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                          LOCKYER’S ASTRONOMY

                       _ELEMENTS OF ASTRONOMY_:

   Accompanied with numerous Illustrations, a Colored Representation
              of the Solar, Stellar, and Nebular Spectra,
                 and Celestial Charts of the Northern
                     and the Southern Hemisphere.

                    By J. NORMAN LOCKYER.

   _American edition_, revised and specially adapted to the Schools
                         of the United States.

                   _12mo. 312 pages. Price, $1.75._

The volume is as practical as possible. To aid the student in
identifying the stars and constellations, the fine Celestial Charts of
Arago, which answer all the purposes of a costly Atlas of the Heavens,
are appended to the work--this being the only text-book, as far as the
Publishers are aware, that possesses this great advantage. Directions
are given for finding the most interesting objects in the heavens at
certain hours on different evenings throughout the year. Every device is
used to make the study interesting; and the Publishers feel assured that
teachers who once try this book will be unwilling to exchange it for any
other.

                D. APPLETON & CO., PUBLISHERS,
                549 & 551 BROADWAY, NEW YORK.




                     SCIENCE PRIMERS, _edited by_
                  _PROFESSORS_ HUXLEY, ROSCOE, _and_
                           BALFOUR STEWART.

                                  VI.
                             _PHYSIOLOGY._

[Illustration: EXPLANATION OF THE PLATE.

FIG. I.--THE HUMAN SKELETON IN PROFILE.

    _Mn._       The Mandible or Lower Jaw.

    _St._       The Sternum.                   }
    _R._        The Ribs.                      } In the Thorax.
    _R´._       The Cartilages of the Ribs.    }

    _Scp._      The Scapula, or Shoulder Blade.

    _Cl._       The Clavicle, or Collar Bone.

    _H._        The Humerus.                   }
    _Ra._       The Radius.                    } In the Arm.
    _U._        The Ulna.                      }

    _F._        The Femur.                     }
    _Tb._       The Tibia.                     } In the Leg.
    _Fb._       The Fibula.                    }

FIG. II.

A front view of the Sternum, _St._, with the Cartilages of the Ribs,
_R´._, and part of the Ribs themselves _R._]




                           Science Primers.

                              PHYSIOLOGY.

                                  BY
                    M. FOSTER, M.A., M.D., F.R.S.,
                 FELLOW OF TRINITY COLLEGE, CAMBRIDGE.

                          WITH ILLUSTRATIONS.

                               NEW YORK:
                       D. APPLETON AND COMPANY,
                         549 AND 551 BROADWAY.
                                 1877.




PREFACE.


This Primer is an attempt to explain in the most simple manner possible
some of the most important and most general facts of Physiology, and may
be looked upon as an introduction to the Elementary Lessons of Professor
Huxley.

In my descriptions and explanations I have supposed the reader to be
willing to handle and examine such things as a dead rabbit and a sheep’s
heart; and written accordingly, I have done this purposely, from an
increasing conviction that actual observation of structures is as
necessary for the sound learning of even elementary physiology, as are
actual experiments for chemistry. At the same time I have tried to make
my text intelligible to those who think reading verbal descriptions less
tiresome than observing things for themselves.

It seemed more desirable in so elementary a work to insist, even with
repetition, on some few fundamental truths, than to attempt to skim over
the whole wide field of Physiology. I have therefore omitted all that
relates to the Senses and to the functions of the Nervous System, merely
just referring to them in the concluding article. These the reader must
study in the “Elementary Lessons.”

M. FOSTER.




TABLE OF CONTENTS.


ART. SECT.

    I. INTRODUCTION.
                                                                    PAGE

 1. “  What Physiology is                                              1

 2. “  Animals move of their own accord                                1

 3. “  Animals are warm                                                3

 4. “  Why animals are warm and move about--they burn                  4

 5. “  The need of Oxygen                                              5

 6. “  The waste matters                                               5


   II. THE PARTS OF WHICH THE BODY IS MADE UP.

 7. “  The Tissues                                                     7

 8. “  The cavities of the Thorax and Abdomen                          9

 9. “  The Vertebral Column                                           12

10. “  Head and Neck                                                  15

11. “  Nerves                                                         18

12. “  General arrangement of all these parts                         19


   III. WHAT TAKES PLACE WHEN WE MOVE.

13. “  The Bones of the Arm                                           21

14. “  The structure of the Elbow Joint                               23

15. “  Other joints in the body                                       25

16. “  The arm is bent by the contraction of the Biceps Muscle        26

17. “  How the will makes the Biceps Muscle contract                  32

18. “  The power of a muscle to contract depends on its being
         supplied with blood                                          35

19. “  It is the food in the blood which gives the muscle strength    37

20. “  The continual need of food                                     38


   IV. THE NATURE OF BLOOD.

21. “  The Blood in the Capillaries                                   40

22. “  The Corpuscles of the Blood                                    42

23. “  The clotting of Blood                                          45

24. “  The substances present in Serum                                48

25. “  The minerals in Blood                                          50


    V. HOW THE BLOOD MOVES.

26. “  The Arteries, Capillaries, and Veins                           51

27. “  The Sheep’s Heart                                              55

28. “  The Course of the Circulation                                  58

29. “  Why the blood moves in one direction only; the Valves
         of the Veins                                                 64

30. “  The Tricuspid Valves of the Heart                              66

31. “  The pulmonary Semilunar Valves                                 71

32. “  The left side of the Heart                                     72

33. “  What makes the blood move at all: The beat of the Heart        76

34. “  The action of the Heart as a whole                             79

35. “  The Capillaries and the Tissues                                82


   VI. HOW THE BLOOD IS CHANGED BY AIR: BREATHING.

36. “  Venous and Arterial Blood                                      85

37. “  The change from Arterial to Venous, and from Venous to
         Arterial Blood                                               87

38. “  The Lungs                                                      88

39. “  The renewal of Air in the Lungs. How the descent of the
         Diaphragm expands the Lungs                                  90

40. “  The natural distension of the Lungs. Inspiration. Expiration   91

41. “  How the Diaphragm descends                                     96

42. “  The Chest is also enlarged by the movements of the Ribs
         and Sternum                                                  98

43. “  Breathing an involuntary act                                  100

44. “  Tidal air; stationary air                                     101


   VII. HOW THE BLOOD IS CHANGED BY FOOD: DIGESTION.

45. “  Why the inside of the mouth is always red and moist           103

46. “  Why the Skin is sometimes moist. Sweat Glands                 106

47. “  The Mucous Membrane of the Alimentary Canal and its Glands    109

48. “  The Salivary Glands, Pancreas and Liver                       111

49. “  Food-stuffs                                                   113

50. “  How proteids and starch are changed                           115

51. “  Lacteals and Lymphatics                                       117

52. “  What becomes of the Food-stuffs                               120


   VIII. HOW THE BLOOD GETS RID OF WASTE MATTERS.

53. “  The need of getting rid of Waste Matters                      123

54. “  The Kidneys get rid of Ammonia in the form of Urea            125


55. IX. THE WHOLE STORY SHORTLY TOLD.                                127


56. X.  HOW WE FEEL AND WILL.                                        130




SCIENCE PRIMERS.




_PHYSIOLOGY._




INTRODUCTION. § I.

=1.= Did you ever on a winter’s day, when the ground was as hard as a
stone, the ponds all frozen, and everything cold and still, stop for a
moment, as you were running in play along the road or skating over the
ice, to wonder at yourself and ask these two questions:--“Why am I so
warm when all things around me, the ground, the trees, the water, and
the air, are so cold? How is it that I am moving about, running,
walking, jumping, when nothing else that I can see is stirring at all,
except perhaps a stray bird seeking in vain for food?”

These two questions neither you nor anyone else can answer fully; but we
may answer them in part, and the knowledge which helps us to the answer
is called =Physiology=.

=2. You can move of your own accord.= You do not need to wait, like the
boughs or the leaves, till the wind blows upon you, or, like the stones,
till somebody stirs you. The bird, too, can move of its own accord, so
can a dog, so can any animal as long as it is alive. If you leave a
stone in any particular spot, you expect to find the stone there when
you come to it again a long time afterwards; if you do not, you say
somebody or something has moved it. But if you put a sparrow or mouse on
the grass plot, you know that directly your back is turned it will be
off.

All animals move of themselves. But only so long as they are alive. When
you find the body of a snake on the road, the first thing you do is to
stir it with a stick. If it moves only as you move it, and as far as you
move it, just as a bit of rope might do, you say it is dead. But if,
when you touch it, it stirs of itself, wriggles about, and perhaps at
last glides away, you know it is alive. Every living animal, of whatever
kind, from yourself down to the tiniest creature that swims about in a
little pool of water and cannot be seen without a microscope, moves of
itself. Left to itself, it moves and rests, rests and moves; stirred by
anything, away it goes, running, flying, creeping, crawling, or
swimming.

Something of the kind sometimes happens with lifeless things. When a
stone is carefully balanced on the top of a high wall, a mere touch will
send it toppling down to the ground. But when it has reached the ground
it stops there, and if you want to repeat the trick you must carry the
stone up to the top of the wall again. You know the toy made like a
mouse, which, when you touch it in a particular place, runs away
apparently of its own accord, as if it were alive. But it soon stops,
and when it has stopped you may touch it again and again without making
it go on. Not until you have wound it up will it go on again as it did
before. And every time you want it to run you must wind it up afresh.
Living animals move again and again, and yet need no winding up, for
they are always winding themselves up. Indeed, as we go on in our
studies we shall come to look upon our own bodies and those of all
animals as pieces of delicate machinery with all manner of springs,
which are always running down but always winding themselves up again.

=3. You are warm=; beautifully warm, even on the coldest winter day, if
you have been running hard; very warm if you are well wrapped up with
clothing, which, as you say, keeps the cold out, but really keeps the
warmth in. The bed you go to at night may be cold, but it is warm when
you leave it in the morning. Your body is as good as a fire, warming
itself and everything near it.

The bird too is warm, so is the dog and the horse, and every four-footed
beast you know. Some animals however, such as reptiles, frogs, fish,
snails, insects, and the like do not seem warm when you touch them. Yet
really they are always a little warm, and some times they get quite
warm. If you were to put a thermometer into a hive of bees when they are
busy you would find that they are very warm indeed. All animals are more
or less warm as long as they are alive, some of them, such as birds and
four-footed beasts, being very warm. But only so long as they are alive;
after death they quickly become cold. When you find a bird lying on the
grass quite still, not stirring when it is touched, to make quite sure
of its being dead you feel it. If it is quite cold, you say it has been
dead some time; if it is still warm, you say it is only just
dead--perhaps hardly dead, and may yet revive.

=4. You are warm, and you move about of yourself. You are able to move
because you are warm; you are warm in order that you may move. How does
this come about?= Just think for a moment of something which is not an
animal, but which is warm and moves about, which only moves when it is
warm, and which is warm in order that it may move. I mean a locomotive
steam-engine. What makes the engine move? The burning coke or coal,
whose heat turns the water into steam, and so works the piston, while at
the same time the whole engine becomes warm. You know that for the
engine to do so much work, to run so many miles, so much coal must be
burnt; to keep it working it must be “stoked” with fresh coal, and all
the while it is working it is warm: when its stock of coal is burnt out
it stops, and, like a dead animal, grows cold.

Well, your body too, just like the steam-engine, moves about and is
warm, because a fire is always burning in your body. That fire, like the
furnace of the engine, needs fresh fuel from time to time, only your
fuel is not coal, but food. In three points your body differs from the
steam-engine. In the first place, you do not use your fire to change
water into steam, but in quite a different way, as we shall see further
on. Secondly, your fire is a burning not of dry coal, but of wet food, a
burning which although an oxidation (Chemistry Primer, Art. 5) takes
place in the midst of water, and goes on without any light being given
out. Thirdly, the food you take is not burnt in a separate part of your
body, in a furnace like that of the engine set apart for the purpose.
The food becomes part and parcel of your body, and it is your whole body
which is burnt, bit by bit.

Thus it is the food burning or being oxidized within your body, or as
part of your body, which enables you to move and keeps you warm. If you
try to do without food, you grow chilly and cold, feeble, faint, and too
weak to move. If you take the right quantity of proper food, you will be
able to get the best work out of the engine, your body; and if you work
your body aright, you can keep yourself warm on the coldest winter day,
without any need of artificial fire.

=5.= But if this be so, in order to oxidize your food, you =have need of
oxygen=. The fire of the engine goes out if it is not fed with air as
well as fuel. So will your fire too. If you were shut up in an air-tight
room, the oxygen in the room would get less and less, from the moment
you entered the room, being used up by you; the oxidation of your body
would after a while flag, and you would soon die for want of fresh
oxygen (see Chemistry Primer, p. 14).

You have, throughout your whole life, a need of fresh oxygen, you must
always be breathing fresh air to carry on in your body the oxidation
which gives you strength and warmth.

=6.= When a candle is burnt (Chemistry Primer, p. 6) it turns into
carbonic acid, and water. When wood or coal is burnt, we get ashes as
well. If you were to take all your daily food and dry it, it too would
burn into ashes, carbonic acid, and water (with one or two other things
of which we shall speak afterwards).

Your body is always giving out carbonic acid (Chemistry Primer, Exp. 7).
Your body is always giving out water by the lungs, as seen when you
breathe on a glass, by the skin, and by the kidneys; and we shall see
that we always give out more water than we take in as food or drink.
Your body too is daily giving out by the kidneys and bowels, matters
which are not exactly ashes, but very like them. We do not oxidize our
food quite into ashes, but very nearly; we burn it into substances which
are no longer useful for oxidation in the body, and which, being
useless, are cast out of the body as waste matters.

The tale then is complete. By the help of the oxygen of the air which
you take in as you breathe, you oxidize the food which is in your body.
You get rid of the water, the carbonic acid, and other waste matters
which are left after the oxidation, and out of the oxidation you get the
heat which keeps you warm and the power which enables you to move.

Thus all your life long you are in constant need of oxygen and food. The
oxygen you take in at every breath, the food at every meal. How you get
rid of the waste matters we shall see further on.

If you were to live, as one philosopher of old did, in a large pair of
delicate scales, you would find that the scale in which you were would
sink down at every meal, and gradually rise between as you got lighter
and hungry. If the food you took were more than you wanted, so that it
could not all be oxidized, it would remain in your body as part of your
flesh, and you would grow heavier and stouter from day to day; if it
were less, you would grow thinner and lighter; if it were just as much
as and no more than you needed, you would remain day after day of
exactly the same weight, the scale in which you sat rising as much
between meals as it sank at the meal time.

What we have to learn in this Primer is--How the food becomes part and
parcel of your body; how it gets oxidized; how the oxidation gives you
power to move; how it is that you are able to move in all manner of
ways, when you like, how you like, and as much as you like.

First of all we must learn something about the build of your body, of
what parts it is made, and how the parts are put together.




THE PARTS OF WHICH THE BODY IS MADE UP. § II.


=7.= When you want to make a snow man, you take one great roll of snow to
make the body or trunk. This you rest on two thinner rolls which serve
as legs. Near the top of the trunk you stick in another thin roll on
either side--these you call the two arms: and lastly, on quite the top
of the trunk you place a round ball for a head. Head, trunk, and limbs,
_i.e._ legs and arms--these together make up a complete body.

In your snow man these are all alike, all balls of snow differing only
in size and form; but in your own body, head, trunk, and limbs are quite
unlike, as you might easily tell on taking them to pieces. Now you
cannot very well take your own body to pieces, but you easily can that
of a dead rabbit. Suppose you take one of the limbs, say a leg, to begin
with.

First of all there is the =skin= with the hair on the outside. If you
carefully cut this through with a knife or pair of scissors and strip it
off, you will find it smooth and shiny inside. Underneath the skin you
see what you call =flesh=, rather paler, not so red as the flesh of beef
or mutton, but still quite like it. Covering the flesh there may be a
little =fat=. In a sheep’s leg as you see it at the butcher’s there is a
good deal of fat, in the rabbit’s there is very little.

This reddish flesh you must henceforward learn to speak of as =muscle=. If
you pull it about a little, you will find that you can separate it
easily into parcels or slips running lengthways down the leg, each slip
being fastened tight at either end, but loose between. Each slip is what
is called =a muscle=. You will notice that many of these muscles are
joined, sometimes at one end only, sometimes at both, to white or bluish
white glistening cords or bands; made evidently of different material
from the muscle itself. They are not soft and fleshy like the muscle,
but firm and stiff. These are =tendons=. Sometimes they are broad and
short, sometimes thin and long.

As you are separating these muscles from each other you will see
(running down the leg between them) little white soft threads, very
often branching out and getting too small to be seen. These are =nerves=.
Between the muscles too are other little cords, red, or reddish black,
and if you prick them, a drop or several drops of blood will ooze out.
These are =veins=, and are not really cords or threads, but hollow tubes,
filled with blood. Lying alongside the veins are similar small tubes,
containing very little blood, or none at all. These are =arteries=. The
=veins= and =arteries= together are called =blood-vessels=, and it will be
easy for you to make out that the larger ones you see are really hollow
tubes. Lastly, if you separate the muscles still more, you will come
upon the hard =bone= in the middle of the leg, and if you look closely you
will find that many of the muscles are fastened to this bone.

Now try to put back everything in its place, and you will find that
though you have neither cut nor torn nor broken either muscle or
blood-vessel or bone, you cannot get things back into their place again.
Everything looks “messy.” This is partly because, though you have torn
neither muscle nor blood-vessel, you have torn something which binds
skin and muscle and fat and blood-vessels and bone all together; and if
you look again you will see that between them there is a delicate
stringy substance which binds and packs them all together, just as
cotton-wool is used to pack up delicate toys and instruments. This
stringy packing material which you have torn and spoilt is called
=connective= because it connects all the parts together.

Well, then, in the leg (and it is just the same in the arm) we have
skin, fat, muscle, tendons, blood-vessels, nerves, and bone all packed
together with connective and covered with skin. These together form the
solid leg. We may speak of them as =the tissues= of the leg.

=8.= If now you turn to the trunk and cut through the skin of the belly,
you will first of all see muscles again, with nerves and blood-vessels
as before. But when you carefully cut through the muscles (for you
cannot easily separate them from each other here), you come upon
something which you did not find in

[Illustration: FIG. 1.--_The Viscera of a Rabbit as seen upon simply
opening the Cavities of the Thorax and Abdomen without any further
Dissection._

     _A_, cavity of the thorax, pleural cavity of either side; _B_,
     diaphragm; _C_, ventricles of the heart; _D_, auricles; _E_,
     pulmonary artery; _F_, aorta; _G_, lungs, collapsed, and occupying
     only the back part of the chest; _H_, lateral portions of pleural
     membranes; _I_, cartilage at the end of sternum; _K_, portion of
     the wall of body left between thorax and abdomen; _a_, cut ends of
     the ribs; _L_, the liver, in this case lying more to the left than
     the right of the body; _M_, the stomach; _N_, duodenum; _O_, small
     intestine; _P_, the cæcum, so largely developed in this and other
     herbivorous animals; _Q_, the large intestine.
]

the leg, a =great cavity=. This is something quite new--there is nothing
like it in the leg--a great cavity, quite filled with something, but
still a great cavity; and if you slit the rabbit right up the front of
its trunk and turn down or cut away the sides as has been done in Fig.
1, you will see that the whole trunk is =hollow= from top to bottom, from
the neck to the legs.

If you look carefully you will see that the cavity is divided into two
by a cross partition (Fig. 1, _B_) called the =diaphragm=. The part =below=
the diaphragm is the larger of the two, and is called the =abdomen= or
belly; in it you will see a large dark red mass, which is the =liver=
(_L_). Near the liver is the smooth pale =stomach= (_M_), and filling up
the rest of the abdomen you will see the coils of the =intestine= or
bowel, very narrow in some parts (_O_), very broad (_P_ _Q_), broader
even than the stomach, in others. If you pull the bowels on one side as
you easily can do, you will find lying underneath them two small
brownish red lumps, one on each side. These are the =kidneys=.

In the smaller cavity =above= the diaphragm, called the =thorax= or chest,
you will see in the middle the =heart= (_C_), and on each side of the
heart two pink bodies, which when you squeeze them feel spongy. These
are the two =lungs= (_G_). You will notice that the heart and lungs do not
fill up the cavity of the chest nearly so much as the liver, stomach,
bowels, &c. fill up the cavity of the belly. In fact, in the chest
there seems to be a large empty space. But as we shall see further on,
the lungs did quite fill the chest before you opened it, but shrank up
very much directly you cut into it, and so left the great space you see.

=9.= The trunk then is really a great chamber containing what are called
the =viscera=, and divided into an upper and lower half, the upper half
being filled with the heart and lungs, the lower with the liver,
stomach, bowels, and some other organs. In front the abdomen is covered
by skin and muscle only. But if all the sides of the trunk were made of
such soft material it would be then a mere bag which could never keep
its shape unless it were stuffed quite full. Some part of it must be
strengthened and stiffened. And indeed the trunk is not a bag with soft
yielding sides, but a box with walls which are in part firm and hard.
You noticed that when you were cutting through the front of the chest
you had to cut through several hard places. These were the =ribs= (Fig. 1,
_a_), made either of hard bone or of a softer gristly substance called
=cartilage=. And if you take away all the viscera from the cavity of the
trunk and pass your finger along the back of the cavity, you will feel
all the way down from the neck to the legs a hard part. This is the
=backbone= or =vertebral column=. When you want to make a straw man stand
upright you run a pole right through him to give him support. Such a
support is the backbone to your own body, keeping the trunk from falling
together.

In the abdomen nothing more is wanted than this backbone, the sides and
front of the cavity being covered in with skin and muscle only. In the
chest the sides are strengthened by the ribs, long thin hoops of bone
which are fastened to the backbone behind and meet in front in a firm
hard part, partly bone, partly cartilage, called the =sternum=.

