252 MOTION AND COORDINATION 



body, or things to be lifted, serve as weights. For these 

 levers to increase the motion of the muscles, it is necessary 

 that the muscles be attached to the bones near the joints, 

 and that the parts to be moved be located at some distance 

 from the joints. In other words the (muscle) power-arm 

 must be shorter than the (body) weight-arm. 1 



Examining Fig. 116, it is seen that the distances 

 moved by the power and weight vary as their respective 



distances from the fulcrum. 

 /C. That is to say, if the weight 



/, --.._ First class lever 



\L -..__ F is twice as far from the ful- 



~~y p i crum as the power, it will 

 move through twice the dis- 



Third class lever 



tance, and if three times as 



UP "---.... far, through three times the 



distance. Thus the muscles, 

 FIG. 116. Motion producing by acting through short dis- 

 levers. Diagrams show relative dis- ^^ , ^ ^^ arms Q 

 tances moved by the power and x 



weight in levers having the power levers), are able to move 



nearer the fulcrum than is the weight, portions of the body (located 

 f. Fulcrum, p, P' . Power. W, W. on tne long arms) through 



long distances. Can all 

 three classes of levers be used in this way in the body? 



Classes of Levers found in the Body. Practically all of 

 the levers of the body belong either to the first class or 

 the third class. In both of these the muscle power can 

 be applied to the short arm of the lever, thereby moving 

 the body weight through a longer distance than the muscle 

 contracts (Fig. 116). In the levers of the second class, 

 however, the weight occupies this position, being situated 

 between the power and fulcrum (Fig. 117). The weight, 



1 The distance from the fulcrum to the power is called the power-arm and the 

 distance from the tulcrum to the weight is called the weight-arm (Fig. 115). 



