MOTION
Our account of the body has now reached the point where we can take up
in detail the special activities of the different kinds of cells. The
first to be considered is motion, both because it is the familiar sign
of life, as pointed out in the first chapter, and because it has so much
to do with everything that enters into life. There are probably no
animals that live out their entire lives without making any active
motions, although some parasites, like the tapeworm, are stationary most
of the time. There are a number of different ways in which movements are
brought about. The very simplest animals, which consist of nothing but a
bit of protoplasm, move by causing the protoplasm to flow bodily in one
direction or another, a projection of part of the protoplasm being
balanced by withdrawal of an equal part on the opposite side, and the
whole mass progresses in the direction of the first projection. Next
beyond this simplest means comes motion by tiny threads of protoplasm
which project beyond the surface of the cell and whip back and forth.
The stroke of these threads or _cilia_, as they are called, is stronger
in one direction than in the other, so the effect of their beating is to
propel the cell of which they are part in one direction through the
water; or if they are on a surface which is stationary they set up a
current in the water itself. This latter is the means by which oysters
and similar animals which are anchored to the rocks get their food
supplies. In some one-celled animals there are only one or two large
cilia at one end; these beat back and forth, propelling the animal much
as a fish swims.
The commonest, as well as the most effective, means of making motions is
by cells specially developed for that purpose. These are called muscle
cells, and every highly organized animal depends on them for most if not
all of the motions which take place in its body. In muscle cells the
functional metabolism takes the form of forcible changes in shape of the
cells by which bodily motions are brought about. A muscle cell might be
described as a mechanical device for transforming the chemical energy of
burning fuel into the energy of motion. We have something comparable in
the automobile cylinder, where the energy obtained from the explosive
burning of the air-gas mixture drives the piston and so propels the car.
Of course the two devices are not even remotely alike in the actual way
in which they operate; their resemblance is purely the general one of
converting one type of energy (chemical) into another type (motion).
There are three kinds of muscle cells in our bodies. The simplest are
those that are found in the wall of the stomach and intestines and other
internal organs that are capable of movements; the next kind is found
only in the heart; the third, and most complex, makes up the bulk of our
muscular tissue; it is the muscle that moves the bones. The first kind,
because it shows no particular markings when examined through the
microscope, is usually called smooth muscle; the second kind is known as
heart muscle; the third kind, because it moves the skeleton, is named
skeletal muscle. We shall devote most of our attention to this third
kind of muscle, because it is a much more efficient machine than the
others, and also because it has to do with our general bodily movements
instead of with motions of internal organs.
A single skeletal muscle cell is an exceedingly slender fiber, much
smaller than the finest thread; it may also be very short, not more than
a twenty-fifth of an inch long, or it may be as much as an inch long. A
muscle is made up of many of these fibers grouped side by side in
bundles, and also, if the muscle is long, placed end to end. The fibers
are held in place and fastened together by connective tissue. Lean meat
consists of thousands of these muscle cells with their connective tissue
fastenings. In coarse meat there is relatively more connective tissue
and less actual muscle tissue than in the finer grades. In every muscle
the connective tissue is loose enough to allow body fluid to penetrate
among the muscle cells. Blood vessels are also distributed through the
mass of the muscles between and among the cells; thus their nutrition is
provided for.
Although not all muscle cells are exactly equal in power, on the whole
the _force_ that muscle can show is the force of one cell multiplied by
the number of cells that can join in the pull. A strong muscle must have
many cells side by side; in other words it must be thick. Also, the
_distance_ through which muscles can make movements depends on their
length, so a muscle that has to pull for a considerable way must be
long, and since single muscle cells are short there will have to be a
good many cells end to end to make the whole muscle long enough for its
task. The actual make-up and arrangement of muscles in the body depends
in part, therefore, on the thickness and length needed for the
particular work to be done, and in part on the architecture of the part
of the body where the muscle is located. For example, the strongest
muscle in the body is that by which one rises on the toes. This muscle
operates by pulling upward at the back of the heel. If it were located
right at the ankle, where it would have to be if attached directly to
the place where its force is exerted, the resulting clumsiness can
easily be imagined. By shifting it up to the middle of the lower leg
room is found for the large mass of muscle needed for the work. The
connection with the heel is made by means of a long and very strong
tendon, known as the tendon of Achilles, because that was the part
Achilles’ mother failed to immerse when she was dipping the infant in
the river Styx to make him invulnerable. Other equally good examples are
the muscles for operating the fingers. If placed in the hands, the
latter would be too bulky and clumsy for any kind of efficient use. By
placing them up in the forearms out of the way, and connecting them with
the fingers by long tendons, delicacy is secured for the hands.
