Contraction is not, however, the natural state of a muscle. In time it is tired, and begins to relax. Even the heart, the hardest-working muscle, has short periods of rest between its beats. Muscles are highly elastic as well as contractile. By this property muscle yields to a stretching force, and returns to its original length if the stretching has not been excessive. Illustration: Fig. 33.—Principal Muscles of the Body. (Anterior view.) 69. The Object of Contraction. The object of contraction is obvious. Like rubber bands, if one end of a muscle be fixed and the other attached to some object which is free to move, the contraction of the muscle will bring the movable body nearer to the fixed point. A weight fastened to the free end of a muscle may be lifted when the muscle contracts. Thus by their contraction muscles are able to do their work. They even contract more vigorously when resistance is opposed to them than when it is not. With increased weight there is an increased amount of work to be done. The greater resistance calls forth a greater action of the muscle. This is true up to a certain point, but when the limit has been passed, the muscle quickly fails to respond. Again, muscles work best with a certain degree of rapidity provided the irritations do not follow each other too rapidly. If, however, the contractions are too rapid, the muscles become exhausted and fatigue results. When the feeling of fatigue passes away with rest, the muscle recovers its power. While we are resting, the blood is pouring in fresh supplies of building material. Experiment 19. _To show how muscles relax and contract_. Lay your left forearm on a table; grasp with the right hand the mass of flesh on the front of the upper arm. Now gradually raise the forearm, keeping the elbow on the table. Note that the muscle thickens as the hand rises. This illustrates the contraction of the biceps, and is popularly called “trying your muscle” Reverse the act. Keep the elbow in position, bring the forearm slowly to the table, and the biceps appears to become softer and smaller,—it relaxes. Experiment 20. Repeat the same experiment with other muscles. With the right hand grasp firmly the extended left forearm. Extend and flex the fingers vigorously. Note the effect on the muscles and tendons of the forearm. Grasp with the right hand the calf of the extended right leg, and vigorously flex the leg, bringing it near to the body. Note the contractions and relaxations of the muscles. 70. Arrangement of Muscles. Muscles are not connected directly with bones. The mass of flesh tapers off towards the ends, where the fibers pass into white, glistening cords known as tendons. The place at which a muscle is attached to a bone, generally by means of a tendon, is called its origin; the end connected with the movable bone is its insertion.