Our last article gave us a complete description of the human
organization. In the present number we will inquire how we move this
complex system of bones, nerves, flesh, and tissues.
We will take a particular motion and see if we can understand that. For
instance, you bend your arm. You know that when your arm is lying on
the table you can bend the forearm on the upper arm (or part above the
elbow) until your fingers touch your shoulder. How is this done?
Look at the arm in a skeleton; you will see that the upper part is
composed of one large bone, called the humerus, the fore part of two
bones, the radius and ulna. 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, is scooped out in such a way that one
fits loosely into the other. 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 as you move the
ulna you will notice that its end and the end of the humerus slide over
each other. But they will slide only one way—up and down. If you try
to slide them from side to side they get locked. At the elbows, then,
we have two bones fitting into each other, so that they will move in a
certain direction; their ends are smoothed with cartilage, kept moist
with a fluid and held in place by ligaments, and this is all called a
joint.
In order that this arm may be bent some force must be used. The radius
and ulna (the two move together) must be pushed or pulled toward the
humerus, or the humerus toward the radius and ulna. How is this done
in your arm? Imagine that a piece of string were fastened to either
the radius or ulna, near the top: let the string be carried through
a little groove, which there is at the upper end of the humerus, and
fastened to the shoulder-blade. Let the string be just long enough to
allow the arm to be straightened out, so that when the arm is straight
the string will be just about tight. Now draw your string up into a
loop and you will bend the fore-arm on the humerus. If this string
could be so made that every time you willed it so, it would shorten
itself, it would pull the ulna up and would bend the arm; every time it
slackened the arm would fall back into a straight position.
In the living body there is not a string, but a band of tissues
placed very much as our string is placed, and which has the power
of shortening itself when required. Every time it shortens the arm
is bent, every time it lengthens again the arm falls back into its
straight position. This body, which can thus lengthen and shorten
itself, is called a muscle. If you put your hand on the front of your
upper arm, half way between your shoulder and elbow, and then bend your
arm, you will feel something rising up under your hand: this is the
muscle shortening or, as we shall now call it, contracting.
But what makes the muscle contract? You willed to move your arm,
and moved it by making the muscle contract; but how did your will
accomplish this? If you should examine, you would find running through
the muscle soft white threads, or cords, which you have already learned
to recognize as nerves. These nerves seem to grow into and be lost in
the muscle. If you trace them in the other direction you would find
that they soon join with other similar nerves, and the several cords
joining together form stouter nerve-cords. These again join others, and
so we should proceed until we came to quite stout white nerve-trunks,
as they are called, which pass between the vertebræ of the neck into
the vertebral column, where they mix in the mass of the spinal cord.
What have these nerves to do with the bending of the arm? Simply this:
If you should cut through the delicate nerves entering the muscle, what
would happen? You would find that you had lost all power of bending
your arm. However much you willed it, the muscle would not contract.
What does this show? It proves that when you will to move, something
passes along the nerves to the muscle, which something causes the
muscle to contract. The nerve, then, is a bridge between your will and
the muscle—so that when the bridge is broken the will can not get to
the muscle.
[Footnote E: Abridged from Foster’s Science Primer of Physiology.]
If, anywhere between the muscle and the spinal cord, you cut the nerve,
you destroy communication between the will and the muscle. If you
injure the spinal cord in your neck you might live, but you would be
paralyzed; you might will to bend your arm, but could not.
In short, the whole process is this: by the exercise of your will a
something is started in your brain. That something passes from the
brain to the spinal cord, leaves the spinal cord and travels along
certain nerves, picking its way along the bundles of nervous threads
which run from the upper part of the spinal cord, until it reaches the
muscle. The muscle immediately contracts and grows thick. The 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.
Why does the muscle contract when that something reaches it? We must
be content to say that it is the property of the muscle. But it does
not always possess this property. Suppose you were to tie a cord very
tightly around the top of the arm close to the shoulder. If you tied it
tight enough the arm would become pale, and would very soon begin to
grow cold. It would get numb, and would seem to be heavy and clumsy.
Your feeling in it would be blunted, and soon altogether lost. You
would find great difficulty in bending it, and soon it would lose all
power. If you untied the cord, little by little the cold and clumsiness
would pass away, the power and warmth would come back, and you would
be able to bend it as you did before. What did the cord do to the arm?
The chief thing was to press on the blood-vessels, and so stop the
blood from moving in them. We have seen that all parts of the body are
supplied with blood-vessels, veins and arteries. In the arm there is a
very large artery, branches from which go into all parts of the muscle.
If, instead of tying the cord about the arm, these branches alone were
tied, the arm, as a whole, would not grow cold or limp, but if you
tried to bend it, you would find it impossible. All this teaches that
the power which a muscle has of contracting may be lost and regained as
the blood is stopped in its circulation, or allowed to circulate freely.
