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.