But this backbone is not made of one long straight piece of bone. If it
were you would never be able to bend your body. To enable you to do this
it is made up of ever so many little flat round pieces of bone, laid one
a-top of the other, with their flat sides carefully joined together,
like so many bungs stuck together. Each of these little round flat
pieces of the backbone is called a =vertebra=, and is of a very peculiar
shape. Suppose you took a bung of bone, and fastened on to one side of
its edge a ring of bone. That would represent a vertebra. The solid bung
is what is called the =body=, and the hollow ring is what is called the
=arch= of the vertebra. Now if you put a number of these bodies together
one upon the top of the other, so that the bodies all came together and
the rings all came together, you would have something very like the
vertebral column (see Frontispiece, also Fig. 2). The bungs or bodies
would make a solid jointed pillar, and the rings or arches would make
together a tunnel or canal. And that is really what you have in the
backbone. Only each vertebra is not exactly shaped like a bung and a
ring; the body is very like a bung, but the arch is rough and jagged,
and the bodies are joined together in a particular way. Still we have
all the bodies of the vertebræ forming together a solid pillar which
gives support to the trunk; and the arches forming together a tunnel or
canal which is called the =spinal canal=, (Fig. 2, _C.S._) the use of
which we shall see

[Illustration: FIG. 2.

     A, a diagrammatic view of the human body cut in half lengthways.
     _C.S._, the cavity of the brain and spinal cord; _N_, that of the
     nose; _M_, that of the mouth; _Al. Al._, the alimentary canal
     represented as a simple straight tube; _H_, the heart; _D_, the
     diaphragm.

     B, a transverse vertical section of the head taken along the line
     _a b_; letters as before.

     C, a transverse section taken along the line _c d_; letters as
     before.
]

directly. The round flat body of each vertebra is turned to the front
towards the cavity of the trunk, and it is the row of vertebral bodies
which you feel as a hard ridge when you pass your fingers down the back
of the abdomen. The arches are at the back of the bodies, so you cannot
feel them in the abdomen; but if you turn the rabbit on its belly and
pass your finger down its back, you will feel through the skin (and you
can feel the same on your own body) a sharp edge, formed by what are
called the spines, _i.e._ the uneven tips of the arches of the vertebræ
(Fig. 2) all the way down the back.

So that what we really have in the trunk is this. In front a large
cavity, containing the viscera, and surrounded in the upper part or
thorax by hoops of bone, but not (or only slightly) in the lower part or
abdomen; behind, a much smaller long narrow cavity or canal formed by
the arches of the vertebræ, and therefore surrounded by bone all the way
along, and containing we shall presently see what; and between these two
cavities, separating the one from the other, a solid pillar formed by
the bodies of the vertebræ. So that if you were to take a cross slice,
or transverse section as it is called, of the rabbit across the chest,
you would get something like what is represented in Fig. 2, C, where
_C.S._ is the narrow canal of the arches and where the broad cavity of
the chest containing the heart _H_ is enclosed in the ribs reaching from
the vertebra behind to the sternum in front. Both cavities are covered
up on the outside with muscles, blood-vessels, nerves, connective, and
skin, just as in the leg.

=10.= We have now to consider the =head and neck=. If you cut through the
skin of the neck of the rabbit, you will see, first of all, muscles and
nerves, and several large blood-vessels; but you will find no large
cavity like that in the trunk. So far the neck is just like the leg. But
if you look carefully you will see two tubes which are not
blood-vessels, and the like of which you saw nowhere in the leg. One of
these tubes is firm, with hardish rings in it; it is the windpipe or
=trachea=; the other is soft, and its sides fall flat together; this is
the gullet or =œsophagus=, leading from the mouth to the stomach.
Behind these and the muscles in which they run you will find, just as in
the trunk, a vertebral column, without ribs, but composed of bodies, and
behind the bodies there is a vertebral canal. This vertebral column and
vertebral canal in the neck are simply continuations of the vertebral
column and canal of the trunk.

=The neck, then, differs from the leg in having a vertebral column and
canal with a trachea and œsophagus, and differs from the trunk in
having no cavity and no ribs.=

The head, again, is unlike all these. Indeed, you will not understand
how the head is made unless you take a rabbit’s skull and place it side
by side with the rabbit’s head. If you do this, you will at once see how
the mouth and throat are formed. =You will notice that the skull is all
in one piece=, except a bone which you will at once recognize as the
jawbone, or, to speak more correctly, the lower =jawbone=; for there are
two jawbones. Both these carry teeth, but the upper one is simply part
of the skull, and does not move; the lower one does move; it can be made
to shut close on the upper jaw, or can be separated a good way from it.
The opening between the two jaws is the gap or gape of the mouth, which
as you know can be opened or shut at pleasure. If you try it on yourself
you will find that, as in the rabbit, it is the lower jaw which moves
when you open or shut your mouth. The upper jaw does not move at all
except when your whole head moves. Underneath the skull at the top of
the neck the mouth narrows into the throat, into the upper part of which
the cavity of the nose opens. So that there are two ways into the
throat, one through the mouth and the other through the nose (Fig. 2).

At the back of the skull you will see a rounded opening, and if you put
a bodkin through this opening you will find it leads into a large hollow
space in the inside of the skull. In the living rabbit this hollow space
is filled up with the =brain=. The skull, in fact, is a box of bone to
hold the brain, a bony brain-case. This bony case fits on to the top of
the vertebræ of the neck in such a way that the rounded opening we spoke
of just now is placed exactly over the top of the tunnel or canal formed
by the rings or arches of the vertebræ. If you were to put a wire
through the arch of the lowest vertebra, you might push it up through
the canal formed by the arches of all the vertebræ, right into the brain
cavity. In fact the brain-case and the row of arches of the vertebræ
form together one canal, which is a narrow tube in the back and in the
neck, but swells out in the head into a wide rounded space (Fig. 2, A
and B, _C.S._) During life this canal is filled with a peculiar white
delicate material, which is called =nervous matter=. The rounded mass of
this material which fills up the cavity of the skull is called the
=brain=; the narrower, rod-like, or band-like mass which runs down the
vertebral canal in the neck and back is called the =spinal cord=. They
have separate names, but they are quite joined together, and the rounded
brain tapers off into the band-like cord in such a way that it is
difficult to say where the one begins and the other ends.

=11.= In the skull, besides the larger openings we have spoken of, you
will find several small holes leading from the outside of the skull into
the inside of the brain-case. Some of these holes are filled up during
life by blood-vessels, but in others run those delicate white threads or
cords which you have already learnt to call nerves. =Nerves are in fact
branches of nervous material running out from the brain or spinal cord.=
Those from the brain pass through holes in the skull, and at first sight
seem to spread out very irregularly. Those which branch off from the
spinal cord are far more regular. A nerve runs out on each side between
every two vertebræ, little rounded gaps being left for that purpose
where the vertebræ fit together, so that when you look at a spinal cord
with portions of the nerves still connected with it, it seems not unlike
a double comb with a row of teeth on either side. The nerves which
spring in this way from the spinal cord are called =spinal nerves=, and
soon after they leave the vertebral canal they divide into branches, and
so are spread nearly all over the body. In any piece of skin or flesh
you examine, never mind in what part of the body, you will find nerves
and blood-vessels. If you trace the nerves out in one direction, you
will find them joining together to form larger nerves, and these again
joining others, till at last all end in either the spinal cord or the
brain. If you try to trace the same nerves in the other direction, you
will find them branching into smaller and smaller nerves, until they
become too small to be seen. If you take a microscope you will find they
get still smaller and smaller until they become the very finest possible
threads.

The blood-vessels in a similar way join together into larger and larger
tubes, which last all end, as we shall see, in the heart. =Every part of
the body, with some few exceptions, is crowded with nerves and
blood-vessels. The nerves all come from the brain or spinal cord--the
vessels from the heart. So that every part of the body is governed by
two centres, the heart, and the brain or spinal cord.= You will see how
important it is to remember this when we get on a little further in our
studies.

=12.= Well, then, the body is made up in this way. First there is the
head. In this is the skull covered with skin and flesh, and containing
the brain. The skull rests on the top of the backbone, where the head
joins the neck. In the upper part of the neck, the throat divides into
two pipes or tubes--one the windpipe, the other the gullet. These
running down the neck in front of the vertebral column, covered up by
many muscles, when they get about as far down as the level of the
shoulders, pass into the great cavity of the body, and first into the
upper part of it, or chest.

Here the windpipe ends in the lungs, but the gullet runs straight
through the chest, lying close at the back on the backbone, and passes
through a hole in the diaphragm into the abdomen, where it swells out
into the stomach. Then it narrows again into the intestine, and after
winding about inside the cavity of the abdomen a good deal, finally
leaves it.

You see the =alimentary canal= (for that is the name given to this long
tube made up of gullet, stomach, intestine, &c.) =goes right through the
cavity of the body without opening into it=--very much as the tall narrow
glass of a lamp passes through the large globe glass. You might pour
anything down the narrow glass without its going into the globe glass,
and you might fill the globe glass and yet leave the narrow glass quite
empty. If you imagine both glasses soft and flexible instead of hard and
stiff, and suppose the narrow glass to be very long and twisted about so
as to all but fill the globe, you will have a very fair idea of how the
alimentary canal is placed in the cavity of the body.

Besides the alimentary canal, there is in the chest, in addition to the
windpipe and lungs, the heart with its great tubes, and in the abdomen
there are the liver, the kidneys, and other organs.

These two great cavities, with all that is inside them, together with
wrappings of flesh and skin which make up the walls of the cavities,
form the trunk, and on to the trunk are fastened the jointed legs and
arms. These have no large cavities, and the alimentary canal goes
nowhere near them.

One more thing you have to note. There is only one alimentary canal, one
liver, one heart--but there are two kidneys and two lungs, the one on
one side, the other on the other, and the one very much like the other.
There are two arms and two legs, the one almost exactly like the other.
There is only one head, but one side of the head is almost exactly like
the other. One side of the vertebral column is exactly like the
other--as are also the two halves of the brain and the two halves of the
spinal cord.

In fact, if you were to cut your rabbit in half from his nose to his
tail, you would find that except for his alimentary canal, his heart,
and his liver, one half was almost exactly the counterpart of the other.

Such is the structure of a rabbit, and your own body, in all the points
I have mentioned, is made up exactly in the same way.




WHAT TAKES PLACE WHEN WE MOVE. § III.


=13.= Let us now go back to the question. =How is it that we can move about
as we do?= And first of all let us take one particular movement and see
if we can understand that.

For instance, you can bend your arm. You know that when your arm is
lying flat on the table, you can, if you like, bend the lower part of
your arm (the fore-arm as it is called, reaching from the elbow to the
hand) on the upper arm until your fingers touch your shoulder. How do
you manage to do that?

Look at the bones of the arm in a skeleton. (Frontispiece; also Fig. 3.)
You will see that in the upper arm there is one rather large bone (_H_)
reaching from the shoulder to the elbow, while in the fore-arm there are
two, one (_U_) being wider and stouter than the other (_Ra_) at the
elbow, but smaller and more slender at the wrist. The bone in the upper
arm is called the =humerus=; the bone in the fore-arm, which is stoutest
at the elbow, is called the =ulna=; the one which is stoutest at the
wrist is called the =radius=. If you look carefully you will see that the
end of the humerus at the elbow is curiously rounded, and the end of the
ulna at the elbow curiously scooped out, in such a way that the one fits
loosely into the other.

[Illustration: FIG 3.--_The Bones of the Upper Extremity with the Biceps
Muscle._

     The two tendons by which this muscle is attached to the scapula, or
     shoulder-blade, are seen at _a_. P indicates the attachment of the
     muscle to the radius, and hence the point of action of the power;
     F, the fulcrum, the lower end of the humerus on which the upper end
     of the radius (together with the ulna) moves; W, the weight (of the
     hand).
]

If you try to move them about one on the other, you will find that you
can easily double the ulna very closely on the humerus without their
ends coming apart, and if you notice you will see that as you move the
ulna up and down, its end and the end of the humerus slide over each
other. But they will only slide one way, what we may call up and down.
If you try to slide them from side to side, you will find that they get
locked. They have only one movement, like that of a door on its hinge,
and that movement is of such a kind as to double the ulna on the
humerus.

Moreover, if you look a little more carefully you will find that, though
you can easily double the ulna on the front of the humerus, and then
pull it back again until the two are in a straight line, you cannot bend
the ulna on the back of the humerus. On examining the end of the ulna
you will find at the back of it a beak-like projection (Fig. 3, also
Frontispiece), which when the bones are straightened out locks into the
end of the humerus, and so prevents the ulna being bent any further
back. This is the reason why you can only bend your arm one way. As you
very well know, you can bend your arm so as to touch the top of your
shoulder with your fingers, but you can’t bend it the other way so as to
touch the back of your shoulder; you can’t bring it any further back
than the straight line.

=14.= Well, then, at the elbow the two bones, the humerus and ulna, are so
shaped and so fit into each other that the arm may be straightened or
bent. In the skeleton the two bones are quite separate, _i.e._ they have
to be fastened together by something, else they would fall apart. Most
probably in the skeleton you have been examining they are fastened
together by wires or slips of brass. But they would hold together if you
took away the wire or brass slips and bound some tape round the two
ends, tight enough to keep them touching each other, but loose enough to
allow them to move on each other. You might easily manage it if you took
short slips of tape, or, better still, of india-rubber, and placed them
all round the elbow, back, front, and sides, fastening one end of each
slip to the humerus and the other to the ulna. If you did this you
would be imitating very closely the manner in which the bones at the
elbow are kept together in your own arm. Only the slips are not made of
india-rubber, but are flat bands of that stringy, or as we may now call
it fibrous stuff, which in the preceding lessons you learnt to call
connective tissue. These flat bands have a special name, and are called
=ligaments=.

At the elbow the two ends of the ulna and humerus are kept in place by
ligaments or flat bands of connective tissue.

In the skeleton, the surfaces of the two bones at the elbow where they
rub against each other, though somewhat smooth, are dry. If you ever
looked at the knuckle of a leg of mutton before it was cooked, you will
have noticed that you have there two bones slipping over each other
somewhat as they do at the elbow, and will remember that where the bones
meet they are wonderfully smooth, and very moist, so as to be quite
slippery. It is just the same in your own elbow; the end of the ulna and
the end of the humerus are beautifully smooth and quite moist, so that
they slip over each other as easily as possible. You know that your eye
is always moist. It is kept moist by tears, though you don’t speak of
tears until your eyes overflow with moisture; but in reality you are
always crying a little. Well, there are, so to speak, tears always being
shed inside the wrapping of ligaments around the elbow, and they keep
the two surfaces of the bones continually moist.

The ends of bones where they touch each other are also smooth, because
they are coated over with what is called gristle or cartilage. Bone is
very hard and very solid; there is not much water in it. Bones dry up
very little. Cartilage is not nearly so hard as bone; there is very much
more water in it. When it is quite fresh it is very smooth, but because
it has a good deal of water in it, it shrinks very much when it dries
up, and when dried is not nearly so smooth as when it is fresh. You can
see the dried-up cartilage on the ends of the bones in the skeleton--it
is somewhat smooth still, but you can form no idea of how smooth it is
in the living body by simply seeing it on the dried skeleton.

=At the elbow, then, we have the ends of two bones fitting into each
other, so that they will move in a certain direction; these ends are
smoothed with cartilage, kept moist with a fluid, and held in place by
ligaments. All this is a called a joint.=

=15.= There are a great many other joints in the body besides the
elbow-joint: there is the shoulder-joint, the knee-joint, the hip-joint,
and so on. These differ from the elbow-joint in the shape of the ends of
the bone, in the way the bones move on each other, and in several other
particulars, but we must not go into these differences now. They are all
like the elbow, since in each case one bone fits into another, the
surfaces are coated with cartilage, are kept moist with fluid (what the
grooms call joint-oil, though it is not an oil at all), and are held in
place by ligaments.

I dare say you will have noticed that though I have been speaking only
of the humerus and ulna at the elbow, the other bone of the
fore-arm--the radius--has something to do with the elbow too. I left it
out in order to simplify matters, but it is nevertheless quite true
that the end of the humerus moves over the end of the radius as well as
over the end of the ulna, and that the end of the radius is also coated
with cartilage and is included in the wrapping of the ligaments. I might
add that the radius also moves independently on the ulna, but I don’t
want to trouble you with this just now. What I wanted to show you was
that the elbow is a joint, a joint so constructed that it allows the
fore-arm to be bent on the upper arm.

=16. In order that the arm may be bent, some force must be used.= The ulna
or radius--for the two move together--must be pushed or pulled towards
the humerus, or the humerus must be pushed or pulled towards the radius
and ulna. How is this done in your own arm?

Take the bones of the arm; fix the top end of the humerus; tie it to
something so that it cannot move. Fasten a piece of string to either the
radius or ulna (it doesn’t matter which), rather near the elbow. Bore a
hole through the top of the humerus and pass the string through it. Your
string must be long enough to let the arm be quite straight without any
strain on the string. Now, taking hold of the string where it comes out
through the humerus, pull it. The fore-arm will be bent on the arm. Why?
=Because you have been working a lever of the third order.=

The radius and ulna form the lever; its fulcrum is the end of the
humerus in the elbow (Fig. 3, F); the weight to be moved is the weight
of the radius and ulna (with that of the bones of the hand if present),
and this may be represented by a weight applied at about the middle of
the fore-arm; the power is the pull you give the string, and that is
brought to bear on your lever at the point where the string is fastened
to the radius, _i.e._ nearer the fulcrum than the point where the weight
is applied; and you know that when you have the fulcrum at one end and
the power between the fulcrum and the weight, you have a lever of the
third order.

Now, in order to make the thing a little more like what takes place in
your own arm, instead of boring a hole through the humerus, let the
string glide in a groove which you will see at the top of the humerus,
and fasten the end of it to the shoulder-blade or anything you like
above the humerus, and let the string be just long enough to let the arm
be quite straightened out, but no longer, so that when the arm is
straight the string is just about tight, or at least not loose.

Now shorten the string by pinching it up into a loop. Whenever you do
this you will bend the fore-arm on the arm. Suppose you used a string
which you had not to pinch up, but which, when you pleased, you could
make to shorten itself. =Every time it shortened itself it would pull the
fore-arm up and would bend the arm--and every time it slackened again,
the arm would fall back into the straight position.=

In your arm there is not a string, but a body, placed very much as our
string is placed, and which has the power of shortening itself when
required. Every time it shortens itself it bends the arm, and when it
has done shortening and lengthens again, the arm falls back into its
straight position. This body which thus can shorten and lengthen itself
is called a muscle.

If you put one hand on the front of your other upper arm, about half-way
between your shoulder and elbow, and then bend that arm, you will feel
something rising up under your hand. This is the muscle, which bends the
arm, shortening, or, as we shall learn to call it, =contracting=.

In your own arm, as in the limb of the rabbit which you studied in your
last lesson, the flesh is arranged in masses or bundles of various sizes
and shapes, and each mass or bundle is called a muscle. There are
several muscles in the arm, but there is in particular a large one
occupying the front of the arm, called the =biceps=. It is a rounded mass
of red flesh, considerably longer than it is broad or thick, and
tapering away at either end. It is represented in Fig. 3.

You may remember that while examining the leg of the rabbit you noticed
that in many of the muscles, the soft flesh, which made up the greater
part of the muscle, at one or both ends of the muscle suddenly left off,
and changed into much firmer material which was white and glistening.
This firmer white part you were told was called the tendon of the
muscle. The rest of the muscle, generally called “the belly,” is made up
of what you are accustomed to call flesh, or lean meat, but which you
must now learn to speak of as =muscular substance=. Every muscle, in fact,
consists in the first place of a mass of muscular substance. =This
muscular substance is made up of an immense number of soft strings or
fibres, all running in one direction and done up into large and small
bundles.= At either end of the muscle these soft muscular fibres are
joined on to firmer but thinner fibres of connective or fibrous tissue.
And these thinner but firmer fibres make up the cord or band of tendon
with which the muscle finishes off at either end.

=It is by these tendons that the soft muscles are joined on to the hard
bones, or to some of the other firm textures of the body.= The tendons
are sometimes round and cord-like, sometimes flat and spread out.
Sometimes they are very long, sometimes very short, so as to be scarcely
visible. But always you have some amount of the firmer fibres of
connective tissue joining the soft muscular fibres on to the bones, and
generally the tendons are not only firmer but much thinner and more
slender than the belly of the muscle.

The muscular belly of the biceps is placed in the front of the upper
arm. Some little way above the elbow-joint it ends in a small round
strong tendon which slips over the front of the elbow and is fastened
to, _i.e._ grows on to, the radius at some little distance below the
joint (Fig. 3, P). The upper part of the muscular belly ends a little
below the shoulder, not in one tendon but in two[1] tendons (Fig. 3,
_a_), which gliding over the end of the humerus are fastened to the
shoulder-blade (or =scapula= as it is called), into which the humerus fits
with a joint.

We have then in the biceps a thick fleshy muscular belly placed in the
front of the arm and fastened by tendons, at one end to the
shoulder-blade, and at the other to the fore-arm. What would happen if
when the arm is straight and the shoulder-blade fixed, the biceps were
suddenly to grow very much shorter than it was? Evidently the same
thing that happened when you pinched up and shortened the string which,
if you look back you will see, we supposed to be placed very much as the
biceps with its tendons is placed. =The radius and ulna would be pulled
up, the fore-arm would be bent on the arm.=

Now tendons have no power of shortening themselves, but muscular
substance does possess this remarkable power of suddenly shortening
itself. Under certain circumstances each soft muscular fibre of which
the muscle is made will suddenly become shorter, and thus the whole
muscle becomes shorter, and so pulls its two tendinous ends closer
together, and if one end be fastened to something fixed, and the other
to something moveable, the moveable thing will be moved.

