LOCKYER’S ASTRONOMY
_ELEMENTS OF ASTRONOMY_:
Accompanied with numerous Illustrations, a Colored Representation
of the Solar, Stellar, and Nebular Spectra,
and Celestial Charts of the Northern
and the Southern Hemisphere.
By J. NORMAN LOCKYER.
_American edition_, revised and specially adapted to the Schools
of the United States.
_12mo. 312 pages. Price, $1.75._
The volume is as practical as possible. To aid the student in
identifying the stars and constellations, the fine Celestial Charts of
Arago, which answer all the purposes of a costly Atlas of the Heavens,
are appended to the work--this being the only text-book, as far as the
Publishers are aware, that possesses this great advantage. Directions
are given for finding the most interesting objects in the heavens at
certain hours on different evenings throughout the year. Every device is
used to make the study interesting; and the Publishers feel assured that
teachers who once try this book will be unwilling to exchange it for any
other.
D. APPLETON & CO., PUBLISHERS,
549 & 551 BROADWAY, NEW YORK.
SCIENCE PRIMERS, _edited by_
_PROFESSORS_ HUXLEY, ROSCOE, _and_
BALFOUR STEWART.
VI.
_PHYSIOLOGY._
[Illustration: EXPLANATION OF THE PLATE.
FIG. I.--THE HUMAN SKELETON IN PROFILE.
_Mn._ The Mandible or Lower Jaw.
_St._ The Sternum. }
_R._ The Ribs. } In the Thorax.
_R´._ The Cartilages of the Ribs. }
_Scp._ The Scapula, or Shoulder Blade.
_Cl._ The Clavicle, or Collar Bone.
_H._ The Humerus. }
_Ra._ The Radius. } In the Arm.
_U._ The Ulna. }
_F._ The Femur. }
_Tb._ The Tibia. } In the Leg.
_Fb._ The Fibula. }
FIG. II.
A front view of the Sternum, _St._, with the Cartilages of the Ribs,
_R´._, and part of the Ribs themselves _R._]
Science Primers.
PHYSIOLOGY.
BY
M. FOSTER, M.A., M.D., F.R.S.,
FELLOW OF TRINITY COLLEGE, CAMBRIDGE.
WITH ILLUSTRATIONS.
NEW YORK:
D. APPLETON AND COMPANY,
549 AND 551 BROADWAY.
1877.
PREFACE.
This Primer is an attempt to explain in the most simple manner possible
some of the most important and most general facts of Physiology, and may
be looked upon as an introduction to the Elementary Lessons of Professor
Huxley.
In my descriptions and explanations I have supposed the reader to be
willing to handle and examine such things as a dead rabbit and a sheep’s
heart; and written accordingly, I have done this purposely, from an
increasing conviction that actual observation of structures is as
necessary for the sound learning of even elementary physiology, as are
actual experiments for chemistry. At the same time I have tried to make
my text intelligible to those who think reading verbal descriptions less
tiresome than observing things for themselves.
It seemed more desirable in so elementary a work to insist, even with
repetition, on some few fundamental truths, than to attempt to skim over
the whole wide field of Physiology. I have therefore omitted all that
relates to the Senses and to the functions of the Nervous System, merely
just referring to them in the concluding article. These the reader must
study in the “Elementary Lessons.”
M. FOSTER.
TABLE OF CONTENTS.
ART. SECT.
