5. THE TENDONS.--Tendons, or sinews, are the extremities of muscles, and are compactly fastened upon bone. They are very strong, and of a silvery whiteness. They may be felt just beneath the skin, in certain parts of the body, when the muscles are being used, as at the bend of the elbow or knee. The largest tendon of the body is that which is inserted into the heel, called the tendon of Achilles, after the hero of the Grecian poet, the fable relating that it was at this point that he received his death-wound, no other part of his body being vulnerable. The muscles which extend into the leg unite to form a single and very powerful tendon, and enclose a small bone called the knee-pan, which, acting like a pulley, greatly increases their power, and at the same time protects the front of the knee-joint (Fig. 10). [Illustration: FIG. 10.--VIEW OF KNEE-JOINT. A, Thigh bone: B, Knee-pan: C, D, Leg bones.] [Sidenote: 6. Contraction of the muscles? Bending of the arm or finger? Other agencies? Automatic movements? In cold-blooded animals?] 6. MUSCULAR CONTRACTION.--The muscles, when acted upon by the appropriate stimulus, contract, or so change {28} their shape, that their extremities are brought nearer together. The bending of the arm, or of a finger, is effected in this manner, by the will; but the will is not the only means of producing this effect. Electricity, a sharp blow over a muscle, and other stimuli, also cause it. Contraction does not always cease with life. In man, after death from cholera, automatic movements of hands and feet have been observed, lasting not less than an hour. In certain cold-blooded animals, as the turtle, contraction has been known to take place for several days after the head has been cut off. [Sidenote: 7. Contractility? Give the illustration. What was supposed? What is the case?] 7. The property which, in muscle, enables these movements to take place is called _contractility_. If we grasp a muscle while in exercise (for example, the large muscle in the front of the arm), we notice the alternate swelling and decrease of the muscle, as we move the forearm to and fro. It was at one time supposed that the muscle actually increased in volume during contraction. This, however, is not the case; for the muscle, while gaining in thickness, loses in length in the same proportion; and thus, the volume remains the same in action and at rest. [Sidenote: 8. What further in relation to contraction? Weariness of a muscle? Beating of the heart? Standing and walking?]