The Chautauquan, Vol. 03, May 1883

by Chautauqua Literary and Scientific Circle; Chautauqua Institution

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.

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.