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{

SCIENCE PRIMERS, edited by

Professors Huxley, Roscoe, and

Balfour Stewart.

PHYSIOLOGY. ,

Sitmu frimers.

PHYSIOLOGY.

M. FOSTER, M.A., M.D.. F.R.S.,

• FELLOW OF TRINITY COLLKOB, CAMBRIDOB.

WITH ILLUSTRATIONS.

®0rOttt0:
JAMES CAMPBELL & SON

1876.

^- +« *\.c. Apt of the Parliament of Canada, in the year one
TsonT in the Office of the Minister of Agriculture.

HUKTIR, ROSB & Co.

Printers,
Toronto.

•t

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 sup-
posed the reader to be willing to handle and examine
such things as a dead rabbit and a sheep's hearty
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 experi-
ments for chemistry. At the same time I have
tried to make my text intelligible to those who think
reading verbal descriptions less tiresome than observ-
ing things for themselves.

It seemed more desirable in so elementary a work
to insist, even with repetition, on some few funda-
mental 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 i

2. ,, Animals move of their own accord . . i

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.

The Tissues 7

The cavities of the Thorax and Abdomen . 9

The Vertebral Column 12

Head and Neck 15

Nerves 18

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 con-

tract 32

18. ,, The power of a muscle to contract depends

on its being supplied with blood . . 35

10.

II.

12.

•• \

TABLE OF CONTENTS,

36.

37-

38-
39-

ART.

SECT.

19.

III.

20.

IV.

21.

22.

23.

»>

24.

>»

25.

26.

>»

27.

>»

28.

29.

J»

30.

»>

31.

>»

32.

>»

33.

34.

35.

>»

PAGE

III. It is the food in the blood which gives the

muscle strength yj

The continual need of food . . . . 38

VI.

THE NATURE OF BLOOD.

The Blood in the Capillaries
The Corpuscles of the Blood
The clotting of Blood .
The substances present in Serum
The minerals in Blood .

HOW THE BLOOD MOVES.

The Arteries, Capillaries, and Veins
The Sheep's Heart ....
The Course of the Circulation
Why the blood moves in one direction only

the Valves of the Veins
The Tricuspid Valves of the Heart
The pulmonary Semilunar Valves
The left side of the Heart
What makes the blood nvove at all : The

beat of the Heart ....
The action of the Heart as a whole
The Capillaries and the Tissues .

HOW THE BLOOD IS CHANGED
BY AIR : BREATHING.

Venous and Arterial Blood ....

The change from Arterial to Venous, and
from Venous to Arterial Blood .

The Lungs

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

45
48

55
58

64
66

79
82

87
88

103

viii TABLE OF CONTENTS.

ART. SECT. PAGE

40. VI. The natural distension of the Lungs. Inspi-

ration. 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

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 . 11 1

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 13a

SCIENCE PRIMERS.

PHYSIOLOGY,

INTRODUCTION. § I.

I. Did you ever on a winter^s day, when the ground
was as hard as a stone, the ponds all frozen, and every-
thing 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 tlie 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

a SCIENCE PRIMERS, [§ i.

■ > ' .■■■ I.I ■ ■ , I

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 sw^'ms about in a little pool of water
and cannot be seen without a microscope, mc^es 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 kmd 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 Jt again and again without

INTRODUCTION.] PHYSIOLOGY.

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 ntiachinery 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 cloth-
ing, 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 v^arm, 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

B 2

SCIENCE PRIMERS. [§ i.

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 wann
in order that it may move. 1 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, Ait. 5) takes
place in the midst of water^ and goes on without any

INTRODUCTION.]

PHYSIOLOGY.

•

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 ke*^p 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 cairy 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

SCIENCE PRIMERS, [§ i.

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 burii 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

THE BODY.]

PHYSIOLOGY

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,
year 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, Le, 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.

8 SCIENCE PRIMERS. [§ ii.

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. Cover-
ing 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, som ^times at both, to white or bluish white
glistening cordb ^r bands, made evidently of diflisrent
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

THE BODY.] PHYSIOLOGY.

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 sepa-
rate 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 can-
not 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 pack-
ing 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 connec-
tive and covered with skia 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

SCIENCE PRIMERS.

[ill*

Fig. I. — The Viscera of a Rabbit as seen upon simply opening thi
Cavities of the Thorax and Abdomen without any further Dissection.

Ay cavity of the thorax, pleural cavity, of either side; B, diaphragm;
C, ventricles of the heart ; /?, auricles ; Ey pulmonary artery ; F^ aorta;

THE BODY.]

PHYSIOLOGY.

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. I, 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. i, 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 (Z). Near the liver is the smooth pale
stomach {M)^ and filHng up the rest of the abdomen
you will see the coils of the intestine or bowel, very
narrow in some parts ((9), very broad (P 0, 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 ((7), 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,

G, lungs, collapsed, and occupying only the back part of the chest ; H, lateral ^
portions of pleural membranes ; /, cartilage at the end of sternum ; K^ por-
tion of the wall of body left between thorax and abdomen ; «, cut ends of the
ribs ; L, the liver, in this case lying mor to the left than the right of the
body ; M, the stomach ; iV, duodenum ; cy, small intestine ; /*, the caecum,
so largely developed in this and, other herbivorous animals ; Q, the large
intestine.

SCIENCE PRIMERS,

[§n.

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 shrark up very
much directly you cut into it, and so left thr great
bpace you sec.

9. The trunk then is really a great chamber contain-
ing what are callea the viscera, and divided into an
upper and lo\/er 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 mei'e 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. i, 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 mn a pole right through
him to give him support. Such a support is the back-
bone 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 BODY.]

