Page:Laboratory Manual of the Anatomy of the Rat (Hunt 1924).djvu/54

40 the muscles attached to them, while adjacent articulations of the bones are fulcra. There are three classes of these levers in the body: (1) those in which the fulcrum lies between the muscular attachment and the resistance; (2) the fulcrum at one end, muscle at the other, the weight between; (3) or the fulcrum may be at one end, with the muscle attached to the middle, and the weight at the other end of the bone. Examples of case 2 are rare in man. The nearer the muscle attachment is to the fulcrum, the greater must be the force of contraction to move the lever. Mechanical advantage is secured in machines by having a small force act through a great distance at the end of a long lever, moving a heavy object a short distance. In the musculo-skeletal system this arrangement is generally reversed, a powerful muscular contraction acting at the end of a shorter lever to cause rapid movement of a small weight at the end of a longer lever. Efficiency is sacrificed for speed.

The extensor muscles straighten a member, like the arm or finger. Flexor muscles bend it. Rotators turn it on its axis. Abductor muscles bend the part away from the median line of the body or limb ; adductor muscles cause the reverse movement. As suggested by the names, a levator muscle lifts a structure, a depressor depresses it. A sphincter muscle surrounds an opening, which is closed when the muscle contracts. The function of dilators is to expand such an orifice.

To prepare a rat for the study of the muscles, lay it on its back and make a midventral longitudinal cut through the skin extending from the external genitalia to the tip of the jaw. Cut no deeper than the skin, to avoid injuring the underlying muscles. In the same way slit the skin on the median surface of the front and hind legs, extending the cuts to the midventral incision. Remove the skin.