The muscles of the arms and legs are arranged in groups about the
joints, and these groups always include opposing sets. Thus if the joint
is a simple hinge, as at the elbow, where the only motion possible is
back and forth, there will be one muscle to bend the joint and another
opposing muscle to straighten it out again. The first is known as a
_flexor_; the second as an _extensor_. In the arm the biceps, on the
upper surface, is the flexor and the triceps, on the under side, the
extensor. Joints that permit of motion in several directions have
correspondingly more opposing sets of muscles acting upon them. The same
scheme applies to the trunk, but since in the trunk instead of a few
very movable joints we have the whole row of slightly movable vertebræ,
the grouping of muscles is more complicated. Not all the skeletal
muscles work about joints. The tongue, the muscles of the lips and about
the eyes, those along the front of the abdomen, and some others are
attached to bones only at one end or not at all, and do their work by
pulling upon one another.
[Illustration: THE BICEPS MUSCLE AND THE ARM BONES
(From Martin’s “Human Body”)]
In earlier paragraphs we have seen that the movements made by muscles
represent their functional metabolism, and also that the actions of
whole muscles are merely the sum of the actions of the individual cells.
Our present task is to see how muscles act; in other words to examine
their functional metabolism. One feature that must be in mind from the
very beginning is that the functional metabolism of muscle cells is
under control; they do not go off at random, but only when started. This
is more or less true of the functional metabolism of all the cells in
highly organized animals. The agency that starts them off is named a
stimulus. To picture how stimuli act we shall have to think for a moment
of the state of affairs in cells at rest. As we have tried to make
clear, cells at rest are not stagnating; a more or less active basic
metabolism goes on within them all the time. This metabolism is of such
a sort that it does not disturb the balance existing within the cell.
The various chemical processes go on, using up material and producing
wastes, but without arousing the additional chemical processes of
functional metabolism. Meanwhile the substances that are required for
this latter are present in the cell, so that when the disturbance that
we call a stimulus comes along there is an increase in the total amount
of metabolism, the extra chemical processes being those which perform
the special function of the cell. In the case of muscle cells the
stimulus ordinarily reaches them by way of the nervous system, although
electric shocks, sharp blows, some irritating chemicals, and perhaps one
or two other kinds of disturbance can act as stimuli. The effect of the
stimulus is to start certain chemical processes; these in turn bring
about the forcible shortening which is the thing that happens in active
muscle. In skeletal muscle the shortening may be very rapid; the muscle
can contract and relax again more quickly than the eye can follow. This
is true at the temperature of our bodies. In cold-blooded animals, like
fish or frogs, muscles become sluggish when they are cold. We see here
one of the advantages we enjoy in having bodies that stay at the same
temperature the year around; if our bodies cooled off in cold weather as
do those of frogs, we should have to do as they do, become inactive
whenever the weather becomes cold. As each muscle cell shortens it pulls
upon the connective tissue that surrounds it; this communicates with the
connective tissue of other cells, and all the connective tissue within
the mass of the muscle fastens to the very stout sheets or cords of the
same at the ends which are called tendons, by which the muscles are
attached to the bones. Thus, although the pull of any single cell is so
feeble as to be scarcely measurable, when hundreds or thousands of them
pull all at once the effect may be very powerful.
We are familiar with the very wide range of effort that our muscles can
show. They may contract with utmost delicacy, as when we hold a humming
bird’s egg in our fingers, or they may pull with a force, in our largest
muscles, of several hundred pounds. Of course this possibility of
variation is of great advantage in our use of our muscles. It depends
upon the very large number of individual fibers of which even our
smallest muscles are made up. Whenever any single fiber contracts, it
pulls to its full extent; if only a few become active, the pull of the
whole muscle will be slight; as more come into action, more force will
be exerted; the muscle will show its utmost power when all the fibers
are contracting at once. We are conscious of greater mental effort when
we make a powerful muscular contraction. This can be explained as due to
the greater nervous discharge required to excite all the muscle fibers
at once.