PHYSIOLOGY,

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 bonc« If it were you would never be able
to bend your body. To eftable 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 togethejr, 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. ITow
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 vertebrae 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

SCIENCE PRIMERS,

[§n.

directly. The round flat body of each vertebra is

.\ I »

Fig. 2.

A, a diagrammatic view of the human body cut in half lengthways.
C.S. the cavity of the brain and spinal cord ; iV, that of the nose ; M^ that
of the mouth ; Al. Al. the alimentary canal represented as a simple straight
tube ; //, the heart ; Z>, the diaphragm.

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

C, a transverse sectioa taken along the line c d; letters as before.

THE BODY.]

PHYSIOLOGY,

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 owrn body) a sharp edge, formed by what
are called the spines, i.e. the uneven tips of the
arches of the vertebrae (Fig. 2) all the way down
the baclc.

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 bonej but not (or only slightly) in the lower part
or abdomen; behind, a much smaller long narrow
cavity 01 canal formed by the arches of the vertebrae,
and theiefore surrounded by bone all the way along,
and coEtaining 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
vertebrae. 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 Hke what
is represented in Fig. 2, C, where C.S. is the narrow
canal of the arches and where the broad cavity of the
ch^t containing the heart H is enclosed in the ribs
re^hing from the vertebra behind to the sternum
in. front. Both cavities are covered up on the out-
side with muscles, blood-vessels, nerves, connective,
aiki skin, just as in the leg.

10. We have now to consider the head and neck.

i6 SCIEMCE PRIMERS, [§ n.

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 "ou saw nowhere in the leg. One of these
tubes is firm, with hardish rings in it ; it is the wind-
pipe or trachea ; the other is soft, and its sides fall
flat together ; this is the gullet or cesophagus, lead-
ing 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 verte-
bral 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 ivith a
trachea and oesophagus, and differs frcm the
trunk in having no cavity and no ribs.

The head, again, is unlike all these. Indep(^ you
will not understand how the head is made unlbss you
take a rabbits skull and place it side by siie 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 ; Ux
there are two jawbones. Both these carry teeth, bit
the upper one is simply part of the skull, and does
not move ; the lower one does movt , it can be mad|
to shut close on the upper jaw, or can be separated \

THE BODY.]

PHYSIOLOGY,

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
thtoat, 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 vertebrae of the
neck in such a way that the round :^d 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 vertebrae. 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
vertebrae, right into the brain cavity. In fact the
brain-case and the row of arches of tlie vertebrae
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, (7.5.) 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

SCIENCE PRIMERS.

[§n.

of the skull is called the brain ; the narrower, rod-like,
or band-like mass which runs down ^^e 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.

II. 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 dr spinal
cord. Those from the brain pass through holes in the
skull, and at first sight seem to spread out very irregu-
larly. Those which branch off from the spinal cord are
far more regular. A nerve runs out on each side be-
tween every two vertebrae, little rounded gaps being left
for that purpose where the vertebrae fit together, so that
when you look at a spinal cord with portions of the
nerves ^till 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 thv^y 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

THE BODY.]

PHYSIOLOGY,

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 micro-
scope you will find they get still smaller and smallei
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 ^- -rp'^^^l cord. You will
see how important ic is to remember Jiis 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 par; of the neck, the
throat divides into two pipes or tubes — one the wind-
pipe, the other the gullet. These running down the
neck m 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

c 2

20 SCIENCE PRIMERS. [§ ik

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.) o;oes 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, zx^^
suppose the narrow glass to be very long and twist '
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 the n,
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

MOVEMENTS. ]

PHYSIOLOGY.

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 (Z7). being wiler and stouter than
the other {Rd) 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

SCIENCE PRIMERS.

[§ HI.

v;hich 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. ' ■ '

F1G3.— /"-^^ Bottes 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 ful-
crum, 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. IS
you try to slide them from side to side, you will find
that they get locked. They have only one movement,

MOVEMENTS.] PHYSIOLOGY, 23

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 jack 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
to xhing 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

24 SCIENCE PRIMERS. [§ m.

did this you would be imitating very closely the
manner in which the bones at the elbo>v 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, \vhich 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 humei*us
are kept in place by ligaments or flat bands of con- {
nective tissue. I

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 slipper}-. 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

MOVEMENTS.]

PHYSIOLOGY.

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-join*: 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

26 SCIENCE PRIMERS. [§ in.

trae 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. "-■

i6. 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 mutter which), rather near the elbow.
Bore a hole through the top of the humerus and pass
the string through it. Your ^ring 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

MOVEMENTS.]

PHYSIOLOGY.

you give the string, and that is brought to bear on
your lever at the point where the string is fastened to
the radius, ue, 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
)f the third order.

Now, in order to make the thing a little more like
rhat takes place in your own arm, instead of boring
fa 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 any-
thing you like above the humerus, and let the string
[be jusflong enough to let the arm be quite straight-
jned out, but no longer, so that when the arm is
rtraigbt 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-
farm 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
I 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.

28 SCIENCE PRIMERS. [§ HI.

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 taper-
ing 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 tnuscle suddenly
left off", and changed into much firmer material which
was white and glistening. This firmer white part you \v
were told was called the tendon of the muscle. The \
rest of the muscle, generally called "the belly," is
made up of what you ?" accustomed to tall 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 sub-
stance. 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

MOVEMENTS.] PHYSIOLOGY. 29

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-Hke, some-
times 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 tht ^ront of the elbow and is fastened
to, Le. 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 ^ tendons (Fig. 3, d)^ which
gliding over the end of the humerus are fastened to
the shoulder-blade (or scapula as it is called), into
which the hum.erus 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 is unusual for muscles to have two tendons at the same end
Hence the name biceps, or " two-headed."

