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MUSCLE-NERVE PHYSIOLOGY 



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 or of the triceps which extends the 

 forearm by action at the elbow, figure 337. 



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 figure 338. In the human body the act of 

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



FIG. 338. 



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

 figure 338. 



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

 raising a pole, as in figure 339, is an example. In the human body there are 

 numerous examples of the employment of this kind of leverage. The act of 

 bending the forearm may be mentioned as an instance, figure 339. The act 

 of biting is another example. 



At the ankle we have examples of all three kinds of lever, ist kind Ex- 

 tending the foot. 3d kind Flexing the foot. In both these cases the foot 



FIG. 339. 



represents the weight, the ankle joint the fulcrum, the power being the gas- 

 trocnemius muscles in the first case and the tibialis anticus in the second 

 case, and kind When the body is raised on tiptoe. Here the tip of the 

 toe is the fulcrum, the weight of the body acting at the ankle joint the 

 weight, and the gastrocnemius muscles the power. 



In the human body, levers are most frequently used at a disadvantage as 

 regards power, the latter being sacrificed for the sake of a greater range of 

 motion. Thus in the diagrams of the first and third kinds it is evident that the 

 power is so close to the fulcrum that great force must be exercised in order to 

 produce motion. It is also evident, however, from the same diagrams, that 



