58 



THE HUMAN MECHANISM 



rather rapidly, then more and more slowly, until at last it 

 becomes parallel with the base line. In other words, the 

 muscle in this case finally finds a constant level of working 

 power. This may be called the fatigue level. 



The broken line b in Fig. 35 gives the result of a fatigue 

 tracing with the isolated muscle. It will be seen that the 

 fall in the height of contraction continues until at last the 

 muscle no longer responds to stimulation. The contrast 

 thus brought out between the effect of work upon muscles 



FIG. 35. Curves of fatigue 

 a, from a muscle with intact circulation ; 6, from an isolated muscle 



with and those without the circulation suggests that the cir- 

 culation of the blood through the working organ in some 

 way maintains the working power. 



The height of the fatigue level in the same muscle at 

 different times is very closely dependent on the rate at 

 which the muscle works. Thus with a contraction every 

 four seconds instead of every three seconds the fatigue 

 level would be higher than in Fig. 34; with a contraction 

 every second it would be much lower. When the contrac- 

 tions come every nine or ten seconds there is usually no 

 falling off in the work done, the time between contractions 

 being sufficient for the complete recovery of working power. 



This picture of fatigue hardly agrees with our feeling of 

 fatigue, for the decline of working power begins at once, or 