30 SCIENCE PRIMERS, [§ in.

it was? Evidently the same thing that happened
when yc u 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 them-
selves, but muscular substance does possess this re*
markable 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 muscled, 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 ks 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

MOVEMENTS.] PHYSIOLOGY, 31

something rise up. You feci the biceps getting
thicker as !^ 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 aij 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 /ore-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

SCIENCE PRIMERS.

l§ HI.

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
01 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 in.^ 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

MOVEMENTS.]

PHYSIOLOGY,

white nerve-trunks as they are called, which we should
find passed at last between the vertebrae, 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 ha\ e 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? I*: proves that when you
will to bend your arm, something passes along the
nerves going to the biceps muscle, which something
causes that musclp 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 gef 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 communica-
tion between the will an ! 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 some-
where in the neck.

If you had the misfortune to have your spinal

34 SCIENCE PRIMERS. [§ iii.

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 unvail-
ing. 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 some-
thing is started in your brain. That some-
thing — 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, stretch-
ing, as it falls, the muscle to its natural length.

MOVEMENTS.] PHYSIOLOGY* 35

i8. 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 sensa-
tions, little by little the power would come back to
you j 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 h^d tied a finer thread round the blood-

36 . SCIENCE PRIMERS. [§ in.

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 bluod-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.

Wliat 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 dejith, 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
s of death in the biceps is the loss of the power to

contract when called upon to do so.
5 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

MOVEMENrS.] PHYSIOLOGY. ' 37

and joints, but the muscle must be supplied with
blood.

19. We can now go a step further and ask the ques-
tion, 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 con- .
tract? 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 con-
tracting ; in the other, little power. Try and find out
the heaviest weight you can raise in this way by bend-
ing your arm, some morning, not too long after break-
fast, 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

38 SCIENCE PRIMERS, [§ in.

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. i 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 bums. The
fuel it bums 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 manu-
facture 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 bums, 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

MOVEMENTS.] PHYSIOLOGY, • 39

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 pro-
duced by muscles pulling sometimes with and some-
times 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 weak-
ness 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 ha^'e to speak later on need
blood too. L^-i^t^^ '^ -^

The whole history of our daily life is shortly this.
The food we eat becomes blood, the blood is carried

SCIENCr PRIMERS,

div.

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 dif-
ferent 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. r •

■■,■_■■'■' , ■ . \ '

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

^ BLOOD.] PHYSIOLOGY, 41

you see are floating in a fluid so clear that yon
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 hav€i 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

SCIENCE PRIMERS.

[§iv.

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 cor-
puscles.

In every part of your flesh, in your brain and
spinal coid, 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 y^u
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 thick-
ness 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 micro-
scope, what do you see? A number of little

BLOOD J

PHYSIOLOGY,

round bodies, the blood discs or blood cor-
puscles (Fig. 4, A), If you look carefully you will
notice that most of them ar^. round, as B ; but every

Fig. 4. — Red and JVhiie 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.

/. Ditto, at the edge only.

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

SCIENCE PRIMERS.

[§iv.

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
cuixant 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
^ ittentively other little bodies, just a little
^ger 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 dump-
ling while you were looking at it gradually changed
into the shape of a three-cornered tart, and then into

BLOO^.J PHYSIOLOGY. 45

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 physiologica!
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. When-
ever 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

^^i__ig_giijg^

SCIENCE PRIMERS.

[8 IV.

it does not ; and if you keep on stirring long enough
you will find that it never clots at all. By conti-
nually stirring it you will prevent its clot-
ting. 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 sub-
stance. 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 after
wards 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

BLOOD.] PHYSIOLOGY, 47

a few hours or for a day, you would find, instead of
the large jelly quite filling the pail, a smaller but
firmer jelly covv^red 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 mailer, 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 sc?m a jelly. But each th. ^d
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 net-
work 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 cor-
puscles have been strained by the process
of clotting. - V '

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

48 SCIENCE PRIMERS, [§ iv.

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
st;ll, 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, ue, blood out of which both fibrin and cor-
puscles 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.
i| It would all go off in steam. But if you were to try
f to boil a basinfiil 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
I serum, which before seemed quite as liquid as water,
only feeling a little sticky if you put your finger in it,
I would all become quite solid. You know the differ-
ence between a raw and a boiled egg. The white of
the raw egg, though very sticky and ropy, or viscid as

BLOOD.] PHYSIOLOGY, 49

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 tgg^ 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 boiUng-point. Both the serum of blood
and white of ^%g even when " set '' are wet, Le. 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 car-
bon 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, sub-
stance, 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.

P 31 .,.-

SCIENCE PRIMERS.

[§iv.

I did mot 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 bum the whole blood
away. Bum as long as you like, you will always have left
a quantity of what you have leamt 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.

CIRCULATION.] PHYSIOLOGY, . 51

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 bupng 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

• * E • 2

52 SCIENCE PRIMERS. [§ V.

rather firm stout walls; and the veins, which are
generally full of blood, and have thinner and fiabby
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, fehere is no pare 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 capil-
laries 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. ^; / :t^

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

CIRCULATION.] PHYSIOLOGY, 53

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 widen-
ing, 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 stout-
ness would tell you that you were already in an artery,
but the inside would be delightfully smooth. As you
went on yoii 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

SCIENCE PRIMERS.

[§v.

