THE TISSUES 59 



on the toes the base of the metatarsals is the fulcrum, the 

 weight comes at the ankle, and the power on the os calcis. 

 3rd Class. Power between fulcrum and weight. In raising 



o o 



a weight placed on the dorsal aspect of the toes by the contrac- 

 tion of the extensors of the foot, we have the weight at the toes, 

 the power at the tarsus, and the fulcrum at the ankle. 



In the other joints, actions involving the principle of each of 

 these levers may be found. 



IV. Work of Muscle 



As a result of the changes in shape, muscle performs its great 

 function of doing mechanical work; and the most important 

 question which has to be considered in 

 regard to muscle, as in regard to other 

 machines, is the amount of work it can 

 do. 



Since the work done depends upon the 

 weight moved and the distance through 

 which it is moved, the work-doing power 

 of muscle is governed by the force of 

 contraction, which determines the weight FlG - 29. Influence of the 

 which can be lifted, and by the amount le " th J ' Mll8cle U P D 



the work done. 



to which the muscle can shorten, for 



this will govern the distance through which the weight may 



be moved. 



It has been already shown that the force of contraction 

 depends chiefly upon the sectional area of a muscle. A thick 

 muscle is stronger than a thinner one. But, on the other hand, 

 the amount of contraction depends upon the length of the 

 muscle, since each muscle can contract only to a fixed proportion 

 of its original length. A glance at the diagram will at once 

 make this plain (fig. 29). 



The size of the muscle is thus the first great factor which 

 governs its work-doing power. But the many factors influenc- 

 ing the force of muscular contraction also influence the work- 

 doing power of the muscle (see p. 53). 



One factor requires special consideration, namely, the Load. 



We have already seen that as the load is increased the 

 extent of contraction is diminished. 



