VARIETIES OF LEVERS. 



597 



poker, as ordinarily used, or the bar in fig. 156, may be 

 cited as an example of this variety of lever, ; while, as an 

 instance in which the bones of the human skeleton are 

 used as a lever of the same kind, may be mentioned the 

 act of raising the body from the stooping posture by 

 means of the hamstring muscles attached to the tuberosity 

 of the isehium (fig. 156). 



Fig. 156, 



In a lever of the second kind, the arrangement is thus : 

 P.W.F. ; and this leverage is employed in the act of 

 raising the handles of a wheelbarrow, or in stretching 



Fig. 157- 

 P -w- P P v w p 



an elastic band as in fig. 157. In the human body the act 

 of opening the mouth by depressing the lower jaw, is an 

 example of the same kind, the tension of the muscles 

 which close the jaw representing the weight (fig. 157). 