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 joume)dng. 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, inio 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 some-
thing, 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 some-
times 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

CIRCULATION.] PHYSIOLOGY. - 55

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 tep would be into the
heart again. So you see, starting fi*om 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 some-
thing 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
ixings out on a table with the heart between them.

teaik

SCIENCE PRIMERS.

[§v.

You will then have something very much like what

Fig. s. — Heart of Sheep, as seen after Removal from the Body^ lying ufon
the Two Lungs. The Pericardimn has been cut away^ but no other
Dissection made.

R.A. Auricular appendage of right auricle \ L.A. auricular appendage of
left auricle; /?.K right ventricle; L.V. left ventricle; S,V,C. superior

CIRCULATION.] PHYSIOLOGY. 57

vena cava ; /. V.C. inferior vena cava : P;A- pulmonary artery ; Ao^ aorta;
A'o\ innominate branch from aorta dividing into subclavian and carotid ,
arteries ; L. lung ; Tr. trachea, i, solid cord often present, the remnant of '^
a once open communication between the pulmonary artery and aorta. 2, 1
masses of fat at the bases of the ventricle hiding from view the greater
part of the auricles. 3, line of fat markintj the division between the two <
ventricles. 4, mass of fat covering the trachea.

is represented in Fig. 5. If you could look through 1
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 A'o' in the figure. Your butcher
perhaps may not have cut them across exactly where
mine has done, but that will not prevent your recogni- !
zing them. You will notice what thick stout walls they 1
have, and how they gape where they are cut. Ao
is the aorta, and A'd 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 A'd you would be able to
bring it out through Ao, or vice versct. But what is
P. A. which looks so much like the aorta, though you
will find that it has no connection with it 1 You
cannot pass a wire from the aorta into it. It also is
an artery, the pulmonary' 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 /. 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 ,.

;-'>-i

* From j^ulmo, Ivais : the artery of the lung.

58 SCIENCE PRIMERS, [§ V.

of the body. S, F.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
hitie Willie ago. / 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 th?.t 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 trans-
parent, 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 /. 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

CIRCULATION.]

PHYSIOLOGY,

is further marked off, but not completely divided, into

ThJk

MXX

Fig. ^.-'Diagram of the Heart and Vessels, -with the Course of the Circu-
lations viewed from behind so that the proper left of the Observer corre-
sponds with the left side of the Heart- in the Diag^-am.
L.A. left auricle ; L. V. left ventricle ; Ao. aorta ; A^. arteries to the upper

1 1

6o SCIENCE PRIMERS. . [§ V.

Ipart of the body ; A^. arteries to the lower part of the body : H.A. hepatic
artery, which supplies the liver with part of its blood ; V^. veins of the upper
part of the body; V^. veins of the lower part of the body; V.P. "ena
jportae; ./". r". 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 ;
1 Lg. lung ; P. V. pulmonary vein ; Let. lacteals ; Ly. lymphatics ; Th.D.
i thoracic duct ; Al. alimentary canal; Lr. liver. The arrows indicate the
1 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 cham-
ber ; 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, K and Z. 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, Z.A,, opens into the left ven-
tricle, Z. 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 gel
round the figure. Suppose we begin with the two tubes
marked V.C.S. and V.C.I., the wall; 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 ven-
tricle ; 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.

' S, F.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,

CIRCULATION.] PHYSIOLOGY, 6i

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 your-
self 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, Ad), It is here drawn for simplicity'^
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^,
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 any-
where, in fact, in the upper part of the body. Oul
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 taker
the other road, A"", we should have passed through
capillaries somewhere in the lower part of the bod>
instead of the upper, and come back by the vens
cava ii. ' rior, F.C/., instead of the vena cava supe
rior. Starting from the right auricle, which
ever way we took we should always come
back to the right auricle again, and in out

?2 SCIENCE PRIMERS. [§ v.

ourney should always pas«=* through the
jbllowing things in the following order ;
ight auricle, right ventricle, pulmonary
[^rtery, arteries, capillaries, and veins of the
tings, pulmonary vein, left auricle, left ven-
tricle, aorta, arteries, capillaries, and veins
iomewhere in the body, and either superior
>r inferior vena cava. That is the course of the
circulation. But there is something still to be added,
f^ong the many large branches, not drawn in the
liagram, given off by the aorta to the lower part of
he body, there are two branches which are drawn
,nd which deserve special notice.
One is a large branch carrying blood to the tube
.Z., which is meant in the diagram to stand for
ihe stomach, intestines, and some other organs. This
•branch, like all other branches of the aorta, divides into
mall arteries, and these into capillaries, which again
re gathered up into veins, forming at last a large vein
inarked in the diagram V.P. and called the vena
)ortae or portal vein. Now the remarkable thing
s that this vein does not, like all the other veins,
/o straight to join the vena cava, but makes for
;he liver, where it divides into smaller and smaller
Veins, until at last it breaks completely up in the liver
3nto a set of capillaries again. These capillaries gather
ibnce more into veins, farming at last the large trunk,
palled the hepatic^ vein, ZT. F!, which does what
ihe 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
^peaking goes straight to the liver, and is called the

•| I From ^/ar, liver J the vein of the liver.

CIRCULATION.] PHYSIOLOGY. 63I

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^, 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, sometinies
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 capil-
laries 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

SCIENCE PRIMERS.

[§v.

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
ci the aorta) all over the body. And you would have
wo 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 re-
member 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 thesj watch-pocket valves
they would simply trample it down flat, and so
pass over it without hardly knowing it was there,

CIRCULATION.] PHYSIOLOGY. 65

and go on their way as if nothing had happened.
But suppose they were journeying the other way,
from the heart to the capii^tries. 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 firil 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

1 ■ r • I

I : C

H "

♦ "

Fig. t .~-Diagrainntatic Sections of Vehis 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 w.iy. C, capillary side ;
H, heart side.

