MOTION. 



In a lever of the third kind the arrangement is F.P.W., 

 and the act of raising a pole, as in fig. 158, is an example. 

 In the human body there are numerous examples of the 

 employment of this kind of leverage. The act of bending 

 the fore-arm may be mentioned as an instance (fig. 158) 

 Fig. 158. 



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 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 assisted or carried out. The combined action 

 of the respiratory muscles, for instance, will be found 

 described in the chapter on " Respiration " ; the action of 

 the heart and blood-vessels, under the head of " Circula- 

 tion " ; while the movements of the stomach and intestines 



