82 THE HUMAN BODY. 



long as W F then half a pound at P- would balance 

 against a pound at W, and just over half a pound laid on 

 the end P would lift a pound on the end W, but W would 

 only go up half as far as P went down. On the other 

 hand, if the weight-arm were longer than the power-arm 

 there would be a loss in force, but a gain in the distance 

 through which the weight was moved. 



Examples of levers of the first order are not numer- 

 ous in the human body. One is found in nodding 

 movements of the head, the fulcrum being where the 

 occipital bone articulates with the atlas (Fig. 20). When 

 the chin is raised the power is applied to the skull behind 

 the fulcrum by muscles passing from the spinal column 



F 



W 



Fro. 37. A lover of the second order. F, f nfcrnm ; P, power; W, weight. 

 The arrows indicate the direction in which the forces act. 



to the back of the head ; the resistance to be overcome is 

 the excess, in weight of the part of the head in front of 

 the fulcrum over that behind it, and is not great, as the 

 head is nearly balanced on the top of the spine. To let 

 the chin drop does not necessitate any muscular effort. 



Levers of the second order. In this form of lever 

 (Fig. 37), the weight or resistance acts between the ful- 

 crum and the power. The power-arm PFis accordingly 



What is lost when power is gained? 



Are there many levers of the first order in the body? Give an 

 ftxample of one, describing the action. 

 Describe a lever of the second order. 