One feature of muscular action calls for an additional word. This is the
temporary loss of power, resulting from too long-continued use, which
is called _fatigue_. We know that a well-constructed machine can operate
day in and day out without having to stop to rest; why cannot our
muscles do the same? Evidently the necessity of resting cuts down the
possibilities of life more than any other one thing; our real life is
only two-thirds as long as it counts up in years because we have to
spend one-third of the time in sleep. Of course muscular fatigue is not
the only kind; there is nervous fatigue, as well, about which something
will be said later. The activity of our muscles is based on functional
metabolism; it follows, therefore, that fatigue is also due to
metabolism. We can think of two ways in which metabolism might cause
fatigue; the first of these is by using up the materials which furnish
energy; clearly no cells can go on working after they have exhausted
their supplies of fuel. The second results from the fact that metabolism
produces waste products. It is a familiar fact of chemistry that when
the substances formed in chemical processes are not removed they
interfere with the processes themselves. In active muscles very rapid
metabolism is going on and large quantities of waste substance are being
formed; these have to be discharged from the cells into the surrounding
fluid, and removed from there in turn by the blood. We can easily
imagine that this might not take place as fast as necessary to keep the
cells from becoming more or less clogged; in fact this clogging is
exactly what happens, so that muscles begin to show fatigue some time
before their supplies of fuel material are used up.
One familiar fact of muscular fatigue is that soreness, which indicates
that fatigue has really
[Illustration: Photo, Metropolitan Museum
MUSCULAR DEVELOPMENT OF AN ATHLETE--THE DISCUS THROWER OF MYRON]
[Illustration: A MODERN “VICTORY”--MISS SABIE AT PRACTICE]
been present in large amount, occurs much more often when we use our
muscles in ways to which we are not accustomed than when they are
exercised according to habit. It is the experience of every one who does
manual labor that when he gets a new job, one that calls for different
use of the muscles than he has been in the habit of, his muscles are
very sore until he is “broken in.” After that, although he does as much
or even more work than at first, he no longer becomes sore. This is
explained as being due to two things. First, whenever we make an
unaccustomed movement we overstimulate our muscles; that is, we call
more fibers into action than are necessary to do the job; as the motion
becomes familiar we cut down the action to that which just meets the
demand. Thus there is a great deal more metabolism than necessary when
unfamiliar motions are being made. Then, secondly, there is a spot in
every muscle cell, just at the point where the nerve makes its
connection with the muscle, that is more easily fatigued than any other
part of the muscle cell. This spot, by becoming fatigued first, tends to
cause metabolism to stop in time to prevent the rest of the cell from
being seriously fatigued. Only when we are so much interested in what we
are doing that we pay no attention to the fatigue of this safety spot,
or when necessity keeps us at work after we would quit if we had our own
way, do we push the metabolism so far that muscular soreness results.
Other types of fatigue, including feelings of exhaustion, are due to
effects on the nervous system, and will be described when we have that
system before us.
Before we leave the subject of the skeletal muscles it will be
interesting to say a word about the different kinds of motions that
they bring about. We have already seen that they work by pulling at the
joints, and we have no intention of enlarging on that topic. What we
want to do here is to group the bodily motions into a few classes,
regardless of what joints are actually moved. First, and most important,
comes _locomotion_; by that we mean any motions that move the body from
one place to another. Under that head we have walking, running,
swimming, jumping; in birds, flying. Next in order comes _grasping_;
this includes all motions by which we take hold of anything. We can
realize the importance of this group of movements when we think that our
fore limbs are specifically grasping organs, while in the great majority
of animals they are organs of locomotion along with the hind limbs.