Our next question is, What is there in the blood that thus gives to
the muscle the power of contracting, or that keeps the muscle alive?
The answer is easy. What is the name given to this power of a muscle
to contract? We call it _strength_. Straighten out your arm upon the
table and put a heavy weight in your hand; then bend your arm. Find
the heaviest weight that you can raise in this way, and try it some
morning after your breakfast, when you are in good condition. Go
without dinner, and in the evening when tired and hungry, try to raise
the same weight in the same way. You will not be able to do so. Your
muscle is weaker than it was in the morning, and you say that the want
of food makes you weak; and that is so, because the food becomes blood.
The things which we eat are changed into other things which form part
of the blood, and this blood going to the muscle gives it strength.
What is true of the relations of the blood to the muscles is true of
all other parts of the body. The brain and nerves and spinal cord have
a more pressing need of pure blood. The faintness which we feel from
want of food is quite as much weakness of the brain and of the nerves
as of the muscles, perhaps even more so.
The whole history of our daily life is, briefly told, 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
through them, the brain, nerves, muscles and 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 work, some pick up what others have thrown
away. There are also scavengers and cleansers to take up things which
are wanted no longer, and to throw them out of the body.
Thus the blood is kept pure as well as fresh. Thus it is through the
blood brought to them, that each part does its work.
But what is blood? It is a fluid. It runs about like water, but while
water is transparent, blood is opaque. Under a microscope you will see
a number of little round bodies—the blood-discs, or blood-corpuscles.
All the redness there is in blood belongs to these. These red
corpuscles are not hard, solid things, but delicate and soft, yet made
to bear all the squeezing they get as they drive around the body.
Besides these red corpuscles, are other little bodies, just a little
larger than the red, not colored at all, and quite round. These are
all that one can see in blood, but it has a strange property which we
will study. Whenever blood is shed from a living body, within a short
time it becomes solid. This change is called coagulation. If a dish
be filled with blood, and you were to take a bunch of twigs and keep
slowly stirring, you would naturally think it would soon begin to
coagulate; but it does not, and if you keep on stirring you find that
this never takes place. Take out your bundle of twigs, and you will
find it coated all over with a thick, fleshy mass of soft substance.
If you rinse this with water you will soon have left nothing but a
quantity of soft, stringy material matted among the twigs. This stringy
material is, in reality, made up of fine, delicate, elastic threads,
and is called fibrin; by stirring you have taken it out. If the blood
had been left in the dish for a few hours, or a day, you would find a
firm mould of coagulated blood floating in a colorless liquid. This
jelly would continue shrinking, and the fluid would remain; this fluid
is called serum, and it is the blood out of which the corpuscles have
been strained by the coagulation. All these various things, fibrin,
serum, corpuscles, etc., make up the blood. This blood must move, and
how does it move? You have had the different organs which assist in its
circulation described, but let us illustrate.
All over the body there are, though you can not see them, networks of
capillaries. All the arteries end in capillaries, and in them begin all
veins. Supposing a little blood-corpuscle be squeezed in the narrow
pathway of a capillary in the muscle of the arm. Let it start in motion
backward. Going along the narrow capillary it would hardly have room to
move. It will pass on the right and left other capillary channels, as
small as the one in which it moves; advancing, it will soon find the
passage widening and the walls growing thicker. This continues until
the corpuscle is almost lost in the great artery of the arm; thence it
will pass but few openings, and these will be large, until it passes
into the aorta, or great artery, and then into the heart. Suppose the
corpuscle retrace its journey and go ahead instead of backward. It will
go through passages similar to the other, and it would learn these
passages to be veins. At last the corpuscle would float into the vena
cava, thence to the right auricle, from there to the right ventricle,
by the pulmonary artery to the lungs; there it, with its attendant
white corpuscles, serum and other substances, would be purified, then
sent by pulmonary vein to the left auricle and ventricle, and then
pumped over the body again. Some one may ask, What is the force that
drives or pumps the blood? Suppose you had a long, thin muscle fastened
at one end to something firm, and a weight attached to it. Every time
the muscle contracted it would pull on the weight and draw it up. But
instead of hanging a weight to the muscle, wrap it around a bladder
of water. If the muscle contract now, evidently the water will be
squeezed through any opening in the bladder. This is just what takes
place in the heart. Each cavity there, each auricle and ventricle is,
so to speak, a thin bag with a number of muscles wrapped about it. In
an ordinary muscle of the body the fibers are placed regularly side by
side, but in the heart, the bundles are interlaced in a very wonderful
fashion, so that it is difficult to make out the grain. They are so
arranged that the muscular fibers may squeeze all parts of each bag
at the same time. But here is the most wonderful fact of all. These
muscles of the auricles and ventricles are always at work contracting
and relaxing of their own accord as long as the heart is alive. The
muscle of your arm contracts only at your will. But the heart is never
quiet. Awake or asleep, whatever you are doing or not doing, it keeps
steadily on.
