactly opposite to the
pupil, that is, in the place where the picture of the objects upon
which we look directly, is made: and that in man it is somewhat
higher, and on the side towards the nose, he had the curiosity to
examine the reason of this structure, by throwing the image of an
object on this part of the eye. In order to do this, he fastened on a
dark wall, about the height of his eyes, a small round paper, to
serve for a fixed point of sight; and he fastened such another paper
on the right hand, at the distance of about two feet, but rather
lower than the former, so that light issuing from it, might strike
the optic nerve of his right eye, while the left was kept shut. He
then placed himself over against the former paper, and drew back by
degrees, keeping his right eye fixed, and very steady upon it, and
when he had retired about ten feet, he found that the second paper
entirely disappeared. This, he says, could not be imputed to the
oblique position of the second paper, with respect to his eye,
because he could see more remote objects on the same side. This
experiment he repeated by varying the distances of the paper and his
eye. He also made it with his left eye, while the right eye was kept
shut, the second paper being fastened on the left side of the point
of sight; so that by the situation of the parts of the eye, it could
not be doubted that this defect of vision is in the place where the
optic nerve enters, where only the choroides isdeficient.

From this he concludes, that the defect of vision is owing to the
want of the choroid coat, and, consequently, that this coat is the
proper organ of vision. A variety of other arguments in favour of the
choroides occurred to him, particularly he observed that the retina
is transparent, which he thought could only enable it to transmit the
rays further, and he could not persuade himself that any substance
could be considered as being the termination of the pencils, and the
proper seat of vision, at which the rays are not stopped in their
progress.

Mr. Pequet, in answer to Mariotte's observation, says, that the
retina is very imperfectly transparent, resembling oiled paper, or
horn: and, besides, that its whiteness demonstrates that it is
sufficiently opaque for stopping the rays of light as much as is
necessary for vision: whereas, if vision be performed by means of
those rays which are transmitted through such a substance as the
retina, it must be very indistinct.

Notwithstanding the plausibility of this opinion of M. Mariotte, and
the number of celebrated men who joined him in it, I must confess,
that none of their arguments, though very ingenious, have been able
to make me a convert to that opinion.

If we argue from the analogy of the other senses, in all of which the
nerves form the proper seat of sensation, we shall be induced to give
judgment in favour of the retina. And this argument from analogy is
much strengthened, by considering that the retina is a large nervous
apparatus, immediately exposed to the impressions of light; whereas
the choroides receives but a slender supply of nerves, and seems no
more fitted for the organ of vision than any other part of the body.
But facts are not wanting which make still more in favour of the
retina. It appears from observations made upon the sea calf and
porcupine, that these animals have their optic nerves inserted in the
axis of the eye, directly opposite the pupil, which renders it very
improbable that the defect in sight, where the optic nerves enter,
can be owing to the want of the choroides in that place; for were
this true, then in those creatures which have the optic nerves
inserted in the axis of the eye, and which by consequence do directly
receive on the extremity of the nerve the pictures of objects, all
objects would become invisible to which their eyes are turned,
because the choroides is wanting in that place where the image falls;
but this is contrary to experience.

M. Le Cat, though he strenuously supports Mariotte's opinion, takes
notice of a circumstance, which, if he had properly considered it,
might have led him to a contrary conclusion: from a beautiful
experiment he obtains data, which enable him with considerable
accuracy to determine the size of the insensible spot in his eye,
which he finds to be about 1/30 or 1/40 of an inch in diameter, and
consequently only about 1/5 or 1/6 of the diameter of the optic
nerve, that nerve being about 1/6 of an inch in diameter. I find that
in my eye likewise, the diameter of the insensible spot is about 1/40
of an inch, or something less. Whence it is evident that vision
exists where the choroid coat is not present, and consequently that
the choroid coat is not the organ of vision.

It is probably owing to the hardness and callosity of the retina
where the nerve enters, that we have this defect of sight, as it has
not yet acquired that softness and delicacy which is necessary for
receiving such slight impressions as those of the rays of light, and
this conjecture is rendered still more probable by an observation of
M. Pequet, who tells us, that a bright and luminous object, such as a
candle, does not absolutely disappear, but one may see its light,
though faint. This not only shows that the defect of sight is not
owing to a want of the choroides, but also that the retina is not
altogether insensible where the nerve enters. These circumstances, in
my opinion, render it certain, that the retina, and not the choroid
coat, is the organ of vision.

_Of our seeing Objects erect by inverted Images._

Another question concerning vision, which has very much perplexed
philosophers, is this; how comes it that we see objects erect, when
it is well known that their images or pictures on the retina are
inverted? The sagacious Kepler, who first made this discovery, was
the first that endeavoured to explain the cause of it.

The reason he gives for our seeing objects erect, is this, that as
the rays from different points of an object cross each other before
they fall on the retina, we conclude that the impulse we feel upon
the lower part of the retina comes from above; and that the impulse
we feel from the higher part, comes from below. Des Cartes afterwards
gave the same solution of this phenomenon, and illustrated it by the
judgment we form of the position of objects which we feel with our
arms crossed, or with two sticks that cross each other. But this
solution is by no means satisfactory: first, because it supposes our
seeing objects erect to be a deduction from reason, drawn from
certain premises, whereas it seems to be an immediate perception; and
secondly, because all the premises from which this conclusion is
supposed to be drawn, are absolutely unknown to far the greater part
of mankind, and yet they all see objects erect.

Bishop Berkeley, who justly rejects this solution, gives another,
founded on his own principles, in which he is followed by Dr. Smith.
This ingenious writer thinks that the ideas of sight are altogether
unlike those of touch; and since the notions we have of an object by
these different senses, have no similitude, we can learn only by
experience how one sense will be affected, by what, in a certain
manner, affects the other. Thus, finding from experience, that an
object in an erect position, affects the eye in one manner, and that
the same object in an inverted position, affects it in another, we
learn to judge, by the manner in which the eye is affected, whether
the object is erect or inverted. But it is evident that Bishop
Berkeley proceeds upon a capital mistake, in supposing that there is
no resemblance between the extension, figure, and position, which we
see, and that which we perceive by touch. It may be further observed,
that Bishop Berkeley's system, with regard to material things, must
have made him see this question, in a very different light from that
in which it appears to those who do not adopt his system.

In order to give a satisfactory answer to this question, we must
first examine some of the laws of nature, which take place in vision;
for by these the phenomena of vision must be regulated.

It is now, I believe, pretty well established, as a law of nature,
that we see every point of an object in the direction of a right
line, which passes from the picture of that point on the retina,
through the centre of the eye. This beautiful law is proved by a very
copious induction of facts; the facts upon which it is founded are
taken from some curious experiments of Scheiner, in his Fundamenta
Optices. They are confirmed by Dr. Porterfield, and well illustrated
by Dr. Reid. The seeing objects erect by inverted images is a
necessary consequence of this law of nature: for from thence it is
evident that the point of the object whose picture is lowest on the
retina, must be seen in the highest direction from the eye; and that
the picture which is on the right side of the retina, must be seen on
the left.

_Of seeing Objects single with two Eyes._

That we should have two pictures of an object, and yet see it single,
has long been looked upon as a curious circumstance by philosophers:
and of consequence, many attempts have been made to account for it,
few of which, however, are satisfactory.

As it would take up too much time to give a view of all the opinions
on this subject, I shall pass over the opinions of Galen, Gassendus,
Baptista Porta, Rohault, and others, which do not deserve a serious
refutation; and shall content myself with making a few observations
on the hypothesis of Bishop Berkeley.

But it seems the most proper way of proceeding, first of all to
consider the phenomena of single and double vision, in order, if
possible, to discover some general principle to which they lead, and
of which they are necessary consequences; and, for the sake of
perspicuity, we shall premise the following definition.

When a small object is seen single with both eyes, those points on
the two retinas on which the pictures of the object fall, may be
called corresponding points: and when the object is seen double, we
shall call such points, non-corresponding points.

Now we find that in sound and perfect eyes, when the axes of both are
directed to one point, an object placed in that point is seen single;
and in this case, the two pictures which show the object single, are
painted on the centres of the retinas. Hence, the centres of the two
retinas correspond.

Other objects at the same distance from the eyes, as that to which
their axes are directed, do also appear single: and in this case, it
is evident to those who understand the principles of optics, that the
pictures of an object to which the eyes are not directed, but which
is at the same distance as that to which they are directed, fall both
on the same side of the centre, that is, both to the right, or both
to the left, and both at the same distance from the centre. Hence it
is plain, that points in the retina, which are similarly situated
with respect to the centres, are corresponding points.

An object which is much nearer, or much more distant from the eyes,
than that to which their axes are directed, appears double. In this
case, it will easily appear, that the pictures of the object which is
seen double, do not fall upon points which are similarly situated.
From these facts, we are led to the following conclusion, viz. that
the points of the two retinas, which are similarly situated with
respect to the centres, correspond with each other, and that the
points which are dissimilarly situated, do not correspond. The truth
of this general conclusion is founded upon a a very full induction,
which is all the evidence we can have for a fact of this nature.

The next thing that seems to merit our attention, is, to inquire,
whether this correspondence between certain points of the two retinas
which is necessary to single vision, is the effect of custom, or an
original property of the human eyes.

We have a strong argument in favour of its being an original
property, from the habit we get of directing our eyes accurately to
an object; we get this habit by finding it necessary for perfect and
distinct vision; because thereby, the two images of the object
falling upon corresponding points, the eyes assist each other in
vision, and the object is seen better by both eyes together, than by
one: but when the eyes are not accurately directed, the two images of
the object fall upon points which do not correspond, whereby the
sight of the one eye disturbs that of the other. Hence it is not
unreasonable to conclude, that this correspondence between certain
points of the retina is prior to the habits we acquire in vision:
and, consequently, natural and original.

We have all acquired the habit of directing our eyes in one
particular manner, which causes single vision; now if the Author of
Nature had ordained that we should see objects single, only when our
eyes are thus directed, there is an obvious reason why all mankind
should agree in the habit of directing them in this manner; but, if
single vision were the effect of custom, any other habit of directing
the eyes would have answered the purpose; we therefore, on this
supposition, can give no reason why this particular habit should be
so universal.

Bishop Berkeley maintains a contrary opinion, and thinks that our
seeing objects single with both eyes, as well as our seeing them
erect, by inverted images, depends upon custom. In this he is
followed by Dr. Smith, who observes, that the question, why we see
objects single with both eyes, is of the same nature with this, why
we hear sounds single with both ears; and that the same answer will
serve for both; whence he concludes, that as the second of these
phenomena is the effect of custom, so also is the first. But I think,
that the questions are not so much of the same sort, as that the same
answer will serve for both; and, moreover, that our hearing single
with both ears is not the effect of custom. No person will doubt that
things which are produced by custom, may be undone by disuse, or by a
contrary custom. On the other hand, it is a strong argument, that an
effect is not owing to custom, but to the constitution of nature,
when a contrary custom, long continued, is found neither to change
nor weaken it. Now it appears, that from the time we are able to
observe the phenomena of single and double vision, custom makes no
change in them, every thing which at first appeared double, appearing
so still in the same circumstances. Dr. Smith has adduced some facts
in favour of his opinion, which, though curious, seem by no means
decisive. But in the famous case of the young man couched by Mr.
Cheselden, after having had cataracts in both his eyes till his
thirteenth year, it appears that he saw objects single from the time
he began to see with both eyes. And the three young gentlemen
mentioned by Dr. Reid, who had squinted, as far as he could learn,
from infancy, as soon as they learned to direct both eyes to an
object, saw it single.

In these cases it is evident that the centres of the retina
corresponded originally, for Mr. Cheselden's young man had never seen
at all before he was couched, and the other three had never been
accustomed to direct the axes of both eyes to the same point. These
facts render it probable, that this correspondence is not the effect
of custom, but of fixed and immutable laws of nature.

With regard to the cause of this correspondence, many theories have
been proposed, but as none of them can be looked on in any other
light than as probable conjectures, I think it would be to little
purpose to notice them. That of the illustrious Newton is the most
ingenious of any, and though it has more the appearance of truth than
any other, that great man has proposed it under the modest form of a
query.

Having given a short account of the principal phenomena of vision, I
proceed next to treat of some of the diseases to which this sense is
subject, I shall first take notice of the deformity called squinting.

_Of Squinting._

Though this is a subject which well deserves our particular
attention, yet having spoken of such a variety of subjects in the
preceding part of this lecture, I have not time for many observations
on this. I shall just mention the principal opinions, concerning the
cause of this deformity, and point out that which seems to me most
probable. This subject is certainly very worthy the attention of the
physician, as it is a case concerning which he may often be
consulted, and which it may be sometimes in his power to cure.

A person is said to squint, when the axes of both his eyes are not
directed to the same object.

This defect consists in the wrong direction of one of the eyes only.
I have never met with an instance in which both eyes had a wrong
direction, neither have I seen one accurately described by any
author.

The generality of writers on this subject have supposed this defect
to proceed from a disease of, or want of proper correspondence in,
the muscles of the eyes, which not acting in proper concert with one
another, as in persons free from this blemish, are not able to point
both eyes to the same object. But this, I think, is very seldom the
cause, for when the other eye is shut, the distorted eye can be moved
by the action of its muscles, in all possible directions, as freely
as that of any other person, which shows that it is not owing to a
defect in the muscles, neither is it owing to a want of
correspondence in the muscles of both eyes; for when both eyes are
open, and the undistorted eye is moved in any direction whatever, the
other always accompanies it, and is turned the same way at the same
instant of time.

I shall next take notice of the hypothesis of M. de la Hire, who
supposes, that in the generality of mankind, that part of the retina
which is seated in and about the axis of the eye, is of a more
delicate sense and perception than what the rest of the coat is
endowed with; and therefore we direct both axes to the same object,
chiefly in order to receive the picture on that part of the retina
which can best perceive it; but in persons who squint, he conceives
the most sensible part of the retina of one eye, not to be placed in
the axis, but at some distance from it: and that, therefore, this
more sensible part of the retina is turned towards the object, to
which the other eye is directed, and thus causes squinting. This
ingenious hypothesis has been followed by Dr. Boerhaave, and many
other eminent physicians. If it be true, then if the sound eye be
shut, and the distorted eye alone be used to look at an object, it
must still be as much distorted as before, for the same reason: but
the contrary is true in fact; for if you desire such a person to
close his other eye and look at you with that which is usually
distorted, he will immediately turn the axis of it directly towards
you. If you bid him open the undistorted eye, and look at you with
both eyes, you will find the axis of this last pointed at you and the
other turned away, and drawn close to the nose, or perhaps to the
upper eye lid. From these facts, and some others mentioned by Dr.
Jurin, I think we may conclude that this defect is seldom, if ever,
occasioned by such a preternatural make of the eye, as M. de la Hire
supposed.

From the most accurate observations it will appear, that by far the
most common cause of squinting, is a defect in the distorted eye. Dr.
Reid examined above twenty people that squinted, and found in all of
them a defect in the sight of one eye; M. Buffon likewise, from a
great number of observations, has found that the true and original
cause of this blemish, is an inequality in the goodness, or in the
limits of distinct vision, in the two eyes. Dr. Porterfield says this
is generally the case with people who squint; and I have found it so
in all that I have had an opportunity of examining.

With regard to the nature of this defect, the distorted eye is
sometimes more convex, and sometimes more flat, than the sound one;
sometimes it does not depend upon the convexity, but upon a weakness,
or some other affection, of the retina, of the nature of which we are
ignorant.

It will be easy to conceive how this inequality of goodness in the
two eyes, when in a certain degree, must necessarily occasion
squinting, and that this blemish is not a bad habit, but a necessary
one, which the person is obliged to learn, in order to see with
advantage. When the eyes are equally good, an object will appear more
distinct and clear when viewed with both eyes than with only one; but
the difference is very little. The ingenious Dr. Jurin, who has made
some beautiful experiments to ascertain this point, has shown, that
when the eyes are equal in goodness, we see more distinctly with both
than with one, by about one thirteenth part only. But M. Buffon has
found that when the eyes are unequal, the case will be quite
different. A small degree of inequality will make the object, when
seen with the better eye alone, appear equally bright or clear, as
when seen with both eyes; a little greater inequality will make the
object appear less distinct when seen with both eyes, than when it is
seen with the stronger eye alone; and a still greater inequality will
render the object, when seen with both eyes, so confused, that in
order to see it distinctly, one will be obliged to turn aside the
weak eye, and put it into a situation where it cannot disturb the
sight of the other. The truth of this may be easily proved by
experiment. Let a person take a convex lens, and hold it about half
an inch before one of his eyes; he will, by these means, render them
very unequal. and if he attempt to read with both eyes, he will
perceive a confusion in the letters, caused by this inequality; which
confusion will disappear as soon as he shuts the eye which is covered
with the lens, and looks only with the other.

Squinting is a necessary consequence of this inequality in the
goodness of the two eyes; for a person whose eyes are to a certain
degree unequal, finds that, when he looks at an object, he sees it
very indistinctly; every conformation, or change of direction of his
eyes which lessens the evil, will be agreeable; and he will acquire a
habit of turning his eye towards the nose, not for the sake of seeing
better with it, but in order to avoid, as much as possible, seeing at
all with the distorted eye; for which purpose it will be drawn either
under the upper eye lid, so that the pupil may be entirely or
partially covered; or directly towards the nose, in which case the
image of the object will fall at a distance from the axis of the eye:
and it is a fact well known to philosophers, that we never naturally
attend to an image which is at a distance from the axis; so that in
this situation it will give little disturbance to the sight of the
other.

It is easy to show that a squinting person very seldom, if ever, sees
an object with the distorted eye. Indeed in above forty cases that I
have examined, I found that when I placed an opaque body between the
undistorted eye and the object, it immediately disappeared, nor were
they able to see it at all, till they directed the axis of the
distorted eye to the object. I find the same observation made by Dr.
Reid and M. Buffon.

M. Buffon takes notice of a fact which I have often observed; viz.
that many persons have their eyes very unequal without squinting.

When the difference is very considerable, the weak eye does not turn
aside, because it can see almost nothing, and therefore cannot
disturb the vision of the good eye. Also, when the inequality is but
small, the weak eye will not turn aside, as it affords very little
disturbance to the sight of the other: when the inequality consists
in the difference of convexity, or difference of the limits of
distinct vision, having the limits of distinct vision in each eye
given, it may be calculated with some degree of accuracy what degree
of inequality is necessary to produce squinting. It seems then that
there are certain limits with regard to the inequality of the eyes,
necessary to produce this deformity; and that if the inequality be
either greater or less than these limits, the person will not squint.

Having now endeavoured to show what is the most common cause of
squinting, I shall briefly attempt to point out those cases in which
we may expect to effect a cure, and afterwards give a very short
account of the most likely methods of doing it.

We cannot have great hopes of success, when there is a very great
defect in the distorted eye. When the eyes are of different
convexities, there is no other way of removing the deformity, than by
bringing them to an equality by means of glasses, and then the person
would only look straight when he used spectacles. When this defect is
owing to a weakness in the distorted eye, it may sometimes be cured:
M. Buffon observes that a weak eye acquires strength by exercise, and
that many persons, whose squinting he had thought to be incurable, on
account of the inequality of their eyes, having covered their good
eye for a few minutes only, and consequently being obliged to
exercise their bad one for that short time, were themselves surprised
at the strength it had acquired, and on measuring their view
afterwards, he found it to be more extended, and judged the squinting
to be curable. In order therefore to judge with any certainty of the
possibility of a cure, it ought always to be tried whether the
distorted eye will grow better by exercise; if it does not, we can
have little hopes of success; but when the eyes do not differ much in
goodness, and it is found that the distorted eye acquires strength by
exercise, a cure may then be attempted: and the best way of doing it,
(according to M. Buffon) is to cover the good eye for some time, for,
in this condition, the distorted eye will be obliged to act, and turn
itself towards objects, which by degrees will become natural to it.

When the eyes are nearly brought to an equality by exercise, but
cannot both be directed to the same point, Dr. Jurin's method may be
practised, which is as follows.

If the person is of such an age, as to be capable of observing
directions, place him directly before you, and let him close the
undistorted eye, and look at you with the other; when you find the
axis of this fixed directly upon you, bid him endeavour to keep it in
that situation, and open the other eye; you will now see the
distorted eye turn away from you towards his nose, and the axis of
the other will be pointed towards you, but with patience and repeated
trials he will, by degrees, be able to keep the distorted eye fixed
upon you, at least for some time after the other is opened, and when
you have brought him to keep the axis of both eyes fixed upon you, as
you stand directly before him, it will be time to change his posture,
and set him, first a little to one side of you, and then to the
other, and so practise the same thing. And when, in all these
situations, he can perfectly and readily turn the axes of both eyes
towards you, the cure is effected. An adult person may practice all
this before a mirror, without a director, though not so easily as
with one: but the older he is, the more patience will be necessary.

With regard to the success of this method, M. Buffon says, that
having communicated his scheme to several persons, and, among others,
to M. Bernard de Jussieu, he had the satisfaction to find his opinion
confirmed by an experiment of that gentleman, which is related by Mr.
Allen, in his Synopsis Universae Medicinae. Dr. Jurin tells us that
he had attempted a cure in this manner with flattering hopes of
success, but was interrupted by the young gentleman's falling ill of
the small pox, of which he died. Dr. Reid likewise tried it with
success on three young gentlemen, and had brought them to look
straight when they were upon their guard. Upon the whole this seems
by much the most rational method of attempting to cure the deformity.

The only remaining morbid affections of the eye which I shall take
notice of in this lecture, are two, which produce the indistinct
vision of an object, by directly opposite means. The first is caused
by the cornea, and crystalline, or either of them, being too convex,
or the distance between the retina and crystalline being too great.
It is evident, that from any of these causes, or all combined, the
distinct picture of an object, at an ordinary distance, will fall
before the retina, and therefore the picture on the retina itself
must be confused, which will render the vision confused and
indistinct; whence, in order to see things distinctly, people whose
eyes are so formed are obliged to bring the object very near their
eyes; by which means the rays fall upon the eye in a more diverging
state, so that a distinct picture will be formed on the retina, by
which the object will be distinctly seen: from the circumstance of
such persons being obliged to hold objects near their eyes, in order
to see them distinctly, they are called short sighted.

If a short sighted person look at an object through a small hole made
in a card, he will be able to see even remote objects, with tolerable
distinctness, for this lessens the circles of dissipation on the
retina, and thus lessens the confusion in the picture. For the same
purpose, we commonly observe short sighted people, when they wish to
see distant objects more distinctly, almost shut their eye lids: and
it is from this, says Dr. Porterfield, that short sighted persons
were anciently called myopes.

The sight of myopes is remedied by a concave lens of proper
concavity, which, by increasing the divergency of the rays, causes
them to be united into a focus on the retina: and they do not require
different glasses for different distances, for, if they have a lens
which will make them see distinctly at the distance most commonly
used by other persons, for example, at the distance at which persons
whose eyes are good generally read, they will, by the help of the
same glass, be able to see distinctly at all the distances at which
good sighted people can see distinctly: for the cause of
shortsightedness, is not a want of power to vary the conformation of
the eye, but is owing to the whole quantity of refraction being too
great for the distance of the retina from the cornea.

The other defect to be mentioned, is of an opposite nature, and
persons labouring under it are called long sighted, or presbytae: it
is caused by the cornea and crystalline, or either of them, being too
flat in proportion to the distance between the crystalline and
retina: whence it follows, that the rays which come from an object at
an ordinary distance, will not be sufficiently refracted, and,
consequently, will not meet at the retina, but beyond it, which will
render the picture on the retina confused, and vision indistinct.
Whence, in order to read, such persons are obliged to remove the book
to a great distance, which lessens the divergency of the rays falling
on the eye, and makes them converge to a focus sooner, so as to paint
a distinct image on the retina.

The presbytical eye is remedied by a convex lens of proper convexity,
which makes the rays converge to a focus sooner, and thus causes
distinct vision: the sight of such persons is even more benefited by
a convex lens, than that of myopes by a concave one; for a convex
lens not only makes the picture of the object on the retina distinct,
but also more bright, by causing a greater quantity of light to enter
the pupil; while a concave one, at the same time that it renders
vision distinct, diminishes the quantity of light.

Long sighted persons commonly become more so as they advance in
years, owing to a waste of the humours of the eye; and even many
people whose sight was very good in their youth, cannot see without
spectacles when they grow old. The same waste in the humours of the
eye, is the reason why shortsighted persons commonly become less so
as they advance in years; so that many who were shortsighted in their
youth, come to see very distinctly when they grow old. Dr. Smith
seems to doubt this, and thinks that it is rather a hypothesis than a
matter of fact. I have however myself seen several instances in
confirmation of it; and it is very natural to suppose, that since
short and long sight depend upon directly opposite causes, and since
the latter is increased by age, the former must be diminished by it.

LECTURE IX.
THE LAWS OF ANIMAL LIFE.

In the preceding lectures I have taken a view, first of the general
structure and functions of the living body, and next of the different
organs called senses, by means of which we become acquainted with
external objects. I shall next endeavor to show that, through the
medium of these different senses, external objects affect us in a
still different manner, and by their different action, keep us alive:
for the human body is not an automaton; its life, and its different
actions, depend continually on impressions made upon it by external
objects. When the action of these ceases, either from their being
withdrawn, or from the organization necessary to perceive them, being
deranged or injured, the body becomes a piece of dead matter; becomes
obedient to the common laws of chemical attraction, and is decomposed
into its pristine elements, which, uniting with caloric, form gases;
which gases, being carried about in the atmosphere, or dissolved in
water, are absorbed by plants, and contribute to their nourishment.
These are devoured by animals, which in their turn die, and are
decompounded; thus, in the living world, as well as in the inanimate,
every thing is subject to change, and to be renewed perpetually.

   "Look nature through, 'tis revolution all,
   All change, no death; day follows night; and night,
   The dying day; stars rise, and set, and rise;
   Earth takes th' example; see the summer gay,
   With her green chaplet, and ambrosial flowers,
   Droops into pallid autumn; winter gray,
   Horrid with frost, and turbulent with storm,
   Blows autumn and his golden fruits away,
   Then melts into the spring; soft spring with breath
   Favonian, from warm chambers of the south
   Recals the first. All to reflourish, fades;
   As in a wheel, all sinks to reascend."

The subject on which we are entering is of the utmost importance;
for, by pointing out the manner in which life is supported and
modified by the action of external powers, it discovers to us the
true and only means of promoting health and longevity; for the action
of these powers is generally within our own direction; and if the
action of heat, food, air, and exercise, were properly regulated, we
should have little to fear from the attacks of diseases.

When we examine the human body, the most curious and unaccountable
circumstance that we observe, is its life, or its power of motion,
sensation, and thought: for though the structure of the different
parts which we have examined must excite our admiration and wonder,
each part being admirably fitted for the performance of its different
functions, yet without the breath of life, all these beautiful
contrivances would have been useless. We have seen that the structure
of the eye indicates in its contriver, the most consummate skill in
optics; and of the ear the most perfect knowledge of sounds; yet if
sensibility had not being given to the nerves which administer to
these organs, the pulses of the air might have been communicated to
the fluid in the labyrinth, and the rays of light might have formed
images in the retina, without our being, in the smallest degree,
conscious of their existence.

Though our efforts to discover the nature of life have hitherto been,
and perhaps always will be, unsuccessful, yet we can, by a careful
induction, or observation of facts, discover the laws by which it is
governed, with respect to the action of external objects. This is
what I shall now attempt to do.

The first observation which strikes us, is that of the very different
effects that are produced when inanimate bodies act on each other,
and when they exert their action on living matter.

When dead matter acts upon dead or inanimate matter, the only effects
we perceive are mechanical, or chemical; that is, either motion, or
the decomposition and new combination of their parts. If one ball
strikes another, it communicates to it a certain quantity of motion,
this is called mechanical action; and if a quantity of salt, or
sugar, be put into water, the particles of salt, or sugar, will
separate from each other, and join themselves to the particles of the
water; these substances in these instances are said to act chemically
on each other, and in all cases whatever, in which inanimate or dead
bodies act on each other, the effects produced are motion, or
chemical attraction; for though there may appear to be other species
of action which sometimes take place, such as electric and magnetic
attraction and repulsion, yet these are usually referred to the head
of mechanical action or attraction.

But when dead matter acts upon those bodies we call living, the
effects produced are much different. There are many animals which
pass the winter in a torpid state which has all the appearance of
death; and they would continue in that state, if deprived of the
influence of heat; now heat if applied to dead matter, will only
produce motion, or chemical combination: in fluids it produces
motions by occasioning a change in their specific gravity; and we
know that it is one of the most powerful agents in chemical
combination and decomposition; but these are the only effects it
produces when it acts upon dead matter. But let us examine its
effects when applied to living organized bodies. Bring a snake or
other torpid animal into a moderately warm room, and observe what
will be the consequence. After a short time the animal begins to
move, to open its eyes and mouth; and when it has been subject to the
action of heat for a longer time, it crawls about in search of food,
and performs all the functions of life.

Here then, dead matter, when applied to the living body, produces the
living functions, sense and motion: for if the heat had not been
applied, the animal would have continued senseless, and apparently
lifeless.

In more perfect animals, the effects produced by the action of dead
matter upon them, are more numerous, and are different in different
living systems; but are in general the following; sense and motion in
almost all animals, and in many the power of thinking, and other
affections of the mind.

The powers, or dead matters, which by their action produce these
functions, are chiefly heat, food, and air. The proof that these
powers do produce the living functions is in my opinion very
satisfactory, for when their action is suspended, the living
functions cease. If we take away, for instance, heat, air, and food,
from animals, they soon become dead matter. This is as strong a proof
that these matters are the cause of the functions, as that heat is
the cause of the expansion of bodies, when we find that by
withdrawing it the expansion ceases. Indeed it is not necessary that
an animal should be deprived of all these powers to put a stop to the
living functions; if any one of them is taken away, the body sooner
or later becomes dead matter: it is found by experience, that if a
man is deprived of air, he dies in about three or four minutes; for
instance, if he is immersed under water: if he is deprived of heat,
or in other words is exposed to a very severe degree of cold, he
likewise soon dies; or if he is deprived of food, his death is
equally certain, though more slow; it is sufficiently evident then
that the living functions are owing to the action of these external
powers upon the body.

What I have here said is not confined to animals, but the living
functions of vegetables are likewise caused by the action of dead
matter upon them. The powers, which by their actions produce the
living functions of vegetables, are principally heat, moisture,
light, and air.

From what has been said, it clearly follows, that living bodies must
have some property different from dead matter, which renders them
capable of being acted on by these external powers, so as to produce
the living functions; for if they had not, it is evident that the
only effects which these powers could produce, would be mechanical,
or chemical.

Though we know not exactly in what this property consists, or in what
manner it is acted on, yet we see that when bodies are possessed of
it, they become capable of being acted on by external powers, so as
to produce the living functions.