I. INTRODUCTION.
PAGE
1. “ What Physiology is 1
2. “ Animals move of their own accord 1
3. “ Animals are warm 3
4. “ Why animals are warm and move about--they burn 4
5. “ The need of Oxygen 5
6. “ The waste matters 5
II. THE PARTS OF WHICH THE BODY IS MADE UP.
7. “ The Tissues 7
8. “ The cavities of the Thorax and Abdomen 9
9. “ The Vertebral Column 12
10. “ Head and Neck 15
11. “ Nerves 18
12. “ General arrangement of all these parts 19
III. WHAT TAKES PLACE WHEN WE MOVE.
13. “ The Bones of the Arm 21
14. “ The structure of the Elbow Joint 23
15. “ Other joints in the body 25
16. “ The arm is bent by the contraction of the Biceps Muscle 26
17. “ How the will makes the Biceps Muscle contract 32
18. “ The power of a muscle to contract depends on its being
supplied with blood 35
19. “ It is the food in the blood which gives the muscle strength 37
20. “ The continual need of food 38
IV. THE NATURE OF BLOOD.
21. “ The Blood in the Capillaries 40
22. “ The Corpuscles of the Blood 42
23. “ The clotting of Blood 45
24. “ The substances present in Serum 48
25. “ The minerals in Blood 50
V. HOW THE BLOOD MOVES.
26. “ The Arteries, Capillaries, and Veins 51
27. “ The Sheep’s Heart 55
28. “ The Course of the Circulation 58
29. “ Why the blood moves in one direction only; the Valves
of the Veins 64
30. “ The Tricuspid Valves of the Heart 66
31. “ The pulmonary Semilunar Valves 71
32. “ The left side of the Heart 72
33. “ What makes the blood move at all: The beat of the Heart 76
34. “ The action of the Heart as a whole 79
35. “ The Capillaries and the Tissues 82
VI. HOW THE BLOOD IS CHANGED BY AIR: BREATHING.
36. “ Venous and Arterial Blood 85
37. “ The change from Arterial to Venous, and from Venous to
Arterial Blood 87
38. “ The Lungs 88
39. “ The renewal of Air in the Lungs. How the descent of the
Diaphragm expands the Lungs 90
40. “ The natural distension of the Lungs. Inspiration. Expiration 91
41. “ How the Diaphragm descends 96
42. “ The Chest is also enlarged by the movements of the Ribs
and Sternum 98
43. “ Breathing an involuntary act 100
44. “ Tidal air; stationary air 101
VII. HOW THE BLOOD IS CHANGED BY FOOD: DIGESTION.
45. “ Why the inside of the mouth is always red and moist 103
46. “ Why the Skin is sometimes moist. Sweat Glands 106
47. “ The Mucous Membrane of the Alimentary Canal and its Glands 109
48. “ The Salivary Glands, Pancreas and Liver 111
49. “ Food-stuffs 113
50. “ How proteids and starch are changed 115
51. “ Lacteals and Lymphatics 117
52. “ What becomes of the Food-stuffs 120
VIII. HOW THE BLOOD GETS RID OF WASTE MATTERS.
53. “ The need of getting rid of Waste Matters 123
54. “ The Kidneys get rid of Ammonia in the form of Urea 125
55. IX. THE WHOLE STORY SHORTLY TOLD. 127
56. X. HOW WE FEEL AND WILL. 130
SCIENCE PRIMERS.
_PHYSIOLOGY._
INTRODUCTION. § I.
=1.= Did you ever on a winter’s day, when the ground was as hard as a
stone, the ponds all frozen, and everything cold and still, stop for a
moment, as you were running in play along the road or skating over the
ice, to wonder at yourself and ask these two questions:--“Why am I so
warm when all things around me, the ground, the trees, the water, and
the air, are so cold? How is it that I am moving about, running,
walking, jumping, when nothing else that I can see is stirring at all,
except perhaps a stray bird seeking in vain for food?”
These two questions neither you nor anyone else can answer fully; but we
may answer them in part, and the knowledge which helps us to the answer
is called =Physiology=.
=2. You can move of your own accord.= You do not need to wait, like the
boughs or the leaves, till the wind blows upon you, or, like the stones,
till somebody stirs you. The bird, too, can move of its own accord, so
can a dog, so can any animal as long as it is alive. If you leave a
stone in any particular spot, you expect to find the stone there when
you come to it again a long time afterwards; if you do not, you say
somebody or something has moved it. But if you put a sparrow or mouse on
the grass plot, you know that directly your back is turned it will be
off.
All animals move of themselves. But only so long as they are alive. When
you find the body of a snake on the road, the first thing you do is to
stir it with a stick. If it moves only as you move it, and as far as you
move it, just as a bit of rope might do, you say it is dead. But if,
when you touch it, it stirs of itself, wriggles about, and perhaps at
last glides away, you know it is alive. Every living animal, of whatever
kind, from yourself down to the tiniest creature that swims about in a
little pool of water and cannot be seen without a microscope, moves of
itself. Left to itself, it moves and rests, rests and moves; stirred by
anything, away it goes, running, flying, creeping, crawling, or
swimming.
Something of the kind sometimes happens with lifeless things. When a
stone is carefully balanced on the top of a high wall, a mere touch will
send it toppling down to the ground. But when it has reached the ground
it stops there, and if you want to repeat the trick you must carry the
stone up to the top of the wall again. You know the toy made like a
mouse, which, when you touch it in a particular place, runs away
apparently of its own accord, as if it were alive. But it soon stops,
and when it has stopped you may touch it again and again without making
it go on. Not until you have wound it up will it go on again as it did
before. And every time you want it to run you must wind it up afresh.
Living animals move again and again, and yet need no winding up, for
they are always winding themselves up. Indeed, as we go on in our
studies we shall come to look upon our own bodies and those of all
animals as pieces of delicate machinery with all manner of springs,
which are always running down but always winding themselves up again.
=3. You are warm=; beautifully warm, even on the coldest winter day, if
you have been running hard; very warm if you are well wrapped up with
clothing, which, as you say, keeps the cold out, but really keeps the
warmth in. The bed you go to at night may be cold, but it is warm when
you leave it in the morning. Your body is as good as a fire, warming
itself and everything near it.
The bird too is warm, so is the dog and the horse, and every four-footed
beast you know. Some animals however, such as reptiles, frogs, fish,
snails, insects, and the like do not seem warm when you touch them. Yet
really they are always a little warm, and some times they get quite
warm. If you were to put a thermometer into a hive of bees when they are
busy you would find that they are very warm indeed. All animals are more
or less warm as long as they are alive, some of them, such as birds and
four-footed beasts, being very warm. But only so long as they are alive;
after death they quickly become cold. When you find a bird lying on the
grass quite still, not stirring when it is touched, to make quite sure
of its being dead you feel it. If it is quite cold, you say it has been
dead some time; if it is still warm, you say it is only just
dead--perhaps hardly dead, and may yet revive.
=4. You are warm, and you move about of yourself. You are able to move
because you are warm; you are warm in order that you may move. How does
this come about?= Just think for a moment of something which is not an
animal, but which is warm and moves about, which only moves when it is
warm, and which is warm in order that it may move. I mean a locomotive
steam-engine. What makes the engine move? The burning coke or coal,
whose heat turns the water into steam, and so works the piston, while at
the same time the whole engine becomes warm. You know that for the
engine to do so much work, to run so many miles, so much coal must be
burnt; to keep it working it must be “stoked” with fresh coal, and all
the while it is working it is warm: when its stock of coal is burnt out
it stops, and, like a dead animal, grows cold.
