arm. Some little way above the elbow-joint it ends in a small round
strong tendon which slips over the front of the elbow and is fastened
to, _i.e._ grows on to, the radius at some little distance below the
joint (Fig. 3, P). The upper part of the muscular belly ends a little
below the shoulder, not in one tendon but in two[1] tendons (Fig. 3,
_a_), which gliding over the end of the humerus are fastened to the
shoulder-blade (or =scapula= as it is called), into which the humerus fits
with a joint.
We have then in the biceps a thick fleshy muscular belly placed in the
front of the arm and fastened by tendons, at one end to the
shoulder-blade, and at the other to the fore-arm. What would happen if
when the arm is straight and the shoulder-blade fixed, the biceps were
suddenly to grow very much shorter than it was? Evidently the same
thing that happened when you pinched up and shortened the string which,
if you look back you will see, we supposed to be placed very much as the
biceps with its tendons is placed. =The radius and ulna would be pulled
up, the fore-arm would be bent on the arm.=
Now tendons have no power of shortening themselves, but muscular
substance does possess this remarkable power of suddenly shortening
itself. Under certain circumstances each soft muscular fibre of which
the muscle is made will suddenly become shorter, and thus the whole
muscle becomes shorter, and so pulls its two tendinous ends closer
together, and if one end be fastened to something fixed, and the other
to something moveable, the moveable thing will be moved.
This way that a muscle or a muscular fibre has of suddenly shortening
itself is called a =muscular contraction=. =All muscles, all muscular
fibres, have the power of contracting.= Now a mass of substance like the
biceps might grow shorter in two ways. It might squeeze itself together
and become smaller altogether, it might squeeze itself as you would
squeeze a sponge into a smaller bulk. Or it might change its form and
not its bulk, becoming thicker as it became shorter, just as you might
by pressing the two ends together squeeze a long thin roll of soft wax
into a short thick one. It might get shorter in either of these two
ways, but it does actually do so in the latter way; it gets thicker at
the same time that it gets shorter, and gets nearly as much thicker as
it gets shorter. And that is why, when you put your hand on the arm
which is being bent, you feel something rise up. =You feel the biceps
getting thicker as it is getting shorter in order to bend the arm.=
The shortening does not last for ever. Sooner or later the muscle
lengthens again, getting thinner once more, and so returns to its former
state. The lengthened condition of the muscle is the natural condition,
the condition of rest. The shortening or contraction is an effort which
can only be continued for a certain time. The contraction bends the arm,
and as long as the muscle remains shortened the arm keeps bent; but as
the muscle lengthens, the weight of the hand and fore-arm, if there is
nothing to prevent, straightens the arm out again.
=It is in the muscle alone, in the belly made up of muscular fibres, that
the shortening takes place.= The tendons do not shorten at all. On the
contrary, if anything they lengthen a little, but only a very little,
when the muscle pulls upon them. Their purpose is to convey to the bone
the pull of the muscle. They are not necessary, only convenient. It
would be possible but awkward to do without them. Suppose the fleshy
fibres of the biceps reached from the shoulder-blade to the fore-arm:
you could bend your arm as before, but it would be very tiresome to have
the muscle swelling up in the inside of the elbow, or on the top of the
shoulder; in either place it would be very much in the way. By keeping
the fleshy, the real contracting muscle, in the arm, and carrying the
thin tendons to the arm and to the shoulder, you are enabled to do the
work much more easily and conveniently.
Well, then, we have got thus far in understanding how the arm is bent.
The biceps muscle contracts and shortens, tries to bring its two
tendinous ends together. The upper tendons, being fastened to the fixed
shoulder-blade, cannot move; but the lower tendon is fixed to the
radius; the radius, with the ulna to which it is fastened, readily moves
up and down on the elbow-joint--the shape of bones in the joint and all
the arrangements of the joint, as we have seen, readily permitting this.
When the muscle, then, pulls on its lower tendon, its pulls on the
radius at the point where the tendon is fastened on to the bone. The
radius thus pulled on forms with the ulna a lever of the third order,
working on the end of the humerus as a fulcrum; and thus as the tendon
is pulled the fore-arm is bent.