We may call this property, with Haller, irritability, or, with Brown,
excitability; or we may use vital principle, or any other term, could
we find one more appropriate. I shall use the term excitability, as
perhaps the least liable to exception, and in using this term, it is
necessary to mention that I mean only to express a fact, without the
smallest intention of pointing out the nature of that property which
distinguishes living from dead matter; and in this we have the
illustrious example of Newton, who called that property which causes
bodies in certain situations to approach each other, gravitation,
without in the least hinting at its nature. Yet though he knew not
what gravitation was, he investigated the laws by which bodies were
acted on by it, and thus solved a number of phenomena which were
before inexplicable: in the same manner, though we are ignorant of
the nature of excitability, or of the property which distinguishes
living from dead matter, we can investigate the laws by which dead
matter acts upon living bodies through this medium. We know not what
magnetic attraction is, yet we can investigate its laws: the same may
be observed with respect to electricity. If ever we should obtain a
knowledge of the nature of this property, it would make no alteration
in the laws which we had before discovered.

Before we proceed to the investigation of the laws by which the
living principle or excitability is acted on, it will be first
necessary to define some terms, which I shall have occasion to use,
to avoid circumlocution: and here it may not be improper to observe,
that most of our errors in reasoning have arisen from want of strict
attention to this circumstance, the accurate definition of those
terms which we use in our reasoning. We may use what terms we please,
provided we accurately define them, and adhere strictly to the
definition. On this depends the excellence and certainty of the
mathematical sciences. The terms are few, and accurately defined; and
in their different chains of reasoning mathematicians adhere with the
most scrupulous strictness to the original definition of the terms.
If the same method were made use of in reasoning on other subjects,
they would approach to the mathematics in simplicity and in truth,
and the science of medicine in particular would be stripped of the
heaps of learned rubbish which now encumber it, and would appear in
true and native simplicity. Such is the method I propose to follow: I
am certain of the rectitude of the plan; of the success of the
reasoning it does not become me to judge.

When the excitability is in such a state as to be very susceptible of
the action of external powers, I shall call it abundant or
accumulated; but when it is found in a state not very capable of
receiving their action, I say it is deficient or exhausted. Let no
one however suppose that by these terms I mean to hint in the least
at the nature of the excitability. I do not mean by them that it is
really at one time increased in quantity or magnitude, and at another
time diminished: its abstract nature is by no means attempted to be
investigated. These or similar terms the poverty or imperfection of
language obliges us to use. We know nothing of the nature of the
excitability or vital principle, and by the terms here used I mean
only to say, that the excitability is sometimes easily acted on by
the external powers, and then I call it abundant or accumulated; at
other times the living body is with more difficulty excited, and then
I say the vital principle or excitability, whatever it may be, is
deficient or exhausted.

On examination we shall find the laws by which external powers act on
living bodies to be the following.

First, when the powerful action of the exciting powers ceases for
some time, the excitability accumulates, or becomes more capable of
receiving their action, and is more powerfully affected by them.

If we examine separately the different exciting powers which act on
the body, we shall find abundant confirmation of this law. Besides
the exciting powers which act on the body, which I mentioned; viz.
heat, food, and air, there are several others, such as light, sound,
odorous substances, &c. which will be examined in their proper
places. These powers, acting by a certain impulse, and producing a
vigorous action of the body, are called stimulants, and life we shall
find to be the effect of these and other stimulants acting on the
excitability.

The stimulus of light, though its influence in this respect is
feeble, when compared with some other external powers, yet has its
proportion of force. This stimulus acts upon the body through the
medium of the organ of vision. Its influence on the animal spirits
strongly demonstrates its connexion with animal life, and hence we
find a cheerful and depressed state of mind in many people, and more
especially in invalids, to be intimately connected with the presence
or absence of the sun. Indeed to be convinced of the effects of light
we have only to examine its influence on vegetables. Some of them
lose their colour when deprived of it, many of them discover a
partiality to it in the direction of their flowers; and all of them
perspire oxygen gas only when exposed to it; nay it would seem that
organization, sensation, spontaneous motion, and life, exist only at
the surface of the earth, and in places exposed to light. Without
light nature is lifeless, inanimate, and torpid.

Let us now examine if the action of light upon the body is subject to
the law that has been mentioned. If a person be kept in darkness for
some time, and then be brought into a room in which there is only an
ordinary degree of light, it will be almost too oppressive for him,
and will appear excessively bright; and if he have been kept for a
considerable time in a very dark place, the sensation will be very
painful. In this case, while the retina or optic nerve was deprived
of light, its excitability accumulated, or became more easily
affected by light: for if a person go out of one room into another,
which has an equal degree of light, he will perceive no effect.

You may convince yourselves of the truth of this law, by a very
simple experiment; shut your eyes, and cover them for a minute or two
with your hand, and endeavour not to think of the light, or what you
are doing; then open them, and the daylight will for a short time
appear brighter.

If you look attentively at a window for about two minutes, then cast
your eyes upon a sheet of white paper, the shape of the window frames
will be perfectly visible upon the paper; those parts which express
the wood work appearing brighter than the other parts. The parts of
the optic nerve on which the image of the frame falls, are covered by
the wood work from the action of the light; the excitability of these
parts will therefore accumulate; and the parts of the paper which
fall upon them must of course appear brighter.

If a person be brought out of a dark room where he has been confined,
into a field covered with snow, when the sun shines, it has been
known to affect him so much as to deprive him of sight altogether.

This law is well exemplified when we come into a dark room in the day
time. At first we can see nothing; but with the absence of light the
excitability accumulates, and we begin to have an imperfect glimpse
of the objects around us; after a while the excitability of the
retina is so far accumulated, and we become so sensible of the feeble
light reflected from the surfaces of bodies, that we can discern
their shapes, and sometimes even their colours.

Let us next consider what happens with respect to heat, which is a
uniform and active stimulus in promoting life. The extensive
influence of heat upon animal life is evident from its decay and
suspension during winter, in certain animals, and from its revival
upon the approach and action of the vernal sun.

If this stimulus is for some time abstracted from the whole body, or
from any part, the excitability accumulates, or, in other words, if
the body has been for some time exposed to cold, it is more liable to
be affected by heat afterwards applied. Of this also you may be
convinced by an easy experiment. Put one of your hands into cold
water, and then put both into water which is considerably warm: the
hand which has been in the cold water will feel much warmer than the
other. If you handle some snow in one hand while you keep the other
in the bosom, that it may be of the same heat with the body, and then
bring both within the same distance of the fire, the heat will affect
the cold hand infinitely more than the warm one. This is a
circumstance of the utmost importance, and ought always to be
carefully attended to. When a person has been exposed to a severe
degree of cold for some time, he ought to be cautious how he comes
near a fire, for his excitability will be so much accumulated that
the heat will act very violently, often producing a great degree of
inflammation, and even sometimes of mortification. This is a very
common cause of chilblains, and other similar inflammations. When the
hands, or any other parts of the body, have been exposed to a violent
cold, they ought first to be put in cold water, or even rubbed with
snow, and exposed to warmth in the gentlest manner possible.

The same law regulates the action of food, or matters taken into the
stomach: if a person have for some time been deprived of food, or
have taken it in small quantity, whether it be meat or drink, or if
he have taken it of a less stimulating quality, he will find that
when he returns to his ordinary mode of life it will have more effect
upon him than before he lived abstemiously.

Persons who have been shut up in a coal work, from the falling in of
the pit, and have had nothing to eat for two or three days, have been
as much intoxicated by a bason of broth, as a person in common
circumstances with two or three bottles of wine.

This circumstance was particularly evident among the poor sailors who
were in the boat with Captain Bligh after the mutiny. The Captain was
sent by government to convey some plants of the bread fruit tree from
Otaheite to the West Indies: soon after he left Otaheite the crew
mutinied, and put the captain and most of the officers, with some of
the men, on board the ship's boat, with a very short allowance of
provisions, and particularly of liquors, for they had only six quarts
of rum, and six bottles of wine, for nineteen people, who were driven
by storms about the south sea, exposed to wet and cold all the time,
for nearly a month; each man was allowed only a teaspoonful of rum a
day, but this teaspoonful refreshed the poor men, benumbed as they
were with cold, and faint with hunger, more than twenty times the
quantity would have done those who were warm and well fed; and had it
not been for the spirit having such power to act upon men in their
condition, they never could have outlived the hardships they
experienced. All these facts, and many others which might be brought
forward, establish, beyond dispute, the truth of the law I mentioned;
viz. that when the powerful action of the exciting powers ceases for
some time, the excitability accumulates, or becomes more capable of
receiving their actions, and is more powerfully affected by them.

When the legs or arms have for some time been exposed to cold, the
slightest exertion, or even the stimulus of a gentle heat, throws the
muscles into an inordinate action or cramp. The glow of the skin, in
coming out of a cold bath, may be explained on the same principle.
The heat of the skin is diminished by the conducting power of the
water, in consequence of which the excitability of the cutaneous
vessels accumulates; and the same degree of heat afterwards applied,
excites these now more irritable vessels to a great degree of action.

On this principle depends the supposed stimulant or tonic powers of
cold, the nature of whose action has been much mistaken by physicians
and physiologists. Heat is allowed to be a very powerful stimulus;
but cold is only a diminution of heat; how then can cold act as a
stimulus? In my opinion it never does; but its effects may be
explained by the general law which we have been investigating. When a
lesser stimulus than usual has been applied to the body, the
excitability accumulates, and is then affected by a stimulus even
less than that which, before this accumulation, produced no effect
whatever. The cold only renders the body more subject to the action
of heat afterwards applied, by allowing the excitability to be
accumulated. No person, I believe, ever brought on an inflammation,
or inflammatory complaint, by exposure to cold, however long might
have been that exposure, or however great the cold; but if a person
have been out in the cold air, and afterwards come into a warm room,
an inflammatory complaint will most probably be the consequence.

Indeed coming out of the cold air into a moderately warm room
generally produces a lively and continued warmth in the parts that
have been exposed.

The second general law is, that when the exciting powers have acted
with violence for a considerable time, the excitability becomes
exhausted, or less fit to be acted on; and this we shall be able to
prove by a similar induction.

Let us first examine the effects of light upon the eye: when it has
acted violently for some time on the optic nerve, it diminishes the
excitability of that nerve, and renders it incapable of being
affected by a quantity of light, that would at other times affect it.
When we have been walking out in the snow, if we come into a room, we
shall scarcely be able to see any thing for some minutes.

If you look stedfastly at a candle for a minute or two, you will with
difficulty discern the letters of a book which you were before
reading distinctly. When our eyes have been exposed to the dazzling
blaze of phosphorus in oxygen gas, we can scarcely see any thing for
some time afterwards, and if we look at the sun, the excitability of
the optic nerve is so overpowered by the strong stimulus of his
light, that nothing can be seen distinctly for a considerable time.
If we look at the setting sun, or any other luminous object of a
small size, so as not greatly to fatigue the eye, this part of the
retina becomes less sensible to smaller quantities of light; hence
when the eyes are turned on other less luminous parts of the sky, a
dark spot is seen resembling the shape of the sun, or other luminous
object on which our eyes have been fixed.

On this account it is that we are some time before we can distinguish
objects in an obscure room, after coming from broad daylight, as I
observed before.

We shall next consider the action of heat. Suppose water to be heated
to 90 degrees, if one hand be put into it, it will appear warm; but
if the other hand be immersed in water heated to 120 degrees, and
then put into the water heated to 90 degrees, that water will appear
cold, though it will still feel warm to the other hand: for the
excitability of the hand has been exhausted, by the greater stimulus
of heat, to such a degree as to be insensible of a less stimulus.

Before we go into a warm bath, the temperature of the air may seem
very warm and pleasant to the body, even though exposed naked to it;
but after we have remained for some time in the warm bath, we feel
the air, when we come out, very cool and chilling, though it is of
the same temperature as before; for the hot water exhausts the
excitability of the vessels of the skin, and renders them less
capable of being affected by a smaller degree of heat. Thus we see
that the effects of the hot and cold bath are different and opposite;
the one debilitates by stimulating, and the other produces stimulant
or tonic effects by debilitating. This seeming paradox may, however,
be easily explained by the principles we have laid down; and though
the hot and cold bath produce such different effects, yet it is only
the same fluid, with a small variation in the degree of temperature;
but these effects depend on the temperature of our body being such,
that a small decrease of it will produce an accumulation of
excitability, while a small increase will exhaust it.

I shall next proceed to examine the effects of the substances taken
into the stomach; and as the effects of spirituous and vinous liquors
are a little more remarkable than those of food, I shall first begin
with them.

A person who is not accustomed to take these liquors, will be
intoxicated by a quantity that will produce no effect upon one who
has been some time accustomed to take them; and when a person has
used himself to these stimulants for some time, the ordinary powers
which in common support life, will not have their proper effects upon
him, because his excitability has been, in some measure, exhausted by
these stimulants.

The same holds good with respect to tobacco and opium; a person
accustomed to take opium, or smoke tobacco, will not be affected by a
quantity that would completely intoxicate one not used to them,
because the excitability has been so far exhausted by the use of
those stimulants, that it cannot be acted on by a smaller quantity.

That tobacco or opium act in the same manner as wine or spirits,
scarce needs any illustration. In Turkey they intoxicate themselves
with opium, in the same way that people in this country do with wine
and spirits; and those who have been accustomed to take this drug for
a considerable time, feel languid and depressed when they are
deprived of it for some time; they repair to the opium houses, as our
dram drinkers do to the gin shops in the morning, sullen, dejected,
and silent; in an hour or two, however, they are all hilarity. This
shows the effects of opium to be stimulant. Tobacco intoxicates those
who are not accustomed to it, and in those who are, it produces a
serene and composed state of mind by its stimulating effects. Like
opium and fermented liquors it exhausts the excitability, and leaves
the person dejected, and all his senses blunted, when its stimulant
effects are over.

That what is more properly called food acts in the same way as the
substances I have just examined, is evident from the fact which I
mentioned some time ago, that persons whose excitability has been
accumulated, by their being deprived of food for some days, have been
intoxicated by a bason of broth.

These facts, with innumerable others which will easily suggest
themselves, prove, beyond doubt, the truth of the second law, namely,
that when the exciting powers have acted violently, or for a
considerable time, the excitability is exhausted, or less fit to be
acted on.

Besides the stimulants which I have mentioned, there are several
others which act upon the body, many of which will hereafter be
considered: but all act according to this law; when their action has
been suspended or diminished, the excitability of the organ on which
they act becomes accumulated, or more easily affected by their
subsequent action; and, on the contrary, when their action has been
violent, or long continued, the excitability becomes exhausted, or
less fit to receive their actions.

Among the stimulants acting on the body, we may mention sound, which
has an extensive influence on human life. I need not mention here its
numerous natural, or artificial sources, as that has been fully done
in a preceding lecture. The effect of music, in stimulating and
producing a state of mind approaching to intoxication, is universally
known. Indeed the influence of certain sounds in stimulating, and
thereby increasing, the powers of life, cannot be denied. Fear
produces debility, which has a tendency to death. Sound obviates this
debility, and restores to the system its natural degree of
excitement. The schoolboy and the clown invigorate their trembling
limbs, by whistling, or singing, as they pass by a country
churchyard, and the soldier feels his departing courage recalled in
the onset of a battle, by the "spirit stirring drum."

Intoxication is generally attended with a higher degree of life or
excitement than is natural. Now sound will produce this effect with a
very moderate portion of fermented liquor; hence we find persons much
more easily intoxicated and highly excited at public entertainments,
where there is music and loud talking, than in private companies,
where no auxiliary stimulus is added to that of wine.

Persons who are destitute of hearing and seeing, possess life in a
more languid state than other people; which is, in a great degree,
owing to the want of the stimulus of light and noise.

Odours have likewise a very sensible effect in promoting animal life.
The effects of these will appear obvious in the sudden revival of
life, which they produce, in cases of fainting. The smell of a few
drops of hartshorn, or even a burnt feather, has frequently, in a few
minutes, restored the system from a state of weakness, bordering upon
death, to an equable and regular degree of excitement.

All these different stimuli undoubtedly produce the greatest effects
upon their proper organs; thus the effect of light is most powerful
on the eye; that of sound on the ear; that of food on the stomach,
&c. But their effects are not confined to these organs, but extended
over the whole body. The excitability exists, one and indivisible,
over the whole system; we may call it sensibility, or feeling, to
enable us to understand the subject. Every organ, or indeed the whole
body, is endowed with this property in a greater or less degree, so
that the effects produced by any stimulus, though they are more
powerful on the part where they are applied, affect the whole system:
odours afford an instance of this; and the prick of a pin in the
finger, produces excitement, or a stimulant effect, over the whole
body.

From what has been said, it must be evident that life is the effect
of a number of external powers, constantly acting on the body,
through the medium of that property which we call excitability; that
it cannot exist independent of the action of these stimuli; when they
are withdrawn, though the excitability does not instantly vanish,
there is no life, no motion, but the semblance of death. Life,
therefore, is constantly supported by, and depends constantly on, the
action of external powers on the excitability; without excitability
these stimulants would produce no effect, and whatever may be the
nature of the excitability, or however abundant it may be, still,
without the action of external powers, no life is produced.

From what has been said, we may see the reason why life is in a
languid state in the morning: It acquires vigour by the gradual and
successive application of stimuli in the forenoon: It is in its most
perfect state about midday, and remains stationary for some hours:
From the diminution or exhaustion of the excitability, it lessens in
the evening, and becomes more languid at bed time; when, from defect
of excitability, the usual exciting powers will no longer produce
their effect, a torpid state ensues, which we call sleep, during
which, the exciting powers cannot act upon us; and this diminution of
their action allows the excitability to accumulate; and, to use the
words of Dr. Armstrong,

   "Ere morn the tonic, irritable nerves
   Feel the fresh impulse, and awake the soul."

LECTURE X.
THE LAWS OF ANIMAL LIFE, CONTINUED.

In the last lecture I began to investigate the laws by which living
bodies are governed, and the effects produced by the different
exciting powers, which support life, upon the excitability, or vital
principle. The facts which we examined led us to two conclusions,
which, when properly applied, we shall find will explain most of the
phenomena of life, both in health, and in disease. The conclusions
alluded to, are these: when the exciting powers have acted more
feebly, or weakly, than usual, for some time; or when their action is
withdrawn, the excitability accumulates, and becomes more powerfully
affected by their subsequent action. And, on the contrary, when the
action of these powers has been exerted with violence, or for a
considerable time, the excitability becomes exhausted, and less fit
to receive their actions.

A number of facts were mentioned in proof of these conclusions, and a
great number more might have been brought forwards, could it have
served any other purpose than to have taken up our time, which I hope
may be better employed.

This exhaustion of the excitability, by stimulants, may either be
final, or temporary. We see animals, while the exciting powers
continue to act, at first appear in their greatest vigour, then
gradually decay, and at last come into that state, in which, from the
long continued action of the exciting powers, the excitability is
entirely exhausted, and death takes place.

We likewise see vegetables in the spring, while the exciting powers
have acted on them moderately, and for a short time, arrayed in their
verdant robes, and adorned with flowers of many mingling hues; but as
the exciting powers, which support their life, continue to be
applied, and some of them, for instance heat, as the summer advances,
become increased, they first lose their verdure, then grow brown, and
at the end of summer cease to live: because their excitability is
exhausted by the long continued action of the exciting powers: and
this does not happen merely in consequence of the heat of the summer
decreasing, for they grow brown, and die, even in a greater degree of
heat than that which in spring made them grow luxuriantly. In some of
the finest days of autumn, in which the sun acts with more power than
in the spring, the vegetable tribe droop, in consequence of this
exhausted state of their excitability, which renders them nearly
insensible of the action, even of a powerful stimulus.

These are examples of the final or irreparable exhaustion of the
excitability; but we find also that it may be exhausted for a time,
and accumulated again. Though the eye has been so dazzled by the
splendour of light, that it cannot see an object moderately
illuminated, yet if it be shut for some time, the excitability of the
optic nerve will accumulate again, and we shall again be capable of
seeing with an ordinary light.

We find also that we are not always equally capable of performing the
functions of life. When we have been engaged in any exertion, either
mental or corporeal, for some hours only, we find ourselves languid
and fatigued, and unfit to pursue our labours much longer.

If in this state several of the exciting powers are withdrawn,
particularly light and noise, and if we are laid in a posture which
does not require much muscular exertion, we soon fall into that state
which nature intended for the accumulation of the excitability, and
which we call sleep. In this state many of the exciting powers cannot
act upon us, unless applied with some violence, for we are insensible
to their moderate action. A moderate degree of light, or a moderate
noise, does not affect us, and the power of thinking, which very much
exhausts the excitability, is in a great measure suspended. When the
action of these powers has been suspended for six or eight hours, the
excitability is again capable of being acted on, and we rise fresh
and vigorous, and fit to engage in our occupations.

Sleep then is the method which nature has provided to repair the
exhausted constitution, and restore the vital energy. Without its
refreshing aid, our worn out habits would scarcely be able to drag on
a few days, or at most, a few weeks, before the vital spring would be
quite run down: how properly therefore has our great poet called
sleep "the chief nourisher in life's feast!"

From the internal sensations, often excited, it is natural to
conclude, that the nerves of sense are not torpid during sleep, but
that they are only precluded from the perception of external objects,
by the external organs being in some way or other rendered unfit to
transmit to them the impulses of bodies during the suspension of the
power of volition; thus the eyelids are closed, in sleep, to prevent
the impulse of light from acting on the optic nerve; and it is very
probable that the drum of the ear is not stretched; it seems likewise
reasonable to conclude, that something similar happens to the
external apparatus of all our organs of sense, which may make them
unfit for their office of perception during sleep.

The more violently the exciting powers have acted, the sooner is
sleep brought on, because the excitability is sooner exhausted, and
therefore sooner requires the means of renewing it: and, on the
contrary, the more weakly these powers have acted, the less are we
inclined to sleep. Instances of the first are, excess of exercise,
strong liquors, or study; and of the latter, an under or deficient
proportion of these.

A person who has been daily accustomed to much exercise, whether
mental or corporeal, if he omit it, will find little or no
inclination to sleep; this state may however be induced by taking
some diffusible stimulus, as a little spirits and water, or opium,
which seem to act entirely by exhausting the excitability, to that
degree which is compatible with sleep, and, when the stimulant effect
of these substances are over, the person soon falls into that state.

But though the excitability may have been sufficiently exhausted, and
the action of external powers considerably moderated, yet there are
some things within ourselves, which often stimulate violently, and
prevent sleep, such as pain, thirst, and strong passions and emotions
of the mind. These all tend to drive away sleep, by their vehement
stimulating effect, which still has power to rouse the excitability
to action, though it has been considerably exhausted. The best method
of inducing sleep, in these cases, is to endeavour to withdraw the
mind from these impressions, particularly from uneasy emotions, by
employing it on something that makes a less impression, and which
does not require much exertion, or produce too much commotion; such
as counting to a thousand, or counting drops of water which fall
slowly; by listening to the humming of bees, or the murmuring of a
rivulet. Virgil describes a situation fitted to induce sleep, most
beautifully, in the following words.

   "Fortunate senex, hic inter flumina nota,
   Et fontes sacros, frigus captabis opacum.
   Hinc tibi, quae semper vicino ab limite sepes
   Hyblaeis apibus florem depasta salicti,
   Seape levi somnum suadebit inire susurro."

In infancy much sleep is required; the excitability, being then
extremely abundant, is soon exhausted by external stimulants, and
therefore soon requires renewing or accumulating; on this account,
during the first five or six months of their life, children require
this mode of renewing their exhausted excitability several times in
the day; as they advance in years, and as this excess of excitability
is exhausted by the application of stimulants, less sleep is
required: in the prime of life least of all is necessary. There is
great difference however, in this respect, in different
constitutions. Some persons are sufficiently refreshed by three or
four hours sleep, while others require eight or ten hours. More
however depends, in my opinion, on the mode of living. Those who
indulge in the use of spirituous or fermented liquors, which exhaust
the excitability to a great degree, require much more sleep than
those who are content with the crystal stream. The latter never feel
themselves stupid or heavy after dinner, but are immediately fit to
engage in study or business. As age advances, more sleep is again
required; and the excitability at last becomes so far exhausted, and
the system so torpid, that the greatest portion of gradually expiring
life is spent in sleep.

Temperance and exercise are the most conducive to sound healthy
sleep, hence the peasant is rewarded, for his toil and frugal mode of
life, with a blessing, which is seldom enjoyed by those whom wealth
renders indolent and luxurious. The poor in the country enjoy sound
and sweet sleep: forced by necessity to labour, their excitability
becomes exhausted in a proper and natural manner, and they retire to
rest early in the evening. Their sleep is generally sound, and early
in the morning they find themselves recruited, and in a state fit to
resume their daily labour. The blooming complexion, strength, and
activity, of these hardy children of labour, who recruit their
wearied limbs on pallets of straw, form a striking contrast with the
pallid and sickly visage, and debilitated constitution of the
luxurious and wealthy, who convert night into day, and court repose
in vain on beds of down. Nature undoubtedly intended that we should
be awake, and follow our occupations, whether of pleasure or
business, during the cheering light of day, and take repose when the
sun withdraws his rays. All other animals, and even vegetables, obey
the command of nature: man alone is refractory; but nature's laws are
never violated with impunity. Dr. Mackenzie very properly observes,
that those who sleep long in the morning, and sit up all the night,
injure the constitution without gaining time: and those who do this
merely in compliance with fashion, ought not to repine at a
fashionable state of bad health.

From what has been said, it is evident that, in order to enjoy sound
sleep, our chambers should be free from noise, dark, and moderately
cold; because the stimulant effects of noise, light, and heat,
prevent the accumulation of excitability: and as we shall afterwards
see that this accumulation depends on free respiration, and the
introduction of oxygen by that means into the system, our bed rooms
ought to be large and airy, and, in general, the beds should not be
surrounded by curtains. We may from this likewise see the reason why
it is so desirable to sleep in the country, even though we are
obliged to spend the day in town.

These observations on sleep have however led me a little from the
direct road; but I thought they could not be better introduced than
here. I shall now return to the subject of our more immediate
inquiry.

By induction we have discovered two of the principal laws by which
living bodies are governed: the first is, that when the ordinary
powers which support life have been suspended, or their action has
been lessened for a time, the excitability, or vital principle,
accumulates, or becomes more fit to receive their actions; and
secondly, when these powers have acted violently, or for a
considerable time, the excitability is exhausted, or becomes less fit
to receive their actions. There are therefore three states in which
living bodies exist. First, a state of accumulated excitability.
Secondly, a state of exhausted excitability. Thirdly, when the
excitability is in such a state as to produce the strongest and most
healthy actions, when acted upon by the external powers.

From what has been said, it must be evident that life depends
continually on the action of external powers on the excitability, and
that by their continued action, if they be properly regulated, the
excitability will be gradually, and insensibly exhausted, and life
will be resigned into the hands of him who gave it, without a
struggle, and without a groan.

We see then that nature operates in supporting the living part of the
creation, by laws as simple and beautiful as those by which the
animated world is governed. In the latter we see the order and
harmony which is observed by the planets, and their satellites, in
their revolution round the great source of heat and light;

   "---------- all combin'd
   And ruled unerring, by that Single Power,
   Which draws the stone projected, to the ground.

In the animated part of the creation, we observe those beautiful
phenomena which are exhibited by an almost infinite variety of
individuals; all depending upon, and produced by one simple law; the
acting of external powers upon their excitability.

I cannot express my admiration of the wisdom of the Creator better
than in the words of Thomson.

   "O unprofuse magnificence divine!
   O wisdom truly perfect! thus to call
   From a few causes such a scheme of life;
   Effects so various, beautiful, and great."

Life then, or those functions which we call living, are the effects
of certain exciting powers acting on the excitability, or property
distinguishing living from dead matter. When these effects, viz. the
functions, flow easily, pleasantly, and completely, from the action
of these powers, they indicate that state which we call health.

We may therefore, as we before hinted, distinguish three states of
the irritable fibre, or three different degrees of excitability, of
which the living body is susceptible.

1. The state of health which is peculiar to each individual, and
which has been called by Haller, and other physiologists, the tone of
the fibre. This is produced by a middle degree of stimulus acting
upon a middle degree of excitability: and the effect produced by this
action, we call excitement.

2. The state of accumulation, produced by the absence or diminished
action of the accustomed stimuli.

3. The state of exhaustion, produced by the too powerful action of
stimuli; and this may be produced either by the too powerful, or long
continued action of the common stimulants which support life, such as
food, air, heat, and exercise; or it may be caused by an application
of stimulants, which act more powerfully on the excitability, and
which exhaust it more quickly, such as wine, spirits, and opium,
musk, camphor, and various other articles used in medicines.

The state of health, or tone, if we use that term, consists therefore
in a certain quantity or energy of excitability necessary to its
preservation. To maintain this state, the action of the stimuli
should be strong enough to carry off from the body the surplus of
this irritable principle. To obtain this end, a certain equilibrium
is necessary between the excitability and the stimuli applied, or the
sum of all the stimuli acting upon it must be always nearly equal,
and sufficient to prevent an excess of excitability, but not so
strong as to carry off more than this excess. It is in this
equilibrium between the acting stimuli and the excitability, that the
health, or tone of the living body consists.

When the sum of the stimuli, acting on the body, is so small, as not
to carry off the excess of excitability, it accumulates, and diseases
of irritability are produced. Of this nature are those diseases to
which the poor are often subject, and which will be particularly
considered hereafter.

When the sum of the stimuli acting on the body, is too great, it is
deprived not only of the excess of excitability, but also of some
portion of the irritable principle necessary for the tone of the
body: or, to speak more distinctly, the body loses more excitability
than it receives, and of course must, in a short time, be in a state
of exhaustion. This gives rise to diseases which afflict drinkers, or
those who indulge in any kind of intemperance, or persons born in
climates where the temperature is moderate, but who emigrate to those
which are much warmer.

Thus we have endeavoured, after the example of Dr. Brown, to
ascertain the cause of the healthy state, before the causes of
diseases were investigated; and though this is contrary to the
general practice, yet it must be evident to every one, that unless we
are acquainted with the causes of good health, it will be impossible
for us to form any estimate of those variations from that state,
called diseases: hence it is that a number of diseases, which have
been brought on merely by the undue action of the exciting powers,
such as gout, rheumatism, and the numerous trains of nervous
complaints, which were by no means understood, may be easily and
satisfactorily explained, and as easily cured, by restoring the
proper action of these powers, and bringing the excitability to its
proper state. As this theory, therefore, is so important, not only in
respect to the preservation of health, which nearly concerns every
individual, but to the cure of diseases, which is the province of the
physician, I have endeavoured to explain it as fully and minutely as
possible; to make it still plainer we may perhaps make use of the
following illustration.

Suppose a fire to be made in a grate or furnace, filled with a kind
of fuel not very combustible, and which could only be kept burning by
means of a machine, containing several tubes placed before it, and
constantly pouring streams of air into it. Suppose also a pipe to be
fixed in the back of the chimney, through which a constant supply of
fresh fuel is gradually let down into the grate, to repair the waste
occasioned by the combustion kept up by the air machine.

The grate will represent the human body; the fuel in it the life or
excitability, and the tube behind, supplying fresh fuel, will denote
the power of all living systems, constantly to regenerate or produce
excitability; the air machine, consisting of several tubes, may
denote the various stimuli applied to the excitability of the body;
the flame produced in consequence of that application, represents
life; the product of the exciting powers acting upon the
excitability.

