THE MUSCULAR SYSTEM. 



423 



At the ankle we have examples of all three kinds of lever. 1st 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. 2d kind When the body is raised on tip-toe. Here the 

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

 the weight, and the calf muscles the power. 



In the human bod} T , 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 ful- 

 crum a great range of movement can be obtained by means of a com- 

 paratively slight shortening of the muscular fibres. 



The greater number of the more important muscular actions of the 

 human body those, namely, which are arranged harmoniously so as to 



FIG. 298. 



subserve some definite purpose or other in the animal economy are de- 

 scribed in various parts of this work, in the sections which treat of the 

 physiology of the processes by which these muscular actions are resisted 

 or carried out. There are, however, one or two very important and 

 somewhat complicated muscular acts which may be described in this 

 place. 



Walking. In the act of walking, almost every voluntary muscle in 

 the body is brought into play, either directly for purposes of progres- 

 sion, or indirectly for the proper balancing of the head and trunk. The 

 muscles of the arms are least concerned; but even these are for the most 

 part instinctively in action also to some extent. ^ 



Among the chief muscles engaged directly in the act of walking are 

 those of the calf, which, by pulling up the heel, pull up also the astrag- 

 alus, and with it, of course, the whole body, the weight of which is 

 transmitted through the tibia to this bone (Fig. 298). When starting 

 to walk, say with the left leg, this raising of the body is not left entirely 

 to the muscles of the left calf, but the trunk is thrown forward in such 

 a way, that it would fall prostrate were it not that the right foot is 