=17.= But now comes the question. What makes the muscle shorten or
contract? You willed to move your arm, and moved it, as we have seen, by
making the biceps contract; but =how did your will make the biceps
contract=?
If you could examine your arm as you did the leg of the rabbit, you
would find running into your biceps muscle, one or more of those soft
white threads or cords, which you have already learnt to recognize as
=nerves=.
These nerves seem to grow into and be lost in the biceps muscle. We need
not follow them any further in that direction, but if we were to trace
them in the other direction, up the arm, we should find that they soon
meet with other similar nerves, and that the several nerves joining
together form stouter and thicker nerve-cords. These again join others,
and so we should proceed until we came to quite stoutish white
nerve-trunks as they are called, which we should find passed at last
between the vertebræ, somewhere in the neck, into the inside of the
vertebral canal, where they became mixed up with the mass of nervous
material we have already spoken of as the spinal cord.
What have these nerves to do with the bending of the arm? Why simply
this. Suppose you were able without much trouble to cut across the
delicate nerves going to your biceps, and did so: what would happen? You
would find that you had lost all power of bending your arm; however much
you willed it, there would be no swelling rise up in your arm. Your
biceps would remain perfectly quiet, and would not shorten at all, would
not contract in obedience to your will.
What does this show? It proves that when you will to bend your arm,
something passes along the nerves going to the biceps muscle, which
something causes that muscle to contract? The nerve, then, is a bridge
between your will and the muscle--so that when the bridge is broken or
cut away, the will cannot get to the muscle.
If anywhere between the muscle and the spinal cord you cut the nerve
which goes, or branches from which go, to the muscle, you destroy the
communication between the will and the muscle.
The spinal cord, as we have seen, is a mass of nervous substance
continuous with the brain; from the spinal cord nearly all the nerves of
the body are given off; those nerves whose branches go to the biceps
muscle in the arm leave the spinal cord somewhere in the neck.
If you had the misfortune to have your spinal cord cut across or
injured in your neck, you might still live, but you would be paralysed.
You might will to bend the arm, but you could not do it. You would know
you were willing, you would feel you were making an effort, but the
effort would be unavailing. The spinal cord is part of the bridge
between the will and the muscle.
When you bend your arm, then, this is what takes place. =By the exercise
of your will a something is started in your brain. That something--we
will not stop now to ask what that something is--passes from your brain
to the spinal cord, leaves the spinal cord and travels along certain
nerves, picking its way among the intricate bundles of delicate nervous
threads which run from the upper part of the spinal cord to the arm
until it reaches the biceps muscle. The muscle, directly that
“something” comes to it along its nerves, contracts, shortens, and grows
thick; it rises up in the arm; its lower tendon pulls at the radius; the
radius with the ulna moves on the fulcrum of the humerus at the
elbow-joint, and the arm is bent.=
You wish to leave off bending the arm. Your will ceases to act. The
something to which your will had given rise dies away in the brain, dies
away in the spinal cord, dies away in the nerves, even in the finest
twigs. The muscle, no longer excited by that something, ceases to
contract, ceases to swell up, ceases to pull at the radius, and the
fore-arm by its own weight falls into its former straightness,
stretching, as it falls, the muscle to its natural length.
=18.= So far I hope you have followed me, but we are still very far from
being at the bottom of the matter. Why does the muscle contract when
that something reaches it through the nerves? We must content ourselves
by saying that it is the property of the muscle to do so. Does the
muscle always possess this property? No, not always.
Suppose you were to tie a cord very tightly round the top of your arm,
close to the shoulder. What would happen? If you tied it tight enough (I
don’t ask you to do it, for you might hurt yourself) the arm would
become pale, and very soon would begin to grow cold. It would get
numbed, and would gradually seem to grow very heavy and clumsy; your
feeling in it would be blunted, and after a while be altogether lost.
