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.]