THE MCSCULAK SYSTEM. 415 



Muscular Work. We have seen that work is estimated by multi- 

 plying the weight raised, by the height through which it has been lifted. 

 It has been found that in order to obtain the maximum of work, a muscle 

 must be moderately loaded: if the weight is increased beyond a certain 

 point, the muscle becomes strained and raises the weight through so 

 small a distance that less work is accomplished. If the load is still fur- 

 ther increased the muscle is completely overtaxed, and cannot raise the 



FIG. 293. Diagram of fatigue muscle-curves. (Ray Lankester.) 



weight. No work is then done at all. Practical illustrations of these 

 facts must be familiar to every one. 



The power of a muscle is usually measured by the maximum 

 weight which it will support without stretching. In man this is readily 

 determined by weighting the body to such an extent, that it can no- 

 longer be raised on tiptoe: thus the power of the calf-muscles is deter- 

 mined. The power of muscle thus estimated depends of course upon its 

 cross section. The power of a human muscle is from two to three times 

 as great as a frog's muscle of the same sectional area. 



Fatigue of Muscle. A muscle becomes rapidly exhausted from re- 

 peated 'stimulation, and the more rapidly, the more quickly the induc- 

 tion-shocks succeed each other. This is indicated by the diminished 

 height of the muscular contractions. 



It will be seen in Fig. 293 that the vertical lines, which indicate the 

 extent of the muscular contraction, decrease in length from left to right. 

 The line A B drawn along the tops of these lines is termed the " fatigue 

 curve." It is usually a straight line. 



In the first diagram the effects of a short rest are shown: there is a 

 pause of three minutes, and when the muscle is again stimulated, it 

 contracts up to A', but the recovery is only temporary, and the fatigue 