When you tried to bend your arm you would find great difficulty in doing
so. Though you tried ever so much, you could not easily make the biceps
contract, and at last you would not be able to do so at all. You would
discover that you had lost all power of bending your arm. And then if
you undid the cord you would find that after some very uncomfortable
sensations, little by little the power would come back to you; the arm
would grow warm again, the heaviness and clumsiness would pass away, the
feeling in it would return, you would be able to bend it, and at last
all would be as it was before.
What did you do when you tied the cord tight? The chief thing you did
was to press on the blood-vessels in the arm and so stop the blood from
moving in them. If instead of tying the cord round the whole arm you had
tied a finer thread round the blood-vessels only, you would have
brought about very nearly the same effect. We saw in the last lesson how
all parts of the body are supplied with blood-vessels, with veins, and
arteries. In the arm there is a very large artery, branches from which
go all over the arm. Some of these branches go to the biceps muscle.
What would happen if you tied these branches only, tying them so tight
as to stop all the blood in them, but not interfering with the
blood-vessels in the rest of your arm? The arm as a whole would grow
neither pale nor cold, it would not become clumsy or heavy, you would
not lose your feeling in it, but nevertheless if you tried to bend your
arm you would find you could not do it. You could not make the biceps
contract, though all the rest of the arm might seem to be quite right.
What does this teach us? =It teaches us that the power which a muscle has
of contracting when called upon to do so, may be lost and regained, and
that it is lost when the blood is prevented from getting to it.= When a
cord is tied round the whole arm, the power of the whole arm is lost.
This loss of power is the beginning of death, and indeed if the cord
were not unloosed the arm would quite die--would mortify, as it is said.
When only those blood-vessels which go to the biceps are tied, the
biceps alone begins to die, all the rest of the arm remaining alive, and
the first sign of death in the biceps is the loss of the power to
contract when called upon to do so.
In order that you may bend your arm, then, you must not only have a
biceps muscle with its nerves, its tendons, and all its arrangements of
bones and joints, but the muscle must be supplied with blood.
=19.= We can now go a step further and ask the question, =What is there in
the blood that thus gives to the muscle the power of contracting, that
in other words keeps the muscle alive?= The answer is very easily found.
What is the name commonly given to this power of a muscle to contract?
We generally call it strength. Lay your arm straight out on the table,
put a heavy weight in your hand, and try to bend your arm. If you could
do it, one would say you were strong; if you could not, one would say
you were weak--all the stronger or weaker, the heavier or lighter the
weight. In the one case your biceps had great power of contracting; in
the other, little power. Try and find out the heaviest weight you can
raise in this way by bending your arm, some morning, not too long after
breakfast, when you are fresh and in good condition. Go without any
dinner, and in the afternoon or evening, when you are tired and hungry,
try to raise the same weight in the same way. You will not be able to do
it. Your biceps will have lost some of its power of contracting, will be
weaker than it was in the morning. What makes it weak? The want of food.
But how can the food affect the muscle? You do not place the food in the
muscle; you put it into your mouth, and from thence it goes into your
stomach and into the rest of your alimentary canal, and there seems to
disappear. How does the food get at the muscle? By means of the blood.
The food becomes blood. =The things which you eat as food become changed
into other things which form part of the blood. Those things going to
the muscle give it strength and enable it to contract.= And that is why
food makes you strong.
=20.= But you are always wanting food day by day, from time to time. Why
is that? Because the muscle in getting strength out of the food changes
it, uses it up, and so is always wanting fresh blood and new food. We
have seen in Art. 1 that food is fuel. We have also seen that muscle
(and other parts of the body do the same) is always burning, burning
without flame but with heat, burning slowly but burning all the same,
and doing the more work the more it burns. The fuel it burns is not dry
wood or coal, but wet, watery blood, a special kind of fuel prepared for
its private use, in the workshop of the stomach or elsewhere, out of the
food eaten by the mouth. This it is always using up; of this it must
always have a proper supply, if it is to go on working. Hence there must
always be fresh blood preparing; hence there must from time to time be
fresh supplies of food out of which to manufacture fresh blood.