Well, your body too, just like the steam-engine, moves about and is
warm, because a fire is always burning in your body. That fire, like the
furnace of the engine, needs fresh fuel from time to time, only your
fuel is not coal, but food. In three points your body differs from the
steam-engine. In the first place, you do not use your fire to change
water into steam, but in quite a different way, as we shall see further
on. Secondly, your fire is a burning not of dry coal, but of wet food, a
burning which although an oxidation (Chemistry Primer, Art. 5) takes
place in the midst of water, and goes on without any light being given
out. Thirdly, the food you take is not burnt in a separate part of your
body, in a furnace like that of the engine set apart for the purpose.
The food becomes part and parcel of your body, and it is your whole body
which is burnt, bit by bit.
Thus it is the food burning or being oxidized within your body, or as
part of your body, which enables you to move and keeps you warm. If you
try to do without food, you grow chilly and cold, feeble, faint, and too
weak to move. If you take the right quantity of proper food, you will be
able to get the best work out of the engine, your body; and if you work
your body aright, you can keep yourself warm on the coldest winter day,
without any need of artificial fire.
=5.= But if this be so, in order to oxidize your food, you =have need of
oxygen=. The fire of the engine goes out if it is not fed with air as
well as fuel. So will your fire too. If you were shut up in an air-tight
room, the oxygen in the room would get less and less, from the moment
you entered the room, being used up by you; the oxidation of your body
would after a while flag, and you would soon die for want of fresh
oxygen (see Chemistry Primer, p. 14).
You have, throughout your whole life, a need of fresh oxygen, you must
always be breathing fresh air to carry on in your body the oxidation
which gives you strength and warmth.
=6.= When a candle is burnt (Chemistry Primer, p. 6) it turns into
carbonic acid, and water. When wood or coal is burnt, we get ashes as
well. If you were to take all your daily food and dry it, it too would
burn into ashes, carbonic acid, and water (with one or two other things
of which we shall speak afterwards).
Your body is always giving out carbonic acid (Chemistry Primer, Exp. 7).
Your body is always giving out water by the lungs, as seen when you
breathe on a glass, by the skin, and by the kidneys; and we shall see
that we always give out more water than we take in as food or drink.
Your body too is daily giving out by the kidneys and bowels, matters
which are not exactly ashes, but very like them. We do not oxidize our
food quite into ashes, but very nearly; we burn it into substances which
are no longer useful for oxidation in the body, and which, being
useless, are cast out of the body as waste matters.
The tale then is complete. By the help of the oxygen of the air which
you take in as you breathe, you oxidize the food which is in your body.
You get rid of the water, the carbonic acid, and other waste matters
which are left after the oxidation, and out of the oxidation you get the
heat which keeps you warm and the power which enables you to move.
Thus all your life long you are in constant need of oxygen and food. The
oxygen you take in at every breath, the food at every meal. How you get
rid of the waste matters we shall see further on.
If you were to live, as one philosopher of old did, in a large pair of
delicate scales, you would find that the scale in which you were would
sink down at every meal, and gradually rise between as you got lighter
and hungry. If the food you took were more than you wanted, so that it
could not all be oxidized, it would remain in your body as part of your
flesh, and you would grow heavier and stouter from day to day; if it
were less, you would grow thinner and lighter; if it were just as much
as and no more than you needed, you would remain day after day of
exactly the same weight, the scale in which you sat rising as much
between meals as it sank at the meal time.
What we have to learn in this Primer is--How the food becomes part and
parcel of your body; how it gets oxidized; how the oxidation gives you
power to move; how it is that you are able to move in all manner of
ways, when you like, how you like, and as much as you like.
First of all we must learn something about the build of your body, of
what parts it is made, and how the parts are put together.
THE PARTS OF WHICH THE BODY IS MADE UP. § II.
=7.= When you want to make a snow man, you take one great roll of snow to
make the body or trunk. This you rest on two thinner rolls which serve
as legs. Near the top of the trunk you stick in another thin roll on
either side--these you call the two arms: and lastly, on quite the top
of the trunk you place a round ball for a head. Head, trunk, and limbs,
_i.e._ legs and arms--these together make up a complete body.
In your snow man these are all alike, all balls of snow differing only
in size and form; but in your own body, head, trunk, and limbs are quite
unlike, as you might easily tell on taking them to pieces. Now you
cannot very well take your own body to pieces, but you easily can that
of a dead rabbit. Suppose you take one of the limbs, say a leg, to begin
with.
First of all there is the =skin= with the hair on the outside. If you
carefully cut this through with a knife or pair of scissors and strip it
off, you will find it smooth and shiny inside. Underneath the skin you
see what you call =flesh=, rather paler, not so red as the flesh of beef
or mutton, but still quite like it. Covering the flesh there may be a
little =fat=. In a sheep’s leg as you see it at the butcher’s there is a
good deal of fat, in the rabbit’s there is very little.
