58 Life and Health 



85. Examples of Levers in the Body. In mechanics three 

 classes of levers are described, according to the relative posi- 

 tion of the power, the fulcrum, and the weight, or the resistance. 

 The movements of the bones can be referred to one or another 

 of these three classes. 



The head is supported on the atlas by a lever of the first 

 class. The joint between the atlas and the skull is the fulcrum, 

 the weight of the front of the head is the resistance. The 

 power is behind, where the muscles from the neck are attached 

 to the back of the skull. The object of this arrangement is to 

 keep the head steady and balanced on the spinal column, 



w FPF WPFP w 



FIG. 34. Diagram illustrating Three Kinds of Lever Action. 



F, fulcrum; P, power; W, weight. In the left-hand diagram the head is tilted 

 back by the muscles of the neck. In the middle figure the toes rest on the 

 ground, and the body is raised by the leg muscles. In the right-hand diagram 

 the forearm is bent up by the biceps muscle. 



and to move it backward and forward. Range and rapidity 

 are here gained at the expense of power. 



Standing on tiptoe is done by means of a lever of the second 

 class. The toes in contact with the ground are the fulcrum, 

 the power is the action of the muscles of the calf, and between 

 these is the weight of the body transmitted down the bones 

 of the leg to the ankle. Power is here gained at the expense 

 of range of movement. 



Levers of the third class are common in the body. In 

 bending the forearm on the arm, familiarly known as "trying 

 your muscle," the power is supplied by the biceps muscle 

 attached to the radius, the fulcrum is the elbow joint at one 

 end of the lever, and the resistance is the weight of the fore- 

 arm and hand at the other end. 



