WOKK AND FATIGUE 59 



at most after a very small number of contractions, whereas 

 we usually notice fatigue only after work has gone on for 

 a considerably longer time. One does not feel tired from 

 walking, for example, during the first ten or twenty minutes 

 of the walk. We need not discuss here just what makes us 

 unconscious of the beginnings of fatigue ; but it is important 

 to understand that whether we are or are not aware of its 

 presence, fatigue is the invariable and immediate result of 

 all muscular work. 



Weariness is simply the conscious feeling of fatigue, but 

 fatigue is a physical condition of living cells and organs. 

 Moreover, its phenomena are by no means confined to 

 muscular work. When a gland is stimulated to vigorous 

 secretion a diminution is sooner or later noted in the amount 

 of the secretion, and there is some reason to believe that 

 nerve cells may also become tired from continued activity. 

 Fatigue, then, in one word, is a natural condition of an 

 organ accompanying work, and we may proceed to inquire 

 into its exact cause. 



2. Waste products as a cause of fatigue. When blood 

 which has been circulating through a fatigued muscle is 

 sent through a resting muscle, the resting muscle shows 

 signs of fatigue, even though it has itself done no work. 

 Apparently the blood has extracted from the working 

 muscle something which has the power of lessening the 

 working capacity of a fresh muscle. 



The same thing is illustrated by another experiment. A 

 muscle which is deprived of its circulation (for example, by 

 clamping its arteries and veins) is fatigued by vigorous work ; 

 it is then found that although when left to itself a slight re- 

 covery takes place, this recovery is much more marked if we 

 first pass through its blood vessels a weak solution of salt 

 Here no food is supplied ; the salt solution has only removed 

 something from the fatigued muscle, which, in consequence of 

 this treatment, recovers some of its working power. 