Here we see, that flame, like life, is drawn forth from fuel by the
constant application of streams of air, poured into it from the
different tubes of the machine. When the quantity of air poured in
through these different tubes is sufficient to consume the fuel as it
is supplied, a constant and regular flame will be produced: but if we
suppose that some of them are stopped, or that they do not supply a
sufficient quantity of air, then the fuel will accumulate, and the
flame will be languid and smothered, but liable to break out with
violence, when the usual quantity of air is supplied.

On the contrary, if we suppose a greater quantity of air to rush
through the tubes, then the fuel will be consumed or exhausted faster
than it is supplied; and in order therefore to reduce the combustion
to the proper degree, the quantity of air supplied must be
diminished, and the quantity of fuel increased.

If we suppose one of the tubes, instead of common air, to supply
oxygen gas, it will represent the action of wine, spirits, ether,
opium, and other powerful stimulants upon the body: a bright and
vivid flame will be produced, which however will only be of short
duration, for the fuel will be consumed faster than it is supplied,
and a state of exhaustion will take place.

We may carry this illustration still further, and suppose that the
air tubes exhaust the fuel every day faster than it can be supplied,
then it will be necessary at night to stop up some of the tubes, so
that the expense of fuel may be less than its supply, in order to
make up for the deficiency. When this is made up, the tubes may in
the morning be opened, and the combustion carried on during the day
as usual. This will illustrate the nature of sleep. In speaking of
this subject, it was observed, that the more violently the exciting
powers have acted, the sooner is sleep brought on; because the
excitability is sooner exhausted. In the same way the more the air
rushes through the tubes, the sooner will the fuel be consumed, and
want replenishing. When the exciting powers have acted feebly, a
person feels no inclination to sleep, because the excitability is not
exhausted to the proper degree, and therefore does not want
accumulating. But any diffusible stimulus, as spirits, or opium, will
soon exhaust it to the proper degree.

In the same way, if the air have not passed rapidly through the
tubes, the fuel will not be exhausted: but it may be brought to a
proper degree of exhaustion by the application of oxygen gas.

When the air which nourishes the flame is so regulated, that it
consumes the fuel as it is supplied, but no faster, a clear and
steady flame will be kept up, which will go on as long as the fuel
lasts, or the grate resists the action of the fire: but at last when
the fuel, which we do not suppose inexhaustible, is burnt out, the
fire must cease.

In the same manner, if the different exciting powers which support
life were properly regulated, all the functions of the body would be
properly performed, and we should pass our life in a state of health,
seldom known to any but savages, and brute animals not under the
dominion of man, who regulate these powers merely by the necessities
of nature.

When air is applied in too great quantity, and especially if some of
the tubes convey oxygen gas, then a violent combustion and flame is
excited, which will, in all probability, consume or burn out the
furnace or grate, or if it do not, it will burn out the fuel, and
thus exhaust itself.

In like manner, if the stimulants which support life be made to act
too powerfully, and particularly if any powerful stimulus, not
natural to the body, such as wine or spirits, be taken in too great
quantity, a violent inflammatory action will be the consequence,
which may destroy the human machine: but if it do not, it will
exhaust the excitability, and thus bring on great debility.

This analogy might be pursued further, but my intention was solely to
illustrate some of the outlines of our theory, by a comparison which
may facilitate the conception of the manner in which external powers
act on living bodies. The different powers which support life, and
without whose action we are unable to exist, such as heat, food, air,
&c. have been very improperly called nonnaturals, a term which is
much more applicable to those substances which we are daily in the
habit of receiving into the system, which excite it to undue actions,
and which nature never intended we should receive; such as spirituous
and fermented liquors, and high seasoned foods. In the preceding
illustration, I have spoken of a tube, as constantly pouring in fresh
fuel, because it was not easy otherwise to convey a familiar idea of
the power which all living systems possess of renewing their
excitability, when exhausted. The excitability is an unknown
somewhat, subject to peculiar laws, some of which we have examined,
but whose different states we are obliged to describe, though,
perhaps, inaccurately, by terms borrowed from the qualities of
material substances.

Though Dr. Brown very properly declined entering into the
consideration of the nature of excitability, or the manner in which
it is produced, the discoveries which have been made in chemistry
since his time, have thrown great light on the subject, and it is now
rendered highly probable that the excitability or vital principle, is
communicated to the body by the circulation, and is intimately
connected with the process of oxidation.

Many circumstances would tend to show, that a strict connexion exists
between the reception of oxygen into the body, and the vital
principle.

When an animal has been killed by depriving it of oxygen gas, the
heart and other muscles, and indeed the whole system, will be found
completely to have lost its excitability. This is not the case when
an animal is killed in a different manner. When an animal is shot, or
killed in the common manner, by bleeding to death, if the heart be
taken out, it will contract for some hours, on the application of
stimulants. But this is not the case with an animal that has been
drowned, or killed by immersion in carbonic acid, azotic, or
hydrogenous gases; in these last instances, the heart either does not
contract at all, or very feebly, on the application of the strongest
stimulants.

We have already seen that oxygen unites with the blood in the lungs,
during respiration: by the circulation of the blood it is distributed
to every part of the system, and we shall find, that in proportion to
its abundance is the excitability of the body. In proof of this, I
shall relate some facts and experiments.

Dr. Girtanner injected a quantity of very pure oxygen gas into the
jugular vein of a dog: the animal raised terrible outcries, breathed
very quickly, and with great difficulty: by little and little his
limbs became hard and stiff, he fell asleep, and died in the course
of a few minutes afterwards. It ought here to be observed, that any
of the gases, or almost any fluid, however mild, when thus suddenly
introduced into the circulating system, generally, and speedily,
occasions death.

On opening the chest, the heart was found more irritable than
ordinary, and its external contractions and dilatations continued for
more than an hour: the right auricle of the heart, which usually
contains black venous blood, contained, as well as the right
ventricle, a quantity of blood of a bright vermilion colour; and all
the muscles of the body were found to be more than usually irritable.
This experiment not only proves that the vermilion colour of the
blood proceeds from oxygen, but likewise seems to show, that oxygen
is the cause of excitability.

A quantity of azotic gas, which had been exposed for some time to the
contact of lime water, in order to separate any carbonic acid gas it
might contain, was injected into the jugular vein of a dog. The
animal died in twenty seconds. Upon opening the chest, the heart was
found filled with black and coagulated blood: this organ, and most of
the muscles had nearly lost the whole of their irritability, for they
contracted but very weakly, on the application of the strongest
stimulants.

A quantity of carbonic acid gas was injected into the jugular vein of
a dog: the animal became sleepy, and died in about a quarter of an
hour: the heart was found filled with black and coagulated blood, and
had lost the whole of its irritability; neither it, nor any of the
muscles producing any contractions, upon the application of
stimulants.

Humboldt likewise mentions a curious fact, which tends strongly to
confirm this idea. When the excitability of the limb of a frog had
been so far exhausted, by the application of zinc and silver, that it
would produce no more contractions, on moistening it with oxygenated
muriatic acid, the contractions were renewed.

After the excitability of the sensitive plant (mimosa pudica) had
been so far exhausted, by irritation, that it ceased to contract,
when further irritated, I restored this excitability, and brought it
to a very high degree of irritability, by moistening the earth in
which it grew with oxygenated muriatic acid. Seeds likewise vegetate
more quickly when moistened with this acid, than when they are not.

In short, we shall find, first, that every thing which increases the
quantity of oxygen in organized bodies, increases at the same time
their excitability.

Secondly, That whatever diminishes the quantity of oxygen, diminishes
the excitability.

The excitability of animals, made to breathe oxygen gas, or to take
the oxygenated muriate of potash, or acid fruits, is very much
increased.

On the contrary, when persons have inspired carbonic acid, or azotic
gas, or have taken into the system substances which have a strong
affinity for oxygen, and therefore tend to abstract it, such as
hydrogen, and spirits, the excitability becomes very much diminished.

When we sleep, in consequence of the excitability being exhausted,
the breathing becomes free, and a great quantity of oxygen is
received by the lungs, and combined with the blood, while very little
of it becomes exhausted by the actions of the body, for none,
excepting those which are called involuntary motions, are carried on
during sound sleep: so that in a few hours the body recovers the
excitability which it had lost: it is again sensible of the
impressions of external objects, and with the return of light we
wake.

These facts afford satisfactory proofs that the excitability of the
body is proportioned to the oxygen which it receives: but in what
manner it produces this state of susceptibility, and how it is
exhausted by stimulants, we have yet to learn.

The following theory may perhaps throw some light upon the subject. I
propose it, however, merely as an hypothesis, for we have no direct
proofs of it, but it seems to account for many phenomena.

It is now well known, that while the limb of an animal possesses
excitability, the smallest quantity of electricity sent along the
principal nerve leading to it, produces contractions similar to those
produced by the will. This is instanced in the common galvanic
experiment with the limb of a frog, which I had formerly occasion to
show.

From the effects produced, when a stream of electricity is sent
through water, I think it not improbable that hydrogen and
electricity may be identical. When a piece of zinc and silver are
connected together, and the zinc is put in a situation to decompose
water, and oxidate, a current of hydrogen gas will separate from the
silver wire, provided this be immersed under water; but when it is
not, a current of electricity passes, which is sensible to the
electrometer.

Now there appears no greater improbability in the supposition that
hydrogen, in a certain state, may be capable of passing through
metals, and animal substances, in the form of electricity, and that
when it comes in contact with water, which is not so good a
conductor, it may combine with caloric, and form hydrogen gas, in
which state it becomes incapable of passing through the conductors of
electricity: I say there appears no greater improbability in this,
than that caloric should sometimes be in such a state, that it will
pass through metals, and animal substances, which conduct it, and at
other times, as when combined with oxygen or hydrogen, it should form
gases, and be then incapable of passing through these conductors of
heat. Galvanic effects may be produced by the oxidation of fresh
muscular fibre without the aid of metals, and contractions have been
thus produced in the limb of an animal; and we have already noticed,
that when this contraction ceases, it may be restored, by moistening
the limb with oxygenated muriatic acid.

The excitability of the body may, most probably, be conveyed by
respiration, and the circulation of the blood, which tend continually
to oxidate the different parts: and hydrogen or electricity may be
secreted by the brain, and sent along the nerves, which are such good
conductors of it, and by uniting with the oxygen of the muscle, may
cause it to contract; but as the oxygen will, by this union, be
diminished, if the contractions be often repeated, the excitability
will thus be expended faster than it can be supplied by the
circulation, and will become exhausted. But will facts bear us out in
this explanation? To see this, we must examine the chemical nature of
the substances which produce the greatest action, and the greatest
exhaustion of the vital principle: namely, those which produce
intoxication.

Fermented liquors differ from water, in containing carbon and more
hydrogen: these produce intoxication: but pure spirits, which contain
still more hydrogen, produce a still higher degree of intoxication,
and consequent exhaustion of the excitability. Ether, which appears
to be little more than condensed hydrogen, probably kept in a liquid
state by union with a small quantity of carbon, and which easily
expands by caloric into a gas, which very much resembles hydrogen
gas, produces a still greater degree of intoxication: so that we see
the action produced by different substances, as well as the
exhaustion of excitability which follows, is proportioned to the
quantity of hydrogen they contain.

There is another circumstance which seems to strengthen this idea.
The intoxicating powers of spirits are diminished by the addition of
vegetable acids, or substances which contain oxygen, which will
counteract the effects of the hydrogen. Thus it is known that the
same quantity of spirit, made into punch, will not produce either the
same ebriety, or the same subsequent exhaustion, as when simply mixed
with water.

Recollect however that I propose this only as a hypothesis: its truth
may be confirmed by future observations and experiments, or it may be
refuted by them: but it is certainly capable of explaining many of
the phenomena, which is one of the conditions required by Newton's
first rule of philosophizing.

Heat, and light, and other stimuli, may perhaps exhaust the
excitability, by facilitating the combination of oxygen in the fibres
with the hydrogen and carbon in the blood.

There are several substances which cause a diminution or exhaustion
of the excitability, without producing any previous increased
excitement. These substances have by physicians been called
sedatives: and though the existence of such bodies is denied by Dr.
Brown, yet we are constrained to admit them; nor do their effects
seem incapable of being explained on the principle laid down,
especially if we call in the aid of chemistry.

Any substance which is capable of combining rapidly with oxygen, and
diminishing its quantity, will be a sedative. But the action of some
of the animal and vegetable poisons is difficult to explain in the
present state of our knowledge; such very minute portions of these
produce great exhaustion of the excitability, and even death, that we
can scarcely explain their action on the supposition that they
combine with the oxygen. They may perhaps act as ferments, and
occasion throughout the whole system a new and rapid combination of
oxygen with the hydrogenous, carbonic, and perhaps azotic parts of
the blood and fluids, and even of the solids, which will speedily
destroy the excitability, and even the organization.

Many of the vegetable narcotics, though they will destroy life when
given in considerable doses, yet when exhibited in less quantities
become very powerful remedies, particularly in cases where the
excitability is accumulated, in consequence of which violent spasms
and inordinate actions take place, which are very quickly calmed by
opium, camphor, musk, asafoetida, ether, &c. medicines that occasion
a speedy exhaustion of the excitability. In diseases of exhaustion,
however, these remedies are improper. The indication here is to
accumulate the irritability, by the introduction of oxygen, and by
the diminution of the action of the stimulants which support life. In
this idea too I dissent from Dr. Brown, who taught that diseases of
exhaustion are to be cured by stimulants, a little less powerful than
those which produced the disease. This subject will however be more
fully discussed hereafter.

This doctrine of animal life, which I have been attempting to
illustrate, and render familiar, exhibits a new view of the manner in
which it is constantly supported. It discovers to us the true means
of promoting health and longevity, by proportioning the number and
force of stimuli to the age, climate, situation, habits, and
temperament, of the human body. It leads us to a knowledge of the
causes of diseases: these we shall find consist either in an
excessive or preternatural excitement in the whole or part of the
human body, accompanied generally with irregular motions, and induced
by natural or artificial stimuli, or in a diminished excitement or
debility in the whole, or in part. It likewise teaches us that the
natural and only efficacious cure of these diseases depends on the
abstraction of stimuli, from the whole, or from a part of the body,
when the excitement is in excess: and in the increase of their number
and force when the contrary takes place.

The light which the discoveries of Galvani, and others who have
followed his steps, begin to throw on physiology, promises, when
aided by the principles of chemistry, and the knowledge of the laws
of life, to produce all the advantages that would result from a
perfect knowledge of the animal functions.

From what has been said, it does not seem improbable that muscular
contraction may depend upon the combination of oxygen with hydrogen
and azote, in consequence of a sort of explosion or discharge
produced by nervous electricity. According to this hypothesis, animal
motion, at least that of animals analogous to man, would be produced
by a beautiful pneumatic structure. This hypothesis, though not
perhaps at this moment capable of strict demonstration, seems
extremely probable, it being countenanced by every observation and
experiment yet made on the subject. It accounts likewise for the
perpetual necessity of inhaling oxygen, and enables us to trace the
changes which this substance undergoes, from the moment it is
received into the system, till the moment it is expelled. By the
lungs it is imparted to the blood; by the blood to the muscular
fibres; in these, during their contraction, it combines with the
hydrogen, and perhaps carbon and azote, to form water and various
salts, which are taken up by the absorbents, and afterwards exhaled
or excreted. We know the necessity of oxygen to muscular motion, and
likewise that this motion languishes when there is a deficiency of
the principle, as in sea scurvy. Thus a boundless region of discovery
seems to be opening to our view: the science of philosophy, which
began with remote objects, now promises to unfold to us the more
difficult and more interesting knowledge of ourselves. Should this
kind of knowledge ever become a part of general education, then the
causes of many diseases being known, and the manner in which the
external powers, with which we are surrounded, act upon us, a great
improvement not only in health, but in morality must be the
consequence.

With respect to its influence on the science of medicine, we may
observe that, from the time of Hippocrates till almost the present
day, medicine has not deserved the name of a science but, as he
called it, of a conjectural art. At present however, by the
application of the laws of life, and of the new chemistry, there is
beginning to appear in physiology and pathology, something like the
simplicity and certainty of truth. In proportion as the laws of
animal nature come to be ascertained, the study of them will excite
more general attention, and will ultimately prove the most popular,
as well as the most curious and interesting branch of philosophy.

This must be productive of beneficial consequences to society, since
these truths, once impressed upon the mind by conviction, will
operate as moral motives, by which the sum of disease and human
misery cannot fail to be greatly diminished.

LECTURE XI.
OF THE NATURE AND CAUSES OF DISEASES.

In the two last lectures I have attempted to investigate the laws of
life. I now proceed to the most important part of our course, and for
which all the preceding lectures were intended to prepare us; I mean
the application of the laws of life to explain the nature and causes
of diseases, and the methods of curing them, which must always be
imperfect, and conjectural, unless the nature of the diseases
themselves be well understood.

We have already seen that life is constantly supported by the action
of the external powers which surround us, and that if the action of
these powers be properly regulated, and at the same time no other
powers be suffered to act on the body, we shall enjoy perfect health,
but if, on the contrary, the exciting powers which support life, act
either too feebly or too powerfully, then the functions will not be
performed with precision and vigour, but irregularly; the mind and
body will become deranged, and death will often take place many years
before the natural period at which that event might be expected.

As health is the greatest blessing which man can enjoy, it is natural
to think, that in the early ages of society, when men began to lose
sight of the dictates of nature, and feel the torture of disease,
they would regard with gratitude those who had contributed towards
their relief, and that they would place their physicians among their
heroes and their gods. In the early ages, however, diseases would be
very few, for it would not be till civilisation had made considerable
progress, that such unnatural modes of life as conduce to their
production, would take place.

As the first professors of physic knew nothing of the animal economy,
and little of the theory of diseases, it is evident that whatever
they did, must have been in consequence of mere random trials. Indeed
it is impossible that this or any other art could originate in any
other manner. Accordingly history informs us that the ancient nations
used to expose their sick in temples, and by the sides of highways,
that they might receive the advice of every one that passed.

It would take up too much time to pursue the history of medicine from
this rude origin, through all its changes and revolutions, till the
present time: let it therefore suffice to say, that after various
theories had been invented and overturned, and after one age had
destroyed the labours of another, though different branches of the
healing art, and particularly anatomy, had been enriched with
valuable discoveries, still a rational theory was wanting; there was
nothing to guide the practitioner in his way, and we may truly say
that till the laws of life, which I have been endeavouring to
illustrate, were investigated by Dr. Brown, medicine could boast of
no theory which had a title to be called philosophical.

The theories of Stahl, Boerhaave, and Cullen, have passed away, and
are almost forgotten, but this, which is founded on nature, and on
fact, will, like the Newtonian philosophy, last for ever. It has
already influenced the practice of medicine, and is taught in almost
all the schools of Europe and America. In this country it seems to
have had less attention paid to it than it deserved, because its
influence was counteracted by the arrogance and profligacy of its
author, as if the grossness of a man's manner affected the
conclusiveness of his arguments; but this influence did not extend
beyond Britain, while the light of his theory illuminated the
opposite hemisphere. And when the manner in which he was persecuted
is recollected, the liberal mind will allow something to the deep
consciousness of neglected merit.

A circumstance much in favour of this doctrine is, that those who
understand its principles thoroughly, are guided by it in their
practice with a certainty and success before unknown. I say those who
understand its principles, for these were not perfectly understood
even by the author himself. He first saw with his mind's eye the
grand outline of the system, from which, for want of proper
reflection, he often drew wrong deductions, and which he often
applied improperly. But whatever errors Brown may have committed in
the application of his system, and however short his doctrines may
fall of a perfect system of medicine, we may venture to predict that
the grand outlines will remain unshaken.

From what has been already shown, it must be evident that if the just
degree of excitement could be kept up, mankind would enjoy continual
health. But it is difficult, if not impossible, to regulate the
action of the exciting powers in this equable manner, and if their
action is increased, the first effect they produce on the functions
is to increase them, and the next is, to render them disturbed or
uneasy; or, in other words, to bring on diseases of increased action,
or what have been called inflammatory or phlogistic, both of which
terms are improper, as they convey false ideas, and are connected
with erroneous theories: Dr. Brown has given the name of sthenic to
these diseases, from their consisting in increased strength or
action, and this is certainly a more appropriate term. On the
contrary, when the action of the exciting powers is diminished more
than is natural, the functions become languid and disturbed, and by a
still further decrease of the action of these powers, they become
irregular and inordinate. This state of the body, which is opposite
to the former, Dr. Brown has denominated asthenic.

But the stimulant powers may act so powerfully, and exhaust the
excitability to such a degree, that they may overstep the bounds of
sthenic or inflammatory disease and bring on debility. Debility may
therefore arise either from the stimuli acting too weakly, or from a
deficient excitability, while the stimulus is not deficient. Debility
produced in the former manner is called direct debility, and in the
latter indirect debility.

To explain this more clearly, let us take a common instance. If a
person by any means be deprived of the proper quantity of food, he
will feel himself enfeebled, and the functions will gradually grow
more and more languid, and at last become irregular, and be performed
with pain. This state is called direct debility. Here is excitability
enough, and even too much, for it has accumulated by the subtraction
of a stimulus; but here is a deficiency of excitement from defect of
stimulus.

If now we suppose that a person, in good health, begins to take a
greater quantity of food than usual, and adds a quantity of wine, all
the functions will at first be increased in vigour, but at last they
will be irregularly performed, and inflammation, with other symptoms
of too great excitement, will be the consequence. This state is
called sthenic diathesis or disease. But if the stimulant power be
pushed still further, the excitability will become gradually
exhausted, till at last there will be too little to produce the
healthy actions, even though there may be plenty of stimulus. This
state of asthenic diathesis is called indirect debility, because it
is not produced by directly subtracting the powers which support
life, but indirectly, by over stimulating. An instance of this latter
state is afforded by that debility which is the consequence of
intoxication.

There is a state however between perfect health and disease, which is
called predisposition; and in which, though the functions are
undisturbed, the slightest cause will bring on disease. Strictly
speaking, there is perhaps only one point, or one degree of
excitement, at which the health is perfect: the first alterations
from this point, on either side, are scarcely perceptible, but if the
morbid causes be continued, the functions will become gradually more
and more disturbed, till at last they become so uneasy or painful
that they are termed disease.

In order to render what has been said still more plain, it may be
proper to make use of an illustration by means of numbers: we must
recollect however that it is merely for the sake of illustration, for
we have not data to enable us to reduce either the excitability, or
excitement, or stimulus, to numerical calculation; if we could do
this, the science of medicine would be perfect, and we could cure
diseases as easily as we could perform any chemical or philosophical
experiment. A very principal object however is to understand the
nature of predisposition, and the kind of diathesis, whether sthenic
or asthenic, to which it inclines: this not only throws light on the
nature of the disease, but affords us the only means of preventing
it. When a slight uneasiness or predisposition is felt, it is almost
impossible to say from our feelings whether it leads to a sthenic or
an asthenic state: here we must be guided chiefly by the exciting
powers. If we find that these have acted too powerfully; that is, if
we have lived freely, been exposed to heat, and perhaps indulged in
some of the unnatural stimuli, such as wine and spirits; and
particularly if we previously to the present time perceived the
functions to go on with more vigour, our spirits and strength
greater, before we experienced the slight disturbance of which we
complain, we are verging towards sthenic or inflammatory disease, and
therefore to prevent the disease we ought immediately to diminish the
action of the exciting powers; the quantity of food ought to be
diminished, wine and other liquors abstained from, heat carefully
avoided; and even the quantity of blood in the circulating system
diminished, if the habit is full and the pulse strong.

On the contrary, if the exciting powers have acted more feebly than
is natural; that is, if we have lived on a less nourishing diet, or
have taken it in less quantity; if we have been long exposed to cold,
without alternating with heat, and other debilitating causes; and if
at the same time we find the vigour of the functions diminished,
though they are not yet become much disturbed, we are verging towards
asthenic disease. To prevent which, we must take a more nutritious
diet, and join a portion of wine, and perhaps take some tonic
medicines. This however ought to be done gradually, for fear of
exhausting the excitability, which in these cases is morbidly
accumulated.

It must be evident that the great difficulty here is to determine the
nature of the predisposition; for if we make a mistake, instead of
preventing, we shall accelerate the disease. For instance, the first
slight disturbance of the functions which rises from a sthenic state,
often resembles those verging towards a state of debility or
asthenia. I have seen various instances arising from plethora, or a
sthenic state, where the patient complained of depression of spirits,
and inability to move; and, in short, from his own account was
labouring under asthenic diathesis: but by inquiring carefully into
the action of the exciting causes, examining minutely the state of
the pulse and of the functions, I have been convinced that the
depression of spirits which he felt, and other symptoms of weakness,
depended on fullness, and they have been quickly removed by lowering
the diet, administering a laxative, or taking a little blood: whereas
if, apprehending from the symptoms that he had laboured under
debility, I had ordered him a more generous diet and tonic remedies,
an inflammatory disease would have been the consequence, which might
have terminated in death.

I have seen various instances where patients have complained of this
unusual depression, and inability to move: they have shown me
prescriptions in which the stimulant or tonic plan was recommended,
but instead of any alleviation the symptoms had become worse from
their use. This hint was generally sufficient, for if the disease of
predisposition had been asthenic, cordials and tonics ought to have
relieved it: if, on inquiry, I found the exciting powers had acted
too powerfully, I then, without hesitation, had recourse to the
debilitating plan, and with the greatest certainty of success. Before
I viewed diseases and their causes in this way, I must confess that I
often felt great hesitation in practice; and judging merely from
symptoms, which are frequently very fallacious, the operation of a
remedy often disappointed me, and I could not pretend to predict the
event with the certainty that I now can. This observation is of the
greatest consequence in the cure both of predisposition and of
disease. Though excitement regulates all the phenomena of life, yet
the symptoms of diseases which either its excess or deficiency
produces, do not of themselves lead to any proper judgment respecting
it. On the contrary their fallacious appearance has proved the source
of infinite error.

As excitement both depends on exciting powers and excitability, it is
evident that when a middle degree of stimulus acts upon a middle
degree of excitability, the most perfect effect will be produced.
This point, could we ascertain it, might be called the point of
health. For the sake of illustration, we may suppose that the
greatest excitability of which the living body is capable is 80
degrees: this may be supposed to be the excitability possessed by the
body at the commencement of its life, because no part has then been
wasted or exhausted by the action of stimuli. Now, if we suppose a
scale of excitability to be formed, and divided into 80 equal parts
or degrees, the excitability will be wasted or exhausted in
proportion to the application of stimuli, from the beginning to the
end of the scale. One degree of exciting power applied takes off one
degree of excitability, and every subsequent degree impairs the
excitability in proportion to its degree of force. Thus a degree of
stimulus or exciting power equal to 10 will reduce the excitability
to 70, 20 to 60, 30 to 50, 40 to 40, 50 to 30, 60 to 20, 70 to 10, 80
to 0; and, on the contrary, the subtraction of stimulant power will
allow the excitability to accumulate.

[DIAGRAM]

The range of good health is ranked from 30 to 50 degrees in the
scale; for perfect health, which consists in the middle point only,
or at 40 degrees, rarely occurs; in consequence of the variation of
the stimuli to which man is continually exposed, such as meat and
drink, heat, exercise, and the emotions of the mind, the excitement
commonly fluctuates between 30 and 50 degrees, and yet no particular
disturbance of the functions takes place. But when at these points,
30 or 50, predisposition commences, the slightest debilitating cause
in the former case, and the slightest stimulating cause in the
latter, brings on disease, in which the functions begin to be
disturbed in various ways, and this disturbance is always in
proportion to the hurting powers which have produced the disease, and
the delicacy or importance of the part affected.

The effect produced on the excitability by any stimulus, must
evidently be in a ratio compounded of the degree of excitability and
the force of the stimulus. The same stimulus will produce greater
contractions upon a fibre that is more irritable than upon one which
possesses less irritability; and the irritability or excitability of
the fibre being given, or remaining the same, the contraction will be
in proportion to the strength of the stimulus. Hence it is evident,
that the effect or excitement must be in a ratio compounded of the
exciting powers and excitability.

Sthenic diathesis and disease is caused by the operation of different
exciting powers, which produce too great a degree of excitement in
the system: this at first increases all the functions, and, when
increased, produces a disturbance and inordinate action of them,
which is communicated to the whole body. In diseases of this kind
there is often an appearance of debility, but this is extremely
fallacious, and arises from the disturbed state of the different
functions. Hence it is evidently of the utmost consequence to
ascertain carefully whether this debility is real, or the effect of
asthenic disease: or whether it is owing to the disturbance of the
functions by over stimulating, and in this case fallacious; for
should a sthenic disease be treated by stimulants and cordials, the
effect would be an aggravation of all the symptoms, and a much higher
degree of disease.

Asthenic diathesis and disease is brought on by the excitement of the
system being diminished: and this may proceed either from a
diminution of common stimulant powers, while the excitability is
sufficiently abundant, or it may proceed from an exhausted
excitability, while the stimulus is sufficiently abundant. The former
is called direct, and the latter indirect debility. The exciting
causes therefore of asthenic disease, first impair the functions,
then occasion a disturbed or inordinate action of them, giving many
of them a false appearance; some of them, for instance, appear to be
increased, for in hysteria and epilepsy, which are both diseases of
debility, the action of the muscles seems to be preternaturally
increased; but this depends chiefly on the accumulated excitability,
which gives such a degree of irritability to the system, that the
smallest irritation, whether external, such as heat, exercise, &c. or
internal, as emotions of the mind, excite a strong spasmodic action,
which brings on the symptoms of epilepsy and hysteria. This
inordinate action however soon exhausts the morbid excitability, and
thus suspends itself, a sleep often follows, from which the patient
wakes with only a general sense of languor and debility: but as the
same cause still remains, the excitability of the body again becomes
morbidly accumulated, and thus the slightest stimulus produces a
recurrence of the fit, and the tendency to return will increase with
its recurrence, so that at last the slightest imaginable cause will
produce it, on account of the power of habit and association.

Gout likewise appears like a sthenic disease, and in inflammation
takes place, which resembles pleurisy or peripneumony; but this
symptom is fallacious, for it depends on debility, and is only to be
cured by means, which in pleurisy and peripneumony, would produce
death.

Hence it must be evident that those phenomena of diseases, which we
call symptoms, are generally fallacious; but this may be owing to our
imperfect knowledge of the animal economy, so that we are not able to
explain or understand the manner in which they are produced: we ought
however carefully to guard against being misled by them in practice.
The great difficulty is to distinguish the nature of the disease,
whether it is sthenic or asthenic, or whether it depends on too great
excitement, or on debility; for this being once clearly ascertained,
we proceed with certainty in our mode of treatment, instead of the
random practice, which must be the consequence of not taking a proper
view of the laws of life, and the causes of diseases.