This way that a muscle or a muscular fibre has of suddenly shortening
itself is called a =muscular contraction=. =All muscles, all muscular
fibres, have the power of contracting.= Now a mass of substance like the
biceps might grow shorter in two ways. It might squeeze itself together
and become smaller altogether, it might squeeze itself as you would
squeeze a sponge into a smaller bulk. Or it might change its form and
not its bulk, becoming thicker as it became shorter, just as you might
by pressing the two ends together squeeze a long thin roll of soft wax
into a short thick one. It might get shorter in either of these two
ways, but it does actually do so in the latter way; it gets thicker at
the same time that it gets shorter, and gets nearly as much thicker as
it gets shorter. And that is why, when you put your hand on the arm
which is being bent, you feel something rise up. =You feel the biceps
getting thicker as it is getting shorter in order to bend the arm.=

The shortening does not last for ever. Sooner or later the muscle
lengthens again, getting thinner once more, and so returns to its former
state. The lengthened condition of the muscle is the natural condition,
the condition of rest. The shortening or contraction is an effort which
can only be continued for a certain time. The contraction bends the arm,
and as long as the muscle remains shortened the arm keeps bent; but as
the muscle lengthens, the weight of the hand and fore-arm, if there is
nothing to prevent, straightens the arm out again.

=It is in the muscle alone, in the belly made up of muscular fibres, that
the shortening takes place.= The tendons do not shorten at all. On the
contrary, if anything they lengthen a little, but only a very little,
when the muscle pulls upon them. Their purpose is to convey to the bone
the pull of the muscle. They are not necessary, only convenient. It
would be possible but awkward to do without them. Suppose the fleshy
fibres of the biceps reached from the shoulder-blade to the fore-arm:
you could bend your arm as before, but it would be very tiresome to have
the muscle swelling up in the inside of the elbow, or on the top of the
shoulder; in either place it would be very much in the way. By keeping
the fleshy, the real contracting muscle, in the arm, and carrying the
thin tendons to the arm and to the shoulder, you are enabled to do the
work much more easily and conveniently.

Well, then, we have got thus far in understanding how the arm is bent.
The biceps muscle contracts and shortens, tries to bring its two
tendinous ends together. The upper tendons, being fastened to the fixed
shoulder-blade, cannot move; but the lower tendon is fixed to the
radius; the radius, with the ulna to which it is fastened, readily moves
up and down on the elbow-joint--the shape of bones in the joint and all
the arrangements of the joint, as we have seen, readily permitting this.
When the muscle, then, pulls on its lower tendon, its pulls on the
radius at the point where the tendon is fastened on to the bone. The
radius thus pulled on forms with the ulna a lever of the third order,
working on the end of the humerus as a fulcrum; and thus as the tendon
is pulled the fore-arm is bent.

=17.= But now comes the question. What makes the muscle shorten or
contract? You willed to move your arm, and moved it, as we have seen, by
making the biceps contract; but =how did your will make the biceps
contract=?

If you could examine your arm as you did the leg of the rabbit, you
would find running into your biceps muscle, one or more of those soft
white threads or cords, which you have already learnt to recognize as
=nerves=.

These nerves seem to grow into and be lost in the biceps muscle. We need
not follow them any further in that direction, but if we were to trace
them in the other direction, up the arm, we should find that they soon
meet with other similar nerves, and that the several nerves joining
together form stouter and thicker nerve-cords. These again join others,
and so we should proceed until we came to quite stoutish white
nerve-trunks as they are called, which we should find passed at last
between the vertebræ, somewhere in the neck, into the inside of the
vertebral canal, where they became mixed up with the mass of nervous
material we have already spoken of as the spinal cord.

What have these nerves to do with the bending of the arm? Why simply
this. Suppose you were able without much trouble to cut across the
delicate nerves going to your biceps, and did so: what would happen? You
would find that you had lost all power of bending your arm; however much
you willed it, there would be no swelling rise up in your arm. Your
biceps would remain perfectly quiet, and would not shorten at all, would
not contract in obedience to your will.

What does this show? It proves that when you will to bend your arm,
something passes along the nerves going to the biceps muscle, which
something causes that muscle to contract? The nerve, then, is a bridge
between your will and the muscle--so that when the bridge is broken or
cut away, the will cannot get to the muscle.

If anywhere between the muscle and the spinal cord you cut the nerve
which goes, or branches from which go, to the muscle, you destroy the
communication between the will and the muscle.

The spinal cord, as we have seen, is a mass of nervous substance
continuous with the brain; from the spinal cord nearly all the nerves of
the body are given off; those nerves whose branches go to the biceps
muscle in the arm leave the spinal cord somewhere in the neck.

If you had the misfortune to have your spinal cord cut across or
injured in your neck, you might still live, but you would be paralysed.
You might will to bend the arm, but you could not do it. You would know
you were willing, you would feel you were making an effort, but the
effort would be unavailing. The spinal cord is part of the bridge
between the will and the muscle.

When you bend your arm, then, this is what takes place. =By the exercise
of your will a something is started in your brain. That something--we
will not stop now to ask what that something is--passes from your brain
to the spinal cord, leaves the spinal cord and travels along certain
nerves, picking its way among the intricate bundles of delicate nervous
threads which run from the upper part of the spinal cord to the arm
until it reaches the biceps muscle. The muscle, directly that
“something” comes to it along its nerves, contracts, shortens, and grows
thick; it rises up in the arm; its lower tendon pulls at the radius; the
radius with the ulna moves on the fulcrum of the humerus at the
elbow-joint, and the arm is bent.=

You wish to leave off bending the arm. Your will ceases to act. The
something to which your will had given rise dies away in the brain, dies
away in the spinal cord, dies away in the nerves, even in the finest
twigs. The muscle, no longer excited by that something, ceases to
contract, ceases to swell up, ceases to pull at the radius, and the
fore-arm by its own weight falls into its former straightness,
stretching, as it falls, the muscle to its natural length.

=18.= So far I hope you have followed me, but we are still very far from
being at the bottom of the matter. Why does the muscle contract when
that something reaches it through the nerves? We must content ourselves
by saying that it is the property of the muscle to do so. Does the
muscle always possess this property? No, not always.

Suppose you were to tie a cord very tightly round the top of your arm,
close to the shoulder. What would happen? If you tied it tight enough (I
don’t ask you to do it, for you might hurt yourself) the arm would
become pale, and very soon would begin to grow cold. It would get
numbed, and would gradually seem to grow very heavy and clumsy; your
feeling in it would be blunted, and after a while be altogether lost.
When you tried to bend your arm you would find great difficulty in doing
so. Though you tried ever so much, you could not easily make the biceps
contract, and at last you would not be able to do so at all. You would
discover that you had lost all power of bending your arm. And then if
you undid the cord you would find that after some very uncomfortable
sensations, little by little the power would come back to you; the arm
would grow warm again, the heaviness and clumsiness would pass away, the
feeling in it would return, you would be able to bend it, and at last
all would be as it was before.

What did you do when you tied the cord tight? The chief thing you did
was to press on the blood-vessels in the arm and so stop the blood from
moving in them. If instead of tying the cord round the whole arm you had
tied a finer thread round the blood-vessels only, you would have
brought about very nearly the same effect. We saw in the last lesson how
all parts of the body are supplied with blood-vessels, with veins, and
arteries. In the arm there is a very large artery, branches from which
go all over the arm. Some of these branches go to the biceps muscle.
What would happen if you tied these branches only, tying them so tight
as to stop all the blood in them, but not interfering with the
blood-vessels in the rest of your arm? The arm as a whole would grow
neither pale nor cold, it would not become clumsy or heavy, you would
not lose your feeling in it, but nevertheless if you tried to bend your
arm you would find you could not do it. You could not make the biceps
contract, though all the rest of the arm might seem to be quite right.

What does this teach us? =It teaches us that the power which a muscle has
of contracting when called upon to do so, may be lost and regained, and
that it is lost when the blood is prevented from getting to it.= When a
cord is tied round the whole arm, the power of the whole arm is lost.
This loss of power is the beginning of death, and indeed if the cord
were not unloosed the arm would quite die--would mortify, as it is said.
When only those blood-vessels which go to the biceps are tied, the
biceps alone begins to die, all the rest of the arm remaining alive, and
the first sign of death in the biceps is the loss of the power to
contract when called upon to do so.

In order that you may bend your arm, then, you must not only have a
biceps muscle with its nerves, its tendons, and all its arrangements of
bones and joints, but the muscle must be supplied with blood.

=19.= We can now go a step further and ask the question, =What is there in
the blood that thus gives to the muscle the power of contracting, that
in other words keeps the muscle alive?= The answer is very easily found.
What is the name commonly given to this power of a muscle to contract?
We generally call it strength. Lay your arm straight out on the table,
put a heavy weight in your hand, and try to bend your arm. If you could
do it, one would say you were strong; if you could not, one would say
you were weak--all the stronger or weaker, the heavier or lighter the
weight. In the one case your biceps had great power of contracting; in
the other, little power. Try and find out the heaviest weight you can
raise in this way by bending your arm, some morning, not too long after
breakfast, when you are fresh and in good condition. Go without any
dinner, and in the afternoon or evening, when you are tired and hungry,
try to raise the same weight in the same way. You will not be able to do
it. Your biceps will have lost some of its power of contracting, will be
weaker than it was in the morning. What makes it weak? The want of food.
But how can the food affect the muscle? You do not place the food in the
muscle; you put it into your mouth, and from thence it goes into your
stomach and into the rest of your alimentary canal, and there seems to
disappear. How does the food get at the muscle? By means of the blood.
The food becomes blood. =The things which you eat as food become changed
into other things which form part of the blood. Those things going to
the muscle give it strength and enable it to contract.= And that is why
food makes you strong.

=20.= But you are always wanting food day by day, from time to time. Why
is that? Because the muscle in getting strength out of the food changes
it, uses it up, and so is always wanting fresh blood and new food. We
have seen in Art. 1 that food is fuel. We have also seen that muscle
(and other parts of the body do the same) is always burning, burning
without flame but with heat, burning slowly but burning all the same,
and doing the more work the more it burns. The fuel it burns is not dry
wood or coal, but wet, watery blood, a special kind of fuel prepared for
its private use, in the workshop of the stomach or elsewhere, out of the
food eaten by the mouth. This it is always using up; of this it must
always have a proper supply, if it is to go on working. Hence there must
always be fresh blood preparing; hence there must from time to time be
fresh supplies of food out of which to manufacture fresh blood.

To understand then fully what happens when you bend your arm, we have to
learn not only what we have learnt about the bones and the joint and the
muscle and the nerves, about the machinery and the engine, we have to
study also how the food is changed into blood, how the blood is brought
to the muscle, what it is in the blood on which the muscle lives, what
it is which the muscle burns, and how the things which result from the
burning, the ashes and the smoke or carbonic acid and the rest of them,
are carried away from the muscle and out of the body.

Meanwhile let me remind you that for the sake of being simple I have
been all this while speaking of one muscle only, the biceps in the arm.
But there are a multitude of muscles in the body besides the biceps, as
there are many bones besides those of the arm, and many joints besides
the elbow. But what I have said of the one is in a general way true of
all the rest. The muscles have various forms, they pull upon the bones
in various ways, they work on levers of various kinds. The joints differ
much in the way in which they work. All manner of movements are produced
by muscles pulling sometimes with and sometimes against each other. But
you will find when you come to examine them that all the movements of
which your body is capable depend at bottom on this--=that certain
muscular fibres, in obedience to a something reaching them through their
nerves, contract, shorten, and grow thick, and so pull their one end
towards the other, and that to do this they must be continually supplied
with pure blood=.

Moreover, what I have said of the relations of muscle to blood is also
true of all other parts of the body. Just as the muscle cannot work
without a due supply of blood, so also the brain and the spinal cord and
the nerves have even a more pressing need of pure blood. The weakness
and faintness which we feel from want of food is quite as much a
weakness of the brain and of the nerves as of the muscles,--perhaps
rather more so. And other parts of the body of which we shall have to
speak later on need blood too.

The whole history of our daily life is shortly this. The food we eat
becomes blood, the blood is carried all over the body, round and round
in the torrent of the circulation; as it sweeps past them, or rather
through them, the muscle, the brain, the nerve, the skin pick out new
food for their work and give back the things they have used or no longer
want. As they all have different works, some use up what others have
thrown away. There are, besides, scavengers and cleaners to pick up
things no longer wanted anywhere and to throw them out of the body. Thus
the blood is kept pure as well as fresh. Through the blood thus ever
brought to them, each part does its work: the muscle contracts, the
brain feels and wills, the nerves carry the feeling and the willing, and
the other organs of the body do their work too, and thus the whole body
is kept alive and well.




THE NATURE OF BLOOD. § IV.


=21. What, then, is this blood which does so much?=

Did you ever look through a good microscope at the thin transparent web
of a frog’s foot, and watch the red blood coursing along its narrow
channels? If not, go and look at it at once; you will never understand
any physiology till you have done so. There you will see a network of
delicate passages far finer than any of your own hairs, and through
those passages a tumbling crowd of tiny oval yellow globules hurrying
and jostling along. Some of the passages are wider than others, and
through some of the wider ones you will see a thick stream of globules
rushing onwards towards the smaller channels, and spreading out among
them. The globules which you see are floating in a fluid so clear that
you cannot see it. Some of the smaller channels are so narrow that only
one globule or =corpuscle=, as we may call it, can pass through at a time,
and very frequently you may see them passing in single file. Watching
them as they glide along these narrow paths, you will note that at last
they tumble again into wider passages, somewhat like those from which
they came, except that the stream runs away from instead of towards the
narrower channels; and in the stream the corpuscle you are watching
shoots out of sight. The finest passages are called =capillaries=; they
are guarded by delicate walls which you can hardly see; they seem to you
passages only, and how fine and small they are will come home to you
when you recollect that all you are looking at is going on in the depths
of a skin which is so thin that perhaps you would be inclined to say it
has no thickness at all.

The larger channels which are bringing the blood down to the capillaries
are the ends of vessels like those which in the rabbit you learnt to
call arteries, and the other larger channels through which the blood is
rushing away from the capillaries are the beginnings of veins.

When you have watched this frog’s foot for some little time, turn away
and reflect that in almost every part of your own body, in every square
inch, in almost every square line, something very similar might be seen
could the microscope be brought to bear upon it, only the corpuscles are
smaller and round, the capillaries narrower and for the most part more
thick-set, and the race a swifter one. In the muscle of which we were
speaking in the last lesson, each of the soft long fibres of which the
muscle is composed is wrapped round with a close network of these tiny
capillaries, through which, as long as life lasts, for ever rushes a
swift stream of blood, reddened by countless numbers of tiny corpuscles.

In every part of your flesh, in your brain and spinal cord, in your
skin, your bones, your lungs, in all organs and in nearly every part of
your body, there is the same hurrying rush through narrow tubes of red
corpuscles and of the clear fluid in which these swim.

If you prick your finger it bleeds. Almost any part of your body would
bleed were you to prick it. So thick-set are the little blood-vessels,
that wherever you thrust a needle, be it as fine a needle as you please,
you will be sure to pierce and tear some little blood channel, either
artery or capillary or vein, and out will come the ruddy drop.

=22. What is blood?= It is a fluid; it runs about like water: yet it is
thicker than water, thicker for two reasons. In the first place, water,
that is pure water, is all one substance. If you were to look at it with
ever so powerful a microscope, you would see nothing in it. It is
exceedingly transparent--you can see very well through ever such a
thickness of clean water. But if you were to try and look through even a
very thin sheet of blood spread out between two glass plates, you would
find that you could see very little; =blood is very opaque=. If again you
examine a drop of your blood with a microscope, what do you see? =A
number of little=

[Illustration: FIG. 4.--_Red and White Corpuscles of the Blood
magnified._

     _A._ Moderately magnified. The red corpuscles are seen lying in
     rows like rolls of coins; at _a_ and _a_ are seen two white
     corpuscles.

     _B._ Red corpuscles much more highly magnified, seen in face; _C._
     ditto, seen in profile; _D._ ditto, in rows, rather more highly
     magnified; _E._ a red corpuscle swollen into a sphere by imbibition
     of water.

     _F._ A white corpuscle magnified same as _B._; _G._ ditto, throwing
     out some blunt processes; _K._ ditto, treated with acetic acid, and
     showing nucleus, magnified same as _D._

     _H._ Red corpuscles puckered or crenate all over.

     _I._ Ditto, at the edge only.
]

=round bodies, the blood discs or blood corpuscles= (Fig. 4, _A_). If you
look carefully you will notice that most of them are round, as _B_; but
every now and then you see something like _C_. That is one of the round
ones seen sideways; for they are not round or spherical like a ball, but
circular and dimpled in the middle, something like certain kinds of
biscuit. When you see one by itself it looks a little yellow in colour,
that is all; but when you see them in a lump, the lump is clearly red.
Remember how small they are: three thousand of them put flat in a line,
edge to edge, like a row of draughts, would just about stretch across
one inch. All the redness there is in blood belongs to them. When you
see one of them, you see so little of the redness that it seems yellow.
If you were to put a drop of blood into a tumbler of water, the water
would not be stained red, but only just turned of a yellowish tint, so
little redness would be given to it by the drop of blood. In the same
way a very very thin slice of currant jelly would look yellowish, not
red.

These red corpuscles are not hard solid things, but delicate and soft,
very tender, very easily broken to pieces, more like the tiniest lumps
of red jelly than anything else, and yet made so as to bear all the
squeezing which they get as they are driven round and round the body.

Besides these red corpuscles, you may see if you look attentively =other
little bodies, just a little bigger than the red corpuscles, not
coloured at all, and not circular and flat, but quite round like a ball=
(Fig. 4, _a_, _F_, _G_). That is to say, these are very often quite
round, only they have a curious trick of changing their form. Imagine
you were looking at a suet dumpling so small that about two thousand
five hundred of them could be placed side by side in the length of one
inch--and suppose the round dumpling while you were looking at it
gradually changed into the shape of a three-cornered tart, and then
into a rounded square, and then into the shape of a pear, and then into
a thing that had no shape at all, and then back again into a round ball,
and kept doing this apparently all of its own accord while you were
looking at it--wouldn’t you think it very curious? Well, one of these
little bodies in the blood of which we are speaking, and which are
called white corpuscles, may be seen, when a drop of blood is watched
under the microscope, to go on in this way, continually changing its
shape. But of these =white corpuscles= of the blood, and of their
wonderful movements, you will learn more as you go on in your
physiological studies.

=23.= Besides these red and white corpuscles there is nothing else very
important in the blood that you can _see_ with the microscope; but their
being in the blood is one reason why blood is thicker than water.

Did you ever see a pig or sheep killed? If so, you would be sure to
notice that the blood ran quite fluid from the blood-vessels in the
neck, ran and was spilt like so much water--but that very soon the blood
caught in the pail or spilt on the stones became quite solid, so that
you could pick it up in lumps. Whenever blood is shed from the living
body, within a short time it becomes solid. This becoming solid is
called the =clotting= or =coagulation of blood=.

What makes it clot? Suppose while the blood was running from the pig’s
neck into the butcher’s pail, and while it was still quite fluid, you
were to take a bunch of twigs and keep slowly stirring the blood round
and round in the pail. You would naturally expect that the blood would
soon begin to clot, would get thicker and thicker and more and more
difficult to stir. But it does not; and if you keep on stirring long
enough you will find that it never clots at all. =By continually stirring
it you will prevent its clotting.= Now take out your bundle of twigs: you
will find it covered all over with a thick reddish mass of some soft
sticky substance; and if you pump on the red mass you will be able to
wash away all its red colour, and will have nothing left but a quantity
of white, soft, sticky, stringy material, all entangled and matted
together among the twigs of your bundle. This stringy material is in
reality made up of a number of fine, delicate, soft, elastic threads or
fibres, and is called =fibrin=.

=You see, by stirring, or, as it is frequently called, whipping the blood
with the bundle of twigs, you have taken the fibrin out of the blood,
and so prevented its clotting.=

If you were to take one of the clotted lumps of blood that were spilt on
the ground or a bit of the clot from a pail in which the blood had not
been whipped, and wash it long enough, you would find at last that all
the colour went away from the lump, and you had nothing left but a small
quantity of white stringy substance. This white stringy substance is
fibrin--exactly the same thing you got on your bundle of twigs.

If the blood is carefully caught in a pail, and afterwards not disturbed
at all, it clots into a solid mass. The whole of the blood seems to have
changed into a complete jelly; and if you turn it out of the pail, as
you may do, it keeps its shape, and gives you quite a mould of the pail,
a great trembling red jelly just the shape of the inside of the pail.

But if you were to leave the blood in the pail for a few hours or for a
day, you would find, instead of the large jelly quite filling the pail,
a smaller but firmer jelly covered by or floating in a colourless or
very pale yellow liquid. This smaller, firmer jelly, which in the course
of a day or so would get still firmer and smaller, would in fact go on
shrinking in size, you may still call the =clot=; the clear fluid in which
it is floating is called =serum=.