This reddish flesh you must henceforward learn to speak of as =muscle=. If
you pull it about a little, you will find that you can separate it
easily into parcels or slips running lengthways down the leg, each slip
being fastened tight at either end, but loose between. Each slip is what
is called =a muscle=. You will notice that many of these muscles are
joined, sometimes at one end only, sometimes at both, to white or bluish
white glistening cords or bands; made evidently of different material
from the muscle itself. They are not soft and fleshy like the muscle,
but firm and stiff. These are =tendons=. Sometimes they are broad and
short, sometimes thin and long.
As you are separating these muscles from each other you will see
(running down the leg between them) little white soft threads, very
often branching out and getting too small to be seen. These are =nerves=.
Between the muscles too are other little cords, red, or reddish black,
and if you prick them, a drop or several drops of blood will ooze out.
These are =veins=, and are not really cords or threads, but hollow tubes,
filled with blood. Lying alongside the veins are similar small tubes,
containing very little blood, or none at all. These are =arteries=. The
=veins= and =arteries= together are called =blood-vessels=, and it will be
easy for you to make out that the larger ones you see are really hollow
tubes. Lastly, if you separate the muscles still more, you will come
upon the hard =bone= in the middle of the leg, and if you look closely you
will find that many of the muscles are fastened to this bone.
Now try to put back everything in its place, and you will find that
though you have neither cut nor torn nor broken either muscle or
blood-vessel or bone, you cannot get things back into their place again.
Everything looks “messy.” This is partly because, though you have torn
neither muscle nor blood-vessel, you have torn something which binds
skin and muscle and fat and blood-vessels and bone all together; and if
you look again you will see that between them there is a delicate
stringy substance which binds and packs them all together, just as
cotton-wool is used to pack up delicate toys and instruments. This
stringy packing material which you have torn and spoilt is called
=connective= because it connects all the parts together.
Well, then, in the leg (and it is just the same in the arm) we have
skin, fat, muscle, tendons, blood-vessels, nerves, and bone all packed
together with connective and covered with skin. These together form the
solid leg. We may speak of them as =the tissues= of the leg.
=8.= If now you turn to the trunk and cut through the skin of the belly,
you will first of all see muscles again, with nerves and blood-vessels
as before. But when you carefully cut through the muscles (for you
cannot easily separate them from each other here), you come upon
something which you did not find in
[Illustration: FIG. 1.--_The Viscera of a Rabbit as seen upon simply
opening the Cavities of the Thorax and Abdomen without any further
Dissection._
_A_, cavity of the thorax, pleural cavity of either side; _B_,
diaphragm; _C_, ventricles of the heart; _D_, auricles; _E_,
pulmonary artery; _F_, aorta; _G_, lungs, collapsed, and occupying
only the back part of the chest; _H_, lateral portions of pleural
membranes; _I_, cartilage at the end of sternum; _K_, portion of
the wall of body left between thorax and abdomen; _a_, cut ends of
the ribs; _L_, the liver, in this case lying more to the left than
the right of the body; _M_, the stomach; _N_, duodenum; _O_, small
intestine; _P_, the cæcum, so largely developed in this and other
herbivorous animals; _Q_, the large intestine.
]
the leg, a =great cavity=. This is something quite new--there is nothing
like it in the leg--a great cavity, quite filled with something, but
still a great cavity; and if you slit the rabbit right up the front of
its trunk and turn down or cut away the sides as has been done in Fig.
1, you will see that the whole trunk is =hollow= from top to bottom, from
the neck to the legs.
If you look carefully you will see that the cavity is divided into two
by a cross partition (Fig. 1, _B_) called the =diaphragm=. The part =below=
the diaphragm is the larger of the two, and is called the =abdomen= or
belly; in it you will see a large dark red mass, which is the =liver=
(_L_). Near the liver is the smooth pale =stomach= (_M_), and filling up
the rest of the abdomen you will see the coils of the =intestine= or
bowel, very narrow in some parts (_O_), very broad (_P_ _Q_), broader
even than the stomach, in others. If you pull the bowels on one side as
you easily can do, you will find lying underneath them two small
brownish red lumps, one on each side. These are the =kidneys=.
In the smaller cavity =above= the diaphragm, called the =thorax= or chest,
you will see in the middle the =heart= (_C_), and on each side of the
heart two pink bodies, which when you squeeze them feel spongy. These
are the two =lungs= (_G_). You will notice that the heart and lungs do not
fill up the cavity of the chest nearly so much as the liver, stomach,
bowels, &c. fill up the cavity of the belly. In fact, in the chest
there seems to be a large empty space. But as we shall see further on,
the lungs did quite fill the chest before you opened it, but shrank up
very much directly you cut into it, and so left the great space you see.
=9.= The trunk then is really a great chamber containing what are called
the =viscera=, and divided into an upper and lower half, the upper half
being filled with the heart and lungs, the lower with the liver,
stomach, bowels, and some other organs. In front the abdomen is covered
by skin and muscle only. But if all the sides of the trunk were made of
such soft material it would be then a mere bag which could never keep
its shape unless it were stuffed quite full. Some part of it must be
strengthened and stiffened. And indeed the trunk is not a bag with soft
yielding sides, but a box with walls which are in part firm and hard.
You noticed that when you were cutting through the front of the chest
you had to cut through several hard places. These were the =ribs= (Fig. 1,
_a_), made either of hard bone or of a softer gristly substance called
=cartilage=. And if you take away all the viscera from the cavity of the
trunk and pass your finger along the back of the cavity, you will feel
all the way down from the neck to the legs a hard part. This is the
=backbone= or =vertebral column=. When you want to make a straw man stand
upright you run a pole right through him to give him support. Such a
support is the backbone to your own body, keeping the trunk from falling
together.