The nature of the disease may be generally ascertained, by attending
to the habits of the patient, and the manner in which he has lived,
as well as to the state of the pulse; but in cases where these
circumstances do not render it clear, it may be ascertained, beyond a
doubt, by a trifling degree of stimulus, as, for instance, by any
cordial, as a little wine or spirits. If the disease be of an
inflammatory or sthenic kind, the symptoms will be aggravated, and
the cordial will not produce its usual pleasant effects on the
system; but on the contrary, if the nature of the disease be
asthenic, then the usual pleasant effects of the cordial will be
perceived, and the pain and other symptoms will be alleviated. This
trial, which is soon made, and without danger, will determine our
plan of cure, and we can then proceed with the most perfect
certainty. Thus you will see that this view we have taken leads to a
very different and much more rational plan of practice than is
generally followed, in which the most judicious physicians confess
that they have no clue to guide them; and complain that the science
of medicine consists merely in a number of insulated facts, not
connected by any theory: that they merely prescribe a remedy because
they have seen it of use in an apparently similar state, but that
they have no certainty of its producing a similar effect in the cases
in which they prescribe it. This all depends on trusting to the
fallacious appearance of symptoms, and not having taken a proper view
of the laws of life, or the manner in which the exciting powers act
on living bodies.

After these observations on the diagnosis, or the method of
distinguishing the nature of diseases, I shall proceed to consider
more particularly the nature of sthenic diseases, and the methods of
curing them, which will occupy the remainder of our time this
evening.

The powers or causes, which by their action produce inflammatory or
sthenic diseases, are, first, heat, which is a very frequent cause,
particularly when it succeeds cold; for the cold accumulates the
excitability, and then renders the whole body, or a part, more liable
to be affected by the heat afterwards applied. In this way is
produced rheumatism, catarrh, or, as it is commonly called, a cold,
and peripneumony. These complaints have been often attributed to
cold, but I believe that there never was a well attested instance
where cold alone, without being either followed by heat or some other
stimulus, produced a real sthenic, or inflammatory disease. This is
not merely a distinction, it is a circumstance of the utmost
importance, because it influences the mode of practice to be pursued.
Heat is one of the exciting or stimulant powers which support life,
and one of the most powerful of these stimulants; but cold is only a
diminution of it: how then can this produce a sthenic state, or a
state of too high excitement? The blood is one of the exciting
powers, which, by its continual circulation supports life; but surely
if we abstracted a quantity of this fluid from the body, no person
will be bold enough to say, that we by that means should produce an
inflammatory disease. Cold renders the body more liable to be
affected by heat, or any other stimulus applied, but does not of
itself produce any stimulant or inflammatory effects.

To see more clearly the manner in which cold acts, let us inquire how
it produces or contributes towards the production of catarrh. When we
go into the cold air, at every respiration we take a quantity of it
into the lungs, which brushes over the surface of the mucous membrane
that lines the nostrils and trachea, and thus, robbing them of their
heat, allows the excitability to accumulate. But we feel no fever, no
sense of tightness or stuffing, nor any other symptom of catarrh, so
long as we continue in the cold. If however we afterwards go into a
warm room, and particularly near a fire, we receive by the act of
respiration the warm air into those very parts which have been
previously exposed to cold, and whose excitability is consequently
accumulated. The first effect we perceive is a glow of the parts,
which is by no means unpleasant, this however increases; and, in the
course of half an hour or an hour, a sense of dryness and huskiness
comes on, with a sensation of stuffing in the nostrils, and a
tendency to a short dry cough: often likewise, if the exposure to
cold has been considerable, and the heat afterwards applied great and
sudden, we experience a shivering, and other symptoms of fever. These
symptoms are all increased by taking into the stomach any liquid that
is either of warm temperature or stimulating quality, or particularly
both; we spend a restless night, and awake with all the symptoms of a
catarrh, or cold, as it is improperly called. For it is evidently an
inflammatory fever, and can be speedily cured by the debilitating
plan, and particularly by keeping in a moderately cool place, where
the temperature is equable, and not subject to alternations of heat
and cold.

But how easily might this complaint have been avoided, were the
person subject to it acquainted with its real nature, and the manner
in which it is brought on. When we come out of a very cold
atmosphere, we should not at first go into a room that has a fire in
it; or, if this cannot be well avoided, we should keep for a
considerable time at as great a distance from the fire as possible,
that the accumulated excitability may be gradually exhausted by the
moderate and gentle action of heat; and then we may bear the heat of
the fire without any danger; but above all, we should refrain from
taking warm or strong liquors while we are hot. In confirmation of
this opinion, numerous instances might be brought, where catarrh was
cured merely by exposure to cold.

When a part of the body only has been exposed to the action of cold,
and the rest kept heated; if, for instance, a person in a warm room
has been sitting so that a current of air, coming through a broken
window, has fallen upon any part of the body, that part will soon be
affected with an inflammation, or what is called a rheumatic
affection. In this case, the excitability of the part exposed to the
action of the cold, becomes accumulated, and the warm blood, rushing
through it, from every other part of the body, excites an
inflammation.

Thus catarrh and rheumatism are inflammatory complaints, or depend on
too great a degree of excitement, and are to be cured by lowering the
excitement, or diminishing the action of the exciting powers; by
bleeding, purging, low diet, and particularly keeping in a moderately
cool place; and these complaints will be as speedily and certainly
cured by these methods, properly and judiciously persevered in, as a
slight cut or wound will be healed by what surgeons call the first
intention.

There are complaints which resemble these, but whose nature, however,
is very different, and which require a very different mode of
treatment. After a part has been long affected with rheumatic
inflammation the excitability of the muscular fibres becomes so far
exhausted, that a state of indirect debility takes place, and an
inflammation, accompanied with pain and redness, which is very
different from that I formerly described, as it depends upon a
debilitated or relaxed state of the parts, instead of too great a
degree of excitement. This instance shows strongly the fallacy of
symptoms; but it may be readily distinguished from the inflammatory
rheumatism, by attention to the effects of the exciting causes. The
inflammatory rheumatism is aggravated by heat, hence it is more
violent in bed than at any other time. The latter complaint, however,
is greatly relieved by heat: the warm bath alleviates all the
symptoms; so does a warm bed. It is evident that these diseases,
though attended by the same symptoms, are as opposite, and require as
different modes of treatment as an inflammation of the brain, and a
dropsy. The inflammatory state has been called the acute rheumatism,
and the other, the chronic rheumatism; I would, however, prefer the
terms sthenic and asthenic rheumatism.

In the same manner, there is a catarrh, which is liable to afflict
persons who have often been subject to an inflammatory cold,
particularly persons advanced in years; and this depends on a state
of indirect debility of the parts, the excitability of which has been
exhausted by frequent and violent inflammatory affections. This
complaint, which I would call asthenic catarrh, requires directly
opposite treatment from the inflammatory or sthenic catarrh. The
latter is aggravated by heat, but relieved by a cool temperature.
Warm air is peculiarly grateful to those who are afflicted with the
former, and if they go into a cool temperature, they are immediately
seized with cough, and expectoration; for the disease being a disease
of debility, the withdrawing the stimulus of heat, must increase it.
The excitability of the parts is so far exhausted, that it requires a
stimulus even more than natural to keep them in tone: hence persons
labouring under asthenic catarrh, and some species of asthma, which
are only varieties of this disease, find themselves best when exposed
to a warm temperature, but on the heat being diminished, and
consequently the parts relaxed, the cough and difficulty of breathing
immediately come on.

Having examined the effect of heat, in producing inflammatory or
sthenic disease, I now proceed to the consideration of the other
powers. Of the articles of diet, the only food in danger of being too
stimulant, is perhaps flesh or land animal food, used in too great
quantity, particularly when seasoned, a preparation which adds much
to its stimulant power. Spirituous and vinous liquors, let them be
ever so weak or much diluted, stimulate more quickly, and more
readily than seasoned food, and their stimulus is in proportion to
the quantity of alcohol which they contain. These substances, when
conjoined with rich food, must bring on a predisposition to sthenic
disease, in almost any constitution, particularly in the young and
healthy, and, in many instances, those diseases actually take place;
or should this not be the case, should the person avoid, or escape
the effects of inflammatory diseases, the excitability will be
exhausted, and diseases of indirect debility, such as gout, apoplexy,
indigestion, palsy, &c. will take place.

These stimulants are never necessary to a good constitution, and
their effects will always, sooner or later, be experienced: for
though a person with a good constitution may continue for years to
indulge in the pleasures of the bottle, or the luxuries of the table,
depend upon it that a continuance of them will sap the vigour of the
strongest constitution that ever existed.

As nothing contributes more to the health of the body than moderate
and frequently repeated exercise, which rouses the muscles to
contraction, and promotes the circulation of the blood in the veins
towards the heart: it thus produces excitement; but an excess of it
will produce sthenic diathesis; and, if carried to great excess, it
will produce a state of indirect debility, or exhausted excitability.

When any, or all of these exciting powers act too strongly on the
body, the first effect they produce is a preternatural acuteness of
all the senses; the motions, both voluntary and involuntary, are
performed with vigour, and there is an acuteness of genius and
intellectual power. In short, every part of the body seems in a state
of complete vigour and strength; that this is the case with the heart
and arteries, appears from the strong and firm pulse; in the stomach
it is shown by the appetite; and, in the extreme parts, by the ruddy
colour and complexion. In short, every appearance marks vigour of the
body, and abundance of blood. Could the body be kept in this state,
nothing could be more to be desired; this, however, is impossible;
the excitement, though still within the bounds of health, has
overstepped the point of good health, and is verging fast to
predisposition to sthenic disease; so that, to secure a permanent
state of health, it is always better to keep the excitement rather
under the middle point, or 40 degrees, than above it. During the
predisposition to sthenic disease, which is produced by the longer
continued, or increased action of these powers, no symptoms of
disease appear; but shortly after, disturbed sleep, depressed
spirits, languor, a sense of fulness, heaviness, particularly after
eating, show that this sthenic state cannot be further increased with
impunity. The least increase of sthenic diathesis now brings on a
disturbance of the functions, or actual disease; the commencement of
which is generally a shivering, and a sense of cold; thirst and heat
succeed; and then generally a pain in some part, either external or
internal: costiveness generally attends this state, the urine is
clear, and secreted in small quantity; memory and imagination become
diminished, and there is generally less appetite for food.

In peripneumony, inflammatory sore throat, and acute rheumatism,
there is an inflamed condition of the lungs, of the parts about the
throat, or of the muscles of the extremities: this shows that the
excitement here is greater than in other parts of the body; but it is
still increased or too great in every part, only those parts which
give the peculiar character to the disease are more affected than
other parts of the body, by being more exposed to the exciting
causes: thus, if a person be in perfect health, or a little below, he
will not be easily affected by any of the exciting causes of sthenic
disease, unless their application be very violent; he will go into a
warm room out of the cold air, and feel no other effect than a
pleasant glow: but if, by high living, or other means, he is brought
near the point of predisposition to sthenic disease, then the
slightest additional stimulus will bring it on, and if the throat has
been exposed to the application of cold, and the person comes
afterwards into a heated room, an inflammation of the parts about the
throat, or an inflammatory sore throat, accompanied by a sthenic
diathesis of the whole system, will be the consequence. This cannot
be cured by merely diminishing the excitement of the part, while the
excitement of the whole system remains: if we apply leeches to the
throat in this state, to diminish the quantity of blood, we only
debilitate the vessels, while fresh quantities of blood are poured
into them from the too full vessels of the body; even if we could
thus remove the sthenic diathesis of the part, we should go but a
little way towards removing the inflammatory disease, which
universally pervades the system.

The mode to be pursued therefore is, to take a quantity of blood from
the body, by opening a vein; to keep the body cool, by remaining in a
room where the temperature is at temperate, or a little below; by
abstaining from animal food, and from spirituous or fermented
liquors; and by the exhibition of purgatives, or at least of
laxatives. Then leeches or blisters applied to the part affected will
produce a good effect; and even stimulant applications to the
inflamed part may be advantageous; for a topical inflammation, as we
shall afterwards have occasion to see, depends on a debilitated state
of the minute vessels of the part, while at the same time the action
of the whole system is increased.

Besides the energy of the exciting hurtful powers, which I have
mentioned, there is in the parts which undergo the inflammation, a
greater sensibility, or an accumulated excitability; by which it
happens that some are more affected than the rest. To this we may
add, that whatsoever part may have been injured by inflammation, that
part in every future sthenic attack is in more danger of being
inflamed than the rest. Hence inflammatory sore throat, rheumatism,
and some other complaints of the kind, when once they have
supervened, are very apt to recur.

Among the sthenic or inflammatory diseases may be enumerated
rheumatism, catarrh, cynanche, or sore throat, scarlet fever,
inflammations of the brain, stomach, lungs, &c. &c.

Many of the contagious diseases, particularly small pox and measles,
produce a sthenic state, and are to be cured, or their action
moderated, by the debilitating plan which has been pointed out; and
particularly by a moderate, constant, and equable diminution of
temperature. Hence the violence of these diseases is greater when
they attack a person already predisposed to sthenic diathesis, but
much more mild when the excitement is rather under par.

LECTURE XII.
ON INFLAMMATION AND ASTHENIC DISEASES.

The last lecture was taken up chiefly with an account of sthenic
diseases, or those depending on too great a degree of excitement, and
which have been generally, but improperly, called inflammatory or
phlogistic. In that lecture I attempted to show, that when the
natural exciting powers, which support life, act with too much power,
or particularly if we employ any stimulants not natural to the body,
the functions both of body and mind become increased in vigour; but
if the exciting causes are continued and increased, the functions
become disturbed, and their action becomes painful and distressing.
This state, which is called sthenic diathesis, is often accompanied
by a redness, swelling, pain, and increased heat of some particular
part: these symptoms constitute what is usually termed an
inflammation of the part.

The method of cure in sthenic diseases was shown to be, by reducing
or moderating the action of the exciting powers; by keeping the body
cool; abstaining from high seasoned, and, in general, from animal
food; by the use of purgatives, and in many cases by diminishing the
quantity of blood in the body. I mentioned likewise, that it would be
but of little use to attempt to subdue the excitement of the inflamed
part, unless the excitement of the whole system was previously
diminished; but that after a general bloodletting, stimulant remedies
applied to the inflamed part, might be employed with success. This is
strictly agreeable to experience, but at first sight seems so very
contrary to the principles that have been advanced, that I shall
endeavour to explain the phenomena of inflammation, which do not seem
to be in general well understood.

All kinds of inflammation agree, in being attended with redness,
increased temperature, pain, and swelling; but they vary according to
the situation and texture of the part affected. All parts of the
body, excepting the cuticle, nails, hardest part of the teeth, and
hair, are subject to inflammation.

Among the causes of these complaints, may be enumerated too full a
diet, particularly too free a use of fermented liquors, and whatever
increases the impetus of the blood towards the part, as mechanical
and chemical irritation, and sudden changes of temperature,
particularly from cold to heat.

To explain the nature of inflammation, it may be observed, that such
is the wise constitution of the animal body, that whatever injures
it, excites motions calculated to correct or expel the offending
cause. Thus if an irritating substance is received into the stomach,
it excites vomiting; if into the lungs, a violent fit of coughing is
excited, and if into the nostrils, sneezing is the consequence. In
such cases we can readily trace the motions excited, and the manner
in which they act; but cannot trace the manner in which the offending
cause excites these motions.

Now if it can be shown that inflammation, like vomiting and coughing,
is an effort of the system to remove an offending cause, and if we
can trace every step of this operation, with the exception of the
changes induced on the nervous system, we shall understand the nature
of inflammation as completely as that of any function of the body.

The circumstance the most difficult to explain, is the increased
redness of the part affected, which can only depend on an increased
quantity of blood in the vessels. This has been supposed to depend
upon an increased action of the vessels of the part; but that this is
not the case, must be evident from what was said when we were
speaking of the circulation of the blood. It was shown, that the
circulation could not be carried on by the mere force of elasticity
alone; this force, were it perfect, would produce no effect; but as
there is no body with which we are acquainted that is perfectly
elastic; so the coats of the arteries are very far from being so,
hence their effect as elastic tubes will be to diminish the force of
the heart, instead of adding to it; for a certain quantity of this
force will be spent in distending the vessels, which, were they
perfectly elastic, would be restored to them, but as this is not the
case, this force is by no means restored. Indeed a variety of
considerations, observations, and experiments, tend to prove, that
the vessels are endowed with a power very different from elasticity,
which differs only in degree from that of the heart; in short, they
are possessed of muscular power.

After each contraction of the muscular coat, the elastic will act as
its antagonist, and enlarge the diameter, till the vessel arrive at a
mean degree of dilatation, but after this there is no further power
of distention inherent in the vessel. The action of the elastic coat
ceases; and no one will assert that a muscular fibre has power to
distend itself.

The only power by which the vessel can be further distended, is the
vis a tergo: after the vessel arrives at its mean degree of
dilatation, both the elastic and muscular coats act as antagonists to
the vis a tergo, or force which propels the blood into, and thus
tends further to dilate the vessel. If then the vis a tergo become
greater than in health, the powers of resistance inherent in the
vessels remaining the same; or if the latter be weakened, the vis a
tergo, or propelling force, remaining the same, the vessel must
suffer a morbid degree of dilatation. These appear to be the only
circumstances under which a vessel can suffer such dilatation.

But if, while the powers of the vessels remain the same, the vis a
tergo, or propelling force, be diminished, or the propelling force
remaining the same, the power of the vessels become increased; then
an opposite condition or state of the vessels, viz. a preternatural
diminution of their area, will take place.

In the one case the distending force bears too great a proportion to
the resisting force; and preternatural distention is the consequence.
In the other the resisting force bears too great a proportion to the
distending force, and preternatural contraction is the consequence.

It is not necessary that the vessels should be in a state of greater
debility than in health, in order that an inflammation or distention
may take place: it is only necessary that the proportion which their
action bears to the propelling force be less than in health. If the
propelling force remain the same, the vessels must be in a state of
debility before an inflammation can take place; but if the propelling
force be increased by a fullness of the vessels and sthenic
diathesis, inflammation may take place, although the vessels of the
part act as powerfully as in health, or more so. But after
inflammation has taken place, as the vessels are preternaturally
distended, they must also be debilitated.

The degree of inflammation is not however proportioned to the
debility of the minute vessels of an inflamed part, but to the
diminished proportion of their power to the propelling force.

When, therefore, inflammation arises from an increased action of the
arterial system, or an increased propelling force, while the force of
the capillaries or minute vessels remains the same, it constitutes
what is called an active inflammation, and is to be cured by general
bleedings, and then by gentle applications of tonics to the part, to
increase its action; but when it arises from a debility of the minute
vessels, without any increase of the propelling force, it forms what
is known by the name of passive inflammation; in which general
bleeding is not required, but the application of stimulants and
tonics to the inflamed part to enable the vessels to recover their
lost tone, and restore the balance between their action and the vis a
tergo. From what has been said, it must be evident, that if
inflammation depend on the diminished proportion of the power of the
capillaries to the propelling force, it will be more apt to supervene
under the three following circumstances.

1. In a state of plethora, because then all the vessels are over
distended, and consequently any cause tending further to distend
them, whether it be a cause which debilitates them, or increases the
propelling force, will be more felt than in health.

2. In a state of general debility, because the vital powers in any
part are more readily destroyed than in health.

3. In a state of general excitement, because then the propelling
force is every where strong, and consequently apt to occasion
distention of the vessels, wherever any degree of debility occurs.
These are the states of the system which are found to predispose to
inflammation. In the first and last, the inflammation is generally of
that kind, which is termed active: the propelling force is
considerable, and the larger arteries are readily excited to
increased action. In the second state the inflammation is of the
passive kind.

This is not merely a useless physiological disquisition; it is of the
greatest use in directing our practice; and teaches us that, in
passive inflammation, which has all the symptoms of active, and
therefore shows in a striking point of view the fallacy of symptoms,
we shall not succeed by applying leeches, and other debilitating
means, to the inflamed part; on the contrary, we shall aggravate the
complaint; and the cure must be effected by stimulants applied to the
part.

As an instance of this kind of inflammation, I may mention that kind
of ophthalmia or inflammation of the eyes, which is of long standing,
and which not only resists the powers of leeches and blisters, but is
increased by them. I have frequently been consulted by patients, who
had for months been under the debilitating plan, without any benefit;
and who have been relieved almost instantly by the application of
electricity and a stimulating lotion, which restored the tone of the
debilitated vessels of the sclerotic coat, and enabled them to expel
their overcharged contents; and the balance between their action and
the propelling force being restored, the inflammation disappeared.

Indeed the effects of electricity in these kinds of inflammations are
wonderful: it seems to act almost by a charm, so quickly does the
inflammation subside; but when we understand the nature of this kind
of inflammation, it is nothing but what we might expect from its
action.

I have been thus minute on the subject of inflammation, because the
theory of it, which I have attempted to defend, differs considerably
from the commonly received opinions. I shall now proceed to consider
the nature of asthenic diseases.

From what has been already said, it must be evident that the causes
of diseases which we have assigned, are very different from those
delivered by physicians who preceded Dr. Brown. Some physicians
imagined that diseases were caused by a change in the qualities of
the fluids, which became sometimes acid, and sometimes alkaline; or
on a change of figure of the particles of the blood: some imagined
diseases to be owing to a rational principle, which they called the
vis medicatrix naturae, which governed the actions of the body, and
excited fever or commotion in the system to remove any hurtful cause,
or expel any morbid matter, which might have insinuated itself into
the body. Others supposed many diseases to arise from a constriction
of the extreme vessels by cold; or from a spasm of them, which was a
contrivance of the vis medicatrix, to rouse the action of the heart
and arteries to remove the debility induced.

We have seen, however, that health and diseases are the same state,
and depending upon the same cause; viz. excitement, but differing in
degree; and that the powers producing both are the same, sometimes
acting with a proper degree of force: at other times either with too
much, or too little.

We shall now examine how the diminished actions of the different
exciting powers produce asthenic disease; and we shall take them in
the same order as when we were speaking of sthenic diseases. It must
be recollected however that an asthenic state, or a state of
debility, may be produced in two ways. First, by directly diminishing
the action of the exciting powers. Secondly, by exhausting the
excitability, by a strong or long continued stimulant action. The
former state is called direct debility, and the latter indirect
debility. This is not merely a distinction without a difference, the
body is in very different states, under these two different forms of
disease. In the former case, the excitability is abundant, and highly
susceptible of the action of stimulants. In the latter, it is
exhausted, and the body has very little susceptibility.

Cold, or a diminution of heat, carried beyond a certain degree, is
unfriendly to all animals. Dr. Beddoes has shown very clearly in his
Hygeia, that it is the cause of a great many diseases which take
place at boarding schools, and that it there gives origin to a great
number of diseases that afterwards arise, and, indeed, not
unfrequently ruins the constitution. It produces relaxation of the
vessels, asthenic or passive inflammation, and even gangrene. He has
shown that in most schools children are afflicted with chilblains
from this cause; this is a case of passive inflammation, but is only
a symptom of the general debility induced, which shows itself
afterwards by the production of other symptoms. Hence it is necessary
for the preservation of health, that the temperature of school rooms
should always be kept equable, and regulated by means of a
thermometer. It should not exceed 50 degrees, nor should it be
allowed to fall much below it. If precautions of this kind are
thought to be necessary, and practised with uncommon attention, in
places where vegetables are reared, surely they ought not to be
neglected in those seminaries where the human species are to be
brought to maturity, and a good constitution established.

But though I have no doubt whatever, that this equable temperature
would prevent a number of diseases, which originate in too low a
temperature, yet I am far from wishing to have it thought that I
would not induce a hardy state of the constitution, which would
enable it to bear the vicissitudes to which it must be exposed in its
journey through life, by every means in my power. Hardiness is the
most enviable of all the attributes of animal nature, and can neither
be acquired, nor recovered when it is lost, but upon certain terms,
to which many people submit with reluctance, because they must give
up many indulgences and gratifications with which it is utterly
inconsistent.

One of the causes that chiefly contributes to reduce persons living
in affluence below the standard of hardiness, is the dependence they
place on a considerable degree of external warmth, for preserving a
comfortable state of sensation. From what has been said again and
again in some of the latest of these lectures, it must be evident
that continued warmth renders the living system less capable of being
excited to strong, healthy, and pleasant action: heat in excess,
whether it may be excess of duration or intensity, constantly
debilitates, by exhausting the excitability of the system, and thus
producing a state of indirect debility. Every muscle steeped in a
heated medium, whether of air or water, loses much of its
contractibility. A heart kept in heated air, or put in hot water,
will not contract on the application of a stimulus; even the limb of
a frog, when heated in this manner, ceases to move on the application
of the galvanic exciters. Every nerve grows languid, and when it does
become excited, it acquires a disposition to throw the moving fibres,
with which it is connected, into starts, twitchings, and other
irregular convulsive motions. Though therefore nothing can more
contribute to the health of the body than a moderate and well
regulated temperature, about 48 or 50 degrees, sometimes for a short
interval a little lower, when exercise is taken at the same time, yet
when we consider the life led by persons of fashion, we should hope
that it proceeded from ignorance of these consequences; so
diametrically opposite is it to the dictates of nature and reason.

Instead of rising from table after dinner, and availing themselves of
the cooling and refreshing qualities of the air, even in the finest
seasons, when every thing which pure and simple nature can offer,
invites them abroad, they do every thing they can, as Dr. Beddoes
observes, to add to the overstimulating operation of a full and
hearty dinner. After taking strong wine with their food, they sit in
rooms rendered progressively warmer, all the afternoon, by the
presence of company, by the increase of fires, and for more than half
the year, by the early closing of the shutters, and letting down of
the window curtains. After a short interval, tea and coffee succeed;
liquors stimulating both by their inherent qualities, and by virtue
of the temperature at which they are often drank. And that nothing
may be wanting to their pernicious effect, they are frequently taken
in the very stew and squeeze of a fashionable mob. The season of
sleep succeeds, and to crown the adventures of the evening, the bed
room is fastened close, and made stifling by a fire: and though the
robust may not quickly feel the effects of this mode of life, with
the feeble it is quite otherwise. These, as they usually manage,
rarely pass a few hours of sleep without feverishness and uneasy
dreams; both of which contribute to their finding themselves by far
more spent and spiritless in the morning, than after their evening
fit of forced excitement, instead of having their spirits and
strength recruited by the "chief nourisher in life's feast," Perhaps
they drink tea before rising, and indulge in a morning nap; this
weakens much more than the greatest muscular exertion they would be
capable of supporting for an equal time. For the sleep at this time
is almost invariably disturbed, and attended by a heat of the skin.
The reason of this must be evident to every one who has attended
these lectures.

The effect of sleep is to accumulate the excitability, or render it
more sensible to the effects of any stimulants applied. This takes
place in every constitution, and much more in the more delicate:
hence the heat of the bed, and of the tea, acts so powerfully on the
surface, as, in general, to produce great perspiration, or, at any
rate, great languor and debility.

Let me ask, can any one, who lives in this manner, expect to enjoy
good health? With as great probability might we expect, that when we
plunged a thermometer into hot water, the mercury would not rise, or
when we applied a lighted match to gunpowder, it would not explode.
The laws of nature are constant and uniform, and the same, or similar
causes, both in the animate and inanimate world, are always
productive of the same, or similar effects.

The cure of these complaints is at least obvious, if not easy. It
consists in deserting crowded and heated rooms, at least for part of
the time they have been usually occupied; in abstaining from strong
wines; in keeping the bed rooms moderately cool; and retiring to rest
at a proper hour.

With respect to the effects of nutriment, in producing asthenic
diseases, we may observe, that all watery vegetable food, too sparing
a use of animal food, as also meat which is too salt, and deprived of
its nutritious juices by keeping, when more nutritious matter is at
the same time withheld, constantly weaken, and thereby tend to
produce asthenic diseases. Hence would appear to arise that
remarkable imbecility of body and mind which distinguishes the
Gentoos. Hence arise the diseases with which the poor are every where
afflicted; hence scrofula, epilepsy, and the whole band of asthenic
diseases.

But intemperance in eating and drinking, or taking nutritious and
highly stimulant substances too freely, will, infallibly, bring on
asthenic disease, or a state of indirect debility, by exhausting the
excitability; and it must be observed, that this species of debility
is much worse to cure than the direct kind; for in the latter we have
abundance of excitability, and a variety of stimuli, by which we can
exhaust it to the proper degree, and thus bring about the healthy
state; whereas, in indirect debility, the vital principle or
excitability is deficient; and we have not the means of reproducing
it, at pleasure, absolutely under our command. Besides, the
subtraction of stimulants, which is one of the most certain means we
have of accumulating excitability, if carried to a great extent, in
diseases of indirect debility, would produce death, before the system
had power to reproduce the lost or exhausted excitability. Hence the
cure, in these two kinds of debility, must be very different: in
cases of direct debility, as in epilepsy, we must begin with gentle
stimulants, and increase them with the greatest caution, till the
healthy state is established: we must, however, guard most carefully
against over doing it; for, if we should once overstep the bounds of
excitement, and convert the direct into indirect debility, we shall
have a disease to combat, in which we have both a want of excitement
and of exciting power.

In cases of violent indirect debility, as, for instance, in gout,
when it affects the stomach: it would be wrong to withdraw the
stimulus, for the excitability is in such an exhausted state as to
produce no action, or very imperfect and diseased, from the effect of
the common exciting powers; we must, therefore, here apply a stimulus
greater than natural, to bring on a vigorous and healthy action, and
this stimulus we should gradually diminish, in order to allow the
excitability to accumulate, by which the healthy state will be
gradually restored.

This method was very judiciously recommended, by a very eminent
physician, in the case of a Highland chieftain, who had brought on
dreadful symptoms of indigestion by the use of whisky, of which he
drank a large silver cup full five or six times in the day. The
doctor did not merely say, diminish the quantity of spirits
gradually, for that simple advice would not have been followed; but
he advised him to drink the cup the same number of times full, but
each morning to melt into it as much wax as would receive the
impression of the family seal. This direction, which had something
magical in it in the mind of the chieftain, was punctually obeyed. In
a few months the cup was filled with wax, and would hold no more
spirits; but it had thus been gradually diminished, and the patient
was cured.

This reminds me of a number of cases, which had been brought on by
drinking porter, and other stimulant liquors, without knowing the
taste of water. In many of these cases if a moderate quantity of
water were drank every day they would be cured; but you would find
few who would follow such plain and simple directions. How then must
a physician proceed? Why, as is generally done by the most judicious:
they direct their patients to Bath or Buxton, and there advise them
to swallow a certain quantity of water every day, which they do most
scrupulously, and, of course, return home cured.

As causes of asthenic disease, we must not omit the undue exercise of
the intellectual functions. Thinking is a powerful exciting cause,
and produces effects similar to those of intoxication. None of the
exciting powers have more influence upon our activity, than the
exercise of the intellectual powers, as well as passion and emotion.
Homer, the great observer and copyist of nature, observes of the
hero, whom he gives for a pattern of eloquence, that, upon his first
address, before he had got into his train of thought, he was awkward
in every motion, and in his whole attitude; he looked down upon the
ground, and his hands hung straight along his sides, as if they had
lost the power of motion; and his whole appearance was a picture of
torpidity. But when he had once fairly entered upon his subject, his
eyes were all on fire, his limbs all motion, grace, and energy.