What has taken place is as follows. Soon after blood is shed there is
formed in it a something which was not present in it before. This
something, which we call =fibrin=, starts as a multitude of fine tender
threads which run in all directions through the mass of blood, forming a
close network everywhere. So the blood is shut up in an immense number
of little chambers formed by the meshes of the fibrin; and it is this
which makes it seem a jelly. But each thread of fibrin as soon as it is
formed begins to shrink, and the blood in each of these little chambers
is squeezed by the shrinking of its walls of fibrin, and tries to make
its way out. The corpuscles get caught in the meshes, but all the rest
of the blood passes between the threads and comes out on the top and
sides of the pail. And this goes on until you have left in the clot very
little besides corpuscles entangled in a network of fibrin, and all the
rest of the blood has been squeezed outside the clot, and is then called
serum. =Serum, then, is blood out of which the corpuscles have been
strained by the process of clotting.=

Now I dare say you are ready to ask the question, If blood clots so
readily when it is shed, why does it not clot inside the body? Why is
our blood ever fluid? This is rather a difficult question to answer.
When blood is shed from the warm body it soon gets cool. But it does not
clot and become solid because it gets cool, as ordinary jelly does. If
you keep it from getting cool it clots all the same, in fact quicker,
and if kept cold enough will not clot at all. Nor does it clot when
shed, because it has become still, and is no longer rushing round
through the blood-vessels. Nor is it because it is exposed to the air.
Perhaps we don’t know exactly why it is, and you will have much to learn
hereafter about the coagulation of blood. All I will say at present is
that as long as the blood is in the body there is something at work to
keep it from clotting. It does clot sometimes in the body, and
blood-vessels get plugged with the clots; but that constitutes a very
dangerous disease.

=24.= Well, blood is thicker than water because it contains solid
corpuscles and fibrin. But even the serum, _i.e._ blood out of which
both fibrin and corpuscles have been taken, is thicker than water.

You know that if you were to take a basinful of pure water and boil it,
it would boil away to nothing. It would all go off in steam. But if you
were to try to boil a basinful of serum, you would find several curious
things happen.

In the first place you would not be able to boil it at all. Before you
got it as hot as boiling water, your serum, which before seemed quite as
liquid as water, only feeling a little sticky if you put your finger in
it, would all become quite solid. You know the difference between a raw
and a boiled egg. The white of the raw egg, though very sticky and ropy,
or viscid as it is called, is still liquid; you will find it hard work
if you try to cut it with a knife. The white of the hard boiled egg, on
the other hand, is quite solid, and you can cut it into ever so thin
slices. It has been “set” by boiling. Well, the serum of blood is in
this respect very like white of egg. In fact they both contain the same
substance, called =albumin=, which has this property of “setting” or
becoming solid when heated nearly to boiling-point. Both the serum of
blood and white of egg even when “set” are wet, _i.e._ contain a great
deal of water. You may dry them in the proper manner into a transparent
horny substance. When quite dry they will readily burn. They are
therefore things which can be oxidized. When burnt they give off
carbonic acid, water, and ammonia; the latter you might easily recognize
by its effect on your nose if you were to burn a piece of dried blood in
a flame. Now, when I say that =albumin= in burning gives off carbonic
acid, water, and ammonia, you know from your Chemistry that it must
contain carbon to form the carbonic acid, hydrogen to form water, and
nitrogen to form ammonia. It need not contain oxygen, for as you know it
could get all the oxygen it wanted from the air; still it does contain
some oxygen. =Albumin, then, is an oxidizable or combustible body made up
of nitrogen, carbon, hydrogen, and oxygen.= It is important you should
remember this; but I will not bother you with how much of each--it is a
very complex substance, built up in a wonderful way, far more complex
than any of the things you had to learn about in your Chemistry Primer.
And this albumin, dissolved in a great deal of water, forms the serum of
blood.

I did not say anything about what fibrin was made of; but it, like
albumin, is made up of nitrogen, carbon, hydrogen, and oxygen. It is not
quite the same thing as albumin, but first cousin to it. There is
another first cousin to both of them, also containing nitrogen, carbon,
hydrogen, and oxygen, which together with a great deal of water forms
muscle; another forms a great part of the red corpuscles; and scattered
all over the body in various places, there are first cousins to albumin,
all containing nitrogen, carbon, hydrogen, and oxygen, all combustible,
and all when burnt giving off carbonic acid, water, and ammonia. All
these first cousins go under one name; they are all called =proteids=.

=25.= Well, then, blood is thicker than water by reason of the proteids in
the corpuscles, in the fibrin, and in the serum, but there is something
else besides. I will not trouble you with the crowd of things of which
there are perhaps just a few grains in a gallon of blood, like the
little pinches of things a cook puts into a savoury dish; though, as you
go on in your studies, you will find that these, like many other little
things in the world, are of great importance.

But I will ask you to remember this. If you take some dried blood and
burn it, though you may burn all the proteids (and some other of the
trifles I spoke of just now) away, you will not be able to burn the
whole blood away. Burn as long as you like, you will always have left a
quantity of what you have learnt from your Chemistry to call =ash, and if
you were to examine this ash you would find it contained ever so many
elements; sulphur, phosphorus, chlorine, potassium, sodium, calcium, and
iron, being the most abundant and most important=.

Blood, then, is a very wonderful fluid: wonderful for being made up of
coloured corpuscles and colourless fluid, wonderful for its fibrin and
power of clotting, wonderful for the many substances, for the proteids,
for the ashes or minerals, for the rest of the things which are locked
up in the corpuscles and in the serum.

But you will not wonder at it when you come to see that the blood is the
great circulating market of the body, in which all the things that are
wanted by all parts, by the muscles, by the brain, by the skin, by the
lungs, liver, and kidney, are bought and sold. What the muscle wants,
it, as we have seen, buys from the blood; what it has done with it sells
back to the blood; and so with every other organ and part. As long as
life lasts this buying and selling is for ever going on, and this is why
the blood is for ever on the move, sweeping restlessly from place to
place, bringing to each part the things it wants, and carrying away
those with which it has done. When the blood ceases to move, the market
is blocked, the buying and selling cease, and all the organs die,
starved for the lack of the things which they want, choked by the
abundance of things for which they have no longer any need.

We have now to learn how the blood is thus kept continually on the move.




HOW THE BLOOD MOVES. § V.


=26.= You have already learnt to recognize the blood-vessels of the
rabbit, and to distinguish two kinds of blood-vessels--the arteries,
which in a dead animal generally contain little or no blood, and have
rather firm stout walls; and the veins, which are generally full of
blood, and have thinner and flabby walls. The arteries when you cut them
generally gape and remain open; the veins fall together and collapse.
The larger the arteries, the stouter and firmer they are, and the
greater the difference between them and the veins.

You have also studied the capillaries in the frog’s foot; you have seen
that they are minute channels, with the thinnest and tenderest walls,
forming a close network in which the smallest arteries end, and from
which the smallest veins begin.

You have moreover been told that all over your own body, in every part,
there are, though you cannot see them, networks of capillaries like
those in the frog’s foot which you can see; that all the arteries of
your body end in capillaries, and all the veins begin in capillaries.
Let me repeat that, one or two structures excepted, there is no part of
your body in which, could you put it under a microscope, you would not
see a small artery branching out and losing itself in a network of
capillaries, out of which, as out of so many roots, a small vein gathers
itself together again.

In some places the network is very close, the capillaries lying closer
together than even in the frog’s foot; in others the network is more
open, and the capillaries wider apart; but everywhere, with a few
exceptions which you will learn by and by, there are capillaries,
arteries, and veins.

Suppose you were a little lone red corpuscle, all by yourself in the
quite empty blood-vessels of a dead body, squeezed in the narrow pathway
of a capillary, say of the biceps muscle of the arm, able to walk
about, and anxious to explore the country in which you found yourself.
There would be two ways in which you might go. Let us first imagine that
you set out in the way which we will call backwards. Squeezing your way
along the narrow passage of the capillary in which you had hardly room
to move, you would at every few steps pass, on your right hand and on
your left, the openings into other capillary channels as small as the
one in which you were. Passing by these you would presently find the
passage widening, you would have more room to move, and the more
openings you passed, the wider and higher would grow the tunnel in which
you were groping your way. The walls of the tunnel would grow thicker at
every step, and their thickness and stoutness would tell you that you
were already in an artery, but the inside would be delightfully smooth.
As you went on you would keep passing the openings into similar tunnels,
but the further you went on, the fewer they would be. Sometimes the
tunnels into which these openings led would be smaller, sometimes
bigger, sometimes of the same size as the one in which you were.
Sometimes one would be so much bigger, that it would seem absurd to say
that it opened into your tunnel. On the contrary, it would appear to you
that you were passing out of a narrow side passage into a great wide
thoroughfare. I dare say you would notice that every time one passage
opened into another the way suddenly grew wider, and then kept about the
same size until it joined the next. Travelling onwards in this way, you
would after a while find yourself in a great wide tunnel, so big that
you, poor little corpuscle, would seem quite lost in it. Had you anyone
to ask, they would tell you it was the main artery of the arm. Toiling
onwards through this, and passing a few but for the most part large
openings, you would suddenly tumble into a space so vast that at first
you would hardly be able to realize that it was the tunnel of an artery
like those in which you had been journeying. This you would learn to be
the =aorta=, the great artery of all; and a little further on you would be
in the heart.

Suppose now you retraced your steps, suppose you returned from the aorta
to the main artery of the arm, and thus back through narrower and
narrower tunnels till you came again to the spot from which you started,
and then tried the other end of the capillary. You would find that that
led you also, in a very similar way, into wider and wider passages. Only
you could not help noticing that though the inside of all the passages
was as smooth as before, the walls were not nearly so thick and stout.
You would learn from this that you were in the veins, and not in the
arteries. You would meet too with something, the like of which you did
not see in the arteries (except perhaps just close to the heart). Every
now and then you would come upon what for all the world looked like one
of those watch-pockets that sometimes are hung at the head of a
bedstead, a watch-pocket with its opening turned the way you were going.
This you would find was called a =valve=, and was made of thin but strong
membrane or skin. Sometimes in the smaller veins you would meet with one
watch-pocket by itself, sometimes with two or even three abreast, and I
dare say you would notice that very frequently, directly you had passed
one of these valves, you came to a spot where one vein joined another.

Well, but for these differences, your journey along the veins would be
very like your journey along the arteries, and at last you would find
yourself in a great vein, whose name you would learn to be the =vena
cava=, or hollow vein (and because, though there is but one aorta, there
are two great “hollow veins,” =the superior vena cava= or =upper hollow
vein=), and from thence your next step would be into the heart again. So
you see, starting from the capillary (you started from a capillary in
the arm, but you might have started from any capillary anywhere),
whether you go along the arteries or whether you go along the veins, you
at last come to the heart.

Before we go on any further we must learn something about the heart.

=27.= Go and ask the butcher for a sheep’s pluck. There will most probably
be one hanging up in his shop. Look at it before he takes it down. The
hook on which it is hanging has been thrust through the windpipe. You
will see that the sheep’s windpipe is, like the rabbit’s, all banded
with rings of cartilage, only very much larger and coarser. Below the
windpipe come the spongy lungs, and between them lies the heart, which
perhaps is covered up with a skin and so not easily seen. Hanging to the
heart and lungs is the great mass of the liver. When you have got the
pluck home, cut away the liver, cut away the skin (pericardium, it is
called) which is covering the heart, if it has not been cut away
already, and lay the lungs out on a table with the heart between them.
You will then have something very much like what

[Illustration:

     FIG. 5.--_Heart of Sheep, as seen after Removal from the Body,
     lying upon the Two Lungs. The Pericardium has been cut away, but no
     other Dissection made._

_R.A._ Auricular appendage of right auricle; _L.A._ auricular appendage
of left auricle; _R.V._ right ventricle; _L.V._ left ventricle; _S.V.C._
superior vena cava; _I.V.C._ inferior vena cava; _P.A._ pulmonary
artery; _Ao_, aorta; _Áó_, innominate branch from aorta dividing into
subclavian and carotid arteries; _L._ lung; _Tr._ trachea. 1, solid cord
often present, the remnant of a once open communication between the
pulmonary artery and aorta. 2, masses of fat at the bases of the
ventricle hiding from view the greater part of the auricles. 3, line of
fat marking the division between the two ventricles. 4, mass of fat
covering the trachea.]

is represented in Fig. 5. If you could look through the front of your
own chest, and see your own heart and lungs in place, you would see
something not so very very different.

If now you handle the heart--and if you want to learn physiology you
must handle things--you will have no great difficulty in finding the
great yellowish tubes marked _Ao_ and _Áó_ in the figure. Your butcher
perhaps may not have cut them across exactly where mine has done, but
that will not prevent your recognizing them. You will notice what thick
stout walls they have, and how they gape where they are cut. _Ao_ is the
=aorta=, and _Áó_ is a great branch of the aorta, going to the head and
neck of one side, perhaps the branch along which we imagined just now
that you, a poor little red blood-corpuscle, were travelling. If you
were to put a wire through _Áó_ you would be able to bring it out
through _Ao_, or _vice versâ_. But what is _P.A._ which looks so much
like the aorta, though you will find that it has no connection with it?
You cannot pass a wire from the aorta into it. It also is an artery, the
=pulmonary[2] artery=. We shall have more to say about it directly.

Now try and find what are marked in the figure as _S.V.C._ and _I.V.C._
You will perhaps have a little difficulty in this; and when you have
found them you will understand why. They are the great veins of the
body. _S.V.C._ is the =superior vena cava=, to form which all the veins
from the head and neck and arms join, the vein in which you were
journeying a little while ago. _I.V.C._ is the =inferior vena cava=, made
out of all the veins from the trunk and the legs. Being veins, they have
thin flabby walls; and their sides fall flat together, so that they seem
nothing more than little folds of skin, and it becomes very hard to find
the passage inside them. But when you have found the opening into them,
you will see that you can stretch them out into quite wide tubes, and
that their walls, though very much thinner than those of the aorta, so
thin indeed that they are almost transparent, are still after a fashion
strong. If you put a penholder or thin rod through either you will find
that they both seem to lead right into the middle of the heart. With a
little care you can pass a rod up _I.V.C._ and bring the end of it out
at the top of _S.V.C._ Of course you will understand that both of these
veins have been cut off short.

=28.= Before we go on any further with the sheep’s heart, let me tell you
something about it, by help of the diagram in Fig. 6, which is meant to
represent the whole circulation. You must remember that this figure is a
=diagram=, and not a picture; it does not represent the way the
blood-vessels are really arranged in your own body. If you had no arms
and no legs, and if you only had a few capillaries at the top of your
head and at the bottom of your body, it might be more like than it is.

In the centre of the figure is the heart. This you will see is
completely divided by an upright partition into two halves, a right half
and a left half. Each half is further marked off, but not completely
divided, into

[Illustration:

     FIG. 6.--_Diagram of the Heart and Vessels, with the Course of the
     Circulation, viewed from behind so that the proper left of the
     Observer corresponds with the left side of the Heart in the
     Diagram._

_L.A._ left auricle; _L.V._ left ventricle; _Ao._ aorta; _A_^{1}.
arteries to the upper part of the body; _A_^{2}. arteries to the lower
part of the body; _H.A._ hepatic artery, which supplies the liver with
part of its blood; _V_^{2}. veins of the upper part of the body;
_V_^{2}. veins of the lower part of the body; _V.P._ vena portæ; _H.V._
hepatic vein; _V.C.I._ inferior vena cava; _V.C.S._ superior vena cava;
_R.A._ right auricle; _R.V._ right ventricle; _P.A._ pulmonary artery;
_Lg._ lung; _P.V._ pulmonary vein; _Lct._ lacteals; _Ly._ lymphatics;
_Th.D._ thoracic duct; _Al._ alimentary canal; _Lr._ liver. The arrows
indicate the course of the blood, lymph, and chyle. The vessels which
contain arterial blood have dark contours, while those which carry
venous blood have light contours.]

two chambers, an upper chamber and a lower chamber; so that altogether
we have four chambers,--two upper chambers, one on each side, marked
_R.A._ and _L.A._, these are called the =right and left auricles=; and two
lower chambers, one on each side, marked _R.V._ and _L.V._, these are
called the =right and left ventricles=. The right auricle, _R.A._, opens
in the direction of the arrow into the right ventricle, _R.V._, the
opening being guarded, as we shall see, by a valve. The left auricle,
_L.A._, opens into the left ventricle, _L.V._, the opening being
likewise guarded by a valve; but you have to go quite a roundabout way
to get from either the right auricle or ventricle to the left auricle or
ventricle. Let us see how we can get round the figure. Suppose we begin
with the two tubes marked _V.C.S._ and _V.C.I._, the walls of which are
drawn with thin lines. These both open into the right auricle. They are
the vena cava superior and inferior, which you have just made out in the
sheep’s heart. From the right auricle you pass easily into the right
ventricle; thence, following the arrow, the way is straight into the
tube marked _P.A._ This is the pulmonary artery, the outside of which
you saw in the sheep’s heart (Fig. 5, _P.A._) Travelling along this
pulmonary artery, you come to the lungs, and after passing through
branches not represented in the figure, picking your way through
arteries which continually get smaller and smaller, you find yourself
at last in the capillaries of the lungs. Squeezing your way through
these, you come out into veins, and gradually advancing through larger
and larger veins, you, still following the arrow, find yourself in one
of four large veins (only one of them is represented in the diagram)
which land you in the left auricle. From the left auricle it is but a
jump into the left ventricle. From the left ventricle the way is open,
as indicated by the arrow, into the tube marked _Ao_. This is intended
to represent the aorta, which you have already seen in the sheep’s heart
(Fig. 5, _Ao_). It is here drawn for simplicity’s sake as dividing into
two branches, but you have already been told, and must bear in mind,
that it does not in reality divide in this way, but gives off a good
many branches of various sizes. However, taking the figure as it stands,
suppose we travel along _A_^{2}. Following the arrow, and shooting
through arteries which continually get smaller and smaller, we come at
last to capillaries somewhere, in the skin or in some muscle, or in a
bone, or in the brain, or almost anywhere, in fact, in the upper part of
the body. Out of the capillaries we pass into veins, which, joining
together and so forming larger and larger trunks, bring us at last to
the point from which we started, the superior vena cava, _V.C.S._ If we
had taken the other road, _A_^{2}, we should have passed through
capillaries somewhere in the lower part of the body instead of the
upper, and come back by the vena cava inferior, _V.C.I._, instead of the
vena cava superior. =Starting from the right auricle, whichever way we
took we should always come back to the right auricle again, and in our
journey should always pass through the following things in the following
order: right auricle, right ventricle, pulmonary artery, arteries,
capillaries, and veins of the lungs, pulmonary vein, left auricle, left
ventricle, aorta, arteries, capillaries, and veins somewhere in the
body, and either superior or inferior vena cava.= That is the course of
the circulation. But there is something still to be added. Among the
many large branches, not drawn in the diagram, given off by the aorta to
the lower part of the body, there are two branches which are drawn and
which deserve special notice.

One is a large branch carrying blood to the tube _A.L._, which is meant
in the diagram to stand for the stomach, intestines, and some other
organs. This branch, like all other branches of the aorta, divides into
small arteries, and these into capillaries, which again are gathered up
into veins, forming at last a large vein marked in the diagram _V.P._
and called the =vena portæ= or =portal vein=. Now the remarkable thing is
that this vein does not, like all the other veins, go straight to join
the vena cava, but makes for the liver, where it divides into smaller
and smaller veins, until at last it breaks completely up in the liver
into a set of capillaries again. These capillaries gather once more into
veins, forming at last the large trunk, called the =hepatic[3] vein=,
_H.V._, which does what the portal vein ought to have done but did not;
it opens straight into the vena cava.

The other branch of the aorta of which we are speaking goes straight to
the liver, and is called the =hepatic artery=, _H.A._: there it breaks up
in the liver into small arteries, and then into capillaries, which
mingle with the capillaries of the portal vein, and form one system, out
of which the hepatic veins spring. So you see it makes a great
difference to a red corpuscle which is travelling along the lower part
of the aorta _A_^{2}, whether it takes a turn into the branch going to
the alimentary canal, or whether it goes straight on into, for instance,
a branch going to some part of the leg. In the latter case, having got
through a set of capillaries, it is soon back into the vena cava and on
its road to the heart. But if it takes the turn to the alimentary canal,
it finds after it has passed through the capillaries and got into the
portal vein, that it has still to go through another set of capillaries
in the liver before it can pass through the hepatic vein into the vena
cava.

This then is the course of the circulation. Right side of the heart,
pulmonary artery, capillaries of the lungs, pulmonary vein, left side of
the heart, aorta, capillaries somewhere, sometimes two sets, sometimes
one, vena cava, right side of the heart again. A little corpuscle cannot
get from the right to the left side of the heart without going through
the capillaries of the lungs. It cannot get from the left side of the
heart to the right without going through some capillaries somewhere in
the body, and if it should happen to take the turn to the stomach, it
has to go through two sets of capillaries instead of one.

You see, you really have two circulations, and you have two hearts
joined together into one. If you were very skilful you might split the
heart in half and pull the two sides asunder, and then you would have
one heart receiving all the veins from the body and sending its arteries
(branches of the pulmonary artery) all to the lungs, and another heart
receiving all the veins from the lungs and sending its arteries
(branches of the aorta) all over the body. And you would have two
circulations, one through the lungs, and another through the rest of the
body, both joining each other. Very often two circulations are spoken
of, and because the lungs are so much smaller than the rest of the body,
the circulation through the lungs is called the lesser circulation, that
through the rest of the body the greater circulation.

=29.= I have described the circulation as if the blood always went in one
direction from the right side of the heart to the left, from arteries to
veins, the way the arrows point in the diagram. And so it does. It
cannot go the other way round. =Why does it go that way? Why cannot it go
the other way round?=

The reasons are to be found partly in the heart, partly in the veins.

