THE NATURE OF FATIGUE 



J 93 



energy, the feeling of fatigue is gone and we turn with freshness to 

 our task; our supposed fatigue was only an illusion. Even with our 

 imperfect experimental methods, however, enough has been discovered 

 to show, among other things, that human beings differ greatly in the 

 rate at which fatigue develops. Mosso demonstrated this many years 

 ago, though his methods are not now regarded as the best. 



I have thus far confined myself to a consideration of the nature of 

 fatigue and the conditions under which it develops. Eecovery from 

 fatigue is perhaps of even greater interest. Both in the isolated muscle 

 and in the intact organism, fatigue may be carried so far that recovery 

 is difficult or even impossible. The later stages of fatigue are often 

 spoken of as exhaustion, but obviously no sharp line can be drawn 

 between fatigue and exhaustion. Exhaustion is probably most common 

 when labor is continued for years without adequate resting periods. 

 Exhaustion from a temporary effort is of rare occurrence, observable 

 occasionally in athletes and in persons upon whom there is made a 

 sudden and unexpected demand for enormous physical or mental exer- 

 tion. Usually, however, when a fatiguing expenditure of energy by a 

 living tissue ceases, recovery begins at once. Even in the excised muscle, 

 with all supply of blood cut off, a few minutes' rest allows for a certain 

 degree of recuperation, due possibly to the absorption of oxygen. If a 

 weak solution of common salt, or, better, a suitable mixture of 

 various salts, be passed through the blood vessels of the muscle for 

 a few minutes and thus the accumulated fatigue substances be, at 

 least partially, washed out, the recuperation is greater. If a small 

 quantity of glucose be added to the solution, or if nutritive oxygenated 

 blood be introduced, there is still greater recovery, and the power of 

 further work is much enhanced (Fig. 7). All of these methods are 



Fig. 7. Series of contractions of a frog's gastrocnemius muscle in situ and 

 stimulated at intervals of two seconds. The flow of blood through the muscle was 

 stopped by ligating the artery, and the record of fatigue was made. At the break in 

 the series, the muscle rested five minutes, during which time the ligature was removed 

 and the blood was allowed to circulate through the muscle. The record of contrac- 

 tions at the right of the break was made immediately after the resting period, and 

 while the blood was still circulating. 



physiological — in them the chemical conditions conducing to fatigue 

 are replaced by reverse conditions and the result is reversed — oxygen 

 and food are introduced, carbon dioxide and lactic acid are removed, 

 and there is a restoration of working power. In the living human body 