Hence, as the exercise of the intellectual functions evidently
stimulates, an excess of thinking must bring on indirect debility, by
exhausting the excitability. But though we do meet with instances of
indirect debility arising from this source, it must be confessed that
they much oftener arise from the use of very different stimulants.

As excessive exercise of the intellectual powers will bring on
indirect debility, so the deficient, weak, or vacant state of mind,
which is unable to carry on a train of thinking, will produce direct
debility. Indeed this debility often occurs to those whose minds have
been all their life actively engaged in business, but who have at
last retired to enjoy themselves, without having a cultivated mind
fit for retirement. They become languid, inert, and low spirited, for
want of the stimulus of mental exertion; and in many cases cannot be
completely restored to health, till they are again engaged in their
usual occupations.

Violent passions of the mind, such as great anger, keen grief, or
immoderate joy, often go to such an extent as to exhaust the
excitability, and bring on diseases of indirect debility. Hence both
epilepsy and apoplexy have been the consequences of violent passion.

On the contrary, when there is a deficiency of exciting passion, as
in melancholy, fear, despair, &c. which are only lower degrees or
diminutions of joy, assurance, and hope, in the same way that cold is
a diminution of heat, this produces a state of direct debility. The
immediate consequences observable are, loss of appetite, loathing of
food, sickness of the stomach, vomiting, pain of the stomach, colic,
and even low fevers.

The effect of impure air, or air containing too small a proportion of
oxygen, is likewise a very powerful cause of debility.

In short, when any or several of these causes, which have been
mentioned, act upon the body, asthenic diseases are the consequence.

Asthenic diseases, as has frequently been hinted, may be divided into
two classes, those of direct debility, and those of indirect
debility.

Among the diseases of direct debility may be enumerated dyspepsia,
hypochondriasis, hysteric complaints, epilepsy, bleeding of the nose,
spitting and other effusions of blood, cholera morbus, chorea,
rickets, scrofula, scurvy, diabetes, dropsy, worms, diarrhoea,
asthma, cramp, intermittent fevers.

Among those of indirect debility, or which are produced by over
stimulating, which exhausts the excitability, may be enumerated,
gout, apoplexy, palsy, jaundice, and chronic inflammation of the
liver, violent indigestion, confluent small pox, typhous fever, and
probably the plague, dysentery, putrid sore throat, tetanus.

Diseases, therefore, according to this system, may be divided into
two classes. First, general diseases, which commence with an
affection of the whole system, and which must be accounted general,
though some part may be more affected than the rest. Secondly, local
diseases, which originate in a part, and which are to be regarded as
local, though they may sometimes in their progress affect the whole
system, like universal diseases: still however they are to be cured
by remedies, applied not to the whole system, but to the part
affected only.

A pleurisy or peripneumony, for instance, is a general disease,
though the chief seat of the symptoms seems confined to a portion of
the thorax: but the affection of this part, though it may be somewhat
greater than that of any other equal part, is vastly less than the
affection or diathesis diffused over the whole body. The exciting or
hurtful causes which produce these diseases, by no means exert their
whole power upon a small portion of the superficial vessels of the
lungs, and leave the rest untouched; on the contrary, they affect
exery part of the system, and the whole body partakes of the morbid
change. Indeed the general or universal affection; viz. a sense of
heaviness and fullness, uneasy sleep, and other symptoms of increased
excitement, are commonly perceived some time before the pain of the
thorax becomes sensible. The remedies which remove the disease; viz.
venesection, abstaining from animal food, and every mode of
debilitating, do not exert their whole efficacy on an inflamed
portion of the lungs; for by removing the affection of the lungs we
should go but a little way towards removing the disease.

Among local diseases we may enumerate wounds, or solutions of the
continuity of the part, bruises, fractures, inflammations from local
irritation, &c. Hence it is evident that the treatment of general
diseases is the province of the physician; and of local ones of the
surgeon. But there are some general diseases which are apt to
degenerate into local, and therefore require the attention both of
the physician and the surgeon. Among these we may reckon suppuration
and gangrene, sphacelus, and some others.

The first class, or general diseases, may be divided into two orders;
sthenic, and asthenic. The asthenic order may be subdivided into two
genera; viz. diseases of direct debility, and diseases of indirect
debility; for debility, according to the system I am explaining, is
that relaxed or atonic state of the system which accompanies a
deficient action of the stimulant or exciting powers; and this
deficient action may arise immediately from the partial or too
sparing application of the exciting powers; the excitability or
capacity of the system to receive their actions, being unaffected or
sufficiently abundant; or it may arise from the excitability being
exhausted, by the violent or long continued action of the exciting
powers.

This arrangement of diseases, which naturally follows from the
fundamental principles of the doctrine, and which is guided by the
state and degree of excitement, is widely different from that of
former nosologists, who have arranged or classed them according to
symptoms, which have already been shown to be fallacious; and which
method of arrangement brings together diseases the most opposite in
their nature, and separates those most nearly allied. This is evident
in every part of the nosology of Sauvages and Cullen. In the genus
cynanche of the latter, are placed the common sthenic or inflammatory
sore throat, or cynanche tonsillaris, and the putrid or gangrenous
sore throat, the cynanche maligna: the former is a sthenic disease;
the latter one of the greatest debility; yet they have the same
generic name.

The mode of classing diseases which I have adopted, after the example
of Dr. Brown, is the consequence of first taking a view of the nature
of life, and the manner in which it is supported; and from thence
observing how those variations from the healthy state, called
diseases, are produced; and this is certainly the proper plan; for,
as every effect will be produced with more accuracy, whilst its cause
is acting in a proper degree, it is certainly right to begin by
drawing our general propositions from the healthy state; by which
means we avoid being misled by those false appearances which the
living system puts on, during a morbid state; and though the contrary
has been the general practice of nosologists and pathologists, I must
confess it appears to me like beginning where the end should be; for
to lay down rules for restoring health, and begin by observing the
phenomena of disease, is like building a house, and beginning with
the roof.

In the last lecture I pointed out the general method of curing
sthenic diseases; I shall now proceed to the cure of asthenic, and
shall begin with those depending on direct debility, as in these
diseases the excitability is morbidly accumulated, and consequently
more liable to be overpowered by the action of a stimulus, we must,
therefore, at first, apply very gentle stimulants, increasing them by
degrees, till the excitement be arrived at the healthy state.

In cases of indirect debility, the excitability is so far exhausted
as not to be sufficiently acted on by the ordinary powers which
support life; we must therefore employ, at first, pretty strong
stimulants, to keep up such a degree of action as is necessary to
preserve life; we should, however, be careful not to overdo it; for
our intention here, in giving these stimuli, is only to keep up life,
while the cure must depend upon the accumulation of the excitability.
That this may take place, therefore, we must gradually lessen the
quantity of stimulus, till the excitability become capable of being
sufficiently acted on by the exciting powers, when the cure will be
affected.

There is, however, an important point, with respect to the cure of
diseases of exhausted excitability, which could not be known to Dr.
Brown; and this depends on the fact which was formerly pointed out;
viz. that the degree of excitability was in proportion to the
oxydation of the system. On this account I have given the oxygenated
muriate of potash in typhus, which is a disease of diminished
excitability, in more than one hundred cases, without the loss of
one, a success which has attended no other mode of practice in this
disease, if we except, perhaps, the affusion of cold water, as
described by Dr. Currie, the effects of which are wonderful, but
which can only be applied at the commencement of the disease. In all
diseases of indirect debility, therefore, it is proper to attempt the
introduction of oxygen into the system, by the oxygenated muriate of
potash, acid fruits, nitre, &c. I do not think that the inhaling of
oxygen gas for a few minutes in the day can do much good; but free
ventilation of apartments, and gentle exercise in the open air, are
highly useful.

In either case of debility, we should by no means rely on the action
of medicines alone; for though there are a variety of stimulants
which will produce excitement, yet this is only temporary, we must
therefore endeavour, by nutritious substances, to fill the vessels
with blood, and employ all the natural exciting powers in due
proportion as soon as possible.

But in the cure of either sthenic or asthenic diseases we shall
seldom succeed by the use of one remedy only: for since no stimulus
exerts its effects equally on all parts of the body, but always acts
more powerfully on some part than on others, we cannot by the use of
one remedy alone obtain an equal increase or diminution of
excitement.

There are few diseases however in which the excitement is equally
increased or diminished over the body; some part being generally more
affected than the rest; and this inequality produces the various
phenomena or forms of disease; indeed no disease but increase or
diminution of strength would take place, on the supposition that an
equal increase or diminution of excitement all over the body, were
produced by the hurtful powers causing the disease.

From what has been said, it necessarily follows, that every stimulus
will not be equally efficacious in curing every form of disease;
which is sufficiently confirmed by experience. Hence there may be
some ground for the appellation of specifics, as some medicines may
act more powerfully upon the part which is the principal seat of the
disease, than others do.

In the cure of diseases we ought always to attend to two things most
carefully: first, to employ the proper kinds of powers, and then not
to overdo them, so as to convert either diathesis into the other; and
by passing over the line of health, instead of the intended cure, to
substitute one disease instead of another, and thereby bring life
itself into danger.

LECTURE XIII.
ON THE GOUT.

There is no disease, with which the human race is afflicted, whose
nature has been more mistaken than that which is to form the subject
of our present consideration. It has been regarded by most
practitioners as a salutary effort of the body to expel some hurtful
cause, and restore health; and therefore has been looked upon as
desirable to the patient. To attempt to cure it, therefore, would
have been wrong, had it been curable; but it has likewise been looked
upon as beyond the reach of medicine, or perfectly incurable; and, on
both these accounts, after having tried a variety of drugs, without
any good effect, the physicians have at last abandoned their
patients, to the care of patience and flannel, which, if the
constitution be not very much shattered, will often see them through
the disease.

But that it is a salutary disease I deny; and I affirm, that it
restores health in no other way, than the indigestion of a habitual
dram drinker would be relieved by a disease in the throat, which
would, for a time, prevent his swallowing any more liquor; the
consequence would be, that his digestive powers would recover their
tone, and he would, after a few weeks, feel himself better.

In the same way the pain and fever, which attend gout, and at the
same time the inability to move, with the weakened stomach, and bad
appetite, prevent the continuance of the mode of life which brought
on the disease; and thus, a truce being obtained, the exhausted
excitability of the body is allowed to accumulate, and the
constitution, of course, feels itself renovated.

Were the disease to be viewed in this light, it is probable that many
patients might in future desist from their former mode of life, which
brought on the disease; and we might venture to promise them, if they
did, that they would have no return of the complaint. But the
misfortune is, they think the gout has restored their constitution,
and that therefore they may return to their old mode of living with
impunity; in consequence of which, after a few months more, the
excitability is again exhausted; symptoms of indigestion come on, and
the stimulant mode of living is increased, with a view to bring on
the disease, which is to cure these symptoms. In this way, each time,
a greater and greater degree of indirect debility is induced, and at
last the system becomes so enfeebled, that the asthenic inflammation
is not confined to the extremities, but attacks the head, the
stomach, the lungs, and often puts a period to the existence of the
patient, which has for some time been miserable.

Besides, the idea, that the gout is incurable, is a false, and a very
dangerous doctrine; this is very far from being the case, and I am
firmly persuaded, not only from the nature of the disease, but from
experience, that it may always be cured, if taken in time, and proper
directions be followed. If, by the cure of gout be meant the
administration of some pill, some powder, or some potion, which shall
drive away the complaint, I firmly believe, that it never was, nor
ever will be cured. Indeed, it is astonishing that such an idea
should have ever entered the mind of any person, who has any
knowledge of nature, or particularly of the human frame; for, if the
gout is a disease of indirect debility, and the effect of
intemperance, as will be shown by and by, then a medicine to cure it
must be something to enable a man to bear the daily effects of
intemperance, during his future life, unhurt by the gout, or any
other disease; that is, it must be something given now, that will
take away the effects of a future cause; as well might a medicine be
given to prevent a man breaking his leg, or his arm, seven years
hence.

But no rational physician, or surgeon, would give a medicine with
this view, in such a case as I have supposed; on the contrary, he
would caution his patient against mounting precipices, scaling walls,
or bringing himself again into a situation, such as produced the
accident; and if he took his advice, he would, in all probability,
escape a broken limb in future.

In the same way a rational physician would advise a person recovering
from gout, to abstain totally and entirely from the course of life
which brought it on; and this being complied with, we might venture
to predict, with as much certainty in the one case as in the other,
that he would in future escape it.

What I have frequently endeavoured to inculcate in the course of
these lectures, always appears to me of the utmost importance: I
mean, the general diffusion of physiological knowledge, or a
knowledge of the human frame; this knowledge ought to form a part of
general education, and is, in my opinion, as necessary for a person
to learn as writing, or accounts, or any other branch of education;
for if it is necessary that a young man should learn these, that he
may be able to take care of his affairs, it surely can be no less
necessary, that he should learn to take care of his health; for to
enjoy good health, as a celebrated practical philosopher observes, is
better than to command the world.

If knowledge of this kind were generally diffused, people would cease
to imagine that the human constitution was so badly contrived, that a
state of general health could be overset by every trifle; for
instance, by a little cold; or that the recovery of it lay concealed
in a few drops, or a pill. Did they better understand the nature of
chronic diseases, and the causes which produce them, they could not
be so unreasonable as to think, that they might live as they chose
with impunity; or did they know any thing of medicine, they would
soon be convinced, that though fits of pain have been relieved, and
sickness cured, for a time, the reestablishment of health depends on
very different powers and principles. Those who are acquainted with
the nature and functions of the living body, well know, that health
is not to be established by drugs; but that if it can be restored, it
must be by nicely adjusting the action of the exciting powers to the
state of the constitution, and the excitability; and thus gently and
gradually calling forth the powers of the body to act for themselves.
And though I believe that most general diseases will admit of a cure,
yet I am confident, that no invalid was ever made a healthy man by
the mere power of drugs. If this is a truth, should it not be
universally known? If it were, there would undoubtedly be an end of
quackery, for all quack medicines, from the balm of Gilead, to the
botanical syrup, are supposed to cure diseases, or at least asserted
to do so, in this mysterious manner.

Dr. Cullen, in his Nosology, gives us the following definition of the
gout.

"Morbus haereditarius, oriens sine causa externa evidente; sed
praeeunte plerumque ventriculi affectione insolita; pyrexia; dolor ad
articulum, et plerumque pedis pollici, certe pedum et manuum
juncturis, potissimum infestus; per intervalla revertens, et saepe
cum ventriculi et internarum partium affectionibus alternans."

Now, though this definition comprises a tolerably good general
character of the disease, it contains some notions, depending on the
prejudice of hypothesis, which, on a careful examination, ought not,
I think, to be admitted.

In the first place, I would deny, that the gout, considered as a
diseased state of the system, is hereditary. This may perhaps excite
some degree of surprise; and, "I had it from my father," is in the
mouth of a great majority of gouty patients.

If the diseased state of the system, which occurs in gout, were
hereditary, it would necessarily be transmitted from father to son;
and no man, whose father had it, could possibly be free from it.
There are, however, many instances to the contrary. Our parents
undoubtedly give us constitutions similar to their own, and there is
no doubt, that if we live in the same manner in which they did, we
shall have the same diseases. This, however, by no means proves the
disease to be hereditary.

We shall hereafter see, that the gout is a disease of indirect
debility, brought on by a long continued use of high seasoned food
and fermented liquors. There is no doubt that particular
constitutions are more liable to be affected by this mode of living
than others; and if my father's constitution be such, I, who probably
resemble him in constitution, shall in all probability be like him,
subject to the gout, provided I live in the same way; this however by
no means proves the disease to be hereditary. The sons of the rich,
indeed, who succeed to their fathers estate, generally succeed also
to his gout, while those who are excluded from the former, are also
exempted from the latter, and for very obvious reasons, unless they
acquire it by their own merit.

So that though the son of a gouty parent may have a constitution
predisposing to the gout; that is, more liable to be affected by
causes, which produce this disease, still, if he regulate the stimuli
to the state of his excitability, he will remain exempt from it.

This distinction is of much greater importance than is generally
imagined; for if a person firmly believes that the gout, as a
disease, is hereditary, what will be his conduct? My father had the
gout, says he, therefore I must have it; well, what cannot be
avoided, must be endured; let me then enjoy a short life, but a merry
one: he therefore abandons himself to a luxurious mode of life, and,
if the gout be the consequence, which most probably it will, he
accuses his stars, and his ancestors, instead of his own misconduct.

On the contrary, if a person be convinced that he has received from
his ancestors a constitution liable to be overpowered by the use of
high seasoned food, and fermented liquors, and excited into gouty
action, what will be his conduct? Surely, if he reason at all, it
must be in this way: my father was dreadfully afflicted with the
gout; I have frequently witnessed his sufferings with the deepest
concern. But is not my constitution, which resembles his, liable to
be affected in the same manner, by similar causes? To avoid his
sufferings, therefore, I must be very temperate; more so than those
who have not the hereditary propensity; for the exciting powers,
which would only keep them in health, would, if applied to me,
infallibly bring on the gout. In consequence of this reasoning, he
adopts a temperate mode of living, and avoids the disease.

From this you must be convinced, that it is not a matter of small
moment to determine, whether the gout is hereditary, and consequently
unavoidable, or not. The next part of Dr. Cullen's definition is
"oriens sine causa evidente". This too, I can have little hesitation
to pronounce erroneous. The cause of gout, namely, the use of highly
seasoned food, and the use of fermented liquors, with, in general, a
luxurious, and indolent mode of living, are quite evident enough in
most gouty cases, and are amply sufficient to produce the disease.

There is another part of the definition, likewise, to which I would
object, as it gives a false idea of the nature of the disease, and
therefore causes the preventative plan to be pursued with less
confidence. I mean that part where he says "per intervalla
revertens."

That the gout, when once cured, is apt to return, if the mode of life
which brought it on be not abandoned, no one will deny; nay, the fits
will increase in violence, because the constitution gets more and
more debilitated. This, however, is not peculiar to the gout, but
common to most diseases.

In describing a broken leg, it would surely be wrong to say, that it
is a disease which returns at intervals, after being cured; yet, it
will return as infallibly as the gout, if a person take the same kind
of leap, or expose himself to the same accidents as those which
brought it on. Let those, therefore, who wish to avoid a return of
the gout, totally change their mode of living: otherwise, if the
attacks return, let them blame themselves, and not the nature of the
complaint.

These observations were thought necessary, with a view to do away
some prejudices, which very much retarded our inquiries into the
nature and cure of this disease. I shall now proceed to give an
account of the symptoms by which it is usually attended.

The gout generally attacks the male sex; but it sometimes, though
more rarely, attacks also the female, particularly those of robust
and full habits. It does not generally make its appearance, till the
period of greatest strength and vigour is past; for instance, about
the fortieth year; but, in some cases, where the exciting causes have
been powerfully applied, or where the hereditary predisposition is
very strong, it attacks much earlier; such cases are, however,
comparatively rare, and can, in general, be easily accounted for.

This disease is seldom known to attack persons employed in constant
bodily labour, and who live temperately; and is totally unknown to
those who use no wine or other fermented liquors.

If then a person of a full strong habit have for several years
accustomed himself to full diet of animal food, and a regular use of
wine, and malt liquor, though he may for a long time find that he can
perform all the functions with vigour, his strength will at last
fail: the mind and body become affected with a degree of torpor and
languor for which he cannot account, and the functions of the stomach
become more or less disturbed. The appetite becomes diminished, and
flatulency, and other symptoms of indigestion are felt. These
symptoms take place for several days, and sometimes for several weeks
before the fit comes on; but often, on the day immediately preceding
it, the appetite becomes greater than usual.

In this state, if the person have fatigued himself by violent
exercise, or if he have exposed the extremities to cold, or if his
mind have been particularly affected by any anxiety, or distressing
event; or in short, if any directly debilitating cause have been
applied, the fit will often follow. It sometimes comes on in the
evening, but more commonly, about two or three o'clock in the
morning; the pain is felt in one foot, most commonly in the ball or
first joint of the great toe; but sometimes in the instep, or other
parts of the foot. With the coming on of this pain there is generally
more or less of a cold shivering, which as the pain increases,
gradually ceases, and is succeeded by heat, which often continues as
long as the pain; from the first attack the pain becomes by degrees
more violent, and continues in this state, with great restlessness of
the whole body, till next midnight, after which it gradually remits,
and after the disease has continued for twenty four hours from the
commencement of the first attack, it often ceases, and with the
coming on of a gentle perspiration allows the patient to fall asleep.
The patient on coming out of this sleep in the morning finds the part
affected with some degree of redness and swelling, which, after
having continued for some days, gradually abate.

Still however, after a fit has come on in this manner, although the
violence of the pain after twenty four hours, by the excitement that
it produces, cures itself, and is considerably abated, the patient is
seldom entirely relieved from it. For several days he has every
evening a return of considerable pain and fever, which continue with
more or less violence till morning. This return is owing to the
exhaustion of the excitability by the stimuli of the day, and its
remission is caused by the accumulation of the excitability, by
sleep.

After having continued in this manner for several days, the disease
often goes off, and generally leaves the person in much better
health, and enjoying greater alacrity in the functions of both body
and mind, than he had for some time experienced. This is owing to the
general excitement produced by the pain, which removes the great
torpor and debility which preceded the fit; and from the inability to
take exercise or food, the excitability accumulates again. This is
the true explanation: it does not depend on any morbid matter, which
the gout hunts from its lurking places, drives to a joint, and thence
out of the body, as has been imagined by many.

At first the attacks of the disease are confined to one foot only:
afterwards both feet become affected, though seldom at the same time;
but when the inflammation appears in one, it generally disappears in
the other, and as the disease continues to recur, it not only affects
both feet at once, but is felt in the other joints, especially those
in the upper and lower extremities, so that there is scarcely a joint
in the body that is not on one occasion or other affected. After
frequent attacks, the pains are commonly less violent than they were
at first, the joints lose their strength and flexibility, and often
become so stiff as to be deprived of all motion.

Concretions of a chalky or calcarious nature are likewise formed upon
the outside of the joints. This arises from an inability of the
capillary vessels, which ought to secrete the calcarious matter, and
deposite it in the bones, to perform their office, from debility:
hence by sympathy other vessels ta ke up the matter and deposite it
in the wrong place. These concretions, though at first fluid, become
at last dry, and firm: they effervesce with acids, and are totally,
or in a great measure, soluble in them.

After this short description of the gout, when it occurs in its
regular form, as it is called, I shall now proceed to inquire how the
exciting causes produce this disease, and what is the state of the
body under which it occurs.

The gout seldom occurs but in those who have for several years lived
upon a full diet of animal food, often highly seasoned, and at the
same time been in the habit of taking daily, or at least very
constantly, a greater or less quantity of fermented liquors, either
in the form of wine, or malt liquor, or both. The affection of the
limb has all the appearance of an active inflammation: the part
becomes swelled, hot, red, and intolerably painful. It is this
circumstance which has misled practitioners, who have supposed it a
case of sthenic, or active inflammation: not only the appearance, but
the causes which produced it, induced them to think so; hence they
were naturally led to employ the debilitating plan: a little time and
observation would, however, be sufficient to convince them of its
inefficacy. They would find that the application of leeches to the
part, and of the lancet to the arm, instead of subduing the
inflammation, would increase it: or if it did not, that the pain
often attacked some internal part, which was ascribed to a
translation of the morbific matter from one part to another, but
which is merely owing to an increased debility: a little attentive
observation would convince practitioners, however mysterious it might
seem to them, that this violent inflammation was not to be cured by
debilitating: on the contrary, they would see cases, in which the
patient, though contrarily to the strict orders of his physicians,
could not forego his old habits; but would take his wine as usual, or
in greater quantity, after a few days abstinence; and this abstinence
having in some degree accumulated the excitability, he would find
himself much relieved by wine, and would exultingly tell them, that
they were mistaken. Circumstances of this kind seem to have staggered
their faith a little, but still the idea of active inflammation which
they believed was visible, and almost palpable, dwelt so upon their
minds, that they were but half convinced. The favourite idea of
increased action of the vessels of the part had so interwoven itself
with every other, that we find it never lost sight of, in the
indications of cure. Hence, though bleeding is not now generally
practised with the lancet, yet leeches are often applied; but the
most usual plan is to consign the patient to patience and flannel;
strictly forbidding wine, or fermented liquors. As an exception to
this general mode, it is however observed, by some practitioners,
that when the stomach is weak, and when the patient has been much
accustomed to the use of strong liquors, a little animal food, and
even wine, may be allowable, and even necessary.

Thus has an erroneous view of the disease been the cause of an inert
practice, which wavers between the suggestions of a favourite
hypothesis, and the conviction of facts.

On inquiry, however, we shall find none of the increased vigour in
the system, which has been suspected, nor increased action in the
part more particularly affected; on the contrary, the whole body is
in a state of indirect debility, or exhausted excitability, and the
part more particularly affected, in a state of asthenic inflammation.

If the gout were of a sthenic or inflammatory nature, might we not
ask, why the causes which produce it, do not produce it in the
meridian of life, when they produce their greatest effect, and when
real sthenic diseases are most apt to occur? or, why the symptoms of
the inflammation, like all other real sthenic inflammations, are not
relieved by the debilitating plan? The contrary, however, points out
to us clearly the nature of the disease: the gout is not a sthenic
disease, or a disease of strength: it does not depend upon increased
vigour of the constitution, and plethora, but is manifestly asthenic,
like all the rest of the asthenic diseases. The mode of living is
such as brings on indirect debility, or exhaustion of the
excitability, such as the use of rich and highly seasoned food, and a
daily use of fermented liquors. These at first certainly produce
vigour, or strength, and will be the cause of sthenic diseases; but
they are generally taken in such a manner, that, though they produce
a degree of excitement above the point of health, still they only
approach the line of sthenic disease, without in general falling into
it. They continue, however, to exhaust the excitability, and by the
time that the vigour of the body begins naturally to decline, the
system of a person who has lived in this manner is unusually torpid;
all the blood vessels, which have hitherto been distended with rich
blood, begin to lose their tone, from their excitability having been
exhausted by the use of these powerful stimulants; but this torpor is
particularly and first experienced in those parts which have been
more immediately subject to the action of the exciting causes; viz.
the stomach and bowels: symptoms of indigestion occur, and the
excitability of these organs having been almost entirely exhausted by
the violent action of the stimulants applied, cannot now be roused to
any healthy action; the food is not properly digested, but runs into
a kind of fermentation, which causes an extrication of gas: this
distends the stomach and bowels, and produces pains, uneasy
eructations, and all the distressing symptoms of indigestion. Nor is
this in the least surprising, when we consider that many people who
have brought on complaints of this kind, have been in the habit of
eating heartily of rich and highly seasoned animal food, and of
drinking from a pint to a bottle of wine, and perhaps a quantity of
malt liquor, almost every day of their lives for years. This mode is
sufficient to wear out the powers of the stomach, were it three times
as capacious as it is, and of the constitution, were it ten times as
strong.

When a torpor, or state of exhausted excitability, of the whole
system, has been induced in this manner, and symptoms of indigestion
produced, any directly debilitating cause applied to the extremities,
adding to the indirect debility, causes a total torpor, or inactivity
of the minute vessels of the part, and thus totally destroys the
balance between the propelling and resisting force; hence the vessels
will be morbidly distended with blood, a swelling and redness will
take place, and an asthenic inflammation, produced in the way which I
fully pointed out in the last lecture, will be established. Hence the
pain, and other symptoms, which accompany a fit of the gout. Hence
likewise we see, why debilitating powers applied to the part will not
reduce the inflammation; and why a warmth, which aggravates every
really sthenic inflammatory affection, is so comfortable in this.

Almost any debilitating cause, when the system has been brought by
intemperance to the torpid state, which I have described, will bring
on a fit of the gout, but nothing more certainly than cold or
moisture: hence if a person have his feet chilled or wet, he will be
almost certain to have an attack.

Hence we see that the asthenic inflammation is not the disease, but
merely a symptom of it; and like other symptoms, fallacious in its
appearance; the disease is a state of indirect debility, to which our
attention ought to be directed.

When this inflammation is violent, and accompanied with great pain,
after several hours continuance, it excites the action of the minute
vessels, enables them to propel the blood, by which they are morbidly
distended, and restores the balance between the resisting and the
propelling force; and thus the inflammatory appearances will for a
time subside, but the torpor of the whole system remaining, and the
debility of the vessels returning, when their excitement, which was
the consequence of their action, has ceased, another asthenic
inflammation will take place, which will again cure itself as before;
so that during a paroxysm, several remissions will take place, as was
mentioned in the description of the disease. As, during the paroxysm,
the pain causes a considerable degree of excitement over the whole
system, the action of the stomach and other parts is roused by it;
during the fit likewise, little nutriment is taken, so that by the
action of the stomach and bowels, they get rid of their load; rest
likewise assists to accumulate the excitability, so that from all
these causes together, the body becomes restored to a state of
vigour, which, compared with its former torpidity, makes the patient
imagine that this friendly disease has restored him to a state of
unusual health, and even renovated the powers of his constitution.
Under this mistaken idea, he does not, when the fit leaves him,
abandon the mode of life, which brought on the disease; highly
seasoned food, and the usual quantity of wine, are again resorted to:
after a time the torpor of the system, and symptoms of indigestion
return, and he again hopes that his friend the gout will come and
cure him.

By a continuance of this plan, the inflammation again appears; but
the system having become more torpid, the inflammatory action is by
no means so great as it was before: if it has power to restore the
equilibrium between the resistance and propelling force, and thus
cure itself, this effect is entirely confined to the inflamed part.
The other foot labouring under similar torpor, or debility, now feels
the effects of the propelling force, and an inflammation takes place
in it, which having cured itself in the same manner, and the torpor
of the foot first affected being returned, or even greater than it
was before, on account of the previous excitement; the inflammation
again attacks this foot, and thus the gout is supposed to emigrate
from one limb to another. The gout, as a disease of general debility,
however, remains the same; and it is only these symptoms, which form
but a small part of the disease, that vary according to
circumstances.

If, during an asthenic inflammation of the lower, or upper
extremities, the torpor and debility of the whole system increase,
then the force of the circulation, or propelling force, being
diminished, the symptoms of inflammation will suddenly disappear; but
as great debility now prevails, the stomach will be apt to be
affected with cramps or convulsions, or an asthenic inflammation of
some internal part will take place: for, though the propelling force
is not sufficient to overdistend the debilitated vessels of the
extremities, it will distend those of the internal parts nearer the
heart, which are now debilitated.

In this case, it has been generally, but absurdly imagined, that the
gout is translated, or recedes from the extremities to some internal
part: the term of retrocedent gout has therefore been applied to
occurrences of this nature. From the explanation which has been
given, it is evident, that this term is improper. The general
debility being increased, the propelling force becomes unable to
produce an inflammation of the extremities, and this is the reason
why it disappears. The disease, however, is not at all altered in its
nature by this variation of symptoms. It is still the same, by
whatever name it may be called.

