WORK AND FATIGUE 61 



undue fatigue can be avoided only by working at a moder- 

 ate rate. It is an old and physiologically true saying that 

 " it is the pace that kills." 



4. Explanation of the fatigue level. In the experiment 

 with the isolated muscle no waste products were removed 

 nor were new food and oxygen supplied; hence the wastes 

 in the muscle increased with each contraction, until at last 

 their accumulation prevented all contraction. In the normal 

 muscle the wastes likewise accumulate for a time ; and this 

 is why the curve of work at first falls (Fig. 34). It does 

 not continue to fall, because as the wastes within the muscle 

 increase in amount, the blood carries more and more of 

 them away in a given time. The quantity of waste removed 

 thus continues to increase until the same quantity is carried 

 away from the muscle between two contractions as the 

 muscle produces with each contraction. When this happens 

 no further accumulation of waste is possible and the fatigue 

 level is established. 



5. General fatigue resulting from muscular activity. Every- 

 one knows that after a day's tramp it is not simply the 

 worked muscles which are unfit for good work, but that 

 the brain, too, is tired, for hard mental work is then dim- 

 cult or well-nigh impossible ; and it is generally the fact 

 that long-continued muscular work fatigues the brain more 

 than brain (mental) work itself. The obvious explanation 

 of this fact is that the waste products of muscular activity 

 have accumulated in the blood more rapidly than the body 

 can get rid of them, and so have fatigued the other tissues, 

 including the nerve cells of the brain, just as the injection 

 of the extract of a tired muscle lessens the working power 

 of a fresh muscle. No doubt these same waste products may 

 similarly fatigue gland cells; for experience seems to show 

 that the secretion of digestive juices is not so active when 

 one is suffering from muscular fatigue and that it is not 

 wise to eat heavy meals when one is tired out. We can also 



