CAUSES AND PHENOMENA OF MOTION. 



41 



second case. 2nd 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 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 evi- 



FIG. 290. 



dent 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 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 

 described 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 some- 

 what 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 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 astraga- 

 lus, and with it, of course, the whole body, the weight of which is trans- 

 mitted through the tibia to this bone (Fig. 291). 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 for- 

 ward and planted on the ground to support 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 inclination forward, 

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

 body will be raised, and pushed obliquely forward and upward. The 



