xv SPECIAL USES OF MUSCLES 209 



at the other, and the load somewhere between the two. 

 When the load is at one end, the fulcrum at the other, 

 and the power between the two, the lever is of the third 

 class. 



All these levers are illustrated in the human body in 

 the arrangement of the bones and muscles. The bones 

 correspond to the crowbar ; they are the levers ; the 

 contracting muscles attached to them furnish the power ; 

 and the body as a whole, any of its parts, or any object 

 moved, serves as the load. 



FIG. 112. The bones of the arm, showing the biceps muscle and its tendons 

 at a and P. The action of the biceps illustrates a lever of the third class, P, 

 power, F, fulcrum, and W, weight. (Huxley.) 



When a weight is lifted by bending the arm at the 

 elbow, the radius and ulna constitute the lever, the 

 object on the hand the load, the elbow the fulcrum, 

 and the muscles of the upper arm (biceps) the power. 

 The muscles are attached to the bones a short distance 

 below the elbow, so that the power is applied between 

 the load and the fulcrum. Here we have a lever of the 

 third class. In raising the body on tiptoe, the tarsal, 