It sometimes happens, that after full living, the stomach becomes
particularly affected, and the patient is troubled with flatulency,
indigestion, loss of appetite, eructations, nausea, and vomiting,
with great dejection of spirits, pain and giddiness of the head,
disturbed recollection, or muddiness of intellect, as it is termed,
with all the symptoms, which usually precede a regular fit of the
gout, yet no inflammatory affection of the joints is produced. This
state has been absurdly enough called the atonic gout, as if there
were a gout accompanied with vigour and sthenic diathesis: but the
absence of inflammation in the extremities may depend on two causes.
First, the powers producing the disease, may have debilitated the
stomach and first passages, while the vessels of the extremities are
not particularly debilitated, and the resisting force is able to
counterbalance the propelling force: in this case, no morbid degree
of distention or inflammation of the extreme vessels can take place.
Secondly, the general debility may be such, and the power of the
circulation so much diminished, that, though the extreme vessels may
be debilitated, no inflammation, or preternatural distention will
take place.

Hence, we see, that this is still the same disease; but that
physicians have erred in their explanation of the symptoms, by
regarding that as the principal part of the disease, which is only a
symptom.

We have seen then, that by the theory which has been unfolded, all
the symptoms of this hitherto mysterious disease are plainly and
naturally explained. We shall next see if the only method of cure
which experience warrants, cannot be explained upon the same
principles.

If, on entering this part of the subject, any one should expect that
I should furnish him with a receipt, consisting of certain drugs,
which swallowed, will cause this terrible disease to disappear, and
health to take its place, he would be very much mistaken; for, can
any person in his senses suppose that a disease, which he has been
almost his whole life in contracting, and an exhausted state of the
excitability, which has been gradually brought on by years of
intemperance, can be dispersed by a pill, a powder, or a julep? Or,
if the symptoms could be relieved by medicine, which they often may,
can he suppose, that they will not return, if the same mode of
living, which first brought them on, be continued?

I shall, however, proceed to give some directions, which if rigidly
persevered in, will not only afford relief in the fit, but will
prevent its return with such violence, and at last totally eradicate
it, provided the constitution be not completely exhausted, and almost
every joint stiffened with calcarious concretions.

The inflammation of the extremities may at any time be relieved by
means of electricity, or by stimulant embrocations applied to the
part, and this without any danger whatever of throwing the complaint
on some more vital part, as has generally been imagined. If I were to
apply any debilitating means to the part, I should then probably
relieve the pain; but, by debilitating the whole system, should cause
an attack of the stomach, or some other internal part, as has been
already explained; but by a stimulant application to the inflamed
part I run no such risk. The inflammation is of the asthenic kind,
depending upon a debility of the small vessels, whereby they do not
afford sufficient resistance to the propelling force, and therefore
become morbidly distended, or inflamed, as it is termed, though this
term is certainly improper, even in a metaphorical view: but a
stimulant application to the part excites the debilitated vessels to
action; their contraction diminishes the morbid quantity of blood;
and the balance between the propelling and resisting forces being
restored, the inflammation of course ceases. This is not a mere
deduction, a priori, from the theory of inflammation, which I have
delivered; it is the result of repeated experience. I have seen
several very violent gouty inflammations very speedily removed by
electricity. Small sparks should be drawn from the part affected, at
first through flannel, and increased as the patient can bear them:
sparks alone are necessary; recourse need never be had to shocks. But
though we thus remove a very painful part of the disease, yet still a
formidable debility remains, and unless this be removed, the
inflammation will be apt to return. In endeavouring to remove this
general debility, we must recollect, that it is of the indirect kind,
or depends upon an exhausted state of the excitability; our great
object therefore, is to allow the excitability to accumulate. But
this accumulation depends as well upon the proper action of the
different functions, as upon the withdrawing of stimulants: we ought
therefore to guard carefully against costiveness, by which the proper
action of the stomach and bowels is very much injured: but we must
use warm laxatives. An infusion of senna and rhubarb in proof
spirits, made still stronger by aromatics, has always seemed to me to
answer the purpose best, and this should be taken of a temperature
rather above blood warm; for instance, about 100 degrees. This is
particularly necessary, when the gout attacks the stomach, and I have
several times seen a severe attack of it removed in half an hour, by
a tincture of this kind. Indeed, the most violent attacks of the
stomach may be relieved; and are only to be relieved by spirits,
ether, and opium.

It is on this organ, that the hurtful powers have produced their
greatest effect; for to it they are immediately applied. It is by no
means surprising, that the constant application of highly seasoned
foods, with fermented and spirituous liquors, should at last wear out
the vital principle of this organ. Indeed it is often so far
exhausted, that the most terrible cramps and convulsions take place,
which would soon end in its total extinction, unless it were roused
to somewhat like a proper action by the most powerful stimulants.
Still, however, their effect is but temporary.

With respect to a regular fit, after the inflammation of the
extremities has been subdued by the means I have mentioned, a
generous, but not full diet should be used. A person who has been for
a long time accustomed to wine, cannot easily be deprived of it at
once; but he should drink Madeira, and those wines, which neither
contain much carbonic acid, nor deposite much tartar. His food should
be of the plainest kind, and generally boiled, instead of roast. The
great thing is to keep the spirits and excitement rather under par,
but not to let the patient sink too low. In this way, the exhausted
excitability will gradually accumulate, and the healthy state be
reestablished. When this is once effected, the gout may be prevented
in future with the greatest certainty, if the patient will have
resolution. The whole secret consists in abstaining, in toto, from
alcohol, in every form, however disguised, or however diluted. He
must not take it, either in the form of liqueurs, cordials, wine, or
even small beer.

I believe there never was an instance of a person having the gout,
who totally abstained from every form of alcohol, however he might
live in other respects: and I doubt very much, if ever the gout
returned after a person had abstained from fermented or spirituous
liquors for two years.

Temperance in eating, and exercise, are, no doubt, powerful
auxiliaries, and tend very much to promote health; but still they
will not secure a person from a return of the gout, without this
precaution. There seems something in alcohol, which peculiarly brings
on this state of the constitution, and without it, it would seem that
gout could not be produced. Here then is an effectual method of
curing the gout, which will no more return, if this method be
strictly persevered in, than the smallpox will attack the
constitution after inoculation.

During the fit therefore, I would say, nearly in the words of Dr.
Darwin, Drink no malt liquor on any account. Let the beverage at
dinner consist of two glasses of Madeira, diluted with three half
pints of water; on no account whatever drink any more wine or
spirituous liquors in the course of the day. Eat meat constantly at
dinner, without any seasoning, but with any kind of tender
vegetables, that are found to agree. When the fit is removed, use the
warm bath twice a week, an hour before going to bed, at about 93
degrees, or 94 degrees of heat. Keep the body open by means of
lenitive electuary and rhubarb; for there is an objection to the
tincture I mentioned, as containing alcohol. Use constant, gentle
exercise; but never so violent as to bring on great fatigue. The
grand secret, however, in the cure, as has been already observed, but
which cannot be too often inculcated, is to abstain, in toto, from
every thing that contains alcohol.

In short, though in acute diseases medicines are highly useful, a
chronic disease can never be cured, and the healthy state
reestablished, by them alone. To effect a cure in such cases, we must
reform our mode of life, change our bad habits into good ones; and
then, if we have patience to wait the slow operations of nature, we
shall have no reason to regret our former luxuries.

LECTURE XIV.
NERVOUS COMPLAINTS, &c.

In this lecture I propose to take a view of some of those affections,
which have been commonly, but improperly known by the appellation of
nervous complaints, because it has been supposed by many that they
are owing to a deranged state of the nerves, which, however, is by no
means the case; for I hope to be able to make it appear, that these
symptoms arise from a general affection of the excitement of the
system. In short, by far the greater number of these complaints,
arise from such a state of the excitement as approaches
predisposition, or perhaps ranges between predisposition and disease,
but does not in general actually reach disease; or rather, it is a
state of the excitement, so far departing from the point of perfect
health, that the functions are not performed with that alacrity, or
vigour, which ought to take place; but labour under that disturbed
and uneasy action, which, though it cannot be called actual disease,
yet deviates considerably from the point of perfect health.

This is a new view of these diseases, but the more I have examined
it, the more I am convinced that it is just. Indeed, the name,
nervous, has generally been given to an assemblage of symptoms, which
the physicians did not understand; and when the patient relates a
history of symptoms, and expects that his physician shall inform him
of the name, and nature of his complaint, he generally receives for
answer, that his complaints are nervous, or bilious; terms which
convey no distinct ideas, but which serve to satisfy the patient, and
to conceal the ignorance of the physician, or spare him the labour of
thinking.

Indeed, the idea of nervous diseases, which I have already pointed
at, is not only new, but could only have arisen from such a view as
we have been taking of the states of excitement and excitability.
This view will not only lead us to form a more just idea of the
manner in which these diseases originate, but will point out a
distinction of them into two classes, of the utmost use in practice,
but which distinction has totally escaped the attention of
practitioners; for though these complaints have been generally
thought to arise from a lowness of the nervous energy, or some kind
of debility, or weakness of the nervous system, and, on this account,
the stimulant and cordial plan of cure has been recommended, I am
convinced, from observation, that nearly one half of them, if not
more, originate from a state of the excitement verging towards
sthenic disease; and in these cases, this general mode of treatment
must be highly improper.

It has been already shown, that when the common exciting powers which
support life, act in such a manner, that a middle degree of exciting
power, acts upon a middle degree of excitability, the most perfect
state of the system, or a state of perfect health, takes place: it
is, however, seldom in our power so to proportion the state of
excitement and excitability to each other. The action of the exciting
powers is continually varying in strength; and the excitability, from
a variety of stimulants, and other circumstances, which are not
entirely under our direction, is sometimes more, and sometimes less
abundant, than this middle degree. There is, however, a considerable
latitude, on each side the point of health, within which the
excitement may vary, and yet no disease, nor any disturbance of the
functions may take place: but this has its limits, beyond which if
the excitement be brought, on either side, it is evident that an
uneasy or unpleasant exercise of the functions must take place. There
is not, however, any precise line or boundary between this state, and
that in which the functions begin to be disturbed; on the contrary,
the law of continuity and gradation seems to extend throughout every
part of nature. This departure from the healthy state, and approach
to disease, in which what has been called the nervous state consists,
is gradual and scarcely perceptible; but is apt to be produced by any
circumstances, which lead the excitement beyond its proper limits.

Nervous complaints may therefore be divided, like all other diseases,
into two classes. First, those in which the excitement is increased,
or in which it verges to, or has actually reached, the point of
predisposition to sthenic disease; Secondly, those in which the
excitement is diminished, or in which it verges towards asthenic
disease. This last class, as has been done before, may be subdivided
into two orders. The first will comprise those diseases in which the
excitability is sufficiently abundant, or even accumulated, but where
the excitement is deficient from a want of energy in the exciting
powers. In the second, there has been no deficiency in the action of
the exciting powers; but on the contrary, probably for a considerable
time, some of the diffusible stimuli not natural to life have been
applied; in this case, the excitability has become exhausted, and a
proper degree of excitement cannot be produced by the action of the
common exciting powers.

No diseases show so clearly the fallacy of trusting to symptoms, as
those of the former class. I have met with innumerable cases of this
kind, in which, if you were to trust to the patients own description,
they laboured under considerable debility; and had it not been for
the particular attention I paid to my own case, I should not probably
have suspected that a directly opposite state of the system may
produce these symptoms.

From inheriting a good constitution, and being brought up in the
country in a hardy manner, I am so much predisposed to the sthenic
state, that I may consider the state of my excitement, as generally,
indeed almost always, above the point of health: and unless I live in
the most temperate, and even abstemious manner, the excitement is
extremely liable to overstep the bounds of predisposition, and fall
into sthenic disease. I have had several attacks of this kind of
disease; and indeed, I never remember to have laboured under any
disease of debility, or diminished excitement.

Health, according to the view we have taken of it, may be compared to
a musical string, tuned to a certain pitch, or note; and though
perhaps in the great bulk of mankind, either from the manner of
living, or from other circumstances, the excitement is a little
below, and requires to be screwed up to the healthy pitch, yet there
are others where it is apt to get constantly above, and where it
requires letting down to this pitch; my constitution is one of these:
but I have this consolation, that if I can for a few years ward off
the fatal effects of some acute sthenic diseases, this tendency to
sthenic diathesis will gradually wear off, and I may probably enjoy a
state of good health, at a time, when most constitutions of an
opposite cast begin to give way. Whenever I have for some time lived
rather fully, though by no means intemperately, after having for some
days, or perhaps some weeks experienced an unusually good flow of
spirits, and taken exercise with pleasure, I begin, first of all, to
have disturbed sleep, I find myself inclined to sleep in the morning,
as if I had not been refreshed by the night's sleep; my spirits
become low, and I am apt to look upon the gloomy side of every thing
I undertake or do. I feel a general sense of languor and debility,
and am ready, as I have heard many patients labouring under the same
state exclaim, to sink into the earth. From the slightest causes, I
am apt to apprehend the most serious evils, and my temper becomes
irritable, and scarcely to be pleased with any thing. If in this
state, I take exercise, I soon feel myself fatigued; a disagreeable
stupor comes on, without, however, the least degree of perspiration,
and I feel an inability to move.

At first, I used to imagine these to be symptoms of debility, or
diminished excitement, nor was it till after several ineffectual
trials to relieve them by the tonic, or stimulant plan, that I was
convinced of my mistake. This plan always caused an aggravation of
every symptom, and if I persevered in it, an inflammatory disease was
sure to be the consequence. Indeed, I might have suspected this, from
considering, that these symptoms had been brought on by full living,
and preceded by good spirits; but my mind had received such a
prejudice from the writings of medical men, who had uniformly
described these as a train of nervous symptoms, as they called them,
depending on a debilitated state of the nervous system, that I was
blind to conviction, till repeated disappointment from the
stimulating plan, convinced me I must be wrong. The only alternative
therefore, was a contrary plan, and the immediate relief I
experienced, was a proof that I had detected the real nature of the
complaint. Since that time, I can at any time prevent these
unpleasant symptoms, by an abstemious course of life, and remove
them, when they have come on, by the debilitating plan; which,
instead of weakening, gives additional elasticity and strength to the
fibres, and alacrity to the spirits. I have described the symptoms in
one case, as this will serve as a general description. We may add,
that persons labouring under this kind of predisposition, are
particularly attentive to the state of their own health, and to every
change of feeling in their bodies; and from every uneasy sensation,
perhaps of the slightest kind, they apprehend great danger, and even
death itself. In cases of this kind, the bowels are generally
costive, and the spirits of the patient are very apt to be affected
by changes in the weather, particularly by a fall of the barometer.
How the diminution of atmospheric pressure acts in increasing the
symptoms, we perhaps do not know; but its effects are experienced
almost universally.

It is evident, that the only mode of cure in cases of this kind is
extreme temperance: animal food should be taken sparingly, and wine
and spirits in general totally abstained from. The bowels should be
kept open by any mild neutral salt. I have generally found magnesia
and lemonade to agree remarkably well in such cases. Exercise on
horseback, is also particularly useful; bark, bitters, and the fetid
and antispasmodic medicines, which are generally prescribed in such
cases, are extremely hurtful.

This view of nervous complaints is, I may venture to say, as new as
it is just. It has never been imagined, that any of them depended
upon too great excitement; on the contrary, they have been
universally considered as originating in debility, and of course,
tonics were prescribed, which, though they produced the greatest
benefit in the other class of nervous complaints, in these they
occasioned the most serious evils, and often brought on real
inflammatory diseases, or even diseases of indirect debility, as I
have repeatedly seen.

These cases cannot at first sight, however, be easily distinguished
from those of the opposite class; the symptoms being nearly alike,
and the patient complaining of languor, debility, and extreme
depression of spirits in both. But by attending carefully to the
effects produced by the exciting powers, they may in general be
distinguished. A patient of this kind will tell you, that he does not
feel pleasant effects from wine, or spirituous liquors; instead of
exhilaration, his spirits become depressed by them; whereas, in the
contrary state, he finds almost instant relief. By attending to
circumstances of this kind, the nature of the complaint may in
general be ascertained.

Highly seasoned, and strongly stimulant foods should in the sthenic
hypochondriasis, if it may be so called, be avoided; but the most
mischievous agent of all, and which contributes to bring on the
greater number of these complaints, is wine. This, I believe,
produces more diseases, than all other causes put together. Every
person is ready to allow, that wine taken to excess is hurtful,
because he sees immediate evils follow; but the distant effects,
which require more attentive observation to perceive, very few see,
and believe; and, judging from pleasant and agreeable feelings, they
say that a little wine is wholesome, and good for every one; and
accordingly take it every day, and even give it to their children;
thus debauching their natural taste in the earliest infancy, and
teaching them to relish what will injure their constitutions; but
which, if properly abstained from, would prove one of the most
valuable cordial medicines we possess.

The idea that wine or spirituous liquors assist digestion, is false.
Those who are acquainted with chemistry, know that food is hardened,
and rendered less digestible by these means; and the stimulus, which
wine gives to the stomach, is not necessary, excepting to those who
have exhausted the excitability of that organ, by the excessive use
of strong liquors. In these, the stomach can scarcely be excited to
action, without the assistance of such a stimulus. If the food wants
diluting, water is the best diluent. Water is the only liquor that
nature knows, or has provided for animals; and whatever nature gives
us, is, we may depend upon it, the best, and safest for us. Wine
ought to be reserved as a cordial in sickness, and in old age; and a
most salutary remedy would it prove, did we not exhaust its power by
daily use.

I am sensible that I am treading on delicate ground, but I am
determined to speak my sentiments with plainness and sincerity, since
the health and welfare of thousands are concerned. Most persons have
so indulged themselves in this pernicious habit of drinking wine,
that they imagine they cannot live without a little every day; they
think that their very existence depends upon it, and that their
stomachs require it. Similar arguments may be brought in favour of
every other bad habit. Though, at first, the violence we do to nature
makes her revolt; in a little time she submits, and is not only
reconciled, but grows fond of the habit; and we think it necessary to
our existence: neither the flavour of wine, of opium, of snuff, or of
tobacco, are naturally agreeable to us: on the contrary, they are
highly unpleasant at first; but by the force of habit they become
pleasant.

It is, however, the business of rational beings to distinguish
carefully, between the real wants of nature, and the artificial calls
of habit; and when we find that the last begin to injure us, we ought
to use the most persevering efforts to break the enchantment of bad
customs; and though it cost us some uneasy sensations at first, we
must learn to bear them patiently; a little time will reward us for
our forbearance, by a reestablishment of health and spirits.

I shall now proceed to examine the opposite class of nervous
complaints: or such as do really depend on debility, or an asthenic
state of the system. These may be divided into two orders; viz. those
of direct, and those of indirect debility. I shall first consider
those of direct debility.

Though these complaints originate from a deficiency of stimulus, yet
it is very seldom from a deficiency of the common stimulant powers.
The only people, who in general labour under this deficiency of the
common stimulants, are the poor; they are seldom troubled with
nervous complaints; their daily exercise, and constant attention to
procure common necessaries, prevent their feeling what so grievously
afflicts the rich and luxurious. These complaints arise chiefly from
a deficiency of mental stimuli. The most common cause of them, and
whose effects are the most difficult to remove, is to be looked for
in the mind.

The passions and emotions, when exercised with moderation, and kept
within proper bounds, are the sources of life and activity; without
these precious affections, we should be reduced to a kind of
vegetation, equally removed from pleasure and from pain. For want of
these elastic springs, the animal spirits lose their regularity and
play; life becomes a lethargic sleep, and we fall into indifference
and languor.

If then the passions are so necessary to the support of the health of
the body, when in a proper degree, can we expect, that when they are
inordinate or excessive, or even deficient, we shall escape with
impunity? tumultuous passions are like torrents, which overflow their
bounds, and tear up every thing before them; and mournful experience
convinces us, that these effects of the mind are easily communicated
to the body. We ought, therefore, to be particularly on our guard
against their seduction.

   "'Tis the great art of life to manage well
   The restless mind."

It is particularly in their infancy, if it may be so called, that we
ought to be upon our guard against their seduction; they are then
soothing and insidious; but if we suffer them to gain strength, and
establish their empire, reason, obscured and overcome, rests in a
shameful dependence upon the senses; her light becomes too faint to
be seen, and her voice too feeble to be heard; and the soul, hurried
on by an impulse to which no obstacle is presented, communicates to
the body its languor and debility. The passions, by which the body is
chiefly affected, are, joy, grief, hope, fear, love, hatred, and
anger. Any others may be reduced to some of these, or are compounded
of them. The pleasurable passions produce strong excitement of the
body, while the depressing passions diminish the excitement; indeed
it would seem that grief is only a diminution of joy, as cold is of
heat; when this passion exists in a proper degree, then we feel no
particular exhilarating sensation, but our spirits and health are
good: we cannot doubt, however, that we are excited by a pleasant
sensation, though we are unable to perceive it. In the same manner,
when heat acts moderately, or is about the degree we call temperate,
we do not perceive its effects on the body, though there can be no
doubt, that the body is under the influence of its stimulus, and
powerfully excited by it; for when it is diminished, or cold applied,
we feel a deficiency of excitement, and become afterwards more
sensible of heat afterwards applied.

The same takes place with respect to joy and grief, and proves, I
think, clearly, that the one is only a diminution of the other, and
that they are not different passions. When the body has been exposed
to severe cold, the excitability becomes so much accumulated with
respect to heat, that if it be afterwards applied too powerfully, a
violent action, with a rapid exhaustion of the excitability, which
ends in mortification, or death of the part, will take place. We
should therefore apply heat in the gentlest manner possible, and
gradually exhaust the morbidly accumulated excitability.

In the same manner, when the body has been under the influence of
violent grief, any sudden joy has been known to overpower the system,
and even produce instant death. We have an instance in history, of a
mother being plunged into the extreme of grief, on being informed
that her son was slain in battle; but when news was brought her, that
he was alive, and well, the effect upon her spirits was such, as to
bring on instant death. This event ought to have been unfolded to her
in the most gradual manner; she should have been told, for instance,
that he was severely wounded; but that it was not certain he was
dead; then that there was a report he was living, which should have
been gradually confirmed, as she could bear it. The same observations
may be made, with respect to hope and fear, or despair; the former is
an exciting passion, the latter, a depressing one; but the one is
only a lower degree of the other; for a moderate degree of hope
produces a pleasant state of serenity of the mind, and contributes to
the health of the body; but a diminution of it weakens; and a great
degree of despair so accumulates the excitability of the system, as
to render it liable to be overpowered by any sudden hope or joy
afterwards applied. What proves that joy and hope act by stimulating,
and grief and despair by withdrawing stimulant action from the body,
is, that the former exhaust excitability, while the latter accumulate
it. Joy, for instance, does not render the system more liable to be
affected by hope, but the reverse; and the same may be said of hope.
In the same way, heat does not render the body more liable to be
affected by food, but the reverse. Both these are stimulants, and
exhaust the excitability. But after heat has been applied, if it be
followed by cold, a great degree of languor or weakness will take
place; because we have here a direct debility, added to indirect
debility. In the same way, grief succeeding joy, or despair
succeeding hope, produce a greater degree of dejection, both of mind
and body, than if they had not been preceded by these stimulant
passions; because here, direct debility is added to indirect. The
excitability is first exhausted, and then the stimulus is withdrawn.

We see then, that the passions of the mind act as stimulants to the
body, that, when in a proper degree, they tend to preserve it in
health; but when their action is either too powerful, or too small,
they produce the same effects as the other powers. We should
therefore naturally expect, that when there is a deficient action of
this kind of mental stimulus, or when the mind is under the influence
of the depressing passions, a predisposition to diseases of direct
debility would take place, and even these diseases be produced.
Accordingly we find a numerous class of nervous complaints
originating from these causes. Indeed, the undue action of the mental
stimulants, produces more quick alterations in the state of the
excitement, than that of the other exciting powers. Violent grief, or
vexation, will immediately suspend the powers of the stomach. If we
suppose a person in the best health, and highest good humour, sitting
down to dinner with his friends, if he suddenly receives any
afflicting news, his appetite is instantly gone, he cannot swallow a
morsel. If the same thing happens after he has made a hearty dinner,
the action of the stomach is suddenly suspended, and the whole
process of digestion stopped, and what he has eaten, lies a most
oppressive load. But this is not all: the whole circulation of the
blood becomes disturbed; the contraction and dilatation of the heart
become irregular; it flutters, and palpitates; hence all the
secretions become irregular, some of the glands acting too
powerfully, others not at all; hence the increased action of the
kidneys, and hence a burst of tears; hysterical affections, epilepsy,
and syncope, frequently succeed, in which every muscle of the body
becomes convulsed. Indeed, many terrible diseases originate from this
source, which were formerly ascribed to witchcraft, and the
possession of devils.

In slower, more silent, but longer continued grief, the effects are
similar, but not so violent. The functions of the stomach are more
gently disturbed, its juices vitiated; and acidity, and other
symptoms of indigestion, will show themselves. Hence no bland and
nutritive chyle is conveyed into the blood; whence emaciation and
general debility must follow; and the patient will at last die, as it
is said, of a broken heart.

Besides the disturbed state of the stomach, and bad digestion, there
can be no sleep in this state of mind; for,

   "Sleep, like the world, his ready visit pays,
   Where fortune smiles; the wretched he forsakes;
   Swift on his downy pinion flies from woe,
   And lights on lids unsullied with a tear."

Hence the animal spirits will not be recruited, nor the worn out
organs restored to vigour.

The minds of patients labouring under this division of nervous
diseases, are likewise in general filled with over anxiety concerning
their health; attentive to every feeling, they find, in trifles light
as air, strong confirmations of their apprehensions.

It is evident, that in these cases, a state of direct debility
prevails, attended with a morbidly accumulated excitability; hence,
those remedies afford relief, which produce a quick exhaustion of
this principle, and thus blunt the feelings, and lull the mind into
some degree of forgetfulness of its woes. Hence opium, tobacco, and
the fetid gums are often resorted to; and in the hands of a judicious
practitioner, they will afford great relief, provided he carefully
watch the patient, and prevent their abuse; for, if left to the
discretion of the patient, he finds that kind of relief which he has
long wished for; his moderation knows no bounds, and he is apt to
take them in such a manner, as to add indirect debility, to direct,
and thus bring on a state of exhausted excitability, while there is
still a diminished state of mental stimulants. This will cause his
spirits to be more depressed than ever; he will therefore increase
the dose, whether it be of opium, tobacco, or spirituous liquors, and
thus he will be hurried on, adding fuel to the flame, till his
exhausted excitability becomes irrecoverable, and he ends his days in
a miserable state of imbecility, if not by suicide. Hence, though
some of these narcotic stimulants, which exhaust the excitability,
and blunt the feelings, may be employed with advantage, in order to
prepare the mind for those changes, which the physician wishes to
produce, they should be used with the greatest caution, and never
left in any degree to the discretion of the patient. The cure,
however, depends chiefly on regulating the state of the mind, or
interrupting the attention of the patient; and diverting it, if
possible, to other objects than his own feeling.

Whatever aversion to application of any kind we may meet with in
patients of this class, we may be assured that nothing is more
pernicious to them than absolute idleness, or a vacancy from all
earnest pursuit.

The occupations of business suitable to their circumstances, and
situations in life, if neither attended with emotion, anxiety, nor
fatigue, are always to be advised to such patients; but occupations
which are objects of anxiety, and more particularly such as are
exposed to accidental interruptions, disappointments, and failures,
are very improper for patients of this class.

To such patients exercise in the open air is of the utmost
consequence. Of all the various methods of preserving health and
preventing diseases, which nature has suggested, there is none more
efficacious than exercise. It puts the fluids all in motion,
strengthens the solids, promotes digestion, and perspiration, and
occasions the decomposition of a larger quantity of air in the lungs,
and thus not only more heat, but more vital energy is supplied to the
body; and of all the various modes of exercise, none conduces so much
to the health of the body, as riding on horseback: it is not attended
with the fatigue of walking, and the free air is more enjoyed in this
way, than by any other mode of exercise. The system of the vena
portarum, which collects the blood from the abdominal viscera, and
circulates it through the liver, is likewise rendered more active, by
this kind of exercise, than by any other, and thus a torpid state,
not only of the bowels, but of this system of vessels, and the
biliary system, is prevented.

When a patient of this class, however, goes out for the sake of
exercise only, it does not in general produce so good an effect, as
might be expected; for he is continually brooding over the state of
his health: there is no new object to arrest his attention, and he is
constantly reminded of the cause of his riding. Exercise will
therefore be most effectual when employed in the pursuit of a
journey, where a succession of pleasant scenes are likely to present
themselves, and new objects arise, which call forth his attention. A
journey likewise withdraws the patient from many objects of
uneasiness and care, which might present themselves at home.

With respect to medicines, costiveness, which often attends these
diseases, ought to be carefully avoided, by some mild laxative.
Calcined magnesia, and lemonade, have always seemed to me to answer
the purpose; but the most effectual method is to acquire a regular
habit, which may be done by perseverance, and strict attention.

Chalybeate waters have been frequently tried, and may in general be
recommended with success, particularly, as the amusement and exercise
generally accompanying the use of these waters, aid the tonic powers
of the iron. The bark may likewise be exhibited with advantage.

There is yet another class of nervous diseases which we have to
notice, which are by no means uncommon; yet they have, like the first
class, escaped the attention of writers on this subject, and of
medical practitioners in general: I mean those where the system is in
a state of torpor, or exhausted excitability.

This state of the system may be brought on by various causes, but
principally by the long continued use of opium, tobacco, or fermented
liquors.

When these substances, which are powerful stimulants, have been taken
for some time, they bring on a state of the system so torpid, that
the usual exciting powers, and the usual occurrences, which in
general produce pleasant sensations, do not occasion a sufficient
degree of excitement, in those whose excitability is thus exhausted.
They therefore feel continual languor and listlessness, unless when
under the influence of the stimulus which brought on the exhaustion.
Every scene, however beautiful, is beheld with indifference by such
patients, and the degree of ennui they feel is insupportable: this
makes them have recourse to the stimulus which has exhausted their
excitability, which in some degree removes this languor for a time;
but it returns with redoubled strength, and redoubled horror, when
the stimulant effect is over: and as this repetition exhausts the
excitability more and more, the stimulus is repeated in greater
quantity, and thus the disease increases to a most alarming degree.

There is no way of curing this state of nervous torpor, but by
leaving off the stimuli which caused the exhaustion; and if the
patient have resolution to do this for a few weeks, though, at first,
he will, no doubt, find his spirits a little depressed, he will
ultimately overcome the habit, and will be rewarded by alacrity of
spirits, such as he never experiences under the most powerful action
of artificial stimulants.

I must not, however, forget to notice, that there is a nervous state,
or ennui, originating from a wrong direction of mental exertion,
which exhausts the excitability to a great degree, and brings on a
state of depression scarcely to be born.

When a person has by habit made his mind constantly dependent on
dissipation, on gaming, and on frivolous, but not inactive pursuits,
in order to produce pleasurable sensations, and at the same time
neglected that culture of the understanding which will enable him to
retire into himself with pleasure, and receive more enjoyment from
the exercise of this cultivated understanding than he does in the
most noisy, or fashionable circle of dissipation: I say, when there
is this vacancy of mind, whenever it is not engaged in such pursuits
as I have mentioned, a languor and weariness is experienced, which is
intolerable, and which prompts the person so circumstanced, to fly
continually to the only scenes which interest his mind. Hence, the
passion for gaming, in which the anxiety attending it causes an
interest in the mind, which takes off the dreadful languor
experienced, when it is not thus employed.