=In the veins the blood will only pass from the capillaries to the heart.=
Why not from the heart to the capillaries? You remember the little
watch-pocket-like valves, here and there, sometimes singly, sometimes
two or three abreast. =You remember that the mouths of the watch-pockets
were always turned towards the heart.= Now suppose a crowd of little
corpuscles hurrying along a vein towards the heart. When they came to
one of these watch-pocket valves they would simply trample it down flat,
and so pass over it without hardly knowing it was there, and go on
their way as if nothing had happened. But suppose they were journeying
the other way, from the heart to the capillaries. When they came to the
open mouth of a watch-pocket valve, some of them would be sure to run
into the pocket, and then the pocket would bulge out, and the more it
bulged out the more blood would run into it, until at last it would be
so full of blood that it would press close against the top of the vein,
as is shown in Fig. 7 (or, if there were two or three, they would all
meet together), and so quite block the vein up. If you doubt this, make
a watch-pocket out of a piece of silk or cotton, fasten it on to a piece
of brown paper, and roll the paper up into a tube, so that the valve is
nicely inside the tube. If you pour some peas down the tube with the
mouth of the valve looking away from you, they will run through at once;
but if you try to pour them the other way, your tube will soon be
choked, and if you carefully unroll the tube you will find the
watch-pocket crammed full of peas.

[Illustration: FIG. 7.--_Diagrammatic Sections of Veins with Valves._

     In the upper, the blood is supposed to be flowing in the direction
     of the arrow, towards the heart; in the lower, the reverse way. C,
     capillary side; H, heart side.
]

=The valves in the veins, then, let the blood pass easily from the
capillaries to the heart, but won’t let it go the other way.= If you bare
your arm you may see some of the veins in the skin, in which the blood
is running up from the hand towards the shoulder. If with your finger
you press one of these veins back towards the hand it will swell up, and
if you look carefully you may see little knots here and there caused by
the bulging out of the watch-pocket valves. If you press it the other
way, towards the elbow, you will empty it easily, and if with another
finger you prevent the blood getting into it from behind, that is from
the hand, the vein will remain empty a very long time.

The presence of valves in the veins, then, is one reason why the blood
moves in one direction, but other reasons, and these the chief ones, are
to be found in the heart.

Let us now go back to the sheep’s heart.

=30.= You know from the diagram that the two great veins, the superior and
inferior vena cava, open into the right auricle. If you slit up these
two veins in the sheep’s heart, you will find that they end by separate
openings in a small cavity, the inside of which is for the most part
smooth, and the walls of which, made, as you will at once see, of
muscle, are not very thick. This small cavity is the right auricle,
shown in Fig. 8, _R.A._, where the great veins have not been slit up,
but the front of the auricle has been cut away. In this auricle, beside
the openings into the two great veins and another one which belongs to a
vein coming from the heart itself (Fig. 8, _b_) there is quite a large
one, leading straight downwards, into which you

[Illustration: FIG. 8.--_Right Side of the Heart of a Sheep._

     _R.A._ cavity of right auricle; _S.V.C._ superior vena cava;
     _I.V.C._ inferior vena cava; (a piece of whalebone has been passed
     through each of these;) _a_, a piece of whalebone passed from the
     auricle to the ventricle through the auriculo-ventricular orifice;
     _b_, a piece of whalebone passed into the coronary vein.

     _R.V._ cavity of right ventricle; _tv_, _tv_, two flaps of the
     tricuspid valve: the third is dimly seen behind them, the _a_,
     piece of whalebone, passing between the three. Between the two
     flaps, and attached to them by _chordæ tendineæ_, is seen a
     papillary muscle, _PP_, cut away from its attachment to that
     portion of the wall of the ventricle which has been removed. Above,
     the ventricle terminates somewhat like a funnel in the pulmonary
     artery, _P.A._ One of the pockets of the semilunar valve, _sv_, is
     seen in its entirety, another partially.

     1, the wall of the ventricle cut across; 2, the position of the
     auriculo-ventricular ring; 3, the wall of the auricle; 4, masses of
     fat lodged between the auricle and pulmonary artery.
]

can put your three fingers. This is the opening into the right
ventricle; and you will have no difficulty in putting your fingers from
the auricle into the ventricle and bringing them out again.

[Illustration: FIG. 9.--_The Orifices of the Heart seen from above, the
Auricles and Great Vessels being cut away._

     _P.A._ pulmonary artery, with its semilunar valves; _Ao._ aorta,
     do.

     _R.A.V._ right auriculo-ventricular orifice with the three flaps
     (_lv._ 1, 2, 3) of tricuspid valve.

     _L.A.V._ left auriculo-ventricular orifice, with _m.v._ 1 and 2,
     flaps of mitral valve; _b_, piece of whalebone passed into coronary
     vein. On the left part of _L.A.V._ the section of the auricle is
     carried through the auricular appendage; hence the toothed
     appearance due to the portions in relief cut across.
]

But hold the heart in one hand with the auricle upwards, and try to pour
some water into the ventricle. The first few spoonfuls will go in all
right, and then you will see some thin white skin or membrane come
floating up into the opening and quite block up the entrance from the
auricle into the ventricle; the

[Illustration: FIG. 10.--_View of the Orifices of the Heart from below,
the whole of the Ventricles having been cut away._

     _R.A.V._ right auriculo-ventricular orifice surrounded by the three
     flaps, _t.v._ 1, _t.v._ 2, _t.v._ 3, of the tricuspid valve; these
     are stretched by weights attached to the _chordæ tendineæ_.

     _L.A.V._ left auriculo-ventricular orifice surrounded in same way
     by the two flaps, _m.v._ 1, _m.v._ 2, of mitral valve; _P.A._ the
     orifice of pulmonary artery, the semilunar valves having met and
     closed together; _Ao._ the orifice of the aorta with its semilunar
     valves. The shaded portion, leading from _R.A.V._ to _P.A._,
     represents the funnel seen in Fig. 8.
]

water will immediately fill the auricle and run over. If you look at the
membrane carefully as it comes bulging up, you will notice that it is
made up of three pieces joined together as is shown in Fig. 9 (_lv._ 1,
_lv._ 2, _lv._ 3). These three pieces form the valve between the right
auricle and ventricle, called the =tricuspid=, or three-peaked valve. Why
it is so called you will understand if you lay open the right ventricle
by cutting with a pair of scissors from the auricle into the ventricle
along the side of the heart, or by cutting away the front of the
ventricle as has been done in Fig. 8. You will then see that the valve
is made up of three little triangular flaps, which grow together round
the opening with their points hanging down into the cavity of the
ventricle (Fig. 10, _t.v._) They do not, however, hang quite loosely.
You will notice fastened to the sides of the flaps, thin delicate
threads, the other ends of which are fastened to the sides of the
ventricle, and often to little fleshy projections called papillary
muscles (Fig. 8, _P.P._)

How do these valves act? In this way. When the ventricle is empty, and
blood or water or any other fluid is poured into it from the auricle,
the valves are pushed on one side against the walls of the ventricle,
and thus there is a great wide opening from the auricle into the
ventricle. But as the ventricle fills, the blood or water gets behind
the flaps and floats them up towards the auricle. The more fluid in the
ventricle the higher they float, until when the ventricle is quite full
they all meet together in the middle of the opening between the auricle
and ventricle and completely block it up. But why do they not turn right
over into the auricle, and so open up again the wrong way? Because of
those little threads (the _chordæ tendineæ_, as they are called) which
fasten them to the walls of the ventricle. The flaps float back until
these threads are stretched quite tight, and the threads are just long
enough to let the flaps reach to the middle of the opening, but no
further. The tighter the threads are stretched the closer the flaps fit
together, and the more completely do they block the way from the
ventricle back into the auricle.

=The tricuspid valve, then, lets blood flow easily from the right auricle
into the right ventricle, but prevents it flowing from the ventricle
into the auricle.=

=31.= Now look at the cavity of the ventricle. Its walls are fleshy, that
is muscular, and you will notice that they are much stouter and thicker
than those of the auricle. Besides the opening from the auricle there is
but one other, which is at the top of the ventricle, side by side with
the former. If you put a penholder or your finger through this second
opening, you will find that it leads into the large vessel which you
have already learnt to recognize as the pulmonary artery (Fig. 5,
_P.A._)

Slit up the pulmonary artery from the ventricle with a pair of scissors,
as has been done in Fig. 8, _P.A._ You will notice at once the line
where the red soft flesh of the muscular ventricle leaves off, and the
yellow firmer material of which the artery is made begins. Just at that
line you will see a row of three (perhaps you may have cut one of the
three with your scissors) most beautiful, watch-pocket valves, made on
just the same principle as those in the veins, only larger, and more
exquisitely finished. These are called =semilunar valves=, because each
pocket is of the shape of a half-moon. Lift them up carefully and see
how tender and yet how strong they are. There is no need to tell you the
use of these. You know it at once. =They are to let the blood flow from
the ventricle into the pulmonary artery, and to prevent the blood going
back from the artery into the ventricle.=

On the right side of the heart we have, then, two great valves, the
tricuspid valve between the auricle and the ventricle, and the semilunar
valve between the ventricle and the pulmonary artery. These let the
blood flow easily one way, but not the other. If you doubt this, try it.
Put a tube into either the superior or inferior vena cava of a fresh
heart, tying the other vena cava and another tube into the pulmonary
artery. If with a funnel you pour water into the tube in the vein, it
will run through auricle and ventricle and out through the tube of the
pulmonary artery as easily as possible; but if you try to pour water the
other way down the pulmonary artery, you will find you cannot do it; the
tube gets blocked directly, and only a few drops come back through the
heart into the vein.

Now slit up the pulmonary artery as far as you can, and note when you
cut it how stout and firm are its walls. You will find that it soon
divides into two branches, one for the right lung, one for the left.
Each of these, when it gets to the lung, divides into branches, and
these again into others, as far as you can follow them. You know from
what you have learnt already that these branches end in capillaries all
over the lungs.

=32.= Not far from the two main branches of the pulmonary artery you will
find, covered up perhaps with fat and other matters, some tubes which
you will at once recognize as veins, and if you open any one of these
you will find that you can put a thin rod into it, and that it leads in
one direction to the lungs, and in the other into the left side of the
heart. These are the _pulmonary veins_, and if you slit them right up
you will find they open (by four openings) into a cavity on the left
side of the heart, almost exactly like that cavity on the right side
which we called the right auricle (Fig. 11). This cavity is, in fact,
the =left auricle=; out of it there is an opening into the left ventricle,
very like the opening from the right auricle into the right ventricle.
It too is guarded by flap valves, exactly like the tricuspid valve, only
there are but two flaps instead of three (Fig. 9, _m.v._ 1, _m.v._ 2).
Hence this valve is called the =bicuspid=, or more frequently the =mitral=
valve. Its flaps have little threads by which they are fastened to the
walls of the ventricle, and in fact, except for there being two flaps
instead of three, the mitral valve is exactly like the tricuspid valve,
and acts exactly the same way.

If you cut with a pair of scissors from the auricle into the ventricle,
you will find the left ventricle (Fig. 11) very much like the right
ventricle, only its walls are very much thicker, so much thicker that
the left ventricle takes up the greater part of the heart. You will see
this if you now look at the outside of a fresh heart.

The auricles are so small and so covered up by fat that from the outside
you can hardly see them at all. What you chiefly see are two little
fleshy corners, one of each auricle (Fig. 5, _R.A._ _L.A._), often
called “the auricular appendages.” By far the greater part is taken up
by the ventricles--and if you look you will see a band of fat slanting
across the heart (Fig. 5, 3). This marks the line of the fleshy
division, or =septum= as it is called, between the two ventricles. You
will notice that the point or apex of the heart belongs altogether to
the left ventricle.

[Illustration: FIG. 11.--_Left Side of the Heart of a Sheep (laid
open)._

     _P.V._ pulmonary veins opening into the left auricle by four
     openings, as shown by the styles or pieces of whalebone placed in
     them: _a_, a style passed from auricle into ventricle through the
     auriculo-ventricular orifice; _b_, a style passed into the coronary
     vein, which, though it has no connection with the left auricle, is,
     from its position, necessarily cut across in thus laying open the
     auricle.

     _M.V._ the two flaps of the mitral valve (drawn somewhat
     diagrammatically): _pp_, papillary muscles, belonging as before to
     the part of the ventricle cut away; _c_, a style passed from
     ventricle in _Ao._ aorta; _Ao_^{2}. branch of aorta (see Fig. 5,
     _Áó_); _P.A._ pulmonary artery; _S.V.C._ superior vena cava.

     1, wall of ventricle cut across; 2, wall of auricle cut away around
     auriculo-ventricular orifice; 3, other portions of auricular wall
     cut across; 4, mass of fat around base of ventricle (see Fig. 5,
     2).
]

To return to the inside of the left ventricle. Up at the top of the
ventricle, close to the opening from the auricle, there is one other
opening, and only one. If you put your finger into this, you will find
that it leads into a tube which first of all dips under or behind the
pulmonary artery and then comes up and to the front again. This tube is
what you already know as the =aorta=. If you slit it up from the ventricle
(and to do this you must cut through the pulmonary artery), you will
find that on the left side, as on the right, the red fleshy wall of the
ventricle suddenly changes into the yellow firm wall of the artery, and
that just at this line there are three semilunar valves exactly like
those in the pulmonary artery.

=On the left side of the heart, then, we have also two valves, the mitral
between the auricle and the ventricle, and the semilunar between the
ventricle and the aorta. These let the blood pass one way and not the
other. You can easily drive fluid from the pulmonary veins through
auricle and ventricle into the aorta, but you cannot send it back the
other way from the aorta.=

These then are the reasons why the blood will only pass one way, the way
I said it did. There are sets of valves opening one way and shutting the
other. These valves are the tricuspid between the right auricle and
right ventricle, the pulmonary semilunar valves between the right
ventricle and the pulmonary artery, the mitral valve between the left
auricle and the left ventricle, the aortic semilunar valves between the
left ventricle and the aorta, and the valves which are scattered among
the veins of the body. Of these by far the most important are the
valves in the heart: they do the chief work; those in the veins do
little more than help.

=33.= Well, then, we understand now, do we not? why the blood, if it moves
at all, moves in the one way only. There still remains the question, =Why
does the blood move at all?=

You know that during life it does keep moving. You have seen it moving
in the web of a frog’s foot--and whenever any part of the body can be
brought under the microscope, the same rush of red corpuscles through
narrow channels may be seen. You know it moves because when you cut a
blood-vessel the blood runs out. If you cut an artery across, the blood
gushes out from the end which is nearest the heart; if you cut a vein
across, the blood comes most from the end nearest the capillaries. If
you want to stop an artery bleeding, you tie it between the cut and the
heart; if you want to stop a vein bleeding, you tie it between the cut
and the capillaries. You understand now why there is this difference
between a cut artery and a cut vein. And you see that this is by itself
a proof that the blood moves in the arteries from the heart to the
capillaries, and in the veins from the capillaries to the heart.

The blood is not only always moving, but moves very fast. It flies along
the great arteries at perhaps ten inches in a second. Through the little
bit of capillaries along which it has to pass it creeps slowly, but
manages sometimes to go all the way round from vein to vein again in
about half a minute.

It is always moving at this rapid rate, and when it ceases to move, you
die.

=What makes it move?=

Suppose you had a long thin muscle, fastened at one end to something
firm, and with a weight hanging at the other end. You know that every
time the muscle contracted it would pull on the weight and draw it up.
But suppose, instead of hanging a weight on to the muscle, you wrapped
the muscle round a bladder full of water. What would happen then each
time the muscle contracted? Why, evidently it would squeeze the bladder,
and if there were a hole in the bladder some of the water would be
squeezed out. That is just what takes place in the heart. You have
already learnt that the heart is muscular. Each cavity of the heart,
each auricle, and each ventricle is, so to speak, a thin bag with a
number of muscles wrapped round it. In an ordinary muscle of the body,
the bundles of fibres of which the muscle is made up are placed
carefully and regularly side by side. You can see this very well in a
round of boiled beef, which is little more than a mass of great muscles
running in different directions. You know that if you try to cut a thin
slice right across the round, at one part your carving-knife will go
“with the grain” of the meat, _i.e._ you will cut the fibres lengthways;
at another part it will go “against the grain,” _i.e._ you will cut the
fibres crossways. In both parts, the bundles of fibres will run very
regularly. But in the heart the bundles are interlaced with each other
in a very wonderful fashion, so that it is very difficult to make out
the grain. They are so arranged in order that the muscular fibres may
squeeze all parts of each bag at the same time.

Each cavity of the heart, then, auricle or ventricle, is a thin bag
with a network of muscles wrapped round it, and each time the muscles
contract they squeeze the bag and try to drive out whatever is in it.
There are more muscles in the ventricles than in the auricles, and more
in the left ventricle than in the right, for we have already seen how
much thicker the ventricles are than the auricles, and the left
ventricle than the right; and the thickness is all muscle.

And now comes the wonderful fact. These muscles of the auricles and
ventricles are always at work contracting and relaxing, shortening and
lengthening, of their own accord, as long as the heart is alive. The
biceps in your arm contracts only when you make it contract. If you keep
quiet, your arm keeps quiet and your biceps keeps quiet. But your heart
never keeps quiet. Whether you are awake or whether you are asleep,
whether you are running about or lying down quite still, whatever you
are doing or not doing, as long as you are alive your heart keeps on
steadily at work. Every second, or rather oftener, there comes a short
sharp squeeze from the auricles, from both exactly at the same time, and
just as the auricles have finished their squeeze, there comes a great
hug from the ventricles, from both at the same time, but a much stronger
hug from the left than from the right; and then for a brief space there
is perfect quiet. But before the second has quite passed away, the
auricles have begun again, and after them the ventricles once more, and
thus the contracting and relaxing of the walls of the heart’s cavities,
this beat of the heart as it is called, this short snap of the two
auricles, this longer, steadier pull of the two ventricles, have gone on
in your own body since before you were born, and will go on until the
moment comes when friends gathering round your bedside will say that you
are “gone.”

=34. But how does this beat of the heart make the blood move?= Let us see.

Remember that you have, or when you are grown up will have, bottled up
in the closed blood-vessels of your body about 12 lbs. of blood. You
have seen that the heart and the blood-vessels form a system of closed
tubes; the walls are in some places, in the capillaries for instance,
very thin, but they are sound and whole--and though the road is quite
open from the capillaries through the veins, heart, and arteries to the
capillaries again, there is no way out of the tubes except by making a
breach somewhere in the walls.

This closed system of heart and tubes is pretty well filled by the 12
lbs. of blood.

What then must happen each time the heart contracts?

Let us begin with the right ventricle. Suppose it is full of blood. It
contracts. The blood in it, squeezed on all sides, tries to go back into
the right auricle, but the tricuspid flaps have been driven back and
block the way. The more the blood presses on them, the tighter they
become, and the more completely they shut out all possibility of getting
into the auricle.

The way into the pulmonary artery is open, the blood can go there. But
stay, the artery is already full of blood, and so are the capillaries
and veins in the lung. Yes, but the artery will stretch ever so much.
Take a piece of pulmonary artery, and having tied one end, pump or pour
water into the other; you will see how much it will stretch. Into the
pulmonary artery, then, goes the blood, stretching it in order to find
room. As the ventricle squeezes and squeezes, until its walls meet in
the middle, all the blood that was in it finds its way out into the
artery. But the beat of the ventricle soon ceases, the squeeze is over
and gone, and back tumbles the blood into the ventricle, or would
tumble, only the first few drops that shoot backwards are caught by the
watch-pocket semilunar valves. Back fly these valves with a sharp click
(for the things of which we are speaking happen in a fraction of a
second), and all further return is cut off. The blood has been squeezed
out of the ventricle, and is safely lodged in the pulmonary artery.

But the pulmonary artery is ever so much on the stretch. It was fairly
full before it received this fresh lot of blood; now it is over-full--at
least that part of it which is nearest to the heart is over-full. What
happens next? What happens when you stretch a piece of india-rubber and
then let it go? It returns to its former size. The ventricle has
stretched the piece of pulmonary artery near it, beyond the natural
size, and then (when it ceased to contract) has let it go. Accordingly
the piece of pulmonary artery tries to return to its former size, and
since it cannot send the blood back to the ventricle, squeezes it on to
the next piece of the artery nearer the capillaries, stretching that in
turn.

This again in turn sends it on the next piece--and so on right to the
capillaries. The over-full pulmonary artery, stretched to hold more than
it fairly can, empties itself through the capillaries into the pulmonary
veins until it is not more than comfortably full. But the pulmonary
veins also are already full,--what are they to do? To empty the surplus
into the left auricle. Oftener than every second there will come a time
when they can do so.

For at the same time that the right ventricle pumped a quantity of blood
into the pulmonary artery and safely lodged it there, the left ventricle
pumped a like quantity into the aorta, safely lodged it there, and was
left empty itself. But just at that moment the left auricle began to
contract and to squeeze the blood that was in it.

Where could that blood go? It could not go back into the pulmonary
veins, for they were already full, and the blood in them was being
pressed behind by the over-full pulmonary arteries. But it could pass
easily into the empty ventricle--and in it tumbled, the mitral flaps
readily flying back and opening up a wide way. And so the auricle
emptied itself into the ventricle. But now the auricle ceases to
contract--its walls no longer squeeze--it is empty and wants filling,
and so comes the moment when the pulmonary veins can pour into it the
blood which has been driven into them by the over-full pulmonary artery.

Thus the right ventricle drives the blood into the over-full pulmonary
artery, the pulmonary artery overflows into the pulmonary veins, the
pulmonary veins carry the surplus to the empty left auricle, the left
auricle presses it into the empty left ventricle, the left ventricle
pumps it into the aorta--(the stretching of the aorta and of its
branches is what we call the pulse)--the over-full aorta overflows just
as did the pulmonary artery, through the capillaries of the body into
the great venæ cavæ--through these the blood falls into the empty right
auricle, the right auricle drives it into the empty right ventricle,
and the full right ventricle is the point at which we began.

