MUSCLE-NEBVE PHYSIOLOGY. 



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. 334, may be cited as an example of 

 this variety of lever ; while, as an instance in which the bones of the human 



Fig. 334. 



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. 334) . 



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 an elastic band, as in fig. 335. In the human body the act of open- 

 ing the mouth by depressing the lower jaw is an example of the same kind 



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Fig. 335. 



the tension of the muscles which close the jaw representing the weight 

 (fig. 335). 



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

 raising a pole, as in fig. 336, 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. 336). The act of 

 biting is another example. 



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

 tending the foot. 3d 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 second case. 3d kind 