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, -

SCIENCE PRIMERS,

[§V.

The valves in the veins, then, let the
hlood 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, intp which you

CIRCULATION.]

PHYSIOLOGY.

can put your three fingers. This is the opening into

AV'C

Fig. 8. — Right Side of the Heart of a Sheep.

R.A. cavity of right auricle ; ^S". V.C. superior vena cava, /. 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 ventiicle through the
auriculo-ventricular orifice ; b, a piece of whalebone passed into the coro-
nary vein.

R. V. cavity of right ventricle ; iv, 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 chordep
tendineie, 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 ven-
tricle 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.

I, 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 betweex)
the auricle and pulmonary artery.

F 2

6i 68

SCIENCE PRIMERS.

[§v.

' the right ventricle ; and you will have no difficulty in

\ putting your fingers from the auricle into the ventricle

\ and bringing them out again.

J But hold the heart in one hand with the auricle

^ upwards, and try to pour some water into the ventricle.

f The first few spoonfuls will go in all right, and then

f you will see some thin white skin or membrane come

1 floating up into the opening and quite block up the

] entrance from the auricle into the ventricle; the

l'V2

'•\

HAV

Fig. ^.—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 {Iv. i, 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,.

CIRCULATION.]

PHYSIOLOGY,

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 (Iv, i,
Iv, 2, Iv, 3). These three pieces form the valve
between the right auricle and ventricle, called the
tricuspid; or three-peaked valve. Why it is so

'i;-

'MVj

nLVA

ucr

m.y.a

Fig. 10,— View 0/ the Orifices of the Heart front below, the whole of the

Ventricles having been cut away.

R.A.V. right auriculo-ventricular orifice surrounded by the three flaps,
t.v.\, t.v. 2, ^.z/. 3, of the tricuspid valve ; these are stretched by weights
attached to the chorcUe tendineee.

L.A.V. left auriculo-ventriculv orifice surrounded m same way by the
two flaps, m.v. i, m.v. 2, of mitral valve ; P. A. the orifice of pulmonary
arter- 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.

70 SCIENCE PRIMERS, . [§ V.

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, /. 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, i^.-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 ven-
tricle 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 to-
gether 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 cordae tendinese, 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

CIRCULATION.] * PHYSIOLOGY, jt

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 ven-
tricle, 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 scissois, 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 semi-
lunar valves, because each pocket is of the shape
of a half-moon. Lift them up carefully f.nd 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

72 SCIENCE PRIMERS. [§ v.

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 triscuspid 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 pul-
monary arterj' 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

CIRCULATION.] PHYSIOLOGY, 73

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. ii). 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. i, 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.
1 1 ) very much like the right ventricle, only its walls are
very much thicker, so much thicker that the left ven-
tricle 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 hardl)' 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.

SCIENCE PRIMERS,

[§v.

Fig. II. — 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 irto tne coionary /ein, ^«; ich, though it haf no connection with the
left auricle, is, from its pos'ti^n, recessarily cut across in thus laying open the
auricle.

M.V. the two ilaps of the mitral valve (drawn somewhat Jiagrammati-
cally) : pp. papillary muscle?, be'onging as before to the part of the ventricle
cut away ; c, a style passed from ventricle in Ao. aorta ; Ao^. branch of
aorta (see Fig. 5, A' d , ', P,A. pulmonary artery; S.V.C superior vena
cava.

T, wall of ventricle cut across; 2, wall of auricle cut .. -vay "around auriculo-
ventri' ular o-ifice ; 3, other portions of auricular wall cut across ; 4, mass of
fat around base of ventricle (see Fig. 5, 2).

CIRCULATION.] . - rHYSIOLOGY, 75

■ To return to the inside of the left ventricle. Up
at the top of the ventiicle, 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 be-
hind 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 ventrick 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 f
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 pul-
monary 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 tlie left auricle and the |
left ventricle, the aortic semilunar valves between the
left ventricle and the aorta, and the valves '.yhich are
scattered among the veins of the body. Of these by

76 SCIENCE PRIMERS, [§ v.

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. Wei], then, we understand now, do we not ? why
I 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 ?

I You know that during life it does keep moving.

. You have seen it moving in the web of a frog's foot^ —

land whenever any part of the body can be brought

i 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 itbelf a

proof that the blood moves in the arteries from the

heart to the capillaries, and in the veins from the

j caDillaries to the heart. ,.., ; , <,

I 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. ri . -. 1 -: i ; .;.

' -.,

CIRCULATION.] PHYSIOLOGY, »i»f

Wh?at 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 muscuj^. 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 |j
placed carefully and regularly side by side. You can Ij
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," />. 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 mus-
cular fibres may squeeze all parts of each bag at the
same time. ' ■

Each cavity of the heart, then, auricle or ventricle,

78 SCIENCE PRIMERS, [§ V.

I lis a thin bag with a network of muscles wrapped
i I round it, and each time the muscles contract they
i ) squeeze the bag and try to drive out whatever is in it.
j jThere are more muscles in the ventricles than in the
liauricles, and more in the left ventricle than In the

I bright, for we have already seen how much thicker the
Iventricles 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

lof the auricles and ventricles are always at work con-

ttracting and relaxing, shortening and lengthening, of

! i ttheir own accord, as long as the heart is alive. The

I I Ibiceps in your ajm contracts only when you make it
^contract. If you keep quiet, your arm keeps quiet
jand your biceps keeps quiet. But your heart never
3keeps 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,

