524 



HANDBOOK OF PHYSIOLOGY. 



When the body is raised on tiptoe. 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 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 



Fig. 338. 



by the closeness of the power to the fulcrum a great range of movement can 

 be obtained by means of a comparatively 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 

 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 best 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 progression, 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 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 astragalus, and with it, 

 of course, the whole body, the weight of which is transmitted through the 

 tibia to this bone (fig. 337), 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 brought forward and planted on the ground to sup- 

 port it. Thus the muscles of the left calf are assisted in their action by those 

 muscles on the front of the trunk and legs which, by their contraction, pull the 

 body forward ; and, of course, if the trunk form a slanting line, with the in- 

 clination forward, it is plain that when the heel is raised by the calf-muscles, 



