422 



HANDBOOK OF PHYSIOLOGY. 



position of the power, the loeight to be removed, and the axis of motion 

 or fulcrum. In a lever of the first kind the poiuer is at one extremity of 

 the lever, the weight at the other, and the fulcrum between the two. 

 If the initial letters only of the power, weight, and fulcrum be used, the 

 arrangement will stand thus: P.F.W. A poker as ordinarily used, or 

 the bar in Fig. 296, 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 



FIG. 296. 



from the stooping posture by means of the hamstring muscles attached 

 to the tuberosity of the ischium (Fig. 296). 



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 wheel- 

 barrow, or in stretching an elastic band, as in Fig. 297. In the human 



El. AST! 



FIG. 297. 



body the act of opening the mouth by depressing the lower jaw is an ex- 

 ample of the same kind the tension of the muscles which close the jaw 

 representing the weight (Fig. 297). 



In a lever of the third kind the arrangement is F.P.W., and the act 

 of raising a pole, as in Fig. 298, is an example. In the human body 

 there are numerous examples of the employment of this kind of leverage. 

 The act of bending the fore-arm may be mentioned as an instance (Fig.. 

 298). The act of biting is another example. 