; las long as you are alive your heart keeps on steadily
lat work. Every second, or rather oftener, there comes

' la short sharp squeeze from the auricles, from both
^exactly at the same time, and just as the auricles

jlhave finished their squeeze, there comes a great hug

I / Ifrom the ventricles, from both at the same time, but a

i ■

I <much stronger hug from the left than from the right ;

I ; and then for a brief space there is perfect quiet. But

^before the second has quite passed away, the auricles

I ! ■ thave begun again, and after them the ventricles once

(more, and thus the contracting and relaxing of the

Iwalls of the heart's cavities, this beat of the heart as

lit is called, this short snap of the two auricles, this

longer, steadier pull of the two ventricles, have gone on

nn your own body since before you were born, and will

CIRCULATION.] PHYSIOLOGY. 79

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 1 2 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 1 2 lbs. of blood.

What then must happen each time the heart con-
tracts?

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 t^ey
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, ^>tA
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

||j|!| 80 SCIENCE PRIMERS. [§ v.

1', 1 .

•

i it in order to find room. As the ventricle squeezes

i and squeezes, until its walls meet in the middle,

8 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

I 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

t 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

Igo. Accordingly the piece of pulmonary artery tries
to Return to its former size, and since it cannot send
i the blood back to the ventricle, squeezes it on to the
i next piece of the artery nearer the capillaries, stretch-
I ing that in turn.

1 This again in turn sends it on the next piece — and
I so on right to the capillaries. The over-full pulmonary
\ artery, stretched to hold more than it fairly can,
i empties itself through the capillaries into the pul-
1 monary veins until it is not more than comfortably full.
1 But the pulmonary veins also are already full, — what

;, t

CIRCULATION.] PHYSIOLOGY. 8i

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 tl^
over-full pulmonary artery, the pulmonary artery over-
flows 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 venae cavae — through these the blood falls into
the empty right auricle, the right auricle drives it into

o *

SCIENCE PRIMERS.

[§v.

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 frag-
ment 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
fc 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 tlie crowds of men and carts jostling through

CIRCULATION.] PHYSIOLOGY 83

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 b; red lines is filled
with living fliesh, 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 factciy, and the manufac-
tured 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. ^^.^ -^^..v^-.:.

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.

-•

G 2

84 SCIENCE PRIMERS, [§ V.

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 soHd 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

BREATHING.] PHYSIOLOGY, 85

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. . \ . . ,.v

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 oxida-
tion 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 hfe in the mouse would flicker and go out,
and after a while its body would be cold.

But suppose you were to nut a fish or a snail in a

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86 ^ SCIENCE PRIMERS. ! [8 vi.

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 tlie 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 v/ould
be bright scarlet, and that in the vein dark purple.
Hence we call the bright scarlet blood which is found

BREATHING.] , . . PHYSIOLOGY. 87

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 5deld 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

-^gg^

88 SCIENCE PRIMERS, ^ [§ vi.

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, >yith 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 iearn 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. , ' '^ 'j

They are bladders of a peculiar constmction.
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

BREATHING.] PHYSIOLOGY, 89

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 ii> the pulmonary
capillaries along the outside of the inflated air-cells,
by loss of carbonic acid and gai% 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,

90 SCIENCE PRIMERS. [§ vi.

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 pulmo-
nary 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. i, G.G,)
The cavity of thqf chest seemed to be a great
empty space, hardly half filled by the lungs and
heart. Bat 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

BREATHING.]

PHYSIOLOGY.

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 abdo-
men, the lungs quite fill the chest ; and when
the diaphragm is drawn down ^nd the cavity
of the chest made biggei^ 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. -v.^ -;-..■::,-?.,. ;-.^ -■..■■ ■ _.,.•■•-.-,„;

92 SCIENCE PRIMERS, [§ vi.

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
sh/unk 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 dia-
phragm 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 per-
fectly 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.

UREATHING.]

PHYSIOLOGY,

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. 1 . ^e chest walls keep the pres-
sure 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 pres-
sure of the atmosphere outside the bhdder. 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 out-
side of the lungs ; there is then the same atmospheric
pressure outside as inside the lungs ; there is nothing

94 SCIENCE PRIMERS, [§ vi.

-■ -- - — - - I

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 at-
mospheric pressure inside and outside, and so remain
perfectly quiet.

When in an air-tight chest the diaphragm
is pulled down, the pressure of the atmo-
sphere drives air into th'e lungs through the
windpipe and swells them up. When the
diaphragm is let go, the stretched lungs
return to their former size, emptying them-
selves 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 tinie it was let go that quantity of air would
come out of the lungs again.

BREATHING.]

PHYSIOLOGY,

This is what does take place in breathing or respira-
tion. 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

Fig, 12. — The Diaphragm of a Dog viewed from the Lower or

Abdominal Side,

y.C.f. the vena cava inferior; O, the oesophagus; Ao. the aorta j, tLe
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 vertebrae.

96 SCIENCE PRIMERS, * [§ vi.

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 dia-
phragm 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, ^ 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 abdo-
men, you will remember, is quite filled by the liver,
stomach, intestines, and other organs), partly pulled ap
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 con-

KREATHING.]

PHYSIOLOGY.

traction is flattened and descends, somewhat
as is shown in Fig. 13, -^.

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,

98 SCIENCE PRIMERS, [§ vi.

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 espe-
cially a woman, breathing, you will notice that with
every breath the chest rises aiid 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 in-
spiration, />. while the diaphragm is descending, and
falls back during expiration. But this coming for-
ward of the sternum means a widening of the chest
from back to front, and the falling back of the ster-
num 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 move-
ment 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 Frontis-
piece). 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

BREATHING.]

