THE TISSUES 61 



the weight comes at the ankle, and the power on the 

 os calcis. 



3rd Class. Power between fulcrum and weight. In 

 raising a weight placed on the dorsal aspect of the toes by 

 the contraction 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. 



IY. Muscle Work. 



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 which can be lifted, and 

 by the amount 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 FlQ - 28. influence of the 

 upon the sectional area of a muscle. ^/^ d a on ^ uscle upon 

 A thick muscle is stronger than a 



thinner one. But on the other hand, the amount of con- 

 traction 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. 28). 



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

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

 fluencing the force of muscular contraction also influence 

 the work-doing power of the muscle (see p. 55). (Practical 

 Physiology, Chap. VI.) 