In the abdomen nothing more is wanted than this backbone, the sides and
front of the cavity being covered in with skin and muscle only. In the
chest the sides are strengthened by the ribs, long thin hoops of bone
which are fastened to the backbone behind and meet in front in a firm
hard part, partly bone, partly cartilage, called the =sternum=.
But this backbone is not made of one long straight piece of bone. If it
were you would never be able to bend your body. To enable you to do this
it is made up of ever so many little flat round pieces of bone, laid one
a-top of the other, with their flat sides carefully joined together,
like so many bungs stuck together. Each of these little round flat
pieces of the backbone is called a =vertebra=, and is of a very peculiar
shape. Suppose you took a bung of bone, and fastened on to one side of
its edge a ring of bone. That would represent a vertebra. The solid bung
is what is called the =body=, and the hollow ring is what is called the
=arch= of the vertebra. Now if you put a number of these bodies together
one upon the top of the other, so that the bodies all came together and
the rings all came together, you would have something very like the
vertebral column (see Frontispiece, also Fig. 2). The bungs or bodies
would make a solid jointed pillar, and the rings or arches would make
together a tunnel or canal. And that is really what you have in the
backbone. Only each vertebra is not exactly shaped like a bung and a
ring; the body is very like a bung, but the arch is rough and jagged,
and the bodies are joined together in a particular way. Still we have
all the bodies of the vertebræ forming together a solid pillar which
gives support to the trunk; and the arches forming together a tunnel or
canal which is called the =spinal canal=, (Fig. 2, _C.S._) the use of
which we shall see
[Illustration: FIG. 2.
A, a diagrammatic view of the human body cut in half lengthways.
_C.S._, the cavity of the brain and spinal cord; _N_, that of the
nose; _M_, that of the mouth; _Al. Al._, the alimentary canal
represented as a simple straight tube; _H_, the heart; _D_, the
diaphragm.
B, a transverse vertical section of the head taken along the line
_a b_; letters as before.
C, a transverse section taken along the line _c d_; letters as
before.
]
directly. The round flat body of each vertebra is turned to the front
towards the cavity of the trunk, and it is the row of vertebral bodies
which you feel as a hard ridge when you pass your fingers down the back
of the abdomen. The arches are at the back of the bodies, so you cannot
feel them in the abdomen; but if you turn the rabbit on its belly and
pass your finger down its back, you will feel through the skin (and you
can feel the same on your own body) a sharp edge, formed by what are
called the spines, _i.e._ the uneven tips of the arches of the vertebræ
(Fig. 2) all the way down the back.
So that what we really have in the trunk is this. In front a large
cavity, containing the viscera, and surrounded in the upper part or
thorax by hoops of bone, but not (or only slightly) in the lower part or
abdomen; behind, a much smaller long narrow cavity or canal formed by
the arches of the vertebræ, and therefore surrounded by bone all the way
along, and containing we shall presently see what; and between these two
cavities, separating the one from the other, a solid pillar formed by
the bodies of the vertebræ. So that if you were to take a cross slice,
or transverse section as it is called, of the rabbit across the chest,
you would get something like what is represented in Fig. 2, C, where
_C.S._ is the narrow canal of the arches and where the broad cavity of
the chest containing the heart _H_ is enclosed in the ribs reaching from
the vertebra behind to the sternum in front. Both cavities are covered
up on the outside with muscles, blood-vessels, nerves, connective, and
skin, just as in the leg.
=10.= We have now to consider the =head and neck=. If you cut through the
skin of the neck of the rabbit, you will see, first of all, muscles and
nerves, and several large blood-vessels; but you will find no large
cavity like that in the trunk. So far the neck is just like the leg. But
if you look carefully you will see two tubes which are not
blood-vessels, and the like of which you saw nowhere in the leg. One of
these tubes is firm, with hardish rings in it; it is the windpipe or
=trachea=; the other is soft, and its sides fall flat together; this is
the gullet or =œsophagus=, leading from the mouth to the stomach.
Behind these and the muscles in which they run you will find, just as in
the trunk, a vertebral column, without ribs, but composed of bodies, and
behind the bodies there is a vertebral canal. This vertebral column and
vertebral canal in the neck are simply continuations of the vertebral
column and canal of the trunk.
=The neck, then, differs from the leg in having a vertebral column and
canal with a trachea and œsophagus, and differs from the trunk in
having no cavity and no ribs.=
The head, again, is unlike all these. Indeed, you will not understand
how the head is made unless you take a rabbit’s skull and place it side
by side with the rabbit’s head. If you do this, you will at once see how
the mouth and throat are formed. =You will notice that the skull is all
in one piece=, except a bone which you will at once recognize as the
jawbone, or, to speak more correctly, the lower =jawbone=; for there are
two jawbones. Both these carry teeth, but the upper one is simply part
of the skull, and does not move; the lower one does move; it can be made
to shut close on the upper jaw, or can be separated a good way from it.
The opening between the two jaws is the gap or gape of the mouth, which
as you know can be opened or shut at pleasure. If you try it on yourself
you will find that, as in the rabbit, it is the lower jaw which moves
when you open or shut your mouth. The upper jaw does not move at all
except when your whole head moves. Underneath the skull at the top of
the neck the mouth narrows into the throat, into the upper part of which
the cavity of the nose opens. So that there are two ways into the
throat, one through the mouth and the other through the nose (Fig. 2).
