I have already told you some things about the muscles. There is no motion in the body that is not made by them. They move the bones, and they move other parts also, as the tongue, the corners of the mouth, the eyes, the eyelids, etc.
How the muscles act.
But you will want to know how they do this. Stretch a strip of India-rubber with your hands. Now let it go, and it will shorten itself. When a muscle pulls a bone, it shortens itself just as this strip of India-rubber does. But the cause of its shortening itself is different. The mind makes the muscle shorten. You think to bend your arm; and, as quick as thought, something goes by nerves to the muscle that can do this, and it shortens itself and bends the arm.
The muscles that bend and straighten the arm.
Here is a figure that shows the muscle that bends the arm, and also the muscle that straightens it out. All the other muscles of the arm are left out, so that you may see just how these operate. Look at the muscle markeda: you can see that when this shortensitself it must pull up the forearm, that is, that part of the arm which is below the elbow. The musclebhas a contrary effect. The end of this muscle is fastened to the point of the elbow, and when it shortens it pulls the forearm down and straightens the arm.
When a muscle shortens itself, it swells out and becomes hard. Straighten your arm, and then take hold of it with your other hand a little above the elbow; now bend up your arm as forcibly as you can, and you will feel the muscle on the front of the arm swell out and harden as you hold your hand upon it.
Color of muscles in different animals.
The muscles are the fleshy part of the body. The meat of animals is made up of muscles. They are not of the same color in all animals. In some they are quite red, while in others they are of a light color. Beef—the meat of the ox or the cow—is, you know, a deep red, and is very different from the meat of a fowl. The muscles of fishes are generally very light in color.
Muscles that move the fingers.
Your arm below the elbow is very fleshy. Most of the muscles that move the fingers, as well as those that move the hand, are there. Take hold of that part of the arm with your other hand while you work the fingers back and forth, and you will feel the muscles as they shorten themselves to pull the fingers. Here is a figure showing the muscles in this fleshy part of thearm. You see that they are quite large. The wrist is very slender. There are no muscles there; there are bright, shining, smooth cords there, that run from the muscles to the fingers. The muscles pull the fingers by these cords just as men pull any thing by ropes. You can see the play of these cords very plainly on the back of the hand of a thin person as the fingers are worked.
Muscles in the hand.
There are only some very small muscles in the hand, as those that spread the fingers out, and those that bring them together again. If you work your fingers in this way, you will see that the muscles, which do such light work, need not be large and strong. The muscles that do the hard work of the hand are up in the arm. They are very large. If they were not, you could not grasp things so tightly, and pull so hard as you sometimes do.
The round fullness of the arm.
Now see why it is that these large muscles are put so far away from where they do their work. If they were put in the hand, they would make it a large and clumsy thing. They are therefore put up in the arm, where there is room for them, and they have small, but very strong cords by which they pull the fingers. They give to the arm that round fullness that makes its shape so beautiful.
Drum-stick of the fowl.
You can see the same kind of arrangement in the drum-stick, as it is called, of the fowl. The large muscles that work the claws are up in the full, round part of the leg, and there are small, stout cords that extend from them down to the claws. Children often amuse themselves with pulling these cords in the drum-stick of a fowl, making the claws move just as they are moved by the muscles of the animal when he is alive.
Muscles of the toes.
It is with the muscles that move the toes as it is with those that move the fingers. They are put mostly up in the leg, and their slender tendons, by which they pull, go down over the ankle to the toes, just as in the arm the tendons go over the wrist to the fingers. If the muscles of the toes were all put in the foot, they would make it very clumsy, and at the same time the leg would be ugly from the want of that fullness which it now has.
Ligaments of the wrist and the ankle.
Both at the wrist and the ankle the tendons are bound down very tightly. If this were not so they would be always flying out of place, stretching out the skin before them in ridges. This would be the case especially with the tendons that go to the toes. Every time that the muscles pulled on them, they would start out very much at the bend of the ankle if they were not firmly held by the ligaments.
The muscles are of many shapes—round, flat, long, short, etc. They are shaped to suit the work which they are to do.
They vary much in size also. Some are very large, and some are exceedingly small. How large are the muscles of the arm that wield the hammer and the axe! But how small are the muscles that work the musical cords in your throat when you speak or sing! These little muscles make all the different notes of the voice by pulling on these cords, and in doing this many of their motions are exceedingly slight.
Muscles in the ear.
You remember that in the chapter on the hearing I told you about the little bones in the ear. These have some very little muscles which move them. The bones and the muscles,aandb, are represented in the following figure. The muscles, you see,have tendons or cords to pull by, in the same way that the muscles in the arm have. Both the bones and the muscles are larger in this figure than they are in the body. As the bones are the smallest ones that we have, so it is with the muscles. Very small machinery is this part of the hearing machinery.
Large and small muscles in birds.
The birds that go swiftly on their wings have very large muscles to work them. This gives them the full, round breast which you see that they have. But the muscles that work the musical cords in their little throats, as they sing so sweetly, are so small that it is difficult to find them.
Questions.—By what is all motion in the body made? What do the muscles move? Explain how the muscles move things. Tell about the two muscles of the arm in the figure. What is said about the swelling out of the muscles as they shorten? What is the meat of animals? What is said about the color of muscles in different animals? What is said of the muscles in the arm below the elbow? What is said of the wrist? What of the muscles in the hand? Why are most of the muscles that move the fingers put up in the arm? What is said about the drum-stick of a fowl? What is said about the muscles of the toes? What about the ligaments of the tendons at the wrist and ankle? What is said of the shapes of muscles? What of their sizes? What are the smallest muscles in the body? What is said about the muscles of birds used in flying and those used in singing?
Questions.—By what is all motion in the body made? What do the muscles move? Explain how the muscles move things. Tell about the two muscles of the arm in the figure. What is said about the swelling out of the muscles as they shorten? What is the meat of animals? What is said about the color of muscles in different animals? What is said of the muscles in the arm below the elbow? What is said of the wrist? What of the muscles in the hand? Why are most of the muscles that move the fingers put up in the arm? What is said about the drum-stick of a fowl? What is said about the muscles of the toes? What about the ligaments of the tendons at the wrist and ankle? What is said of the shapes of muscles? What of their sizes? What are the smallest muscles in the body? What is said about the muscles of birds used in flying and those used in singing?