To understand then fully what happens when you bend your arm, we have to
learn not only what we have learnt about the bones and the joint and the
muscle and the nerves, about the machinery and the engine, we have to
study also how the food is changed into blood, how the blood is brought
to the muscle, what it is in the blood on which the muscle lives, what
it is which the muscle burns, and how the things which result from the
burning, the ashes and the smoke or carbonic acid and the rest of them,
are carried away from the muscle and out of the body.
Meanwhile let me remind you that for the sake of being simple I have
been all this while speaking of one muscle only, the biceps in the arm.
But there are a multitude of muscles in the body besides the biceps, as
there are many bones besides those of the arm, and many joints besides
the elbow. But what I have said of the one is in a general way true of
all the rest. The muscles have various forms, they pull upon the bones
in various ways, they work on levers of various kinds. The joints differ
much in the way in which they work. All manner of movements are produced
by muscles pulling sometimes with and sometimes against each other. But
you will find when you come to examine them that all the movements of
which your body is capable depend at bottom on this--=that certain
muscular fibres, in obedience to a something reaching them through their
nerves, contract, shorten, and grow thick, and so pull their one end
towards the other, and that to do this they must be continually supplied
with pure blood=.
Moreover, what I have said of the relations of muscle to blood is also
true of all other parts of the body. Just as the muscle cannot work
without a due supply of blood, so also the brain and the spinal cord and
the nerves have even a more pressing need of pure blood. The weakness
and faintness which we feel from want of food is quite as much a
weakness of the brain and of the nerves as of the muscles,--perhaps
rather more so. And other parts of the body of which we shall have to
speak later on need blood too.
The whole history of our daily life is shortly this. The food we eat
becomes blood, the blood is carried all over the body, round and round
in the torrent of the circulation; as it sweeps past them, or rather
through them, the muscle, the brain, the nerve, the skin pick out new
food for their work and give back the things they have used or no longer
want. As they all have different works, some use up what others have
thrown away. There are, besides, scavengers and cleaners to pick up
things no longer wanted anywhere and to throw them out of the body. Thus
the blood is kept pure as well as fresh. Through the blood thus ever
brought to them, each part does its work: the muscle contracts, the
brain feels and wills, the nerves carry the feeling and the willing, and
the other organs of the body do their work too, and thus the whole body
is kept alive and well.
THE NATURE OF BLOOD. § IV.
=21. What, then, is this blood which does so much?=
Did you ever look through a good microscope at the thin transparent web
of a frog’s foot, and watch the red blood coursing along its narrow
channels? If not, go and look at it at once; you will never understand
any physiology till you have done so. There you will see a network of
delicate passages far finer than any of your own hairs, and through
those passages a tumbling crowd of tiny oval yellow globules hurrying
and jostling along. Some of the passages are wider than others, and
through some of the wider ones you will see a thick stream of globules
rushing onwards towards the smaller channels, and spreading out among
them. The globules which you see are floating in a fluid so clear that
you cannot see it. Some of the smaller channels are so narrow that only
one globule or =corpuscle=, as we may call it, can pass through at a time,
and very frequently you may see them passing in single file. Watching
them as they glide along these narrow paths, you will note that at last
they tumble again into wider passages, somewhat like those from which
they came, except that the stream runs away from instead of towards the
narrower channels; and in the stream the corpuscle you are watching
shoots out of sight. The finest passages are called =capillaries=; they
are guarded by delicate walls which you can hardly see; they seem to you
passages only, and how fine and small they are will come home to you
when you recollect that all you are looking at is going on in the depths
of a skin which is so thin that perhaps you would be inclined to say it
has no thickness at all.
The larger channels which are bringing the blood down to the capillaries
are the ends of vessels like those which in the rabbit you learnt to
call arteries, and the other larger channels through which the blood is
rushing away from the capillaries are the beginnings of veins.
When you have watched this frog’s foot for some little time, turn away
and reflect that in almost every part of your own body, in every square
inch, in almost every square line, something very similar might be seen
could the microscope be brought to bear upon it, only the corpuscles are
smaller and round, the capillaries narrower and for the most part more
thick-set, and the race a swifter one. In the muscle of which we were
speaking in the last lesson, each of the soft long fibres of which the
muscle is composed is wrapped round with a close network of these tiny
capillaries, through which, as long as life lasts, for ever rushes a
swift stream of blood, reddened by countless numbers of tiny corpuscles.
