290 MUSOIiES 



us to get the maximum strength of the muscle. (See 

 Ex. LVI.) 



All classes of levers are illustrated in the muscle attach- 

 ments to the bones in our bodies. (See Fig. 124.) Levers 

 of the first class are least numerous. The action when the 

 toes are pressed downward, and the head lifted are ex- 

 amples of this type. Those of the second class are com- 

 paratively few also. The action of the foot when the body 

 is raised on the toes is a case in point. Here the Achilles' 

 tendon, which connects the calf muscle with the heel, exerts 

 the pull; the ankle supports the weight of the body, and 

 the toes are the pivot. Levers of the third class are most 

 abundant. The lifting of the forearm, as already described, 

 'the movement of the jaws in eating, the bending of the 

 toes and fingers, and the bending of the knee, are all ex- 

 amples of this class. It is to be noted that while this form 

 of lever requires relatively greater muscle pull to the 

 amount of weight raised, it is of advantage in the body 

 not only in the range of motion it permits, but also because 

 it allows bulky muscle bellies to be placed where they will 

 least interfere with freedom of action. 



Standing, walking, and running. — It is a law of physics 

 that a rigid body will remain in a given position of rest as 

 long as a line drawn from its center of gravity toward the 

 center of the earth falls inside of the supporting surface. 

 If such a body is tipped it -will return to this position of 

 rest, provided it is not tipped so far that the line falls out- 

 side of this basal support. It is this second fact which 

 makes it possible to tip a broad-based body farther than a 

 narrow-based one without danger of upsetting it. Now, 

 in the body, the center of gravity lies in the pelvic region 

 near where the sacrum connects with the last lumbar ver- 