=Thus the alternate contractions of auricles and ventricles, thanks to
the valves in the heart and in the veins, pump the blood, stroke by
stroke, through the wide system of tubes; and thus in every capillary
all over the body we find blood pressed upon behind by over-full
arteries, with a way open to it in front, thanks to the auricles, which
are, once a second or oftener, empty and ready to take up a fresh supply
from the veins.= Thus it comes to pass that every little fragment of your
body is bathed by blood, which a few moments ago was in your heart, and
a few moments before that was in some other part of your frame. Thus it
is that no part of your body can keep itself to itself; the blood makes
all things common as it flies from spot to spot. The red corpuscle that
a minute ago was in your brain, is now perhaps in your liver, and in
another minute may be in a muscle of your arm or in a bone of your leg:
wherever it goes it has something to bring, and something to fetch. A
restless heart is for ever driving a busy blood, which wherever it goes
buys and sells, making perhaps an occasional bargain as it shoots along
the great arteries and great veins, but busiest of all as it lingers in
the narrow pathways of the capillaries.

=35.= When you look down upon a great city from a high place, as upon
London from St. Paul’s, you see stretching below you a network of
streets, the meshes of which are filled with blocks of houses. You can
watch the crowds of men and carts jostling through the streets, but the
work within the houses is hidden from your view. Yet you know that, busy
as seems the street, the turmoil and press which you see there are but
tokens of the real business which is being carried on in the house.

So is it with any piece of the body upon which you look through the
microscope. You can watch the red blood jostling through the network of
capillary streets. But each mesh bounded by red lines is filled with
living flesh, is a block of tiny houses, built of muscle, or of skin, or
of brain, as the case may be. You cannot _see_ much going on there,
however strong your microscope; yet that is where the chief work goes
on. In the city the raw material is carried through the street to the
factory, and the manufactured article may be brought out again into the
street, but the din of the labour is within the factory gates. In the
body the blood within the capillary is a stream of raw material about to
be made muscle, or bone, or brain, and of stuff which, having been
muscle, or bone, or brain, is no longer of any use, and is on its way to
be cast out. The actual making of muscle, or of bone, or of brain, is
carried on, and the work of each is done, outside the blood, in the
little plots of tissue into which no red corpuscle comes.

The capillaries are closed tubes; they keep the red corpuscles in their
place. But their walls are so thin and delicate that they let the watery
plasma of the blood, the colourless fluid in which the corpuscles float,
soak through them into the parts inside the mesh. You probably know that
many things will pass through thin skins and membranes in which no holes
can be found even after the most careful search. If you put peas into a
bladder and tie the neck, the peas will not get out until the bladder is
untied or torn. But if you were to put a solution of sugar or of salt
into the bladder, and place the bladder with its neck tied ever so
tightly in a basin of pure water, you would find that very soon the
water in the basin would begin to taste of sugar or salt--and that
without your being able to discover any hole, however small, in the
bladder. By putting various substances in the bladder, you will find
that solid particles and things which will not dissolve in water keep
inside the bladder, whereas sugar and salt, and many other things which
dissolve in water, will make their way through the bladder into the
water outside, and will keep on passing until the water in the basin is
as strong of sugar or salt as the water in the bladder. This property
which membranes such as a bladder have of letting certain substances
pass through them is called =osmosis=. You will at once see how important
a part it plays in your own body. It is by osmosis chiefly that the raw
nourishing material in the blood gets into the little islets of flesh
lying, as we have seen, in the meshwork of the capillaries. It is by
osmosis chiefly that the worn-out stuff from the same islets gets back
into the blood. It is by osmosis chiefly that food gets out of the
stomach into the blood. It is by osmosis chiefly that the waste,
worn-out matters are drained away from the blood, and so cast out of the
body altogether. By osmosis the blood nourishes and purifies the flesh.
By osmosis the blood is itself nourished and kept pure.

There are two chief things by which the blood, and through the blood
the body, is nourished. These are food and air. The air we have always
with us, we have no need to buy it or toil for it; hence we take it as
we want it, a little at a time, and often. We gather up no store of it;
and cannot bear the lack of it for more than a few moments.

For our food we have to labour; we store it up in our bodies from time
to time, at intervals of hours, in what we call meals, and can go hours
or even days without a fresh supply.

Let us first of all see how the blood, and, through the blood, the body,
is nourished by air.




HOW THE BLOOD IS CHANGED BY AIR: BREATHING. § VI.


=36.= I have already said, perhaps more than once, that our muscles burn,
burn in a wet way without giving light. And when I say our muscles, I
might say our whole body, some parts burning more fiercely than others.

You have learnt from your Chemistry Primer (Art. 2, p. 2) what happens
when a candle is placed in a closed jar of pure air. The oxygen gets
less, carbonic acid comes in its place, and after a while the candle
goes out for want of oxygen to carry on that oxidation which is the
essence of burning. You also know that exactly the same thing would
happen if you were (only you need not do it) to put a bird or a mouse in
the jar instead of a candle. The oxygen would go, carbonic acid would
come, and the little flame of life in the mouse would flicker and go
out, and after a while its body would be cold.

But suppose you were to put a fish or a snail in a jar of pure fresh
water, and cork the jar tight. There seems at first sight to be no air
in the jar. But there is. If you were to take that fresh water, and put
it under an air-pump, you could pump bubbles of air out of it; and if
you were to examine these bubbles you would find them to contain oxygen
and nitrogen, with very little carbonic acid. =The water contains
dissolved air.= After the fish or the snail had been some time in the
jar, you would see its flame of life flicker and die out, just like that
of the bird in air; and if you then pumped the air out of the water you
would find that the oxygen was nearly gone and that carbonic acid had
come in its place.

You see, then, that air can be breathed, as we call it, even when it is
dissolved in water.

Now to return to our muscle. When you were watching the circulation in
the frog’s foot, you could tell the artery from the vein, because in the
artery the blood was flowing _to_ the capillaries, and in the vein
_from_ them. Both artery and vein were rather red, and of about the same
tint of colour. But if you could see in your own body a large artery
going to your biceps muscle, and a large vein coming away from it, you
would be struck at once with the difference of colour between them. The
artery would look bright scarlet, the vein a dark purple; and if you
were to prick both, the blood would gush from the artery in a bright
scarlet jet, and bubble from the vein in a dark purple stream. And
wherever you found an artery and a vein (with a great exception of which
I shall have to speak directly), the blood in the artery would be bright
scarlet, and that in the vein dark purple. Hence we call the bright
scarlet blood which is found in the arteries =arterial blood=, and the
dark purple blood which is found in the veins =venous blood=.

What is the difference between the two? If you were to pump away at some
arterial blood, as you did at the water in which you put your fish, you
would be able to obtain from it some air, or, more correctly, some gas;
a great deal more gas, in fact, than you did from the water. A pint of
blood would yield you half a pint of gas. This gas you would find on
examination not to be air, _i.e._ not made up of a great deal of
nitrogen and the rest oxygen. (Chemical Primer, Art. 9.) There would be
very little nitrogen, but a good deal of oxygen, and still more carbonic
acid.

If you were to pump away at some venous blood you would get about as
much gas, but it would be very different in composition. The little
nitrogen would remain about the same, but the oxygen would be about half
gone, while the carbonic acid would be much increased.

This, then, is one great difference (for there are others) between
venous and arterial blood, that while =both contain, dissolved in them,
oxygen, nitrogen, and carbonic acid, venous blood contains less oxygen
and more carbonic acid than arterial blood=.

=37. In passing through the capillaries on its way to the vein, the blood
in the artery has lost oxygen and gained carbonic acid.= Where has the
oxygen gone to? Whence comes the carbonic acid? To and from the islets
of flesh between the capillaries, to the bloodless muscular fibre or bit
of nerve or skin which the blood-holding capillaries wrap round. The
oxygen has passed from the blood within the capillaries to the flesh
outside; from the flesh outside the carbonic acid has passed to the
blood within the capillaries. And this goes on all over the body.
Everywhere the flesh is breathing blood, is breathing gas dissolved in
the blood, just as a fish breathes water, _i.e._ breathes the air
dissolved in the water.

Goes on everywhere with one great exception. There is one great artery,
with its branches, in which blood is not bright, scarlet, arterial, but
dark, purple, venous. There are certain great veins in which the blood
is not dark, purple, venous, but bright, scarlet, arterial. You know
which they are. The pulmonary artery and the pulmonary veins. The blood
in the pulmonary veins contains more oxygen and less carbonic acid than
the blood in the pulmonary artery. =It has lost carbonic acid and gained
oxygen, as it passed through the capillaries of the lungs.=

=38.= What are the lungs? As you saw them in the rabbit, or as you may see
them in the sheep, they are shrunk and collapsed. We shall presently
learn why. But if you blow into them through the windpipe, which divides
into branches, one for either lung, you can blow them out ever so much
bigger. They are in reality bladders which can be filled with air, but
which, left to themselves, at once empty themselves again.

They are bladders of a peculiar construction. Imagine a thick short bush
or tree crowded with leaves; imagine the trunk and the branches, small
and great, down to the veriest twigs, all hollow; imagine further that
the leaves themselves were little hollow bladders, stuck on to the
smallest hollow twigs, and made of some delicate, but strong and
exceedingly elastic, substance. If you blew down the trunk you might
stretch and swell out all the hollow leaves; when you left off blowing
they would all fall together, and shrink up again.

Around such a framework of hollow branches called =bronchial-tubes=, and
hollow elastic bladders called =air-cells=, is wrapped the intricate
network of pulmonary arteries, veins, and capillaries, in such a way
that each air-cell, each little bladder, is covered by the finest and
most close-set network of capillaries, very much as a child’s
india-rubber ball is covered round with a network of string. Very thin
are the walls of the air-cell, so thin that the blood in the capillary
is separated from the air in the air-cell by the thinnest possible sheet
of finest membrane. As the dark purple blood rushes through the crowded
network, its carbonic acid escapes through this thin membrane, from the
blood into the air, and oxygen slips from the air into the blood.

Thus the dark purple venous blood coming along the pulmonary artery, as
it glides in the pulmonary capillaries along the outside of the inflated
air-cells, by loss of carbonic acid and gain of oxygen is changed into
the bright scarlet blood of the pulmonary veins.

This then is the mystery of our constant need of air. =The flesh of the
body of whatever kind, everywhere all over the body, breathes blood,
making pure arterial blood venous and impure, all over the body except
in the lungs, where the blood itself breathes air, and changes from
impure and venous to pure and arterial.=

=39.= Through the capillaries of the muscle a stream of blood is ever
flowing so long as life lasts and the heart has power to beat; every
instant a fresh supply of bright, pure, arterial blood comes to take the
place of that which has become dark, venous, and impure. Without this
constant renewal of its blood the muscle would be choked, and its vital
flame would flicker and die out.

In the lungs, the air filling the air-cells would if left to itself soon
lose all its oxygen and become loaded with carbonic acid; and the blood
in the capillaries of the lungs would no longer be changed from venous
to arterial, but would travel on to the pulmonary vein as dark and
impure as in the pulmonary artery. =Just as the blood in the muscle must
be constantly renewed, so must the air in the lungs be continually
changed.=

How is this renewal of the air in the lungs brought about?

In the dead rabbit you saw the lungs, shrunk, collapsed, emptied of much
of their air, and lying almost hidden at the back of the chest (Fig. 1,
_G.G._) The cavity of the chest seemed to be a great empty space, hardly
half filled by the lungs and heart. But this is quite an unnatural
condition of the lungs. Take another rabbit, and before you touch the
chest at all, open the abdomen and remove all its contents--stomach,
liver, intestines, &c. You will then get a capital view of the
=diaphragm=, which as you already know forms a complete partition between
the chest and the belly. You will notice that it is arched up towards
the chest, so that the under surface at which you are looking is quite
hollow. If you hold the rabbit up by its hind legs with its head hanging
down, and pour some water into the abdomen, quite a little pool will
gather in the shallow cup of the diaphragm.

In the rabbit the diaphragm is very transparent; you can see right
through it into the chest, and you will have no difficulty in
recognizing the pink lungs shining through it. =You will notice that they
cover almost all the diaphragm--in fact they fill up the whole of the
cavity of the chest that is not occupied by the heart.=

If you seize the diaphragm carefully in the middle with a pair of
forceps, and pull it down towards the abdomen, you will find that you
cannot create a space between the lungs and the diaphragm, but that the
lungs follow the diaphragm, and are quite as close to it when it is
pulled down as when it is drawn up.

=In other words, when the diaphragm is arched up as you find it on
opening the abdomen, the lungs quite fill the chest; and when the
diaphragm is drawn down and the cavity of the chest made bigger, the
lungs swell out so that they still fill up the chest.=

=40.= How do they swell out? By drawing air in through the windpipe. If
you listen, you will perhaps hear the air rush in as you pull the
diaphragm down--and if you tie the windpipe, or quite close up the nose
and mouth, you will find it much harder to pull down the diaphragm,
because no fresh air can get into the lungs.

Now prick a hole through the diaphragm into the cavity of the chest,
without wounding the lungs. You will hear a sudden rush of air, and the
lungs will shrink up almost out of sight. They are no longer close
against the diaphragm as they were before; and if you open the chest you
will find that they have shrunk to the back of the thorax as you saw
them in the first rabbit. The rush of air is partly a rush of air out of
the lungs, and partly a rush of air into the chest between the chest
walls and the outside of the lungs.

But before you lay open the chest, pull the diaphragm up and down as you
did before you made the hole in the diaphragm. You will find that you
have no effect whatever on the lungs. They remain perfectly quiet, and
do not swell up at all. By working the diaphragm up and down, you only
drive air through the hole you have made, =in and out of the cavity of
the chest=, not =in and out of the lungs= as you did before.

We see then that the =chest is an air-tight chamber=, and that the lungs,
when the chest walls are whole, =are always on the stretch=, are on the
stretch even when the diaphragm is arched up as high as it can go.

Why is it that the lungs are thus always on the stretch? Because the
chest is air-tight, so that no air can get in between the outside of the
lungs and the inside of the chest wall. You know from your Physics
Primer (Art. 29, p. 34) that the atmosphere is always pressing on
everything. It is pressing on all parts of the rabbit; it presses on the
inside of the windpipe and on the inside of the lungs. It presses on
the outside of the abdomen, and so presses on the under surface of the
diaphragm, and drives it up into the chest as far as it will go. But it
will not go very far, because its edges are fastened to the firm walls
of the chest. The air also presses on the outside of the chest, but
cannot squeeze that much, because its walls are stout.

If the walls of the chest were soft and flabby, the atmosphere would
squeeze them right up, and so through them press on the outside of the
lungs; since they are firm it cannot. The chest walls keep the pressure
of the atmosphere off the outside of the lungs.

The lungs then are pressed by the atmosphere on their insides and not on
their outsides; and it is this inside pressure which keeps them on the
stretch or expanded. When you blow into a bladder, you put it on the
stretch and expand it because the pressure of your breath inside the
bladder is greater than the pressure of the atmosphere outside the
bladder. If, instead of making the pressure inside _greater_ than that
outside, you were to make the pressure outside _less_ than that inside,
as by putting the bladder under an air-pump, you would get just the same
effect; you would expand the bladder. That is just what the chest walls
do; they keep the pressure outside the lungs less than that inside the
lungs, and that is why the lungs, as long as the chest walls are sound,
are always expanded and on the stretch.

When you make a hole into the chest, and let the air in between the
outside of the lungs and the chest wall, the pressure of the atmosphere
gets at the outside of the lungs; there is then the same atmospheric
pressure outside as inside the lungs; there is nothing to keep them on
the stretch, and so they shrink up to their natural size, just as does
the bladder when you leave off blowing into it, or when you take it out
of the air-pump.

When before you made the hole in the diaphragm you pulled the diaphragm
down, you =still further lessened the pressure on the outside of the
lungs=; hence the pressure inside the lungs caused them to swell up and
follow the diaphragm. But this put the lungs still more on the stretch,
so that when you let go the diaphragm and ceased to pull on it, the
lungs went back again to their former size, emptying themselves of part
of their air and pulling the diaphragm up with them. When there is a
hole in the chest wall, pulling the diaphragm down does not make any
difference to the pressure outside the lungs. They are then always
pressed upon by the same atmospheric pressure inside and outside, and so
remain perfectly quiet.

=When in an air-tight chest the diaphragm is pulled down, the pressure of
the atmosphere drives air into the lungs through the windpipe and swells
them up. When the diaphragm is let go, the stretched lungs return to
their former size, emptying themselves of the extra quantity of air
which they had received.=

Suppose now the diaphragm were pulled down and let go again regularly
every few seconds: what would happen? Why, every time the diaphragm went
down a certain quantity of air would enter into the lungs, and every
time it was let go that quantity of air would come out of the lungs
again.

[Illustration: FIG. 12.--_The Diaphragm of a Dog viewed from the Lower
or Abdominal Side._

     _V.C.I._ the vena cava inferior; _O._ the œsophagus; _Ao._ the
     aorta; the broad white tendinous middle (_B_) is easily
     distinguished from the radiating muscular fibres (_A_) which pass
     down to the ribs and into the pillars (_C_ _D_) in front of the
     vertebræ.
]

This is what does take place in breathing or respiration. Every few
seconds, about seventeen times a minute, the diaphragm does descend, and
a quantity of air rushes into the lungs through the windpipe. This is
called =inspiration=. As soon as that has taken place, the diaphragm
ceases to pull downwards, the stretched lungs return to their former
size, carrying the diaphragm up with them, and squeeze out the extra
quantity of air. This is called =expiration=.

As the diaphragm descends it presses down on the abdomen; when it ceases
to descend, the contents of the abdomen help to press it up. If you
place your hand on your stomach, you can feel the abdomen bulging out
each time the diaphragm descends in inspiration, and going in again each
time the diaphragm returns to its place in expiration.

=41.= But what causes the diaphragm to descend?

If you look at the diaphragm of the rabbit (or of any other animal) a
little carefully, you will see that it is in reality a flat thin muscle,
rather curiously arranged; for the red fleshy muscular fibres are on the
outside all round the edge (Fig. 12, _A_ and _C_), while the centre _B_
is composed of a whitish transparent tendon. These muscular fibres, like
all other muscular fibres, have the power of contracting. What must
happen when they contract and become shortened?

When these muscular fibres are at rest, as in the dead rabbit, the whole
diaphragm is arched up, as we have seen, towards the thorax, somewhat as
is shown in Fig. 13, _B_. It is partly pushed up by all the contents of
the abdomen (for the cavity of the abdomen, you will remember, is quite
filled by the liver, stomach, intestines, and other organs), partly
pulled up by the lungs, which, as we know, are always on the stretch.
When the muscular fibres contract, they pull at the central tendon (just
as the biceps pulls at its lower tendon), =and pull the diaphragm flat=;
and some of the fibres, such as those at _C_, Fig. 12, also pull it
=down=. =The diaphragm during its contraction is flattened and descends=,
somewhat as is shown in Fig. 13, _A_.

[Illustration: FIG. 13.--_Diagrammatic Sections of the Body in_

     _A._ inspiration; _B._ expiration. _Tr._ trachea; _St._ sternum;
     _D._ diaphragm; _Ab._ abdominal walls. The shading roughly
     indicates the stationary air. The unshaded portion at the top of
     _A_ is the tidal air.
]

=The descent of the diaphragm in inspiration is caused by a contraction
of its muscular fibres. During expiration the diaphragm is at rest; its
muscular fibres relax; and it goes up because it is partly drawn up by
the lungs, partly pushed up by the contents of the abdomen.=

=42.= Other structures besides the diaphragm assist in pumping air in and
out of the lungs. By the action of the diaphragm the chest is
alternately lengthened and shortened. But if you watch anyone, and
especially a woman, breathing, you will notice that with every breath
the chest rises and falls; the front of the chest, the sternum, as you
have learnt to call it, comes forward and goes back; and a little
attention will convince you that it comes forward during inspiration,
_i.e._ while the diaphragm is descending, and falls back during
expiration. But this coming forward of the sternum means a widening of
the chest from back to front, and the falling back of the sternum means
a corresponding narrowing. So that while the chest is being lengthened
by the descent of the diaphragm, it is also being widened by the coming
forward of the sternum. In inspiration the lungs are expanded not only
downwards, by the movement of the diaphragm, but also outwards, by the
movement of the walls of the chest.

What thrusts forward the sternum? If you were to watch closely the sides
of the chest of a very thin person, you would be able to notice that at
every breathing in, at every inspiration, the ribs are pulled up a
little way. Now, each rib is connected with the backbone behind by a
joint, and is firmly fastened to the sternum in front by cartilage (see
Frontispiece). If you were to fasten a piece of string to the middle of
one of the ribs and to pull it, you would find you were working on a
lever, with the fulcrum at the backbone, with the weight acting at the
sternum, and the power at the point where your string was tied. Every
time you pulled the string the rib would move on its fulcrum at the
backbone, in such a way that the front end of the rib would rise up, and
the sternum would be thrust out a little. When you left off pulling, the
sternum, which in being thrust forward had been put on the stretch,
would sink back, and the rib would fall down to its previous position.