At the back of the skull you will see a rounded opening, and if you put
a bodkin through this opening you will find it leads into a large hollow
space in the inside of the skull. In the living rabbit this hollow space
is filled up with the =brain=. The skull, in fact, is a box of bone to
hold the brain, a bony brain-case. This bony case fits on to the top of
the vertebræ of the neck in such a way that the rounded opening we spoke
of just now is placed exactly over the top of the tunnel or canal formed
by the rings or arches of the vertebræ. If you were to put a wire
through the arch of the lowest vertebra, you might push it up through
the canal formed by the arches of all the vertebræ, right into the brain
cavity. In fact the brain-case and the row of arches of the vertebræ
form together one canal, which is a narrow tube in the back and in the
neck, but swells out in the head into a wide rounded space (Fig. 2, A
and B, _C.S._) During life this canal is filled with a peculiar white
delicate material, which is called =nervous matter=. The rounded mass of
this material which fills up the cavity of the skull is called the
=brain=; the narrower, rod-like, or band-like mass which runs down the
vertebral canal in the neck and back is called the =spinal cord=. They
have separate names, but they are quite joined together, and the rounded
brain tapers off into the band-like cord in such a way that it is
difficult to say where the one begins and the other ends.
=11.= In the skull, besides the larger openings we have spoken of, you
will find several small holes leading from the outside of the skull into
the inside of the brain-case. Some of these holes are filled up during
life by blood-vessels, but in others run those delicate white threads or
cords which you have already learnt to call nerves. =Nerves are in fact
branches of nervous material running out from the brain or spinal cord.=
Those from the brain pass through holes in the skull, and at first sight
seem to spread out very irregularly. Those which branch off from the
spinal cord are far more regular. A nerve runs out on each side between
every two vertebræ, little rounded gaps being left for that purpose
where the vertebræ fit together, so that when you look at a spinal cord
with portions of the nerves still connected with it, it seems not unlike
a double comb with a row of teeth on either side. The nerves which
spring in this way from the spinal cord are called =spinal nerves=, and
soon after they leave the vertebral canal they divide into branches, and
so are spread nearly all over the body. In any piece of skin or flesh
you examine, never mind in what part of the body, you will find nerves
and blood-vessels. If you trace the nerves out in one direction, you
will find them joining together to form larger nerves, and these again
joining others, till at last all end in either the spinal cord or the
brain. If you try to trace the same nerves in the other direction, you
will find them branching into smaller and smaller nerves, until they
become too small to be seen. If you take a microscope you will find they
get still smaller and smaller until they become the very finest possible
threads.
The blood-vessels in a similar way join together into larger and larger
tubes, which last all end, as we shall see, in the heart. =Every part of
the body, with some few exceptions, is crowded with nerves and
blood-vessels. The nerves all come from the brain or spinal cord--the
vessels from the heart. So that every part of the body is governed by
two centres, the heart, and the brain or spinal cord.= You will see how
important it is to remember this when we get on a little further in our
studies.
=12.= Well, then, the body is made up in this way. First there is the
head. In this is the skull covered with skin and flesh, and containing
the brain. The skull rests on the top of the backbone, where the head
joins the neck. In the upper part of the neck, the throat divides into
two pipes or tubes--one the windpipe, the other the gullet. These
running down the neck in front of the vertebral column, covered up by
many muscles, when they get about as far down as the level of the
shoulders, pass into the great cavity of the body, and first into the
upper part of it, or chest.
Here the windpipe ends in the lungs, but the gullet runs straight
through the chest, lying close at the back on the backbone, and passes
through a hole in the diaphragm into the abdomen, where it swells out
into the stomach. Then it narrows again into the intestine, and after
winding about inside the cavity of the abdomen a good deal, finally
leaves it.
You see the =alimentary canal= (for that is the name given to this long
tube made up of gullet, stomach, intestine, &c.) =goes right through the
cavity of the body without opening into it=--very much as the tall narrow
glass of a lamp passes through the large globe glass. You might pour
anything down the narrow glass without its going into the globe glass,
and you might fill the globe glass and yet leave the narrow glass quite
empty. If you imagine both glasses soft and flexible instead of hard and
stiff, and suppose the narrow glass to be very long and twisted about so
as to all but fill the globe, you will have a very fair idea of how the
alimentary canal is placed in the cavity of the body.
Besides the alimentary canal, there is in the chest, in addition to the
windpipe and lungs, the heart with its great tubes, and in the abdomen
there are the liver, the kidneys, and other organs.
These two great cavities, with all that is inside them, together with
wrappings of flesh and skin which make up the walls of the cavities,
form the trunk, and on to the trunk are fastened the jointed legs and
arms. These have no large cavities, and the alimentary canal goes
nowhere near them.
One more thing you have to note. There is only one alimentary canal, one
liver, one heart--but there are two kidneys and two lungs, the one on
one side, the other on the other, and the one very much like the other.
There are two arms and two legs, the one almost exactly like the other.
There is only one head, but one side of the head is almost exactly like
the other. One side of the vertebral column is exactly like the
other--as are also the two halves of the brain and the two halves of the
spinal cord.