In every part of your flesh, in your brain and spinal cord, in your
skin, your bones, your lungs, in all organs and in nearly every part of
your body, there is the same hurrying rush through narrow tubes of red
corpuscles and of the clear fluid in which these swim.
If you prick your finger it bleeds. Almost any part of your body would
bleed were you to prick it. So thick-set are the little blood-vessels,
that wherever you thrust a needle, be it as fine a needle as you please,
you will be sure to pierce and tear some little blood channel, either
artery or capillary or vein, and out will come the ruddy drop.
=22. What is blood?= It is a fluid; it runs about like water: yet it is
thicker than water, thicker for two reasons. In the first place, water,
that is pure water, is all one substance. If you were to look at it with
ever so powerful a microscope, you would see nothing in it. It is
exceedingly transparent--you can see very well through ever such a
thickness of clean water. But if you were to try and look through even a
very thin sheet of blood spread out between two glass plates, you would
find that you could see very little; =blood is very opaque=. If again you
examine a drop of your blood with a microscope, what do you see? =A
number of little=
[Illustration: FIG. 4.--_Red and White Corpuscles of the Blood
magnified._
_A._ Moderately magnified. The red corpuscles are seen lying in
rows like rolls of coins; at _a_ and _a_ are seen two white
corpuscles.
_B._ Red corpuscles much more highly magnified, seen in face; _C._
ditto, seen in profile; _D._ ditto, in rows, rather more highly
magnified; _E._ a red corpuscle swollen into a sphere by imbibition
of water.
_F._ A white corpuscle magnified same as _B._; _G._ ditto, throwing
out some blunt processes; _K._ ditto, treated with acetic acid, and
showing nucleus, magnified same as _D._
_H._ Red corpuscles puckered or crenate all over.
_I._ Ditto, at the edge only.
]
=round bodies, the blood discs or blood corpuscles= (Fig. 4, _A_). If you
look carefully you will notice that most of them are round, as _B_; but
every now and then you see something like _C_. That is one of the round
ones seen sideways; for they are not round or spherical like a ball, but
circular and dimpled in the middle, something like certain kinds of
biscuit. When you see one by itself it looks a little yellow in colour,
that is all; but when you see them in a lump, the lump is clearly red.
Remember how small they are: three thousand of them put flat in a line,
edge to edge, like a row of draughts, would just about stretch across
one inch. All the redness there is in blood belongs to them. When you
see one of them, you see so little of the redness that it seems yellow.
If you were to put a drop of blood into a tumbler of water, the water
would not be stained red, but only just turned of a yellowish tint, so
little redness would be given to it by the drop of blood. In the same
way a very very thin slice of currant jelly would look yellowish, not
red.
These red corpuscles are not hard solid things, but delicate and soft,
very tender, very easily broken to pieces, more like the tiniest lumps
of red jelly than anything else, and yet made so as to bear all the
squeezing which they get as they are driven round and round the body.
Besides these red corpuscles, you may see if you look attentively =other
little bodies, just a little bigger than the red corpuscles, not
coloured at all, and not circular and flat, but quite round like a ball=
(Fig. 4, _a_, _F_, _G_). That is to say, these are very often quite
round, only they have a curious trick of changing their form. Imagine
you were looking at a suet dumpling so small that about two thousand
five hundred of them could be placed side by side in the length of one
inch--and suppose the round dumpling while you were looking at it
gradually changed into the shape of a three-cornered tart, and then
into a rounded square, and then into the shape of a pear, and then into
a thing that had no shape at all, and then back again into a round ball,
and kept doing this apparently all of its own accord while you were
looking at it--wouldn’t you think it very curious? Well, one of these
little bodies in the blood of which we are speaking, and which are
called white corpuscles, may be seen, when a drop of blood is watched
under the microscope, to go on in this way, continually changing its
shape. But of these =white corpuscles= of the blood, and of their
wonderful movements, you will learn more as you go on in your
physiological studies.