[Illustration: FIG. 14.--_View of Four Ribs of the Dog with the
Intercostal Muscles._

     _a._ The bony rib; _b_, the cartilage; _c_, the junction of bone
     and cartilage; _d_, unossified; _e_, ossified, portions of the
     sternum. _A._ External intercostal muscle. _B._ Internal
     intercostal muscle. In the middle interspace, the external
     intercostal has been removed to show the internal intercostal
     beneath it.
]

Between the ribs are certain muscles called =intercostal muscles= (Fig.
14). The exact action of these you will learn at some future time.
Meanwhile it will be enough to say that they act like the piece of
string we are speaking of. =When they contract, they pull up the ribs and
thrust out the sternum; when they leave off contracting, the ribs and
sternum fall back to their previous position.=

There are many other muscles which help in breathing, especially in hard
or deep breathing, but it will be sufficient for you to remember that in
ordinary breathing there are two chief movements taking place exactly at
the same time, by means of which air is drawn into the chest, both
movements being caused by the contraction of muscles. First, the
diaphragm contracts and flattens itself, making the chest deeper or
longer; secondly, at the same time the ribs are raised and the sternum
thrust out by the contraction of the intercostal muscles, making the
chest wider. But as the chest becomes wider and longer, the lungs become
wider and longer too. In order to fill up the extra room thus made in
the lungs, air enters into them through the windpipe. This is
=inspiration=. But soon the diaphragm and the intercostal muscles cease to
contract; the diaphragm returns to its arched condition, the ribs sink
down, the sternum falls back, and the extra air rushes back again out of
the lungs through the windpipe. This is =expiration=. An inspiration and
an expiration make up a whole breath; and thus we breathe some seventeen
times in every minute of our lives.

=43.= But what makes the diaphragm and intercostal muscles contract and
rest in so beautifully regular a fashion? The biceps of the arm, we saw,
was made to contract by our will. It is not our will, however, which
makes us breathe. We breathe often without knowing it; we breathe in our
sleep when our will is dead; we breathe whether we will or no, because
we cannot help it. We can quicken our breathing, we can take a short or
deep breath as we please, we can change our breathing by the force of
our will; but the breathing itself goes on without, and in spite of, our
will. It is =an involuntary act.=

Though breathing is not an effort of the will, it is an effort of the
brain; an effort, too, of one particular part of the brain, that part
where the brain joins on to the spinal cord. Nerves run from the
diaphragm and the intercostal and other muscles through the spinal cord,
to this part of the brain. And seventeen times a minute a message comes
down along these nerves, from the brain, bidding them contract; they
obey, and you breathe. Why and how that message comes, you will learn at
some future time. When your head is cut off, or when that part of the
brain which joins on to the spinal cord is injured by accident or made
powerless by disease, the message ceases to be sent, and you cease to
breathe.

=44.= At every breath, then, a certain quantity of air goes in and out of
the chest; but only a small quantity. =You must not think the lungs are
quite emptied and quite filled at each breath.= On the contrary, you only
take in each time a mere handful of air, which reaches about as far as
the large branches of the windpipe, and does not itself go into the
air-cells at all. This is often called =tidal= air; and the rest of the
air in the lungs, which does not move, is often called the =stationary=
air (see Fig. 13).

How then does the carbonic acid at the bottom of the lungs get out? How
do the capillaries in the air-cells get their fresh oxygen?

The stationary air mingles with the tidal air at every breath. If you
want to ventilate a room, you are not obliged to take a pair of bellows
and drive out every bit of the old air in the room, and supply its place
with new air: it will be enough if you open a window or a door and let
in a draught of pure air across one corner, say, of the room. That
current of pure air flowing across the corner will mingle with all the
rest of the air until the whole air in the room becomes pure; and the
mingling will take place very quickly. So it is in the lungs. The tidal
air comes in with each inspiration as pure air from without; but before
it comes out at the next expiration it gives up some of its oxygen to
the stationary air, and robs the stationary air of some of its carbonic
acid. For each breath of tidal air the stationary air is so much the
better, having lost some of its carbonic acid and gained some fresh
oxygen. The tidal air rapidly purifies the stationary air, and the
stationary air purifies the blood.

Thus it comes to pass that the tidal air, which at each pull of the
diaphragm and push of the sternum goes into the chest as pure air with
twenty-one parts oxygen to seventy-nine parts nitrogen in every hundred
parts, comes out, when the diaphragm goes up and the sternum falls back,
as impure air with only sixteen parts oxygen, but with five parts
carbonic acid to seventy-nine of nitrogen. That lost oxygen is carried
through the stationary air to the blood in the capillaries, and the
gained carbonic acid came through the stationary air from the blood in
the capillaries. So each breath helps to purify the blood, and the
pumping of air in and out of the chest changes the impure, hurtful,
venous, to pure, refreshing, arterial blood; the blood breathes air in
the lungs, that all the body may in turn breathe blood.




HOW THE BLOOD IS CHANGED BY FOOD: DIGESTION. § VII.


=45.= The blood is not only purified by air, it is also renewed and made
good by food. The food we eat becomes blood. But our food, though
frequently moist, is for the most part solid. We cut it into small
pieces on the plate, and with our teeth we crush and tear it into still
smaller morsels in our mouth. Still, however well chewed, a great deal
of it, most of it in fact, is swallowed solid. In order to become blood
it must first be dissolved. It is dissolved in the alimentary canal, and
we call the dissolving =digestion=. Let us see how digestion is carried
on.

Your skin, though sometimes quite moist with perspiration, is as
frequently quite dry. The inside of your mouth is always moist--very
frequently quite filled with fluid; and even when you speak of it as
being dry, it is still very moist. Why is this? The inside of your mouth
is also very much redder than your skin. The redness and the moisture go
together.

In speaking of the capillaries, I said that almost all parts of the body
were completely riddled with them, but =not quite all=. A certain part of
the skin, for instance, has no capillaries or blood-vessels at all. You
know that where your skin is thick, you can shave off pieces of skin
without “fetching blood;” if your

[Illustration: FIG. 15.--_Section of Skin, highly magnified._

     _a_, horny epidermis; _b_, softer layer; _c_, dermis; _d_,
     lowermost vertical layer of epidermic cells; _e_, cells lining the
     sweat duct continuous with epidermic cells; _h_, corkscrew canal of
     sweat duct. To the right of the sweat duct the dermis is raised
     into a papilla, in which the small artery, _f_, breaks up into
     capillaries, ultimately forming the veins, _g_.
]

knife were very sharp and you very skilful, you might do the same in
every part of your skin. If you were to put some of the skin you had
thus cut off under the microscope, you would find that it was made up of
little scales. And if you were to take a very thin upright slice running
through the whole thickness of the skin, and examine that under a high
power of the microscope, you would find that the skin was made up of two
quite different parts or layers, as shown in Fig. 15. The upper layer,
_a_, _b_, is nothing but a mass of little bodies packed closely
together. At the top they are pressed flat into scales, but lower down
they are round or oval, and at the same time soft. They are called
=cells=. As you advance in your study of Physiology you will hear more and
more about cells. This layer of cells, either soft and round, or
flattened and dried into scales, is called the =epidermis=. No
blood-vessel is ever found in the epidermis, and hence, when you cut it,
it never bleeds. As long as you live it is always growing. The top
scales are always being rubbed off. Whenever you wash your hands,
especially with soap, you wash off some of the top scales; and you would
soon wash your skin away, were it not that new round cells are always
being formed at the bottom of the epidermis, along the line at _d_ (Fig.
15), and always moving up to the top, where they become dried into
scales. Thus the skin, or more strictly the epidermis, is always being
renewed. Sometimes, as after scarlet fever, the new skin grows quickly,
and the old skin comes away in great flakes or patches.

The lower layer below the epidermis is what is called the =dermis=, or
=true skin=. This is full of capillaries and blood-vessels, and when the
knife or razor gets down to this, you bleed. It is not made up of cells
like the epidermis, but of that fibrous substance which you early learnt
to call connective tissue (see p. 9). Its top is rarely level, but
generally raised into little hillocks, called =papillæ=, as in the figure;
the epidermis forming a thick cap over each papillæ, and filling up the
hollows between them. Most of the papillæ are full of blood-vessels.

Now, then, I think you will understand why your skin is not red, but
flesh-coloured, and why it is generally dry. The true skin under the
epidermis is always moist, because of the blood-vessels there; the waste
and fluid parts of the blood pass readily through the walls of the
capillaries, as you have learnt, by osmosis, and so keep everything
round them moist. But this moisture is not enough to soak through the
thick coating of epidermis, and so the top part of the epidermis remains
dry and scaly.

The true skin underneath the epidermis is always red; you know that if
you shave off the surface of your skin anywhere, it gets redder and
redder the deeper you go down, even though you do not fetch blood. It is
red because of the immense number of capillaries, all full of red blood,
which are crowded into it. When you look at these capillaries through a
great thickness of epidermis, the redness is partly hidden from you, as
when you put a sheet of thin white paper over a red cloth, and the skin
seems pink or flesh-coloured; and where the epidermis is very thick, as
at the heel, the skin is not even pink, but white or yellow, more or
less dirty according to circumstances.

=46.= But if the moist true skin is thus everywhere covered by a thick
coat of epidermis, which keeps the moisture in, how is it that the skin
is nevertheless sometimes quite moist, as when we perspire?

[Illustration: FIG. 16.--_Coiled end of a Sweat Gland, Epithelium not
shown._

_a_, the coil; _b_, the duct; _c_, network of capillaries, inside which
the duct gland lies.]

If you look at Fig. 15, you will see that the epidermis is at one point
pierced by a canal (_h_) running right through it. You will notice that
this canal is not closed at the bottom of the epidermis, but runs right
into the dermis or true skin, where the canal becomes a tube, with just
one layer (_e_) of cells, like the cells of the epidermis, for its
walls. There is no room in Fig. 15 to show what becomes of this tube,
but it runs some way down under the skin all among the blood-vessels,
and then twisting itself up into a knot, ends blindly, as is shown in
Fig. 16, where _b_ is a continuation on a smaller scale of the same
tube which is seen in Fig. 15. This knot is covered by a close network
of capillaries, which at _c_ are supposed to be unravelled and taken
away from the knotted tube in order to show them. The capillaries, you
will understand, though inside the knot, are always outside the tube. If
you were to drop a very diminutive marble in at _h_ (Fig. 15), it would
rattle down the corkscrew passage through the thick epidermis, shoot
down the straight tube _b_ (Fig. 16), and roll through the knot _a_,
until it came to rest at the blind end of the tube. Along its whole
course it would touch nothing but cells, like the cells of the
epidermis, a single layer of which forms the walls of the tube where it
runs below the epidermis. If it got lodged at _h_ (Fig. 15), or got
lodged in the knot at _a_ (Fig. 16), it would in both cases be touching
epidermic cells. But there would be this great difference. At _h_ it
would be ever so far removed from any blood capillary; at _a_ it would
only have to make its way through a thin layer of single cells, and it
would be touching a capillary directly. At _h_ it might remain dry for
some time; at _a_ it would get wet directly, for there is nothing to
prevent the fluid parts of the blood oozing out through the thin wall of
the capillaries, and so through the thin wall of the tube into the canal
of the tube, on to the marble.

In fact, the inside of the knot is always moist and filled with fluid.
When the capillaries round the knot get over-full of blood, as they
often do, a great deal of colourless watery fluid passes from them into
the tube. The tube gets full, the fluid wells up right into the
corkscrew portion in the thickness of the epidermis, and at last
overflows at the mouth of the tube over the skin. We call this fluid
sweat or =perspiration=. We call the tube with its knotted end =a gland=;
and we call the act by which the colourless fluid passes out of the
blood capillaries into the canal of the tube, =secretion=. We speak of the
=sweat gland secreting sweat out of the blood brought by the capillaries
which are wrapped round the gland=.

=47.= Now we can understand why the inside of the mouth is red and moist.
The mouth has a skin just like the skin of the hand. There is an outside
epidermis, made up of cells and free from capillaries, and beneath that
a dermis or true skin crowded with capillaries. Only the epidermis of
the mouth is ever so much thinner than that of the hand. The red
capillaries easily shine through it, and their moisture can make its way
through. Hence the mouth is red and moist. Besides there are many glands
in it, something like the sweat gland, but differing in shape; these
especially help to keep it moist.

Because it is always red and moist and soft, the skin of the inside of
the mouth is generally not called a skin at all, but =mucous membrane=,
and the upper layer is not called epidermis, but =epithelium=. You will
remember, however, that a mucous membrane is in reality a skin in which
the epidermis is thin and soft, and is called epithelium.

The mouth is the beginning of the alimentary canal. Throughout its whole
length the alimentary canal is lined by a skin or mucous membrane like
that of the mouth, only over the greater part of it the epithelium is
still thinner than in the mouth, and indeed is made up of a single
layer only of cells. The whole of the inside of the canal is therefore
red and moist, and whatever lies in the canal is separated by a very
thin partition only from the blood in the capillaries, which are found
in immense numbers in the walls of the canal. The alimentary canal is,
as you know, a long tube, wide at the stomach but narrow elsewhere. In
all parts of its length the tube is made up of mucous membrane on the
inside, and on the outside of muscles, differing somewhat from the
muscles of the body and of the heart, but having the same power of
contracting, and by contracting of squeezing the contents of the tube,
just as the muscles of the heart squeeze the blood in its cavities. The
muscles, and especially the mucous membrane, are crowded with
blood-vessels.

Though the epithelium of the mucous membrane is very thin, the mucous
membrane itself is thick, in some places quite as thick as the skin of
the body. This thickness is caused by its being =crowded with glands=. In
the skin the sweat glands are generally some little distance apart, but
in the mucous membrane of the stomach and of the intestines they are
packed so close together, that the membrane seems to be wholly made up
of glands.

These glands vary in shape in different parts. Nowhere are they exactly
like the sweat glands, because none of them are long thin tubes coiled
up at the end in a knot, and none of them have a great thickness of
epidermis to pass through. Most of them are short, rather wide tubes;
some of them are branched at the deep end. They all, however, resemble
the sweat glands in being tubes or pouches closed at the bottom but open
at top, lined by a single layer of cells, and wrapped round with blood
capillaries. From these capillaries, a watery fluid passes into the
tubes, and from the tubes into the alimentary canal. This watery fluid
is, however, of a different nature from sweat, and is not the same in
all parts of the canal. The fluid which is, as we say, secreted by the
glands in the walls of the stomach is an =acid fluid=, and is called
=gastric juice=; that by the glands in the walls of the intestines is =an
alkaline fluid=, and is called =intestinal juice=.

=48.= But besides these glands in the mucous membrane of the mouth, the
stomach, and the intestines, there are other glands, which seem at first
sight to have nothing to do with the mucous membrane.

Beneath the skin, underneath each ear, just behind the jaw, is a soft
body, which ordinarily you cannot feel, but which, when inflamed by what
is called “the mumps,” swells up into a great lump. In a sheep’s head
you would find just the same body, and if you were to examine it you
would notice fastened to it a fleshy cord running underneath the skin
across the cheek towards the mouth. By cutting the cord across you would
discover that what seemed a cord was in reality a narrow tube coming
from the soft body we are speaking of and opening into the mouth. Just
close to the soft body this tube divides into two smaller tubes, these
divide again into still smaller ones, or give off small branches; all
these once more divide and branch like the boughs of a tiny tree; and so
they go on branching and dividing, getting smaller and smaller, until
they end in fine tubes with blind swollen ends. All the tubes, great and
small, are lined with epithelium and wrapped round with blood-vessels,
and being packed close together with connective tissue, make up the soft
body we are speaking of. This body is in fact a =gland=, and is called a
=salivary gland=; as you see it is not a simple gland like a sweat gland,
but is made up of a host of tube-like glands all joined together, and
hence is called a =compound gland=. Being placed far away from the mouth,
it has to be connected with the cavity of the mouth by a long tube,
which is called its =duct=. You cannot fail to notice how like such a
gland is, in its structure, to a lung. The lung is in fact a gland
secreting carbonic acid: and the duct of the two lungs is called the
trachea. The salivary gland beneath the ear is called the =parotid gland=;
there is another very similar one underneath the corner of the jaw on
either side, called the =submaxillary gland=. By each of them a watery
fluid is secreted, which, flowing along their ducts into the mouth and
being there mixed with the moisture secreted by the other glands in the
mouth, is called =saliva=.

In the cavity of the abdomen lying just below the stomach is a much
larger but altogether similar compound gland called =the pancreas=, which
pours its secretion called =pancreatic juice= into the alimentary canal
just where the small intestine begins (Fig. 17, _g._)

That large organ the liver, though the plan of its construction is not
quite the same as that of the pancreas or salivary glands, as you will
by and by learn, is nevertheless a huge gland, secreting from the blood
capillaries into which the portal vein (see p. 62) breaks up, a fluid
called =bile= or =gall=, which by a duct, the =gall duct=, is poured into the
top of the intestine (Fig. 17, _e_). When bile is not wanted, as when
we are fasting, it turns off by a side passage from the duct into the
gall-bladder (Fig. 17, _f_), to be stored up there till needed.

=49.= What are the uses of all these juices and secretions? To dissolve
the food we eat.

[Illustration: FIG. 17.--_The Stomach laid open behind._

     _a_, the œsophagus or gullet; _b_, one end of the stomach; _d_,
     the other end joining the intestine; _e_, gall duct; _f_, the
     gall-bladder; _g_, the pancreatic duct; _h_, _i_, the small
     intestine.
]

We eat all manner of dishes, but in all of them that are worth eating we
find the same kind of things, which we call =food-stuffs=.

We eat various kinds of meat; but all meats are made up chiefly of two
things: the substance of the muscular fibre, which you have already
learnt is a =proteid= matter containing nitrogen, and the =fat= which wraps
round the lean muscular flesh. Now, proteids are, when cooked, insoluble
in water (see p. 49); and fat, you know, will not mingle with water.
Both these parts of meat, both these food-stuffs, must be acted upon
before they can pass from the inside of the alimentary canal, through
the epithelium of the mucous membrane, into the blood capillaries.

Besides meat we eat bread. Bread is chiefly composed of =starch=; but
besides starch we find in it a substance containing nitrogen,
exceedingly like the proteid matter of muscle or of blood.

Potatoes contain a very great deal of starch with a very small quantity
of proteid matter; and nearly all the vegetables we eat contain starch,
with more or less proteid matter.

Then we generally eat more or less sugar, either as such or in the form
of sweet fruits. We also take salt with our meals, and in almost
everything we eat, animal or vegetable, meat, bread, potatoes or fruit,
we swallow a quantity of mineral substances, that is, various kinds of
salts, such as potash, lime, magnesia, iron, with sulphuric,
hydrochloric, phosphoric, and other acids.

=In everything on which we live we find one or more of the following
food-stuffs:--Proteid matter, starch or sugar, and fat, together with
certain minerals and water. It is on these we live: any article which
contains either proteid matter, or starch, or fat, is useful for food.
Any article which contains none of them is useless for food, unless it
be for the sake of the minerals or water it holds.=

We are not obliged to eat all these food-stuffs. Proteid matter we must
have always. It is the only food-stuff which contains nitrogen. It is
the only substance which can renew the nitrogenous proteid matter of the
blood and so the nitrogenous proteid matter of the body.

We might indeed manage to live on proteid matter alone, for it contains
not only nitrogen but also carbon and hydrogen, and out of it, with the
help of a few minerals, we might renew the whole blood and build up any
and every part of the body. But, as you will learn hereafter, it would
be uneconomical and unwise to do so. Starch, sugar, and fats, contain
carbon and hydrogen without nitrogen; and hence, if we are to live on
these we must add some proteid matter to them.

=50.= Of these food-stuffs, putting on one side the minerals, sugar (of
which, as you know, there are several kinds, cane sugar, grape sugar,
and the like) is the only one which is really soluble, and will pass
readily by osmosis through thin membranes (see p. 84). If you take a
quantity of white of egg, or blood serum, or meat, or fibrin, or a
quantity of starch boiled or unboiled, or a quantity of oil or fat,
place it in a bladder, and immerse the bladder in pure water, you will
find that none of it passes through the bladder into the water outside,
as sugar or salt would do. In the same way a quantity of meat, or of
starch, or of fat, placed in your alimentary canal, would never get
through the membrane which separates the inside of the canal from the
inside of the capillaries, and so would remain perfectly useless as food
unless something were done to it. While the food is simply inside the
alimentary canal, it is really outside your body. It can only be said
to be inside your body when it gets into your blood.

In the things we eat, moreover, these food-stuffs are mixed up with a
great many things that are not food-stuffs at all; they are packed away
in all manner of little cases, which are for the most part no more good
for eating than the boxes or paper in which the sweetmeats you buy are
wrapped up. The food-stuffs have to be dissolved out of these boxes and
packing.

=The juices secreted by the glands= of which we have been speaking,
dissolve the food-stuffs out of their wrappings, act upon them so as to
make them fit to pass into the blood, and leave all the wrappings as
useless stuff which passes out of the alimentary canal without entering
into the blood, and therefore without really forming part of the body at
all.

This preparation and dissolving of food-stuffs is called digestion.

Different food-stuffs are acted upon in different parts of the
alimentary canal.

The saliva of the mouth has a wonderful power of =changing starch into
sugar=. If you take a mouthful of boiled starch, which is thick, sticky,
pasty, and tasteless, and hold it in your mouth for a few moments, it
will become thin and watery, and will taste quite sweet, because the
starch has been changed into sugar. Now sugar, as you know, will readily
pass through membranes, though starch will not.

The gastric juice in the stomach does not act much on starch, but it
=rapidly dissolves all proteid matters=.

If you take a piece of boiled meat, put it in some gastric juice and
keep the mixture warm, in a very short time the meat will gradually
disappear. All the proteid matter will be dissolved, and only the
wrappings of the muscular fibre and the fat be left. You will have a
solution of meat--a solution, moreover, which, strange to say, will
easily pass through membranes, and is therefore ready to get into the
blood.

The pancreatic juice and the juice secreted by the intestine act both on
starch as saliva does, and on proteids very much as gastric juice does.

=51.= The bile and the pancreatic juice together act upon all fats in a
very curious way.