In fact, if you were to cut your rabbit in half from his nose to his
tail, you would find that except for his alimentary canal, his heart,
and his liver, one half was almost exactly the counterpart of the other.
Such is the structure of a rabbit, and your own body, in all the points
I have mentioned, is made up exactly in the same way.
WHAT TAKES PLACE WHEN WE MOVE. § III.
=13.= Let us now go back to the question. =How is it that we can move about
as we do?= And first of all let us take one particular movement and see
if we can understand that.
For instance, you can bend your arm. You know that when your arm is
lying flat on the table, you can, if you like, bend the lower part of
your arm (the fore-arm as it is called, reaching from the elbow to the
hand) on the upper arm until your fingers touch your shoulder. How do
you manage to do that?
Look at the bones of the arm in a skeleton. (Frontispiece; also Fig. 3.)
You will see that in the upper arm there is one rather large bone (_H_)
reaching from the shoulder to the elbow, while in the fore-arm there are
two, one (_U_) being wider and stouter than the other (_Ra_) at the
elbow, but smaller and more slender at the wrist. The bone in the upper
arm is called the =humerus=; the bone in the fore-arm, which is stoutest
at the elbow, is called the =ulna=; the one which is stoutest at the
wrist is called the =radius=. If you look carefully you will see that the
end of the humerus at the elbow is curiously rounded, and the end of the
ulna at the elbow curiously scooped out, in such a way that the one fits
loosely into the other.
[Illustration: FIG 3.--_The Bones of the Upper Extremity with the Biceps
Muscle._
The two tendons by which this muscle is attached to the scapula, or
shoulder-blade, are seen at _a_. P indicates the attachment of the
muscle to the radius, and hence the point of action of the power;
F, the fulcrum, the lower end of the humerus on which the upper end
of the radius (together with the ulna) moves; W, the weight (of the
hand).
]
If you try to move them about one on the other, you will find that you
can easily double the ulna very closely on the humerus without their
ends coming apart, and if you notice you will see that as you move the
ulna up and down, its end and the end of the humerus slide over each
other. But they will only slide one way, what we may call up and down.
If you try to slide them from side to side, you will find that they get
locked. They have only one movement, like that of a door on its hinge,
and that movement is of such a kind as to double the ulna on the
humerus.
Moreover, if you look a little more carefully you will find that, though
you can easily double the ulna on the front of the humerus, and then
pull it back again until the two are in a straight line, you cannot bend
the ulna on the back of the humerus. On examining the end of the ulna
you will find at the back of it a beak-like projection (Fig. 3, also
Frontispiece), which when the bones are straightened out locks into the
end of the humerus, and so prevents the ulna being bent any further
back. This is the reason why you can only bend your arm one way. As you
very well know, you can bend your arm so as to touch the top of your
shoulder with your fingers, but you can’t bend it the other way so as to
touch the back of your shoulder; you can’t bring it any further back
than the straight line.
=14.= Well, then, at the elbow the two bones, the humerus and ulna, are so
shaped and so fit into each other that the arm may be straightened or
bent. In the skeleton the two bones are quite separate, _i.e._ they have
to be fastened together by something, else they would fall apart. Most
probably in the skeleton you have been examining they are fastened
together by wires or slips of brass. But they would hold together if you
took away the wire or brass slips and bound some tape round the two
ends, tight enough to keep them touching each other, but loose enough to
allow them to move on each other. You might easily manage it if you took
short slips of tape, or, better still, of india-rubber, and placed them
all round the elbow, back, front, and sides, fastening one end of each
slip to the humerus and the other to the ulna. If you did this you
would be imitating very closely the manner in which the bones at the
elbow are kept together in your own arm. Only the slips are not made of
india-rubber, but are flat bands of that stringy, or as we may now call
it fibrous stuff, which in the preceding lessons you learnt to call
connective tissue. These flat bands have a special name, and are called
=ligaments=.
At the elbow the two ends of the ulna and humerus are kept in place by
ligaments or flat bands of connective tissue.
In the skeleton, the surfaces of the two bones at the elbow where they
rub against each other, though somewhat smooth, are dry. If you ever
looked at the knuckle of a leg of mutton before it was cooked, you will
have noticed that you have there two bones slipping over each other
somewhat as they do at the elbow, and will remember that where the bones
meet they are wonderfully smooth, and very moist, so as to be quite
slippery. It is just the same in your own elbow; the end of the ulna and
the end of the humerus are beautifully smooth and quite moist, so that
they slip over each other as easily as possible. You know that your eye
is always moist. It is kept moist by tears, though you don’t speak of
tears until your eyes overflow with moisture; but in reality you are
always crying a little. Well, there are, so to speak, tears always being
shed inside the wrapping of ligaments around the elbow, and they keep
the two surfaces of the bones continually moist.
The ends of bones where they touch each other are also smooth, because
they are coated over with what is called gristle or cartilage. Bone is
very hard and very solid; there is not much water in it. Bones dry up
very little. Cartilage is not nearly so hard as bone; there is very much
more water in it. When it is quite fresh it is very smooth, but because
it has a good deal of water in it, it shrinks very much when it dries
up, and when dried is not nearly so smooth as when it is fresh. You can
see the dried-up cartilage on the ends of the bones in the skeleton--it
is somewhat smooth still, but you can form no idea of how smooth it is
in the living body by simply seeing it on the dried skeleton.