PHYSIOLOGY.

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,

Fig. 14. — View of Four Ribs of the Dog with the Intercostal Muscles.

a. The bony rib ; b, the cartilage ; tf, the junction of bone and cartilage ;
d, unossified, ^, ossified, portions of the sternum, yl. External intercostal
muscle. B. Internal intercostal muscle. In the middle interspace, the
external intercostal has been removed to show the internal intercostal
beneath it.

would sink back, and the rib would fall down to its
previous position.

Between the ribs are certain muscles called inter-
costal muscles (Fig. i). The exact action of these
you will learn at some future time. Meanwhile it

H z

loo SCIENCE PRIMERS, [§ vi.

will be enough to say that they act like the piece
of string we are speaking of. When they con-
tract, they pull up the ribs and thrust out the
sternum ; when they leave off contracting,
the ribs and sternum fall back to their pre-
vious 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 ordi-
nary 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 dia-
phragm 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 contrac-
tion 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 inter-
costal muscles cease to contract; the diaphragm re-
turns to its arched condition, the ribs sink down, the
stemum 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,

BREATHING.] PHYSIOLOGY, lOf

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 par-
ticular 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 dowr
along these nerves, from the brain, bidding them con-
tract ; 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 hand-
ful 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 j
move, is often called the stationary air (see Fig. 13). |

■MUMMia

I02 SCIENCE PRIMERS. [§ vi.

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 comer 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 inspi-
ration 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, a. ^ 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 ioFt oxygen is carried
through the stationary air to the blood in the capil-
laries, and the gained carbonic acid came through the
Stationary air from the blood in the capillaries. So

DIGESTION.] PHYSIOLOGY. 103

each breath helps to purify the blood, and the pump-
ing of air in and out of the chest changes the impure,
hurtful, venous, to pure, refreshing, arterial blood ; tlje
blood breathes air in the lungs, that all the body may
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 swal-
lowed 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 per-
spiration, 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

104

SCIENCE r/UMERS.

[§vn

Fig. JS.-Section of Skin, highly magnified.

a, homy epidermis ; 3, 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, /, breaks up into
capillaries, ultimately forming the veins, g.

DIGESTION.]

PHYSIOLOGY,

105

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. te;. The upper layer, ^, ^, is nothing but a mass
of little bodies packed closely together. At the top
they are pressed fiat 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 cellSj 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 aS what is
called the dermis, or true skin. This is full of
capillaries and blood-vessels, and when the knife or

1 N

lo6 SCIENCE PRIMERS. [§ vii.

razor gets down to this, you bleed. It is not made up
of cells like the epidermis, bvt of thai fibrous sub-
stance which you early learnt to call connective tissue
(see p. 9). Its top is rarely level, but generally
raised irto little hillocks, called papillae, as in tlie
figure; the epidermis forming a thick cap over each
papillae, and filling up the hollows ^between them.
Most of the papillae 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 gene-
rally 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
wails 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

DIGESTION.]

PHYSIOLOGY.

107

moisture in, how is it that the skin is nevertheless
sometimes quito moist, as v^hen we perspire ?

If you I'^ok at Fig. 15, you will see that the epi-
dermis is a one point pierced by a canal (h) running
right through it. You will notice that this canal is not
closed at the bottom of ihe epidermis, but runs right
into the dermis or true skin, where the canal becomes

Fig. 16. — Coiled end of a Sweat Gland, Epithelium not sJunun.

. ^e coil ; b, the duct ; c, network of capillaries, inside which the duct

gland lies.

M tube, with just one layer (e) of cells, like the cells
of the epidermis, for its walls. There is no room in
x^ig. 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. i^, where b is

io8 SCIENCE PRIMERS, [§ vii.

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 unravdled and taken away from the knotted
tube in order to show them. The capillaries, you
will understand, though inside the knot, are always out-
side 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 ^, 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. 1 5 ), 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 « 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 tune ;
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 tlie 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

DIGESTION.] PHYSIOLOGY, 109

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 epi-
dermis, 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, some-
thing 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

no SCIENCE PRIMERS. [§ VII.

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 artition
only from the blood in the capillaries, which are found
in immense numbers in the walls of the canal. The
alimentary canal is, as yci 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 mem-
brane of the stomach and of the intestines Ihey are
packed so close together, that the membrane seems
to be wholly made up of glands.

These glands vary in shape in different parts. No-
where 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 thick-
ness 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

DiGK ON.] PHYSIOLOGY, ill

cells, and wrapped round with blood capillaries. From
these capillaries, a watery fluid passes into the tubes,
and from the tubes into thq alimentary canal. This
watery fluid is, however, of a difl*erent 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 he intestines is an alkaline fluid, and is called
iii;Ustinal juice.

48. But besides these glands in the mucous mem-
brane 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 be-
hind 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 s^,me
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 speakhig 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 tiity
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

% •

112 SCIENCE PRIMERS. [§ Vll.

blood-vessels, and being packed close together with
connective tissue, make up the soft body we are
speaking of. This bo4y 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 com-
pound gland called the pancreas, which pours its
secretion called pancreatic juice into the alimentary
canal just where the small intestine begins (Fig. 17,^.)

That large organ the liver, though the plan of its
construction is not quite the same as that of the pan-
creas 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.

DIGESTION.]

PHYSIOLOGY.

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, /), to be stored up
there till needed.