You know that if you shake up oil and water together, though by violent
shaking you may mix them a good deal, directly you leave off they
separate again, and all the oil is seen floating on the top of the
water. If, however, you shake up oil with pancreatic juice and bile, the
oil does not separate. You get a sort of creamy mixture, and will have
to wait a very long time before the oil floats to the top. Milk, you
know, contains fat, the fat which is generally called butter. If you
examine milk under the microscope, you will find that the fat is all
separated into the tiniest possible drops. So also, when you shake up
oil or butter, or any other fat, with bile and pancreatic juice, you
will find on examination that the fat or oil is all separated into the
tiniest possible drops. What is the purpose of this?

If you look at the inside of the small intestine of any animal, you will
find that it is not smooth and shiny like the outside of the intestine,
but shaggy, or, rather, velvety. This is because the mucous membrane is
crowded all over with little tags, like very little tongues, hanging
down into the inside of

[Illustration: FIG. 18.--_Semi-diagrammatic View of Two Villi of the
Small Intestines._ (Magnified about 50 diameters.)

     _a_, substance of the villus; _b_, its epithelium, of which some
     cells are seen detached at _b_^{2}; _c d_, the artery and vein,
     with their connecting capillary network, which envelopes and hides
     _e_, the lacteal which occupies the centre of the villus and opens
     into a network of lacteal vessels at its base.
]

the intestine. These are called =villi=; they are not unlike the papillæ
of the skin (Fig. 15), if you suppose all the epidermis stripped except
the bottom row of cells (_d_), and the papilla itself pulled out a good
deal. Fig. 18 is a sketch to illustrate the structure of a =villus=. The
epithelium (_b_), you see, is made up of a single row of cells. Beneath
the epithelium, just as in the papilla of the skin, is a network of
blood capillaries, shown, for convenience, in the right-hand villus
only. But besides the blood capillaries, there is in each villus, what
there is not in a papilla of the skin, another capillary (shown, for
convenience, in the left-hand villus only) which does not contain blood,
which is not connected with any artery or with any vein, but which
begins in the villus. This is a =lacteal=. I have said nothing of these at
present. In most parts of the body we find, besides blood capillaries,
fine passages very much like capillaries, except that they contain a
colourless fluid instead of blood, and do not branch off from any larger
vessels like arteries. They seem to start out of the part in which they
are found, like the roots of a plant in the soil. But though unlike
blood capillaries in not branching off from larger trunks, they resemble
capillaries in joining together to form larger trunks corresponding to
veins, and the colourless fluid flows from the fine capillary channels
towards these larger trunks. This colourless fluid is called =lymph=; it
is very much like blood without the red corpuscles, and the channels in
which it flows are called =lymphatics=.

The lymphatics from nearly all parts of the body join at last into a
great trunk called the =thoracic duct=, which empties itself into the
great veins of the neck, as is shown in the diagram, Fig. 6, _Lct._,
_Ly._, _Th. D._

Now, many of the lymphatics start from the innumerable villi of the
intestine, and are there called =lacteals= (Fig. 6, _Lct._); so that
lacteals may be said to be those lymphatics which have their roots in
the villi of the intestine.

But what has all this to do with the digestion of fat? Lacteal means
=milky=, and the lymphatics coming from the villi are called lacteals
because, when digestion is going on, the fluid in them, instead of being
transparent as in the rest of the lymphatics, =is white and milky=. Why is
it thus white and milky? Because it is crowded with minute particles of
fat, and those minute particles of fat come from the inside of the
intestine. They are the same minute particles into which the bile and
pancreatic juice have divided the fat taken as food. We know this
because when no fat is eaten the lacteals do not get milky; and when for
any reason bile and pancreatic juice are prevented from getting into the
intestine, though ever so much fat be eaten, it does not get into the
lacteals at all, it remains in the intestine in great pieces, and is
finally cast out as useless.

=52.= This, then, is what becomes of the food-stuffs:--

The fats are broken up by the bile and pancreatic juice into minute
particles. These minute particles, we do not exactly know how, pass
through the epithelium of the villus into the lacteal vessels, from the
lacteals into the thoracic duct, and from the thoracic duct into the
vena cava. Thus the fats we eat get into the blood.

The starch is changed into sugar in the mouth by saliva, and in the
intestine by the pancreatic juice; but sugar passes readily through
membranes, and so slips into the blood capillaries of the walls of the
alimentary canal. Thus all the sugar we eat, and all the goodness of the
starch we eat, pass into the blood.

The proteids are dissolved in the stomach by the gastric juice, and what
passes the stomach is dissolved in the intestine, dissolved in such a
way that it can pass through membranes; and thus proteids pass into the
blood.

Probably some of the sugar and proteids pass into the lacteals as well.

The minerals are dissolved either in the mouth, or in the stomach, or in
the intestine, and pass into the blood.

And water passes into the blood everywhere along the whole length of the
canal.

When we eat a piece of bread, while we are chewing it in our mouth it is
getting moistened and mixed with saliva. Part of its starch is thereby
changed into sugar, and all of it is softened and loosened. Passing into
the stomach, some of the proteids are dissolved out by the gastric
juice, and pass into the blood, and all the rest of the bread breaks up
into a pulpy mass. Passing then into the intestine, what is left of the
starch is changed by the pancreatic juice into sugar, and is at once
drained off either into the lacteals or straight into the blood. In the
intestine what remains of the proteids is dissolved, till nothing is
left but the shells of the tiny chambers in which the starch and
proteids were stored up by the wheat-plant as it grew.

When we eat a piece of meat, it is torn into morsels by the teeth and
well moistened by saliva, but suffers else little change in the mouth.
In the stomach, however, the proteids rapidly vanish under the action of
the gastric juice. The morsels soften, the fibres of the muscle break
short off and come asunder; the fat is set free from the chambers in
which it was stored up by the living ox or sheep, and, melted by the
warmth of the stomach, floats in great drops on the top of the softened
pulpy mass of the half-digested food. Rolled about in the stomach for
some time by the contraction of the muscles which help to form the
stomach walls, losing much of its proteids all the while to the hungry
blood, the much-changed meat is squeezed into the intestine. Here the
bile and the pancreatic juice, breaking up the fat into tiny particles,
mix fat, and broken meat, and empty wrappings, and salts, and water, all
together into a thick, dirty, yellowish cream. Squeezed along the
intestine by the contraction of the muscular walls, the goodness of this
cream is little by little sucked up. The fat goes drop by drop, particle
by particle, into the lacteals, and so away into the blood. The
proteids, more and more dissolved the further they travel along the
canal, soak away into blood-vessel or into lacteal. The salts and the
water go the same way, until at last the digested meat, with all its
goodness gone, with nothing left but indigestible wrappings, or perhaps
as well some broken bits of fibre or of fat, is cast aside as no longer
of any use.

=Thus all food-stuffs, not much altered, with all their goodness
unchanged, pass either at once into the blood, or first into the
lacteals and then into the blood, and the useless wrappings of the
food-stuffs are cast away.=

While we are digesting, the blood is for ever rushing along the branches
of the aorta, through the small arteries and capillaries of the stomach
and intestine, along the branches of the portal vein, and so through the
liver back to the heart; and during the few seconds it tarries in the
intestine, it loads itself with food-stuffs from the alimentary canal,
becoming richer and richer at every round. While we are digesting, the
thoracic duct is pouring, drop by drop, into the great veins of the neck
the rich milky fluid brought to it by the lacteals from the intestine,
and as the blood sweeps by the opening of the thoracic duct on its way
down from the neck to the heart, it carries that rich milky fluid with
it, and the heart scatters it again all over the body.

=Thus the blood feeds on the food we eat, and the body feeds on the
blood.=




HOW THE BLOOD GETS RID OF WASTE MATTERS. § VIII.


=53.= But if the blood is thus continually being made rich by things, it
must also as continually be getting rid of things. The things with which
it parts are not, however, the same as those which it takes. The blood,
as we have said, is fuel for the muscles, for the brain, and for other
parts of the body. These burn the blood, burn it with heat but without
light. But, as you have learnt from your Chemistry Primer, Art. 4,
burning is only change, not destruction; in burning nothing is lost. If
the muscle burns blood, it burns it into something; that something,
being already burnt, cannot be burnt again, and must be got rid of.

Into what things does the body burn itself while it is alive?

I have already said that if you were to take a piece of meat or some
blood, and dry it and burn it, you would find that it was turned into
four things--water, carbonic acid, ammonia, and ashes. The body is made
up of nitrogen, carbon, hydrogen, and oxygen, with sulphur, phosphorus,
and some other elements. The nitrogen and hydrogen go to form ammonia;
the hydrogen, with the oxygen of combustion, forms water; the carbon,
carbonic acid; the phosphorus, sulphur, and other elements go to form
phosphates, sulphates, and other salts.

=In whatever way the body be oxidized, whether it be rapidly burnt in a
furnace, whether it be slowly oxidized after death, as when it moulders
away either above ground or in the soil, whether it be quickly oxidized
by living arterial blood while still alive--in all these several ways
the things into which it is burnt, into which it is oxidized, are the
same. Whatever be the steps, the end is always water, carbonic acid,
ammonia, and salts.=

These are the things which are always being formed in the blood through
the oxidation of the body, these are the things of which the body has
always to be getting rid.

In addition to the water which comes from the oxidation of the solids of
the body, we are always taking in an immense quantity of water; partly
because it is absolutely necessary that our bodies within should be kept
continually moist, partly because food cannot pass into the blood except
when dissolved in water, and partly because we need washing inside quite
as much as outside; if we had not, so to speak, a stream of water
continually passing through our bodies to wash away all impurities, we
should soon be choked, just as an engine is choked with soot and ashes
if it be not properly cleaned. We have, then, to get rid daily of a
large quantity of washing water over and above that which comes from the
burning of the hydrogen of our food.

We have already seen that a great deal of the carbonic acid goes out by
the lungs at the same time that the oxygen comes in. A large quantity of
water escapes by the same channel. You very well know that however dry
the air you breathe, it comes out of your body quite wet with water.

We have also already seen how the blood secretes sweat into the
sweat-glands, and so on to the skin. Perspiration is little more than
water with a little salt in it. The skin, therefore, helps to purify the
blood through the sweat-glands, by getting rid of water with a little
salt. You must remember that a great deal of water passes away from your
skin without your knowing it. Instead of settling on the skin in drops
of sweat, it passes off at once as vapour or steam. Some carbonic acid
also makes its way from the blood through the skin.

=54.= It only remains for us to inquire, In what way does the blood get
rid of the ammonia and the rest of the saline matters that do not pass
through the skin?

These are secreted from the blood by the kidney, dissolved in a large
quantity of water in the form of =urine=.

What is the kidney? You will learn more about this organ by and by.
Meanwhile it will for our present purpose be sufficient to say that a
kidney is a bundle of long tubular glands, not so very unlike
sweat-glands, all bound together into the rounded mass whose appearance
is familiar to you. =Into these glands the blood secretes urine just as
it secretes sweat into the sweat-glands=. The glands themselves unite
into a common tube or duct which carries the urine into the receptacle
called the urinary bladder, from whence it is cast out when required.

What is urine? Urine is in reality water holding in solution several
salts, and in particular containing a quantity of ammonia. The ammonia
in urine is generally in a particular condition, being combined with a
little carbonic acid, in the form of what is called =urea=. If urea is not
actually ammonia, it is at least next door to it.

The three great channels, then, by which the blood purifies itself, by
which it gets rid of its waste, are the lungs, the kidneys, and the
skin. Through the lungs, carbonic acid and water escape; through the
kidneys, water, ammonia in the shape of urea, and various salts; through
the skin, water and a few salts. As the blood passes through lung,
kidney, and skin, it throws off little by little the impurities which
clog it, one at one place, another at another, and returns from each
purer and fresher. The need to get rid of carbonic acid and to gain a
fresh supply of oxygen is more pressing than the need to get rid of
either ammonia or salts. Hence, while all the blood which leaves the
left ventricle has to pass through the lungs before it returns to the
left ventricle again, only a small part of it passes through the
kidneys, just enough to fill at each stroke the small arteries leading
to those organs. The blood craves for great draughts of oxygen, and
breathes out great mouthfuls of carbonic acid, but is quite content to
part with its ammonia and salts in little driblets, bit by bit.

The three channels manage between them to keep the blood pure and fresh,
working hard and clearing off much when much food or water is taken or
much work is done, and taking their ease and working slow when little
food is eaten or when the body is at rest.




THE WHOLE STORY SHORTLY TOLD. § IX.


=55.= And now you ought to be able to understand how it is that we live on
the food we eat.

Food, inasmuch as it can be burnt, is a source of power. In burning it
gives forth heat, and heat is power. If we so pleased, we might burn in
a furnace the things which we eat as food, and with them drive a
locomotive or work a mill; if we so pleased, we might convert them into
gunpowder, and with them fire cannon or blast rocks. Instead of doing
so, we burn them in our own bodies, and use their power in ourselves.

Food passing into the alimentary canal is there digested; the nourishing
food-stuffs are with very little change dissolved out from the
innutritious refuse; they pass into and become part and parcel of the
blood.

The blood, driven by the unresting stroke of the heart’s pump, courses
throughout the whole body, and in the narrow capillaries bathes every
smallest bit of almost every part. Kept continually rich in combustible
material by frequent supplies of food, the blood as well at every round
sucks up oxygen from the air of the lungs; and thus arterial blood is
ever carrying to all parts of the body, to muscle, brain, bone, nerve,
skin, and gland, stuff to burn and oxygen to burn it with.

Everywhere oxidation, burning, is going on, in some spots or at some
times fiercely, in other spots or at other times faintly, changing the
arterial blood rich in oxygen to venous blood poor in oxygen. From most
places where oxidation is going on, the venous blood goes away hotter
than the arterial which came; and all the hot blood mingling together
and rushing over the whole body keeps the whole body warm. Sweeping as
it continually does through innumerable little furnaces, the blood must
needs be warm. This is why =we= are warm. But from some places, as from
the skin, the venous blood goes away cooler than the arterial which
came, because while journeying through the capillaries of the skin it
has given up much of its heat to whatever is touching the skin, and has
also lost much heat in turning liquid perspiration into vapour. This is
why so long as we are in health we never get hotter than a certain
degree of temperature, the so-called blood-heat, 98° Fahr., and why we
make warm the clothes which we wear and the bed in which we sleep.

Everywhere oxidation is going on, oxidation either of the blood itself
or of the structures which it bathes, and whose losses it has to make
good. Everywhere change is going on. Little by little, bit by bit, every
part of the body, here quickly, there slowly, is continually mouldering
away and as continually being made anew by the blood. Made anew
according to its own nature. Though it is the same blood which is
rushing through all the capillaries, it makes different things in
different parts. In the muscle it makes muscle; in the nerve, nerve; in
the bone, bone; in the glands, juice. Though it is the same blood, it
gives different qualities to different parts: out of it one gland makes
saliva, another gastric juice: out of it the bone gets strength, the
brain power to feel, the muscle power to contract.

When the biceps muscle contracts and raises the arm, it does work. The
power to do that work, the muscle got from the blood, and the blood from
the food. All the work of which we are capable comes, then, from our
food, from the oxidation of our food, just as the power of the
steam-engine comes from the oxidation of its fuel. But you know that in
the steam-engine only a very small part of the power, or energy, as it
is called, of the fuel goes to move the wheel. By far the greater part
is lost in heat. So it is with our bodies: all the force we can exert
with our bodies is but a small part of the power of our food; all the
rest goes to keep us warm.

Visiting all parts of the body, rebuilding and refreshing every spot it
touches, the blood current also carries away from each organ the waste
matters of which that organ has no longer any use. Just as each part or
organ has different properties and different work, so also is the waste
of each not exactly the same, though all are alike inasmuch as they are
all the results of oxidation. The waste of the muscle is not exactly the
same as the waste of the brain or of the liver. Possibly the waste
things which the blood bears from one organ may be useful to another,
and so be made to do double work, just as the tar which the gasworks
throw away makes the fortune of the colour manufacturer.

Be this as it may, the waste products of all parts, travelling hither
and thither in the body, come at last to be brought down to very simple
things, with all their virtue gone out of them, with all, or all but
all, their power of burning lost, fit for nothing but to be cast away,
come at last to be urea or ammonia, carbonic acid, and salts. In this
shape, the food, after a longer or shorter sojourn in the body, having
done its work, having built up this or that part, having helped the
muscle to contract or the liver to secrete, having by its burning given
rise to work or to heat, goes back powerless to the earth and air from
which it came. And so the tale is told.




HOW WE FEEL AND WILL. § X.


=56.= One other matter we have to note before we have given the full
answer to the question why we move.

We have seen that we move by reason of our muscles contracting, and that
in a general way a muscle contracts because a something started in the
brain by our will passes down from the brain through more or less of the
spinal cord, along certain nerves till it reaches the muscle. It is this
something, which we may call a =nervous impulse=, which causes the muscle
to contract.

But what leads us to exercise our wills? What starts the nervous
impulse?

All the nerves in the body do not end in muscles. Many of them end, for
instance, in the skin, in those papillæ of which I spoke a little while
ago. These nerves cannot be used for carrying nervous impulses from the
brain to the skin. By an effort of the will you can make your muscles
contract; but try as much as you can, you cannot produce any change in
your skin.

What purpose do these nerves serve, then? If you prick or touch your
finger, you feel the prick or touch; you say you have =sensation= in your
finger. Suppose you were to cut across the nerves which lead from the
skin of your finger along your arm up to your brain. What would happen?
If you pricked or touched your finger, you would not feel either prick
or touch. You would say you had lost all sensation in your finger. These
nerves ending in the finger then, have a different use from those ending
in the muscle. =The latter carry impulses from the brain to the muscle,
and so, being instruments for causing movements, are called motor
nerves. The former, carrying impulses from the skin to the brain, and
being instruments for bringing about sensations, are called sensory
nerves.= All parts of the skin are provided with these sensory nerves,
but not to the same extent. The parts where they abound, as the fingers,
are said to be very sensitive; the parts where they are scanty, as the
back of the trunk, are said to be less sensitive. Other parts besides
the skin have also sensory nerves.

Motor nerves are of one kind only; they all have one kind of work to
do--to make a muscle contract. But there are several kinds of sensory
nerves, each kind having a special work to do. The several works which
these different kinds of sensory nerves have to do are called =the
senses=.

The work of the nerves of the skin, all over the body, is called the
=sense of touch=. By touch you can learn whether a body is rough or
smooth, wet or dry, hot or cold, and so on.

You cannot, however, by touch distinguish between salt and sugar. Yet
directly you place either salt or sugar on your tongue you can recognize
it, because you then employ sensory nerves of another kind, the nerves
which give us the =sense of taste=. So also we have nerves of =smell=,
nerves of =hearing=, and nerves of =sight=.

The nerves of touch, where they end, or rather where they begin in the
skin, sometimes have and sometimes have not, little peculiar structures
attached to them, little =organs of touch=. So also the nerves of taste,
and smell, end or rather begin in a peculiar way. When we come to the
nerves of hearing and of seeing, we find these beginning in most
elaborate and complicated organs, the ear and the eye.

Of all these =organs of the senses= you will learn more hereafter;
meanwhile, I want you to understand that by means of these various
sensory nerves, we are, so long as we are alive and awake, receiving
impressions from the external world, sensations of touch, sensations of
roughness and smoothness, of heat and cold, sensations of good and bad
odours, sensations of tastes of various kinds, sensations of all manner
of sounds, sensations of the colours and forms of things.

By our skin, by our nose, by our tongue and palate, by our ears, and
above all by our eyes, impressions caused by the external world are for
ever travelling up sensory nerves to the brain; thither come also
impressions from within ourselves, telling us where our limbs are and
what our muscles are doing. Within the brain these impressions become
sensations. They stir the brain to action; and the brain, working on
them and by them, through ways we know not of, governs the body as a
conscious intelligent will.

                   *       *       *       *       *

                         NICHOLSON’S GEOLOGY.

           _Text-Book of Geology, for Schools and Colleges._

      By H. ALLEYNE NICHOLSON, M. D., D. SC., M. A., PH. D.,
    F. R. S. E., F. G. S., etc., Professor of Natural History
              and Botany in University College, Toronto.

                   _12mo. 266 pages. Price, $1.50._

This work is thoroughly adapted for the use of beginners. At the same
time the subject is treated with such fulness as to render the work
suitable for advanced classes, while it is intended to serve as an
introduction to a larger work which is in course of preparation by the
author.

                   *       *       *       *       *

                         NICHOLSON’S ZOOLOGY.

           _Text-Book of Zoology, for Schools and Colleges._

                       BY SAME AUTHOR AS ABOVE.

                   _12mo. 353 pages. Price, $1.75._

In this volume much more space has been devoted, comparatively speaking,
to the Invertebrate Animals, than has usually been the case in works of
this nature: upon the belief that all teachings of Zoology should, where
possible, be accompanied by practical work, while the young student is
much more likely to busy himself practically with shells, insects,
corals, and the like, than with the larger and less attainable
Vertebrate Animals.

Considerable space has been devoted to the discussion of the principles
of Zoological classification, and the body of the work is prefaced by a
synoptical view of the chief divisions of the animal kingdom.

⁂ A copy of any of the above works, for examination, will be sent by
mail, post-paid, to any Teacher or School-Officer remitting one-half its
price.


                  D. APPLETON & CO., PUBLISHERS,
                  549 & 551 BROADWAY, NEW YORK.


FOOTNOTES:

 [1] It is unusual for muscles to have two tendons at the same end.
 Hence the name =biceps=, or “two-headed.”

 [2] From _pulmo_, =lung=; the artery of the lung.

 [3] From _hepar_, =liver=; the vein of the liver.


Typographical errors corrected by the etext transcriber:

would be unvailing=> would be unavailing {pg 34}

cordæ tendineæ=> chordæ tendineæ {pg 70}

the triscuspid valve between=> the tricuspid valve between {pg 72}

the body may may=> the body may {pg 103}