40 ANATOMY OF THE RAT 



the muscles attached to them, while adjacent articula- 

 tions 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 sacri- 

 ficed for speed. 



The extensor muscles straighten a member, like the arm 

 or finger. Flexor muscles bend it. Batators 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. 



