The peculiar property of the cells which compose the muscles is their elasticity, no other cells of the body having this property. At Fig. 49 is a diagram representing a microscopic muscular fibre, in which the cells are relaxed, as in the natural state of rest. But when the muscle contracts, each of its numberless cells in all its small fibres becomes widened, making each fibre of the muscle shorter and thicker, as at Fig. 50. This explains the cause of the swelling out of muscles when they act. Every motion in every part of the body has a special muscle to produce it, and many have other muscles to restore the part moved to its natural state. The muscles that move or bend any part are called _flexors_, and those that restore the natural position are called _extensors_. [Illustration: Fig. 51.] Fig. 51 represents the muscles of the arm after the skin and flesh are removed. They are all in smooth, silky cases, laid over each other, and separated both by the smooth membranes that encase them and by layers of fat, so as to move easily without interfering with each other. They are fastened to the bones by strong tendons and cartilages; and around the wrist, in the drawing, is shown a band of cartilage to confine them in place. The muscle marked 8 is the extensor that straightens the fingers after they have been closed by a flexor on the other side of the arm. In like manner, each motion of the arm and fingers has one muscle to produce it and another to restore to the natural position. The muscles are dependent on the brain and nerves for power to move. It has been shown that the gray matter of the brain and spinal marrow furnishes the stimulating power that moves the muscles, and causes sensations of touch on the skin, and the other sensations of the several senses. The white part of the brain and spinal marrow consists solely of conducting tubes to transmit this influence. Each of the minute fibrils of the muscles has a small conducting nerve connecting it with the brain or spinal marrow, and in this respect each muscular fibril is separate from every other. When, therefore, the mind wills to move a flexor muscle of the arm, the gray matter sends out the stimulus through the nerves to the cells of each individual fibre of that muscle, and they contract. When this is done, the nerve of sensation reports it to the brain and mind. If the mind desires to return the arm to its former position, then follows the willing, and consequent stimulus sent through the nerves to the corresponding muscle; its cells contract, and the limb is restored.