VARIETIES OF LEVERS. 



469 



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 com- 

 paratively slight shortening of the muscular fibres. 



FIG. 165. 



The greater number of the more important muscular actions 

 of the human body those, namely, which are arranged har- 

 moniously 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 pro- 

 cesses by which these muscular actions are resisted or carried 

 out. The combined action of the respiratory muscles, for in- 

 stance, will be found described in the chapter on " Respira- 

 tion ;" the action of the heart and bloodvessels, under the 

 head of " Circulation ;" while the movements of the stomach 

 and intestines are too intimately associated with the function 

 of " Digestion," to be described apart from it. 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 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 astragalus, and with it, of course, the whole 

 body, the weight of which is transmitted through the tibia to 

 this bone (Fig. 166). 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 



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