Originally grasping had to do, undoubtedly, with the taking of food and
not much else. In civilized man we have in addition the use of all kinds
of tools from the coarsest to the finest. In most four-legged animals
the chief organ of grasping is the mouth. We still use our mouths to
some extent as grasping organs, and could probably learn to make even
more use of them in that direction if forced to it. _Chewing_ and
_swallowing_ make up a group of movements concerned primarily with the
handling of food after it has been grasped. Not much need be said about
them. Of small extent but great importance are the motions connected
with _sense perception_; these include chiefly the motions of the body,
neck, and eyes in _vision_; we are constantly turning to look at
something; in such animals as horses movements of the ears help greatly
in _hearing_; and both man and animals make sniffing motions to increase
the keenness of _smelling_. There is a group of motions devoted to
_voice production_ (including breathing). In man the vocal cords,
tongue, and muscles of the cheeks are the chief muscles that have to do
with the voice, not including the muscles of breathing, which, of
course, are essential. The interesting things about the vocal cords are
the excessive fineness of their operation, enabling expert singers to
produce tones that vary by only a few vibrations a second, and the
amazing exactitude of the control that the nerves have over them, so
that good singers can set them at the tension needed for producing a
particular tone with absolute certainty. The tongue is not a single
muscle, but a mass of several muscles working one upon the other. It
plays a part both in voice production and in the chewing of food. As an
organ of voice production it helps by changing the shape of the mouth
cavity. Speech depends very largely on this, since not the tension of
the vocal cords but the shape of the mouth and throat determines the
making of letters and syllables.
In addition to these familiar uses of the muscles there is a use which
is just as important but about which we are apt to think less. This is
their use in connection with posture, the taking and holding of
particular bodily positions. Posture is unlike other muscular activities
in several things. In the first place there is a steady, but rather
feeble, tension which can be held without marked fatigue for long
periods; all other forms of muscular contraction become severely
fatiguing rather quickly if held steadily. In the second place the
nervous control of posture seems to be different in some respects from
our ordinary control of our muscles. Finally there is some doubt as to
whether the contractions of the muscles themselves are the same.
Measurements of the functional metabolism of posture show that it is
much less than would be expected if the muscular action were of the
ordinary type. This, of course, explains why posture is less fatiguing
than other forms of activity.
The other two kinds of muscle, heart muscle and smooth muscle, must have
a word of description. Heart muscle contracts quickly and powerfully, as
does skeletal. It differs from skeletal in not depending on nervous
stimulation to make it contract; the heart can be cut clean out of the
body and will go on beating for a short time; in cold-blooded animals,
like frogs or turtles, for a long time. This could not be true if the
heart muscle had to be aroused to activity by nerves. Besides being
automatic, heart muscle shows the peculiarity that whenever it contracts
all the fibers join. We do not have a varying strength of pull shown by
heart muscle as we do in skeletal. As we shall see, it would be a
serious disadvantage rather than an advantage if heart muscle were to be
like skeletal in this respect.
Smooth muscle has the duty of operating the internal organs. For this
function no great strength is required; the motions do not have to be
powerful. Nor is rapid motion important. Smooth muscle does not have to
be so highly developed, then, as is skeletal. It is sluggish and rather
feeble in its actions. There are, however, two points of superiority
about smooth muscle, which fit in well with its special task. The first
of these is its freedom from fatigue. There are in the body numerous
smooth muscle masses that are in contraction practically all the time.
This would be impossible if fatigue were to develop. These masses make
up what are called the _sphincters_, rings of muscle surrounding
openings like that from the esophagus to the stomach or from the stomach
to the small intestine. It is the duty of these sphincters to hold the
openings closed all the time except occasionally when they open for just
an instant to let material through. The second point about smooth muscle
which fits it for its work is that it is capable of stretching out
greatly or contracting sharply without much difference in the force with
which it is pulling. For example, at the beginning of a meal the walls
of the stomach are drawn up, so that the food that is swallowed enters a
small space. With the progress of the meal the stomach enlarges, so that
at the end it has a much greater bulk than at the beginning. But the
actual pressure of the stomach upon its contents is about the same as at
the beginning. If the stomach were an ordinary elastic bag this could
not happen; the walls would have to stretch as the stomach filled, and
the stretching would mean greater pressure. Since the stomach walls are
of smooth muscle they adjust themselves to the progress of the meal. It
is important to note that there is a limit to this possibility of
adjustment. If one is so greedy as to keep on stuffing after the stomach
has reached its full size, stretching does occur, and if this is
repeated it may lead to a diseased condition known as “dilated stomach,”
which will cause much digestive trouble.