It is owing to wealth, admitting of indolence, and yielding to the
pursuit of transitory and unsatisfying amusements, or to that of
exhausting pleasures only, that the present times exhibit to us so
many instances of persons suffering under this state: it is a state
totally unknown to the poor, who labour for their daily bread, and to
those whose minds are actively employed in study or business. It can
only be cured by cultivating the understanding, and applying to some
art or science, which will engage and interest the attention. I have
received the thanks of many for recommending the study of philosophy,
and particularly of chemistry, to their attention. This affords a
rational and interesting pursuit, which, if entered into with ardour,
and if the person actually works, or makes experiments himself, he
will soon experience an enjoyment and an interest, such as he never
experienced at the gaming table, or at any other place of fashionable
amusement. Nay, I will venture to say, that all elegant amusements
will be enjoyed with much greater relish by one who employs himself
in some rational pursuit, and only resorts to such amusements as a
relaxation, than by one who makes these amusements a business.

From the view we have taken of these complaints, it is evident, that
they are like other general diseases of the sthenic, or asthenic
kind; they seem to constitute a state of the body between
predisposition and disease; and they differ from most diseases in
this, that in most complaints the increase, or diminution of the
excitement is unequal in different parts of the body, and this gives
rise to the different forms of disease; but in nervous complaints the
excitement seems much more equably affected in different parts. These
complaints, as we have seen, may be divided into three classes;
sthenic; those of accumulated excitability; and those of exhausted
excitability: but though they are evidently distinguishable in this
manner, and require different modes of cure, I have never seen any
account of more than one kind in any medical writer: the same
remedies were prescribed for all, however different they might be.

Though medicines may relieve complaints of this kind, and
particularly those of the second class, yet from what has been said,
it must be evident, that much more may be done by regulating the
action of the common exciting powers. Indeed, this is the case in
most chronic diseases. Exercise and temperance will do infinitely
more than medicine. By their means, most diseases may be overcome;
but without them we may administer drugs as long as we please.

Voltaire sets this advice, which I have frequently inculcated, in so
strong a light, that it may perhaps carry more conviction than any
thing I can say. Ogul was a voluptuary, ambitious of nothing but good
living: he thought that God had sent him into the world for no other
purpose than to eat and drink: his physician, who had but little
credit with him, when he had a good digestion, governed him with
despotic sway, when he had eaten too much.

On feeling himself much and seriously indisposed by indolence and
intemperance, he requested to know what he was to do, and the doctor
ordered him to eat a basilisk, stewed in rose water, which he
asserted would effect a complete cure. His slaves searched in vain
for a basilisk; at last they met with Zadig, who was introduced to
this mighty lord, and spoke to him in the following terms.

"May immortal health descend from Heaven to bless all thy days! I am
a physician: at the report of thy indisposition, I flew to thy
castle, and have now brought thee a basilisk, stewed in rose water.
But, my lord, the basilisk is not to be eaten; all its virtue must
enter through thy pores. I have enclosed it in a little ball, blown
up and covered with a fine skin. Thou must strike this ball, with all
thy might, and I must strike it back for a considerable time: and by
observing this regimen for a few days, thou wilt see the effects of
my art." The first day Ogul was out of breath, and thought he should
have died with fatigue; the second he was less fatigued, and slept
better. In eight days he recovered all the strength, all the health,
all the agility and cheerfulness of his most agreeable years. Zadig
then said unto him, "there is no such thing in nature as a basilisk;
but thou hast taken exercise, and been temperate, and hast recovered
thy health." In the same manner I say, that temperance and exercise
are the two great preservers of health, and restorers of it when it
is lost; and that the art of reconciling intemperance and health is
as chimerical, as washing the Ethiopian white.

It will easily be perceived that the system of animal life which I
have investigated, may be applied to all other general diseases, as
well as the gout and those called nervous: I have merely given a view
of these by way of specimen of its application.

Should these lectures contribute in any degree to lessen the future
sufferings of my hearers, or any of their friends, I shall not have
delivered them in vain. To be assured of this, would be the greatest
pleasure that I could receive.

THE END.

From the Press of the Royal Institution of Great Britain,
Albemarle Street, London: W. Savage, Printer.

INDEX.

A.
_Air_, its properties--its components
_Animals_, specific temperature of
_Appetite_, artificial
_Arteries_, their structure and office
_Assimilation_, from the blood
_Attention_, fixed on new objects

B.
_Banks, Sir Joseph_, his almost fatal experience of cold
_Beddoes, Dr._ his remarks on temperature,
_Bile_, its properties
_Blood_, difference between arterial and venous
   contains iron
   changes produced on, by the different gases
   assimilation from
_Bones_, mechanism of
   structure of
_Brown, Dr. John_, his example followed
   declines a definition of excitability
   denies the existence of sedatives
   his cure of diseases of exhaustion objected to
   his theory will be as durable as Newton's philosophy
   not aware of the extent of his own theory

C.
_Camera obscura_
_Colour_ of different nations
_Cooper, Mr. Astley_, successfully perforates the tympanum of the ear
_Circulation_ of the blood
   through the lungs
   through the liver
   affected by centrifugal force
   discovered by Harvey, and derided
_Cullen, Dr._ his system defective
_Currie, Dr._ his treatment of typhus

D.
_Darwin, Dr._ quoted
_Digestion_, organs of
   process of
   diseases affecting
_Diseases_
   sthenic
   asthenic
   fallacy of symptoms
 _Diseases_, method of cure
   spasmodic, of extreme vess.
   classification of
   nervous and bilious (so called)
   of the poor
_Dollond, Mr._ his achromatic lenses, first suggested by Dr. David
Gregory

E.
_Ear_, description of
   diseases of
   tympanum perforated
_Electricity_, phenomena of
   useful in gout
_Excitability_, or vital principle
   how affected by heat, food, air, &c.
   accumulated by sleep
   three states of
   its accumulation, and exhaustion illustrated
   an unknown indefinable somewhat
   connected with oxygen
   hypothesis respecting this connexion
   scale of
   how to be regulated in gout
_Exercise_
  on horseback, best
_Eye_, description of
   vertical section of
   its accommodating power

F.
_Food_, animal and vegetable
   in gout

G.
_Galvanism_, its identity with electricity
   accompanies oxydation
_Gaming_, &c. deplorable effects of, on the mind
_Gases_, their proportions in the atmosphere
   changes they produce on the blood
_Gastric Juice_, dissolves food
   proved by experiment
_Gout_
   denied to be salutary, or incurable
   not to be cured by drugs
   not hereditary
   mode of attack
   depends not on morbific matter
   its inflammation asthenic
   its symptoms and description
   its remissions accounted for
   the terms retrocedent, atonic, &c. misapplied
   mode of cure
   electricity useful
   Dr. Darwin's advice
   Diet and medicine in
_Gravity_, the circulation affected by it

H.
_Habit_, its power
_Harvey_, discovers the circulation of the blood, and is opposed
_Health_
   previous to disease, investigated
   point of, on the scale
_Heart_, description of
_Heat_, its combinations
   animal, accounted for
   its effects on the system
   how it affects vegetables
   its debilitating effects exemplified
_Hunger_ and _thirst_
_Hunter, Mr. John_, dissects torpedo, and electrical eel

I.
_Inflammation_, illustrated
   sthenic
   asthenic
   of gout asthenic
_Iron_, contained in the blood

L.
_Lacteals_, described
_Life_, principle of
_Light_, its properties
   its exciting power
_Lungs_, circulation through
_Lymphatics_ described

M.
_Muscles_, mechanical action of
_Music_, its effects

N.
_Nerves_, their distribution
_Nervous_ and _bilious_, terms sometimes used without ideas
   diseases (so called)
   diseases really so
_Newton_, his query about elastic fluid misapplied
   even his conjectures important
   discovers the laws of sound
   his reason why the crystalline is densest in the middle
   knew not the cause of gravitation
_Nonnaturals_, misapplication of that term
_Nosology_

O.
_Odours_, extreme subtilty of
   Haller, &c. fail in classing them
   convey nourishment
_Opium_, its intoxicating effects
   its use in gout
_Organs_, digestive
_Oxygen_, experiments with
   connected with excitability

P.
_Pain_
   encreases mental energy
_Passions of the mind_, their effects
_Pendulums_, laws of
_Peristaltic motion_
_Physiology_, its importance
_Pulse_, the phenomena of

R.
_Respiration_, organs of
   how performed
   analogous to combustion
   chemically explained

S.
_Schools_, their temperature ill regulated
_Sensation_
   organs of
   more acute by pain
_Senses_, general account of
   external and internal
_Skin_
_Sleep_, accumulates excitability
_Smell_
   different in different animals
   Blumenbach's opinion on
   diseases of
_Sound_, production of
   media of
   illustrated
   velocity of
   musical
   reflection of
_Spallanzani_, his experiments on digestion
_Spirituous liquors_, their effects
   a singular mode of correcting the abuse of
_Squinting_, &c.
   method of cure
_Stomach_
   diseases of
_Study_, debilitating effects of

T.
_Taste_
   diseases of
_Temperancc_
_Temperature_ of animals, specific and uniform
_Touch_
   the basis of the other senses
_Typhus_, Dr. Garnett's treatment of
   Dr. Currie's ditto

V.
_Veins_, their structure and office
_Vision_, manner of
   opinions on
   seat of
   why objects appear erect
   why seen single
   diseases and cure of
_Voltaire_, his story of Ogul, the epicure

W.
_Water_, the best diluent
_Wine_, its use and abuse


LIST OF SUBSCRIBERS.
                                       _L.  s._
A.
Henry Ainslie, M.D.                     10  10
James Peter Auriol, Esq. P.R.I.          5   5
James Acheson, Esq. _Oaks, Londonderry_  1   1
W. Dacre Adams, Esq.                     1   1
Miss Adams                               1   1
Mr. Adams, _Optician_                    5   5
------Addington, Esq.                    1   1
Mr. Thomas Addison, _Preston_            2   2
Mrs. Addison                             2   2
John Aikin, M.D.                         1   1
Mr. Arthur Aikin                         1   1
Charles R. Aikin, Esq. _Surgeon_         1   1
William Alers, Esq.                      2   2
Dr. Alexander, _Hallifax_                2   2
Henry Alexander, Esq.                    1   1
E. Alexander, M.D. _Leicester_           1   1
William Allen, Esq. F.L.S.               2   2
Robert Anderson, Esq.                    1   1
Dr. Anderson                             1   1
Dr. Anderson, _Hammersmith_              1   1
A. Apsley, Esq.                          1   1
Mr. D. Archer, _Bath_                    1   1
Mr. J. T. Armiger                        1   1
Nicholas Ashton, Esq. _Liverpool_        1   1
Mr. Thomas Aston                         1   1
Abram Atkins, Esq.                       3   3
William James Atkinson, Esq.             3   3
Mr. Accum                               10  10
Mr. Noah Ayton                           1   1

B.
Rt. Hon. Sir Joseph Banks, Bart. K.B.
   Pr. R.S. F.A.S. P.R.I. F.L.S.        10  10
Sir C. W. Rouse Boughton, Bart. P.R.I.   1   1
Sir William Blizard, F.R.S. F.A.S.       2   2
Mr. B. _Lancaster_                       1   1
Dr. Babington, _Physician to Guy's
   Hospital._                            3   3
Miss Bacon                               1   1
Miss Mary Bacon                          1   1
John Bailey, Esq.                        1   1
Andrew Bain, M.D.                        5   5
John Baker, Esq.                        10  10
William Baker, Esq.                     10  10
Mr. Joseph Ball                          1   1
Rev. Edward Balme                        1   1
Mr. Bamford                              1   1
Rev. John Banks, P.R.I. _Boston,
   Lincoln._                             1   1
Mr. Banks, _Leeds_                       1   1
Charles Baratty, Esq. F.A.S.             1   1
Robert Barclay Esq. _Southwark_
   P.R.I. F.L.S.                         1   1
Mr. William Barclay                      1   1
R. J. Barlow, Esq.                       3   3
Dr. Barrow                               1   1
Rev. William Barton, _Whalley_           1   1
Mr. Barton                               1   1
Thomas Baskerfeild, Esq. F.A.S.          1   1
Jacob Bath, Esq. _Surgeon, 66th Reg._    1   1
Robert Batty, M.D. P.R.I. F.L.S.         5   5
Mr. Charles Baumer                       1   1
E. Bayley, Esq. _St. Petersburg_     1  11   6
Mr. John Baynes                          1   1
Mr. Beckett                              1   1
Hugh Bell, Esq.                          1   1
William Bell, Esq. _St. Petersburg_  1  11   6
Mr. Bell                                 5   5
Christopher K. Bellew, Esq.              2   2
Rev. N. Benezer, _Swaffham_              1   1
Nathaniel Barnardiston, Esq.             1   1
Thomas Bernard, Esq. P.R.I.              5   5
W. Berwick, Esq. _St. Petersburg_    1  11   6
Richard Best, Jun. Esq.                  1   1
Henry Bickersteth, Esq. _Kirkby
   Lonsdale_                             2   2
Henry Bickersteth, Jun. Esq. _Kirkby
   Lonsdale_                             1   1
Mr. John Bickersteth                     2   2
Mr. E. Bickersteth                       1   1
T. Bigge, Esq. _Newcastle upon Tyne_     1   1
Mr. George Biggin                        1   1
W. Bindsale, Esq. _Pickering, Surgeon_
Charles Binney, Esq.                     1   1
Joseph Birch, Esq. M.P.                  3   3
Dr. Birkbeck                             5   5
Morris Birkbeck, Jun. Esq.
   _Wanborough, Surry_                   1   1
Thomas Blacker, Esq. P.R.I.              5   5
Mr. Blades                               1   1
Alexander Blair, Esq. P.R.I.             5   5
William Blair, Esq.                      3   3
Dr. Blake, _Taunton_                     1   1
Dr. Blane                                5   5
Mr. Henry Blatch, _Surgeon_              1   1
Ditto, _2nd Subscription_                4   4
Ralph Blegborough, M.D. P.R.I.           2   2
John Bliss, Esq.                         1   1
Robert Blundell, Esq.                    1   1
Samuel Boddington, Esq. P.R.I.           1   1
George Bodley, Esq.                      1   1
Thomas Bodley, Esq.                      1   1
John Bolton, Esq.                        1   1
Dr. Bolton                               5   5
Thomson Bonar, Esq. P.R.I.              10  10
George Booth, Esq.                       2   2
John Bostock, M.D.                       2   2
Josiah Boydell, Esq.                     3   3
Daniel Braithwaite, Esq. F.R.S.
   F.A.S.                                5   5
Miss Braithwaite                         2   2
Mr. James Braithwaite                    1   1
------Brande, Esq.                       1   1
Joseph Brandreth, M.D.                   5   5
Mr. Brandreth, _Surgeon_                 3   3
Mr. Brandreth, _Attorney_                2   2
Messrs. Brash and Reid, _Glasgow_        2   2
John Breare, Esq.                        1   1
Martin Bree, Esq.                        1   1
J. H. Brehmer, Esq. _St. Petersburg_ 1  11   6
Brentford Book Society                   1   1
William Bridgman, Esq. P.R.I.
   F.L.S.                                1   1
Mr. Richard Briggs                       2   2
Mrs. Richard Briggs                      1   1
Lowbridge Bright, Esq. _Bristol_
British Library, _St. Petersburg_        1   1
Theodore Henry Broadhead, Esq.
   F.A.S. P.R.I.                         2   2
Messrs. Broderip                         5   5
James Brogden, Esq. M.P.                 3   3
Mr. E. Brook                             1   1
Joshua Brooks, Esq. _Lecturer on
   Anatomy, Surgery, &c._                5   5
James Brown, Esq. _Leeds_                1   1
J. Brown, M.D. _Islington_               1   1
Mr. Brown                                1   1
------Bryant, Esq. _Lynn Regis,
   Surgeon_                              1   1
Dr. Buchan                               1   1
Rev. Gilbert Buchanan, LL.D.
   _Rector of Woodmanstow, Surry_        1   1
Mr. Buchenson, _Surgeon_                 1   1
Buckingham Book Society                  1   1
George Buckle, Esq.                      2   2
William Buckley, Esq.                    1   1
Mr. Henry Budd                           1   1
Mr. John Buddle                          1   1
Mr. J. Buncomb, _Taunton, Somerset._     1   1
Mr. A. S. Burkitt, _Chemist_         3   5   6
Charles Burney, LL.D. F.R.S.
   F.A.S. P.R.I. _Greenwich_             1   1
Robert Burns, Esq.                       1   1
Mrs. Butts                               1   1

C.
Sir John Chetwode, Bart.                 1   1
Henry Cavendish, Esq. F.R.S.
   F.A.S. P.R.I.                        10  10
Thomas Coutts, Esq. P.R.I.              10  10
Mr. John Callow                          1   1
Dr. Alexander Campbell, _Secretary
   to the Medical Board, Calcutta_       1   1
Dr. Campbell, _Lancaster_                1   1
Colonel James Campbell                   1   1
Mr. Card                                 1   1
Anthony Carlisle, Esq. F.L.S.            1   1
Mr. John Carpenter, Senr. _Lyme_
J. C. Carpue, Esq.                       2   2
George Carr, Esq. _St. Petersburg_   1  11   6
Thomas W. Carr, Esq. P.R.I.              1   1
Mr. W. Cary                              1   1
William Cass, Esq.                       1   1
Dr. Cassels, _Lancaster_                 1   1
D. Cassidy, Esq.                         1   1
John Cates, Esq.                         1   1
H. Cayley, Esq. _St. Petersburg_     1  11   6
William Chamberlain, Esq.                1   1
L. B. Chateauneauf, Esq.                 1   1
Chester Public Library                   1   1
Henry Chevalier, Esq. F.L.S.             2   2
Mrs. Chevalier                           1   1
James Chew, M.D. _Blackburn_             1   1
Mr. Christian, _Attorney_                1   1
------Clapham, Esq. _Thorney
   Abbey, Surgeon_                       1   1
John Clarke, Esq.                        1   1
J. Calvert Clarke, Esq.                  5   5
Ralph Clarke, Esq.                       1   1
Richard Hall Clarke, Esq. _Devon._       1   1
Mr. John Clay                            1   1
S. P. Cockerell, Esq. P.R.I.             2   2
Miss Codrington                          2   2
Mr. John Cohen                           1   1
Mr. Collier                              1   1
Mr. Coltman, _Surgeon_                   2   2
Mr. Colton                               1   1
Mr. Cooke                                1   1
B. Coombe, Esq.                          2   2
Astley Cooper, Esq. F.R.S.               5   5
Rev. Stuart Corbett
Adam Cottam, Esq. _Whalley, near
   Blackburn_                            1   1
Mr. Cotter, _Godstone, Surry_            1   1
Mr. Cotter                               1   1
John Cowan, Esq.                         1   1
Theodore Cox, Esq.                       2   2
John Craig, Esq. _Glasgow_               1   1
Mr. Cregg                                1   1
Thomas Creser, Esq.                      1   1
William Cresswell, Esq.                  1   1
Alexander Crichton, M.D. F.R.S.
   F.L.S.                                2   2
Mr. Crigg                                1   1
George Crooke, Esq.
Hugh Cross, Esq.                         2   2
C. Cuppage, Esq.                         1   1
Mr. William Cuppage                      2   2
James Currie, M.D. F.R.S.
   _Physician to Guy's Hospital_         2   2
------Currie, M.D.                       5   5
------Cuthbert                       1  11   6
Mr. John Cuthbertson                 1  11   6

D.
Sir George Duckett, Bart. P.R.I.         1   1
Peter Denys, Esq. P.R.I.                10  10
Lady C. Denys                            5   5
George William Denys, Esq. P.R.I.        5   5
David Dale, Esq.                         5   5
J. P. Dale, Esq.                         5   5
Thomas Dale, M.D.                        1   1
Mr. Daniel Dale                          5   5
Mr. Daltera, _Attorney_                  1   1
Mr. Darbyshire                           1   1
Mr. Davie, _St Thomas's Hospital_        1   1
Dr. Davies, _Bath_                       1   1
Dr. Davison, _Harewood_
Humphry Davy, Esq. F.R.S.                5   5
Mr. Joseph Dawson                        1   1
Thomas Dawson, Esq. _Surgeon_            1   1
Mr. Thomas Dawson                        1   1
Mr. Dawson                               2   2
Thomas Day, Esq. P.R.I.                  5   5
Miss Day                                 2   2
Mrs. Delamore                            1   1
Thomas Demson, Esq.                      1   1
R. Dennison, M.D. and F.A.S.             2   2
Osbert Denton, Esq. _Chelsea_           10   0
Mrs. Denton                             10   0
Miss Denton                              5   0
Mr. Benjamin Devaynes                    1   1
N. Devy, Esq.                            1   1
Charles Dibdin, Esq.                     5   5
William Dick, Esq.                       1   1
W. Dickson, LL.D. _Clipstone St._        2   2
Charles Dilly, Esq.                      2   2
Mr. William Dinsley, _Leeds_             1   1
------Doig, Esq.                         1   1
Mr. Doig                                 1   1
George Dominicus, Esq.                   2   2
Rev. Thomas Donald, _Anthorne,
   Cumberland_                           1   1
Andrew Douglas, Esq. F.R.S. P.R.I.       5   5
Mr. James Dove                           1   1
Henry Downer, Esq.                       1   1
Mr. Drewe                                1   1
F. Duboulay, Esq.                        1   1
Dr. Andrew Duncan, _Edinburgh_
Dr. Andrew Duncan, Jun. _Do._
Mrs. Dunlop, _Hammersmith_               1   1
Miss Dunlop, _Hammersmith_               1   1
James Dupre, Esq. P.R.I.                 1   1

E.
Marquis of Exeter, F.R.S. F.A.S.         5   5
Marchioness of Exeter                    5   5
The Earl of Egremont, F.R.S.
   F.A.S. P.R.I.                        10  10
Sir H. C. Englefield, Bart. F.R.S.
   F.A.S. P.R.I.                         3   3
Sir James Earle                          2   2
Hon. William Elphinstone                 1   1
R. G. Ealand, Esq.                       1   1
Willis Earle, Esq.                       2   2
------Eccles, Esq. _Clithero_            1   1
D. B. Plantagenet Eccleston, Esq.        1   1
Library of the University of
   Edinburgh
Mr. Edmondson, _Surgeon, Lancaster_      1   1
Mr. Richard Elgar                        1   1
George Ellis, Esq. F.R.S. F.A.S.
   P.R.I.                                1   1
John Fullerton Elphinstone, Esq.
   _East Lodge, Enfield_                 1   1
Alexander Erskine, Esq. F.L.S.           1   1
George Erving, Esq.                      1   1
John Esdaile. Esq. P.R.I.                1   1
Isaac Espinasse, Esq.                    2   2
William Etty, Esq.                       1   1
Mr. Evans                                1   1

F
Col. the Hon. T. W. Fermor, P.R.I.      10  10
James Fallofeild, Esq. P.R.I.            2   2
Vere Fane, Esq.                          1   1
Thomas Farquhar, Esq.                    1   1
Robert Farquhar, Esq. _Rochester_        1   1
Mr. Faulder                              1   1
W. Fawkes, Esq. _Farnley, near Leeds_    3   3
John Feltham, Esq.                       2   2
Richard Firmin, Esq.                     3   3
Mr. M. Fitzgerald                        1   1
Lewis Flannagan, Esq.                    2   2
Arch. Fletcher, _Advocate, Glasgow_      2   2
Caleb Fletcher, Esq.                     2   2
Mr. Benjamin Flockton
------Forbes, Esq.                       1   1
Henry Fock, Esq. _St. Petersburg_    1  11   6
Mr. Ford, _Pimlico_                      1   1
------Ford, Esq.                         2   2
R. Forrester, Esq. _St. Petersburg_  1  11   6
B. M. Forster, Esq.                      1   1
Rev. Edward Forster, F.R.S. F.A.S.
   P.R.I.                                1   1
Thomas Foster, Esq.                      1   1
Thomas Fothergill, Esq.                  2   2
John Foulks, Esq.                        2   2
Joseph Fox, Esq.                         2   2
Rev. M. Foxcroft                         1   1
James Fraser, Esq. P.R.I.                1   1
F. Freelong, Esq.                        1   1
------French, Esq.                       1   1
A Friend                                 2   2
A friend, W. M.                          1   1
A friend, J. C.                          1   1
A friend, T. K.                          2   2
A friend, E.                             4   4
A friend, T.                            10   0
A friend, J. G.                          5   5
A friend, A.                             2   2
A friend, R. W. by Mr. Lawson            1   1
A friend, W. W.                          1   1
A friend, I. A. W.                       1   1
A friend, I. S.                          1   1
A friend, A. B.                          2   2
A friend, I. B.                          5   5
A friend, W. D.                          2   2
A friend, D. D.                          2   2
A friend, J. L. Esq.                     3   3
A friend, E. A.                          1   1
A friend, I. S.                          1   1
A friend, I. B.                          1   1
A friend, M. W.                          1   1
A friend, M. B.                          5   0
A friend, I. A. W.                       1   1
A friend, E. D.                          1   1
A friend, J. D.                          1   1
A friend, J. P. D.                       1   1
A friend, S. B. D.                       4   4
A friend, S. D.                          5   5
A friend, Mr. B.                         1   1
A friend, Mr. B.                         1   1
A friend, S. E. D.                       1   1
A friend, S. E. D.                       1   1
A friend, S. M. D.                       1   1
A friend, I. T. by Mr. I. J. Armiger     1   1
A friend, by Dr. Batty                   3   3
A friend, I. H. D. by Dr. Batty          3   3
A friend, by D. Braithwaite, Esq.        3   3
A friend, by Miss Braithwaite            2   2
A friend, by Mr. Gear                    1   1
A friend, H. L. by Mr. Thomas            1   1
A friend, S. H. by Mr. Lawson            2   2
A friend, by Mr. Rathbone                5   5
Edmund Fry, M.D.                         1   1

G
Sir James Whalley Gardiner, Bart.
   _Clerk Hill, near Blackburn_          3   3
Mr. William Gaitskell                    3   3
Peter Garforth, Esq.                     5   5
John Gaythorne, Esq.                     2   2
Robert Gear, Esq.                        2   2
Do. for a friend, _no Book_              1   1
John Geddes, Esq.                        2   2
James Gerard, Esq.                       3   3
Mr. Robert Gibson                        2   2
Mr. W. Gilbert, _Optician_               2   2
Collin Gillespie, Esq.                   1   1
William Gillespie, Esq.                  5   5
Robert Gillespie, Esq.                   1   1
Thomas Gisborne, M.D. F.R.S.             1   1
Henry Glossop, Esq. _Cambridge_          2   2
Mrs. Gooch                               1   1
------Good, Esq.                         1   1
Mr. Good                                 1   1
Mr. Alexander Gordon, _Surgeon_          1   1
Mr. Alexander Caudleraig Gordon,
   _Edinburgh_                           1   1
Mrs. Maxwell Gordon, _Edinburgh_         2   2
Thomas T. Gorsuch, Esq.                  1   1
Mr. Samuel Gosnell, _Printer_            1   1
Joseph Gough, Esq.                       1   1
Archibald Graham, Esq. _Glasgow_         1   1
Robert Graham, Esq.                      1   1
Mrs. Henry Grant                         2   2
Alex. Grant, Esq. _St. Petersburg_   1  11   6
Edward Whitaker Gray, M.D.
   Sec. R.S.                             1   1
R. Greaves, M.D. _Reading, Berks._       2   2
Mr. Griffenhof, _Hampton_                1   1
Thomas Green, Esq.                       3   3
Mrs. Green                               2   2
------Greenwood, Esq. _Lancaster,
   Surgeon_                              1   1
John Grenfell, Esq. _Chelsea_            1   1
Mr. Griffin                              1   1
T. Grimston, Esq. _Kilnwick, York._      1   1
Henry Grimston, Esq. F.L.S.              1   1
John Guthrie, Esq.                       2   2

H
Sir John Cox Hippisley, Bart.
   F.R.S. F.A.S. P.R.I.                  5   5
Hon. Mr. Justice Heath, P.R.I.           1   1
Messrs. Hoare                           21   0
Mr. Hadwen, _Lancaster_                  1   1
G. H. C. Hahn, Esq. _Wimbledon_          1   1
Samuel Hailstone, Esq.                   1   1
John Hale, Esq. P.R.I.                   2   2
Mr. T. Hall                              2   2
Mr. Halliday, _Everton, near
   Liverpool_                            1   1
Dr. James Hamilton, _Professor of
   Midwifery, University, Edinburgh_
Mr. John Hamilton                        1   1
Colonel Hamilton, _His Majesty's
   Consul, Norfolk, Virginia_            5   0
G. H. Hancock, Esq.                      1   1
Mr. Hankey                               5   5
Mr. John Harding                         1   1
Robert Hardy, Esq. _Whalley, near
   Blackburn, Surgeon_                   1   1
Mr. John Hardy                           1   1
Rev. Richard Harrington, _Hayley,
   Worcestershire_                       1   1
Rev. Robert Harris, _Preston_            1   1
J. Harrison, Esq. _Kirkby Lonsdale_      5   5
Thomas Harrison, Esq.                    1   1
Mr. Joseph Harrison, _Bury_              3   3
Mr. William Harrison, _Manchester_       1   1
Harrowgate Book Club                     1   1
Enoch Harvey, Esq. _Liverpool_           2   2
Joseph Haskins, Esq. P.R.I.              3   3
William Huskisson, Esq.                  1   1
Charles Hatchett, Esq. F.R.S.
   F.L.S.  P.R.I.                       10  10
Dr. Hawes, (W. H. S.)                    1   1
Mr. B. Hawes                             1   1
Richard Haworth, Esq.                    2   2
Mr. Joseph Hawthorne, _Reading_          1   1
Thomas Hay, Esq.                         1   1
------Haygarth, M.D. _Bath_              2   2
William Headington, Esq.                 1   1
John Heath, M.D. _Fakenham, Nor._        1   1
Mr. Richard Heathfield                   1   1
Mr. Henbest                              2   2
William Henderson, Esq.                  2   2
Messrs. Thomas and William Henry         5   5
Messrs. T. and W. Henry                  5   5
Benjamin Arthur Heywood, Esq.            2   2
B. A. Heywood, Esq.                      2   2
Mr. Hicks, _Surgeon_                     1   1
Dr. Higgins                              2   2
John Benton Higgon, Esq.                 1   1
Henry Hinckley, Esq.                     2   2
John Hinckley, Esq. P.R.I.               2   2
Elias Hocker, Esq. _Brook House,
   Devon._                               1   1
Hoddesdon Book Society                   2   2
Mrs. Hodge                               1   1
Henry Hodgson, Esq.                      1   1
Rev. Dr. Hodgson, _Market Rasen,
   Lincolnshire_                         1   1
Charles Holford, Esq. _Hampstead_        1   1
Mr. W. Holland                           1   1
Edward Holme, M.D.                       2   2
Mr. Jacob Holme                          1   1
Everard Home, Esq. F.R.S.  P.R.I.        5   5
Mr. Hommey, _Military Academy,
   Woolwich_                             1   1
Mr. Hooke, _Optician_, P.R.I.            2   2
Dr. Hooper, F.L.S.                       5   5
Dr. Thomas C. Hope, _Edinburgh_
Rev. J. J. Hornby                        1   1
Mr. J. D. Hose, Jun.                     1   1
Mr. Houseman                             1   1
Luke Howard, Esq.                        3   3
S. R. Howard, Esq.                       1   1
T. M. Hubert, Esq.                       1   1
Henry Hughs, Esq.                        3   3
Richard Hughes, Esq.                     5   5
Hull Subscription Library                1   1
John Hull, M.D.                          2   2
Dr. Hull                                 2   2
Dr. Hulme                                2   2
N. Hulme, M.D. F.R.S. and F.A.S          3   3
Mr. N. Hume                              3   3
Mr. J. Hume                              1   1
Mrs. Hunt                                3   3
Robert Hunter, Esq. _Kew_                1   1
Dr. Hunter, _York_
Mr. John Hustler                         1   1
Walter Hutton, Esq.                      2   2
Mr. Hutton, _Sheffield_

