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HAND-BOOK OF PHYSIOLOGY. 



letters Only of the power, iveiglit, and fulcrum be used, the arrangement 

 will stand thus: r.F.W. A poker, as ordinarily used, or the bar in 

 Pig. 288, 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 ischium (Fig. 288). 



FIG. 288. 



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. 289. 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. 289). 



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

 act of raising a pole, as in Fig. 290, is an example. In the human body 



FIG. 289. 



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. 

 290). The act of biting is another example. 



At the ankle we have examples of all three kinds of lever. 1st kind 

 Extending the foot. 3rd kind Flexing the foot. In both these cases 

 the foot represents the weight: the ankle joint the fulcrum, the power 

 being the calf muscles in the first case, and the tibialis anticus in the 



