476 



MUSCLE-NERVE PHYSIOLOGY 



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 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 



FIG. 338. 



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 Extending 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, ad kind When the body is raised on tiptoe. Here 



F 3? 



FIG. 339- 



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

 and the calf 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 by the closeness of the power to the fulcrum a great range of move- 

 ment can be obtained by means of a comparatively slight shortening of the muscular fibers. 



In the act of walking, almost every voluntary muscle in the body is brought 

 into play, either directly for purposes of progression, or indirectly for 

 the proper balancing of the head and trunk. The muscles of the arms are 