Each time the heart contracts what happens? Let us begin with the right
ventricle full of blood. It contracts; the pressure comes on all sides,
and were it not for the flaps that close and shut the way, some of the
blood would be forced back into the right auricle. As it is, there is
but one way,—through the pulmonary artery. This is already full of
blood, but, because of its wonderful elasticity, it stretches so that
it holds the extra fluid. The valve at its mouth closes, and the blood
is safely shut in, but the artery so stretched contracts and forces
the blood along into the veins and capillaries of the lungs, in turn
stretching them so that they must force ahead the blood which they
already contain. This blood is forced into the pulmonary vein, thence
to the left auricle; the auricle forces it into the ventricle, and the
latter pumps it into the aorta; the aorta overflows as the pulmonary
artery did, and the blood goes through every capillary of the body into
the great venæ cavæ, which forces it into the right auricle; thence
to the ventricle where we started. In this passage every fragment of
the body has been bathed in blood. This stream rushing through the
capillaries contains the material from which bone, muscle, and brain
are made, and carries away all the waste material which must be thrown
off.
The actual work of making bone or muscle is performed outside of the
blood in the tissues. You say, the capillaries are closed, and how
can the blood get to the tissues? It will be necessary here to speak
of a certain property of membranes in order that you understand how
the tissues are built up by the blood apparently closed within the
veins. If a solution of sugar or salt be placed in a bladder with the
neck tied tightly, and this placed in a basin of pure water, you will
find that the water in the basin will soon taste of sugar or salt and
after a time will taste as strong as the water in the bladder. If you
substitute solid particles, or things that will not dissolve, you will
find no change. This property which membranes, such as a bladder, have,
is called osmosis. It is by osmosis chiefly that the raw, nourishing
material in the blood gets into the flesh lying about the capillaries.
It is by osmosis chiefly that food gets out of the stomach into the
blood. It is by this property that the worn-out materials are drained
away from the blood, and so cast out of the body. By osmosis the blood
nourishes and purifies the flesh. By osmosis the blood is itself
nourished and kept pure.
We must now understand how we live on this food we eat. Food passing
into the alimentary canal is there digested. The nourishing food-stuffs
are dissolved out of the innutritious and pass into the blood. The
blood thus kept supplied with combustible materials, draws oxygen from
the lungs, and thus carries to every part of the body stuff to burn and
oxygen to burn it with. Everywhere this oxidation is going on, changing
the arterial blood to venous.
From most places where there is oxidation, the venous blood comes away
hotter than the arterial, and all the hot venous blood mingling, keeps
the whole body warm. Much heat is given up, however, to whatever is
touching the skin, and much is used in turning liquid perspiration into
vapor. Thus, as long as we are in health, we never get hotter than a
certain degree.
Everywhere this oxidation is going on. Little by little, every part of
the body is continually burning away and continually being made anew by
the blood. Though it is the same blood, it makes very different things:
in the nerves it makes nerve; in the muscle, muscle. It gives different
qualities to different parts: out of one gland it makes saliva, another
gastric juice; out of it the bone gets strength and the muscle power to
contract. But the far greater part of the power of the blood is spent
in heat, or goes to keep us warm.
One thing more we have to note before we answer 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 our brain by our will, passes through the spinal cord,
through certain nerves, until it reaches the muscle, and this something
we may call a nervous impulse. But what starts this?
All the nerves do not end in the muscles, but many in the skin. These
nerves can not be used to carry nervous impulses from the brain to
the skin. By no effort of yours can you make the skin contract. For
what purpose then are these nerves? If you prick your finger, you feel
the touch, or say that you have sensation in your finger. If you were
to cut the nerve leading to the finger you would lose this power of
feeling. These nerves, then, ending in the fingers have a different use
from those ending in the muscles. The latter carry an impulse from the
brain to the muscles, and are called motor nerves. The former carry
impulses from the skin to the brain, and are called sensory nerves, or
those which bring about sensations. Motor nerves are of but one kind,
but there are several kinds of sensory nerves, each kind having a
special work to do. The several works which these nerves do are called
the senses. By means of these sensory nerves we receive impressions
from the external world, sensations of heat, cold, roughness, good
and bad odors, taste, sound, and the color and form of things. Thus
impressions of the external world are made upon the brain, and it is
these impressions that stimulate the brain to action. The brain worked
on by them, through ways that we know not of, governs the muscles,
sends commands by the motor nerves, and rules the body as a conscious,
intelligent will.