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

Fig. 17. — The Stomach laid open behind.

tf, the oesophagus or gullet ; b, one end of the stomach ; rf, the other end
joining the intestine ; ^, gall duct \/, the gall-bladder ; gy the pancreatic
duct; hy 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. v

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

114 SCIENCE PRIMERS. [§ vil.

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 fr-^m 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 com-
posed 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, lim6, 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.

DIGESTION. ;(

PHYSIOLOGY.

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
V Ti hereafter, it would be uneconomical and unwise
tv 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 mine-
rals, 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 os-
nosis through thin membranes (see p. 84). If you take
a quantity of white of tgg^ cr 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 im-
merse 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 alimen-
tary cp" al, 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 tae alimentary canal, it is really

I 2

Ii6 SCIENCE PRIMERS. [§ vii.

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 sweet-
meats 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
psirts 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

DIGESTION.] PHYSIOLOGY, 1,7

short time the meat wll gradually disappear. All the
proteid matter will be dissolved, and only the wrap-
pings 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 mem-
branes, 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 to-
gether, 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 pan-
creatic 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 gene-
rally 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

lUiiKMMMMili

SCIENCE PRL^ERS,

t§ VII.

SCIENCE t^i^u^^— .

iiS , — —

— ' n ^A ^/lUi • thev are not

the intestine. .'^^'^^'^^.^Xv^'^ yousuppose

unlike thepapilte "f *^ f^^^p? the bottom row of
all the epide^js stnpped «cep ^^^ ^ ^^^^

cells (4 and *« P?^^ ? Ulustrate the structure
deal. Fig. i8 « a sketch '/M .^ ^^^^ up

of a villus. The eP'J^XShV epithelium, just
of a single row of cell^ f^^^^^ network of blood
S^S:^.t^^::ience> the ri^tW

.'Si-'-

^ , . :.u.i;„m. of wWch some cells aic

'^^ ,, -Joe ty,e blood capillaries, there

viUus only. But besides die btood^ P ^^ ^^^

is in each vUlus, what Aere is no ^JJ^^^^^ -^ the

skin, another <^P^llfYi*°h doe not contain blood,
left-hand villus only) 'f"?' '^^ery or with any
which is not connected ^* any^ery ^^ .^ ^

Ser f^eS^nlii^rLse at present

DIGESTION.]

PHYSIOLOGY,

119

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 capil-
laries in joining together to form larger trunks corre-
sponding 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 chan-
nels 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, Z^., Zj.,
Th, D,

Now, many of the lymphatics start from the innu-
merable villi of the intestine, and are there called
lacteals (Fig. 6, Let,) \ 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

I20 SCIENCE PRIMERS, [§ vii.

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 pancre-
atic juice are prevented from getting into the intes-
tine, 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 epitheUum
of the villus into the lacteal vessels, from the lac-
teals 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 dis-
solved 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 :n the stomach, or in the intestine, and pass into
the blood.

DIGESTION.) PHYSIOLOGY. I2i

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

When we eat a piece of bread, while we are chew-
ing 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. Iti
the intestine what remains of the proteids is dis-
solved, 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 mor-
sels 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 siqueezed into the intestine. Here the bile and
the pancreatic juice, breaking up the fat into tiny

122 SCIENCE PRIMERS. [§ vil

particles, mix fat, and broken meat, and empty wrap-
pings, and salts, and water, all together into a thick,
dirty, yellowish creamt 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,
anore 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 !irst 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 .xie 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.

WASTE.] PHYSIOLOGY. 123

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, bum it with heat but without light. But,
as you have learnt from your Chemistry Primer, Art.
4, burning is only change, not destruction ; in burn-
ing nothing is lost. If the muscle burns blood, it
bums it into something ; that something, being already
bumt, cannot be butnt again, and must be got rid of

Into what things does the body bum 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,
phosphoms, 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

134 SCIENCE PRIMERS. [§ viii.

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 can-
not 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 car-
bonic 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.

WASTE.] PHYSIOLOGY. 125

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 know-
ing 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 iX\t 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 put 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

. jLV^M ** V.l.«A ^

126 SCIENCE PRIMERS. [§ viii.

generally in a particular condition, being combined
ivith 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 an-^ salts in
llctle 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 rist.

SUMMARY.] PHYSIOLOGY. . 127

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, wc
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 innutritions 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 capillar! is bathes every smallest bit of
almost every part. Kept continually rich in combus-
tible 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, stufif 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

MMHik

i28 SCIENCE PRIMERS. [§ ix.

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. Sweep-
ing as it continually does through innutnerable
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 perspira-
tion 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 itstflf 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 .^f the body, here quickly, there slowly, is con-
tinually mouldering away and as continually being
made anew by the blood. Made anew according
to itF ovTi 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 gastr.'^
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

•• -31

SUMMARY.] PHYSIOLOGY. 129

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.
Bv 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 re-
freshing every spot it touches, the blood current also
carries away from each organ the waste matters of
which that organ has no longer any usei 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 thr«w 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, car-
bonic acid, and salts. In this shape, the food, after a

130 SCIENCE PRIMERS. ' [§ x.

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 caro'^.
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
papillae 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 mal ^ 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
pricfc or touch your finger, you feel the prick or touch ;
you say you have sensation in your finger. Suppose

FEELING.] PHYSIOLOGY. 131

you were to cut across the nervos 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
J , \, 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

132 SCIENCE PRIMEKS, [§ x.

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, 1 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, sensa-
tions of touch, sensations of roughness and smooth-
ness, of heat and cold, sensations of goo^ and bad
odours, sensations of tastes of various kinds, sensa-
tions 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 impres-
sions 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.

■1

J *