J
G. Jackson, Esq. _St. Petersburg_    1  11   6
Mr. Jackson, _Lancaster_                 1   1
Miss Sarah Jackson                       1   1
William James, Esq.                      1   1
Thomas I'anson, Esq.                     2   2
------Jaques, M.D. _Harrowgate_          2   2
------Jardine, M.D.                      2   2
George Jeffery, Esq.                     1   1
Edward Jenner, M.D.  F.R.S.  F.L.S.     10  10
Edward Jenings, Esq. _Kensington_        1   1
William Ingham, Esq. _Newcastle
   upon Tyne_                            2   2
Rt. Innes, Esq. _Newcastle upon Tyne_    1   1
Edward Johnson, Esq. _Mile End_          1   1
Christopher Johnson, Esq.                1   1
R. Johnson, Esq.                         1   1
------Jones, Esq. _Chelsea_              1   1
W. Jones, Esq. _St. Petersburg_      1  11   6
Thomas Jones, Esq. _Llandisillio
   Hall, Oswestry_                       1   1
Mr. John Jones, _Surgeon_                2   2
Stephen Jones, Esq.                      1   1
Gibbs Walker Jordan, Esq. F.R.S.         3   3
Major Jourdan                            1   1
Rev. J. Joyce                            1   1
Thomas Irving, Esq.                      2   2
Thomas Irwin, Esq.                       1   1
Mr. Ives, _Surgeon, Chertsey_

K
The Hon. George Knox, F.R.S.            10  10
Frederick Kanmacher, Esq. P.R.I.
   F.L.S.                                1   1
Mr. B. A. Keck, _Leeds_
Miss Keene                               2   2
Mr. P. Kelly                             1   1
Dr. E. Kentish, _Bristol_
Mr. Jonathan Key                         1   1
T. King, Esq. _Carshalton, Surry_        1   1
James King, Esq.                         2   2
J. King, Esq. _Finsthwaite, Kendal_      1   1
Messrs. Knipe and Mower                  2   2
Kirkby Lonsdale Book Club                1   1
John Knox, Esq.                          1   1
William Knox, Esq.                       1   1

L
Right Hon. Lady Lyttelton                3   3
Sir James Lake, Bart.                    1   1
George Lackington, Esq.                  2   2
Mr. W. Laforest                          2   2
H. Lamatte, Esq.                         1   1
Mrs. Lappan                              1   1
Samuel Lawford, Esq. P.R.I. _Peckham_    1   1
Thomas Wright Lawford, Esq.              1   1
Mr. Samuel Lawford, jun.                 1   1
Johnson Lawson, Esq. P.R.I.              5   5
Henry Lawson, Esq.                       5   5
Rev. G. Lawson                           1   1
James Inglish Lawson, Esq.               1   1
William Leake, Esq.                      2   2
Mr. Joseph Leay                          1   1
Mr. William Leay                         1   1
Rev. Francis Lee                         2   2
John Leeds, Esq. _Chelsea_               1   1
James Leighton, Esq.                     2   2
Dr. Lempriere                            1   1
J. Lestourgeon, Esq.                     1   1
Thomas Lewis, Esq. P.R.I.                2   2
William Lewis, Esq. F.L.S.               5   5
Mr. Lewthwaite                           1   1
Library of the Sheffield Infirmary
Henry Lider, Esq. _Stilton_              1   1
John Lightbody, Esq. _Liverpool_         5   5
A. S. Lillingston, Esq. _Llyne_          1   1
Lincoln Medical Society
Literary Fund                           30   0
Literary Society at Newcastle            2   2
Mr. Littlewood                           1   1
Liverpool Library                        1   1
John Lloyd, Esq. _Bashall Lodge,
   Yorkshire_                            1   1
H. E. Lloyd, Esq.                        1   1
Mr. James Long, _Optician_               3   3
C. Lormar, Esq. _Bath_                   1   1
Mr. R. Low, _Surgeon_                    1   1
James Lowe, Esq.                         2   2
Mrs. W. Lowry                            5   5
Captain John Lucas                       1   1
Mr. Lucas                                2   2
James Ludlam, Esq. _Homerton_            1   1
Mr. Lumley                               1   1
William Luxmoore, Esq. P.R.I.            1   1
T. Luxmoore, Esq.                        1   1
Lynn Subscription Library                1   1
John Lyon, M.D.                          3   3

M
Right Hon. Lord Muncaster
John Mac Arthur, Esq. P.R.I.             1   1
John McCartney, M.D.                     2   2
A. McCausland, Esq. _St. Petersburg_ 1  11   6
Angus McDonald, Esq.                     2   2
Angus McDonald, M.D.                     1   1
Dr. Macdonald _Taunton, Somerset._       1   1
Duncan Macfarlane, Esq.                  5   5
George McIntosh, Esq.                    4   4
Dr. McIntosh                             2   2
Mr. Mackender                            1   1
Alexander McLeay, Esq.                   1   1
Mr. Maddon, jun.                         1   1
John Mair, Esq.                          2   2
Mr. Malcolm                              1   1
F. H. Maltz, jun. Esq. _Hamburgh_        1   1
Alexander Marcet, M.D. P.R.I.
   _Assistant Physician to Guy's
   Hospital_                             2   2
Charles Marsh, Esq. F.A.S. P.R.I.        1   1
Dr. Marshal                              1   1
Mr. William Marshall                     1   1
William Maud, Esq.                       1   1
P. W. Mayow, Esq.                        1   1
W. W. Mayow, Esq.                        1   1
Samuel Mellish, Esq. P.R.I.              1   1
W. Menish, Esq. P.R.I. _Bromley,
   Kent_                                 2   2
Richard Meux, jun. Esq.                  5   5
John Middleton, Esq.                     1   1
John Milford, Esq.                       2   2
James Millar, Esq. _College, Glasgow_    1   1
Captain Millar                           1   1
Miss Millar                              1   1
------Miller, Esq. _Peterborough,
   Surgeon_                              1   1
James Milne, _College, Glasgow_          2   2
Charles Minier, Esq.                     2   2
William Minier, Esq.                     2   2
Mr. Minchell, _Surgeon_                  3   3
Ed. Moberly, Esq. _St. Petersburg_   1  11   6
Mr. Mogg                                 1   1
Dr. Alex Monro, Sen. _Edinburgh_
Matthew Montagu, Esq. F.R.S.             1   1
Charles Montague, Esq. _Surgeon_         1   1
Robert Monteith, Esq.                    1   1
Jon Monteith, Esq.                       1   1
Robert Montgomerie, Esq.                 1   1
G. Morgan, Esq. _Norfolk, Virginia_      5   0
Mr. Joseph Morgan                        1   1
William Morley, jun. Esq. P.R.I.         1   1
John Morris, Esq.                        2   2
Mr. James Moss                           1   1
P. S. Munn, Esq.                         1   1
Sir John Chardin Musgrave, Bart.         1   1

N
Edward Nairne, Esq. F.R.S. P.R.I.        5   5
Rev. Archdeacon Nares, F.A.S.            2   2
Mr. Charles Newby                        2   2
Mr. J. Nicholas                          1   1
George Nicholson, Esq. _Ponchnill,
   near Ludlow_                          1   1
Mr. Francis Nicholson, _Artist,
   Somers Town_                          2   2
Mr. William Nicholson                    5   5
Mrs. Norman, _Chelsea_                   1   1
William Norris, Esq.                     2   2
Henry Norris, Esq. _Davy-Hulme-
   Hall, near Manchester_                5   5
Mr. Norris                               5   5
William North, Esq.                      1   1
Thomas Northmore, Esq.                   5   5
Messrs. Norton and Son, _Bristol_

O
James Ogilvy, Esq.                       1   1
Edward Ogle, Esq.                        1   1
Henry Okey, Esq.                         1   1
Mr. Oliver                               1   1
Mr. Serjeant Onslow                      1   1
Rev. Richard Ormerod, _Kensington_       1   1
Alexander Oswald, Esq.                   5   5
R. Oswin, Esq.                           2   2
Mr. Otley                                1   1
------Owen, Esq.                        10  10

P
Royal College of Physicians             20   0
The Earl of Pomfret, P.R.I.             10  10
Sir William Pepperell, Bart.             2   2
Sir Christopher Pegge, Knt. _Regius
   Professor of Physic, Oxford_
Philip Palmer, Esq.                      5   5
Felix Palmer, Esq.                       3   3
Mrs. Palmer                              3   3
Executors of Mrs. Palmer                10   0
Thomas Park, Esq. F.A.S.                 1   1
Mr. Park, _Surgeon_                      3   3
Mr. J. C. Parker                         5   5
Thomas Parker, Esq.                      5   5
Mr. Robert Parker                        1   1
G. Parkinson, Esq.                       3   3
------Parkinson, Esq. _Clitheroe_        1   1
Dr. Parkinson, _Lancaster_               1   1
John Parry, Esq.                         5   5
John Pattison, Esq.                      7   7
Mrs. Paulin, _Chelsea_                   1   1
William Payne, Esq. _Bradford_           1   1
Thomas Paytherus, Esq.                   2   2
John Peake, Esq. _Battersea_             1   1
C. Pears, Esq. F.L.S.                    2   2
Rev. William Pearson, P.R.I.             3   3
William Pearson                          3   3
Thomas Pearson, Esq.                     1   1
William Scott Peckham, Esq.              1   1
Edmund Peckover, Esq. _Bradford_         1   1
W. H. Pepys, Esq.                        5   5
W. H. Pepys, jun. Esq. P.R.I.            5   5
Edmund Pepys, Esq.                       1   1
Thomas Perceval, M.D. F.R.S. F.A.S.      2   2
Dr. Percival                             2   2
Mr. B. D. Perkins                        1   1
W. Perrin, Esq.                          2   2
James Perry, Esq. P.R.I.                 3   3
Louis Hayes Petit, Esq. F.A.S.           2   2
Mr. C. H. Pfeffel                        1   1
Rev. W. G. Phillips, P.R.I.              1   1
William Phillips, Esq.                   1   1
Mr. R. Phillips, _Bookseller_            5   5
Mr. Richard Phillips                     1   1
Mrs. Charles Phillott, _Bath_            1   1
Johnson Phillott, Esq. _Bath_            1   1
Philosophical Society, _Hythe, Kent_     1   1
Physical Society, _Guy's Hospital_       6   6
Library of the College of Physicians,
   Edinburgh
J. K. Picard, Esq. _Summergang's
   House, near Hull_                     2   2
Charles Pieschell, Esq. P.R.I.           2   2
Gillery Pigott, Esq.                     2   2
T. Pilkington, Esq, _Bewdley,
   Surgeon_                              1   1
William Pilkington, Esq.                 2   2
Mrs. Pilkington                          1   1
William Pim, Esq.                        2   2
I. Pinchard, Esq. _Taunton_              1   1
Dr. Pinckard                             2   2
Thomas Pitt, Esq. F.A.S. P.R.I.          2   2
William Plasted, Esq. _Chelsea_          1   1
------Pocock, Esq.                       1   1
Plymouth Literary Society                1   1
R. Podmore, Esq.                         1   1
Mr. William Pollard, _Bradford_          1   1
Thomas C. Porter, Esq.                   1   1
Portsmouth Book Society                  2   2
The Rev. Josiah Pratt, F.A.S.            1   1
J. Prescott, Esq. _St. Petersburg_   1  11   6
The Rev. I. Preston, _Flasby Hall,
   Yorkshire_                            1   1
Joseph Price, Esq.                       1   1
Rev. William Price, _Bath_               1   1
Benjamin Price, Esq.                     1   1
Mr. Joseph Priestly, _Bradford_          1   1
Mr. Prince                               1   1
Mr. Pritt                                1   1
Mr. Pullen                               1   1
T. Pulley, Esq.                          1   1

R
The Royal Institution of Great
   Britain                              50   0
Earl of Radnor, F.R.S. F.A.S.            4   0
Hon. Dr. Francis Rigby, _Jamaica_
The Hon. Mrs. Rollo                      2   2
Rev. Thomas Rackett, F.R.S. F.A.S.
   P.R.I. F.L.S.                         1   1
Mrs. Rackett                             1   1
Miss Rackett                         0  10   6
Richard Radford, Esq.                    1   1
Rev. Dr. Raine, F.R.S. F.A.S.            2   2
Richard Raley, Esq. _Clare Hall,
   Cambridge_                            1   1
George Ranking, Esq. P.R.I.              2   2
John Ranking, Esq. _St. Petersburg_  1  11   6
Mr. Rathbone                             5   5
Thomas Rawson, jun. Esq.                 2   2
Mr. W. L. Reay, _Surgeon_                2   2
Rev. Dr. Rees, F.R.S.                    1   1
Joshua Reeve, Esq. P.R.I.                5   5
T. R. Reid, Esq.                         1   1
James Remnant, Esq.                      2   2
Thomas Renwick, M.D.                     3   3
George Reveley, Esq.                     1   1
S. W. Reynolds, Esq.                     2   2
Dr. Reynolds, F.R.S. F.A.S.              2   2
D. Ricardo, Esq.                         1   1
Mr. Rich, _Fryston, Yorkshire_           5   0
Mrs. Richards                            5   5
Isaac Ried, Esq.                         1   1
J. Ring, Esq.                            2   2
John Risdon, Esq. _Peckham_              1   1
Mr. Rittson, _Lancaster, Surgeon_        1   1
Thomas Roberts, Esq.                     1   1
James Robertson, Esq.                    1   1
Arthur Robinson, Esq.                    1   1
Mr. Robinson, _York_                     1   1
------Robinson, Esq.                     1   1
Jonathan Rogers, M.D.                    3   3
Dr. Rogerson                             2   2
John Rollo, M.D.                         1   1
Thomas Rolph, Esq. _Peckham_             1   1
Samuel Romilly, Esq.                     2   2
William Rosco, Esq.                      3   3
Dr. Rowley                               5   5
------Rowley, M.D.                       5   5
J. Rowlat, Esq. _St. Petersburg_     1  11   6
Edward Rudge, Esq. F.L.S.                1   1
Rev. Joshua Ruddock, A.M.                2   2
Mr. L. T. Rupp                           2   2
Mr. Rupp                                 2   2
T. Rutt, Esq.                            1   1
John Rutter, M.D.                        2   2

S
Earl Stanhope, F.R.S. P.R.I.             1   1
Sir Richard Joseph Sullivan, Bart.
   F.R.S. F.A.S. P.R.I.                  5   5
Sir John St. Aubyn, Bart.
Mr. William Salter                       1   1
Dr. Satterley, _Tunbridge Wells_         1   1
George Saunders, Esq. F.A.S.             1   1
Thomas Scott, Esq.                       1   1
Mrs. P. Scott                            2   0
Mr. G. M. Sears                          1   1
John Semple, Esq.                        2   2
Miss Senior, _Sittingbourne_             1   1
Thomas Serjeant, Esq.                    1   1
Dr. Serney, _Boston, Lincolnshire_       1   1
Henry Settrey, Esq.                      1   1
Mr. Edmund Shadwick                      1   1
George Grinley Sharpe, Esq. _Peckham,
   Surgeon_                              1   1
Mr. Shaw, _Surgeon_                      2   2
Townley Rigby Shawe, Esq. _Preston_      1   1
Miss Shee                                1   1
Miss E. Shee                             1   1
Miss J. Shee                             1   1
Miss H. Shee                             1   1
Library of Sheffield Infirmary
W. Shepherd, Esq.                        1   1
Stephen Shute, Esq.                      1   1
Mr. William Silver, _Portsmouth_         1   1
Richard Simmons, Esq. _Surgeon_          2   2
Mr. John Simmons                         1   1
Mr. Thomas Simpkin                       1   1
Mr. John Simpson                         1   1
Mr. Robert Simpson, jun.                 1   1
Samuel Everingham Sketchley, Esq.        1   1
Mr. Alexander Sketchley                  1   1
Mr. Alexander Sketchley, _Clapham_       1   1
------Slade, Esq.                        1   1
R. Smales, Esq.                          1   1
Rev. Mr. Smirnove                        1   1
John Stafford Smith, Esq.                1   1
J. R. Smith, Esq.                        5   5
Nathan Smith, Esq.                       1   1
------Smith, Esq. _Lancaster,
   Surgeon_                              1   1
James Smith, Jun. Esq. _Glasgow_         2   2
Dr. Smith                                1   1
Rev. Thomas Jenys Smith _Dulwich_        1   1
Mr. Egerton Smith                        1   1
Mr. Frederick Smith                      1   1
Mr. Smithson, _Heath, Wakefield_         2   2
Society for the Encouragement of
   Arts, Manufactures, and Commerce      5   5
Samuel Solly, Esq. P.R.I.                2   2
Archibald Sorley, Esq.                   2   2
James Sowerby, Esq. _Lambeth_            1   1
John Spence, Esq. _Greenock_             3   3
Knight Spencer, Esq.                     2   2
------Spencer, Esq. _Surgeon_            2   2
John Spencer, Esq.                       2   2
John Sperling, Esq.                      1   1
M. Spratt, Esq.                          1   1
William Stanistreet, Esq.                3   3
William Stanniforth, Esq. _Sheffield,
   Surgeon_
Rev. T. Starkie, A.M. _Blackburn_        1   1
S. F. Statham, Esq. _Arnold, Notting._   1   1
Mr. William Standley, _Bradford_         1   1
Mr. Thomas Steel                         1   1
J. Steers, Esq.                          1   1
Francis Stephens, Esq.                   1   1
Mr. J. Stephenson                        1   1
Rev. W. Stevens                          1   1
William Stodart, Esq. P.R.I.             5   5
Mr. James Stodart, P.R.I.                5   5
Mr. M. Stodart                           1   1
Miss Strickland, _Boynton_
Sunderland Subscription Library          1   1
John Swale, Esq.                         1   1
Mr. Swan                                 1   1
Mr. Sydenham, _no Book_                  1   1
Godfrey Sykes, Esq.                      1   1

T
Peter Tahourdin, Esq.                    1   1
John Tate, jun. Esq.                     1   1
C. H. Tatham, Esq.                       1   1
B. Tayle, Esq.                           1   1
John Taylor, Esq.                        3   3
James Taylor, Esq.                       2   2
R. S. Taylor, Esq.                       1   1
Mr. I. Taylor, _Gomersall_
Arthur Tegart, Esq.                      1   1
Dr. Temple                               5   5
Mrs. Tennant                             2   2
Mr. Frederick Thackerey, _Emanuel
   College, Cambridge                    1   1
H. L. Thomas, Esq.                       1   1
Robert Thompson, Esq. F.A.S.             2   2
Robert Thompson, jun. Esq.               1   1
Mrs. Thompson                            2   2
John P. Thompson, Esq.                   1   1
Messrs. Thomson and Son, _Manchester_    1   1
Robert Thornton, Esq. M.P. F.A.S.
   P.R.I.                                5   5
Samuel Thornton, Esq. F.A.S. P.R.I.      1   1
Samuel Thornton, Esq.                    1   1
Robert John Thornton, M.D.               1   1
Anthony Thorp, Esq. _York_               1   1
William Hall Timbrel, Esq.               1   1
Mr. Cornelius Tongue                     1   1
John Townshend, Esq. _Chelsea_           5   5
John Townsend, Esq. _Wandsworth_         5   5
Samuel Turner, Esq.                      2   2
Thomas Turner, M.D.                      2   2
Sharon Turner, Esq. F.A.S.               1   1
Dawson Turner, Esq. F.A.S.               2   2
J. Turton, Esq.                          1   1
Thomas Tyndall, Esq. _Bristol_

U
Peter Vere, Esq. F.A.S.                  1   1
Mr. Underwood                            1   1
James Upton, Esq.                        1   1
J. Upton, Esq.                           1   1
T. Upward, Esq.                          2   2
Miss S. Vranken, _Bristol_
Gordon Urquhart, Esq.                    1   1

W
Sir William E. Welby, Bart.              2   2
Honourable J. S. Wortley, P.R.I.         1   1
M. F. Wagstaff, Esq.                     1   1
Daniel Walker, Esq.                      1   1
Samuel Walker, Esq. _Masbrough_
Mr. J. Walker                            1   1
Mr. Joseph Walker                        1   1
Martin Wall, M.D. _Clinical Professor,
   Oxford_
T. Walshman, M.D. _Physician to the
   Western and Surry Dispensaries, and
   President of the Physical Society     1   1
John Walter, Esq. _Pimlico_              1   1
D. Walters, Esq.                         1   1
Olaus Warberg, Esq. _Copenhagen_         1   1
Mrs. Ward, _Hampstead_                   1   1
Gilbert Wardlaw, Esq. _Glasgow_          1   1
James Ware, Esq. F.R.S. F.A.S.           5   5
Rev. John Ware, _York_
John Warren, Esq. P.R.I.                 1   1
Mr. Watkins                              2   2
Mr. Watson                               2   2
W. H. Watts, Esq.                        5   5
David Pike Watts, Esq. P.R.I.            2   2
Mrs. Wawn                                1   1
Rev. George Wearing, _Whalley_           1   1
Mr. James Weatherby                      1   1
Mr. Matthew Wedeson                      1   1
J. Wedgwood, Esq. P.R.I.                 1   1
J. C. Weguelin, Esq.                     2   2
Mr. J. Welbank                           1   1
William Earle Welby, Esq. _Carlton
   House, Nottinghamshire_               3   3
Richard Earle Welby, Esq.                2   2
George Welch, Esq.                       1   1
Mrs. Welch                               2   2
Thomas West, Esq. _Bath_                 1   1
Mrs. West                                5   5
Westminster Library                      1   1
Lawson Whalley, Esq. _Lancaster_         1   1
Lawson Whalley, Esq. _Lancaster_         1   1
The Rev. John Wheeler                    1   1
Samuel Whitbread, Esq. M.P.              3   3
Edward White, Esq.                       1   1
Rev. Samuel White                        1   1
Mrs. White, _Bury St. Edmunds_           2   2
Miss White                               1   1
Rev. Walter Whiter                       1   1
Robert Whitfield, Esq. _Surgeon_         1   1
William Whittington, Esq.                5   5
Miss Whittington                         3   3
Miss E. Whittington                      2   2
Edward Athenry Whyte, Esq. _Dublin_      1   1
Rev. Thomas Wigan                        1   1
Mr. John Wiglesworth                     1   1
Lieut. Col. Wilder                       1   1
Mr. Wilkes                               1   1
Thomas Willan, Esq.                      5   5
Mrs. Willlan                             5   5
John Willan, Esq.                        5   5
John Williams, Esq. _Caermarthen,
   Surgeon_                              1   1
------Williams, Esq.                     1   1
Mr. William Henry Williams, _Caius
   College, Cambridge_                   1   1
Dr. A. F. M. Willich                     1   1
Dr. Robert Willis                        5   5
H. W. C. Wilson, Esq.                    2   2
James Wilson, Esq.                       2   2
Dr. Wilson                               2   2
Rev. Thomas Wilson, _Clitheroe_          1   1
Miss Winckworth                          1   1
Joseph Windham, Esq. F.R.S. F.L.S.       2   2
Alexander Wood, Esq. _Edinburgh,
   Surgeon_
R. Wood, Esq.                            1   1
Robert Wood, Esq. _Tamworth_             1   1
William Wood, Esq. _Hackney_             1   1
Mr. James Wood                           1   1
Samuel Woods, Esq.                       3   3
Mr. Henry Worboys                        1   1
Mr. John Worboys                         1   1
Peter Wright, Esq. P.R.I.                2   2
Mr. James Wright, _Liverpool_            2   2
Dr. Wright                               1   1
Mr. Wright, _Optician_                   1   1
William Wynch, Esq. P.R.I.               2   2
W. Wynch, Esq. _2nd Subscription_        1   1

Y
Joseph Yallowley, Esq.                   1   1
Rev. John Yates                          3   3
Matthew Yatman, Esq. F.A.S.              1   1
Dr. Yelloly                              2   2
J. W. Yonge, Esq.                        2   2
Thomas Young, M.D. For. Sec. R.S.
   F.L.S.                                5   5
G. Young, Esq.                           1   1
William Younge, M.D. _Sheffield_

From the Press of the Royal Institution of Great Britain,
Albemarle Street, London: William Savage, Printer.

ADDENDA.
                                        _L.  s._
A
The Right Hon. Lord Altamont             1   1
Nicholas Ashton, Esq.                    5   5
J. J. Angerstein, Esq.                  10  10
Mr. James Asperne                        1   1
J. B. Austin, Esq.                       1   1
Mr. Abernethy                            5   5

B
Scrope Barnard, Esq.                     1   1
C. F. Bamwell, Esq.                      1   1
Rev. Dr. Balward                         1   1
Rev. T. Browne                           5   5
Rev. B. Bridge                           1   1
Rev. T. Brooke                           1   1
W. S. Bruere, Esq.                       1   1
J. D. Borton, Esq.                       1   1
William Breese, Esq.                     2   2
Mr. Bradner                              1   1
James Bureau, Esq.                       3   3
Mrs. Mary Bowyer                         1   1
Rev. G. Butler                           1   1
Rev. J. Brown                            1   1
Mr. Bryant                               1   1
John Butts, Esq.                         1   1
Mr. Bruce Boswell                        1   1

C
James Collins, Esq.                      1   1
William Cass, Esq.                       2   2
M. Da Costa                              1   1
F. Carter, Esq.                          1   1
Rev. T. Cautley                          1   1
G. Caldwell, Esq.                        1   1
Rev. T. Clarkson                         1   1
E. D. Clarke, Esq.                       1   1
T. M. Cripps, Esq.                       1   1
Rev. B. Chapman                          1   1

D
The Lord Bishop of Durham               10  10
P. Denys, Esq.                           4   4
Rev. F. Drake                            1   1
J. Dearman, Esq. _Champion Hill_         1   1
M. Dobson, Esq.                          1   1
Dr. Denman                               2   2
Mr. Durham                               2   0
Rev. Dr. Dealtry                         1   1
Martyn Davy, M.D.                        1   1
Philip Derbishire, Esq.                  1   1

E
Sir James Esdaile                        1   1
Dr. G. B. Eaton                          1   1

F
John Fuller, Esq.                        1   1
Edward Foster, Esq.                      1   1
Edward Foster, jun. Esq.                 1   1
John Fellowes, Esq.                      1   1
A friend, W. H. S. by Mr. Cuppage        1   1
A friend                                 5   5
A friend, S. B.                          2   2
A friend, R. R.                          1   1
A friend, C. A. Esq.                     1   1

G
Stephen Gaselee, Esq.                    2   2
W. B. Garlike, M.D.                      2   2
D. Garnett, Esq.                         1   1
Mrs. Garnett                             1   1
J. R. Gladdow, Esq.                      1   1

H
The Hon. Major Henniker                  1   1
J. Hall, Esq.                            1   1
------Hill, Esq.                         2   2
Richard Hussey, Esq.                     2   2
Mark Huish, jun. Esq.                    1   1
Nathaniel Hubbins, Esq.                  1   1
Dr. Heberden                             2   2
T. Holmes, Esq.                          1   1
John Holkyard, Esq. _Surgeon_            1   1
Professor Harwood                        2   2
E. Hibgame, Esq.                         1   1
T. P. Hackhouse, Esq.                    1   1
B. Harvey, Esq.                          1   1
Rev. T. Holden                           1   1
Rev. Mr. Holme                           1   1
William Hutton, Esq.                     1   1
Dr. Halliday                             2   2
Thomas Halliday, Esq.                    1   1
William Holden, Esq.                     1   1
------Van Hoorst, Esq.                   1   1

J
Rev. T. Jones                            1   1
Mr. S. Jones                             1   1

K
L. Knight, Esq.                          1   1
Rev. J. Kenrick                          1   1

L
Sir John Leggart, Bart.                  1   1
William Lewis, Esq.                      1   1
A. S. Lillingstone, Esq.                 2   2
Dr. Latham                               5   5
M

Basil Montague, Esq.                     1   1
James Mackintosh, Esq.                   1   1
H. Mayers, Esq.                          1   1
Lieutenant Col. McDonald                 1   1
M. Marshall, Esq.                        1   1
Rev. Mr. Maule                           1   1
Rev. Mr. Manning                         1   1
Rev. W. Miller                           1   1
Dr. John William Minier                  2   2
Mr. Miller                               1   1
Mr. Marshall                             1   1

N
Richard Nixon, Esq.                      1   1
George Norman, Esq.                      3   3
------Nottage, Esq.                      2   2
Mr. Richard Nicholls                     1   1

O
Benjamin Oakley, Esq.                    1   1
Rev. W. Otter                            1   1

P
Hon. Mr. Pakenham                        1   1
N. Palmer, Esq.                          1   1
Dr. David Pitcairn                       5   5
Dr. Pennington                           1   1
The Rev. G. Pollen                      10  10
St. Peter's College                      6   6

R
W. Robinson, Esq.                        1   1
William Rathbone, Esq.                   5   5
Rev. H. R. Rouney                        1   1
Rev. Mr. Renouard                        1   1
Mr. Rawlins                              1   1

S
A. Smith, Esq.                           2   2
James Skirrow, Esq.                      1   1
Dr. Sloper                               5   5
John Simpson, M.D.                       1   1
Rev. Mr. Simeon                          1   1
Rev. J. Satterthwaite                    1   1

T
The Right Hon. John Trevor               2   2
Rev. Dr. Towitt                          1   1
Mr. Thomas Teasdale                      1   1

V
Rev. T. Vickers                          1   1

W
M. S. Wakefield, Esq.                    1   1
Thomas Wilson, Esq.                      2   2
R. S. Wells, Esq.                        1   1
William White, Esq.                      1   1
Joseph Walker, Esq.                      1   1
John Welbank, Esq.                       1   1
Rev. J. Wood                             1   1
Rev. Mr. Wheatear                        1   1
Rev. J. Walker                           1   1
Mrs. Ward                                1   1

Y
J. A. Young, Esq.                        2   2
Dr. Young                                1   1






End of Project Gutenberg's Popular Lectures on Zoonomia, by Thomas Garnett