=At the elbow, then, we have the ends of two bones fitting into each
other, so that they will move in a certain direction; these ends are
smoothed with cartilage, kept moist with a fluid, and held in place by
ligaments. All this is a called a joint.=
=15.= There are a great many other joints in the body besides the
elbow-joint: there is the shoulder-joint, the knee-joint, the hip-joint,
and so on. These differ from the elbow-joint in the shape of the ends of
the bone, in the way the bones move on each other, and in several other
particulars, but we must not go into these differences now. They are all
like the elbow, since in each case one bone fits into another, the
surfaces are coated with cartilage, are kept moist with fluid (what the
grooms call joint-oil, though it is not an oil at all), and are held in
place by ligaments.
I dare say you will have noticed that though I have been speaking only
of the humerus and ulna at the elbow, the other bone of the
fore-arm--the radius--has something to do with the elbow too. I left it
out in order to simplify matters, but it is nevertheless quite true
that the end of the humerus moves over the end of the radius as well as
over the end of the ulna, and that the end of the radius is also coated
with cartilage and is included in the wrapping of the ligaments. I might
add that the radius also moves independently on the ulna, but I don’t
want to trouble you with this just now. What I wanted to show you was
that the elbow is a joint, a joint so constructed that it allows the
fore-arm to be bent on the upper arm.
=16. In order that the arm may be bent, some force must be used.= The ulna
or radius--for the two move together--must be pushed or pulled towards
the humerus, or the humerus must be pushed or pulled towards the radius
and ulna. How is this done in your own arm?
Take the bones of the arm; fix the top end of the humerus; tie it to
something so that it cannot move. Fasten a piece of string to either the
radius or ulna (it doesn’t matter which), rather near the elbow. Bore a
hole through the top of the humerus and pass the string through it. Your
string must be long enough to let the arm be quite straight without any
strain on the string. Now, taking hold of the string where it comes out
through the humerus, pull it. The fore-arm will be bent on the arm. Why?
=Because you have been working a lever of the third order.=
The radius and ulna form the lever; its fulcrum is the end of the
humerus in the elbow (Fig. 3, F); the weight to be moved is the weight
of the radius and ulna (with that of the bones of the hand if present),
and this may be represented by a weight applied at about the middle of
the fore-arm; the power is the pull you give the string, and that is
brought to bear on your lever at the point where the string is fastened
to the radius, _i.e._ nearer the fulcrum than the point where the weight
is applied; and you know that when you have the fulcrum at one end and
the power between the fulcrum and the weight, you have a lever of the
third order.
Now, in order to make the thing a little more like what takes place in
your own arm, instead of boring a hole through the humerus, let the
string glide in a groove which you will see at the top of the humerus,
and fasten the end of it to the shoulder-blade or anything you like
above the humerus, and let the string be just long enough to let the arm
be quite straightened out, but no longer, so that when the arm is
straight the string is just about tight, or at least not loose.
Now shorten the string by pinching it up into a loop. Whenever you do
this you will bend the fore-arm on the arm. Suppose you used a string
which you had not to pinch up, but which, when you pleased, you could
make to shorten itself. =Every time it shortened itself it would pull the
fore-arm up and would bend the arm--and every time it slackened again,
the arm would fall back into the straight position.=
In your arm there is not a string, but a body, placed very much as our
string is placed, and which has the power of shortening itself when
required. Every time it shortens itself it bends the arm, and when it
has done shortening and lengthens again, the arm falls back into its
straight position. This body which thus can shorten and lengthen itself
is called a muscle.
If you put one hand on the front of your other upper arm, about half-way
between your shoulder and elbow, and then bend that arm, you will feel
something rising up under your hand. This is the muscle, which bends the
arm, shortening, or, as we shall learn to call it, =contracting=.
In your own arm, as in the limb of the rabbit which you studied in your
last lesson, the flesh is arranged in masses or bundles of various sizes
and shapes, and each mass or bundle is called a muscle. There are
several muscles in the arm, but there is in particular a large one
occupying the front of the arm, called the =biceps=. It is a rounded mass
of red flesh, considerably longer than it is broad or thick, and
tapering away at either end. It is represented in Fig. 3.
You may remember that while examining the leg of the rabbit you noticed
that in many of the muscles, the soft flesh, which made up the greater
part of the muscle, at one or both ends of the muscle suddenly left off,
and changed into much firmer material which was white and glistening.
This firmer white part you were told was called the tendon of the
muscle. The rest of the muscle, generally called “the belly,” is made up
of what you are accustomed to call flesh, or lean meat, but which you
must now learn to speak of as =muscular substance=. Every muscle, in fact,
consists in the first place of a mass of muscular substance. =This
muscular substance is made up of an immense number of soft strings or
fibres, all running in one direction and done up into large and small
bundles.= At either end of the muscle these soft muscular fibres are
joined on to firmer but thinner fibres of connective or fibrous tissue.
And these thinner but firmer fibres make up the cord or band of tendon
with which the muscle finishes off at either end.
=It is by these tendons that the soft muscles are joined on to the hard
bones, or to some of the other firm textures of the body.= The tendons
are sometimes round and cord-like, sometimes flat and spread out.
Sometimes they are very long, sometimes very short, so as to be scarcely
visible. But always you have some amount of the firmer fibres of
connective tissue joining the soft muscular fibres on to the bones, and
generally the tendons are not only firmer but much thinner and more
slender than the belly of the muscle.