340 MUSCULAR ACTIVITY AND FATIGUE 



acid is, however, not the cause of fatigue, for there is a 

 Hmit to which recovery can be obtained by mere removal 

 of the acid. Under these conditions recovery can only 

 be induced when the perfusing fluid contains food substances 

 such as carbohydrates. 



Two factors, then, are concerned in fatigue of the isolated 

 muscle — accumulation of lactic acid and deficiency of food 

 material. As a necessary consequence of the absence of 

 circulating fluid, the former is the more important. To 

 what extent is lactic acid a cause of fatigue in the muscle 

 in situ? 



Now lactic acid, as we have seen, is not an abnormal 

 product of muscular metabohsm but a normal inter- 

 mediate product, its ultimate oxidation providing the 

 necessary energy for subsequent activity. What is ab- 

 normal is not the formation of the acid but its accumulation 

 due to failure of oxidation. We may therefore look to 

 deficiency of oyxgen as a cause of this accumulation. This 

 is borne out by experimental findings in cases of muscular 

 exercise. The appearance of lactic acid in the urine de- 

 pends not upon the duration of exercise nor upon the 

 amount of mechanical work involved, but upon the coinci- 

 dent respiratory distress — that is to say, owing to the call 

 for oxygen by the tissues not being satisfied. 



Provided, then, that the supply of oxygen is sufiicient, 

 there is no accumulation of lactic acid, at any rate in 

 sufiicient amount to cause overflow into the general circu- 

 lation. But in the measure that oxygen fails, acid accumu- 

 lates. In so far, then, as fatigue can be located in muscle, 

 we may attribute its occurrence to accumulation of lactic 

 acid. 



As regards fatigue of the nervous system, this may be 

 induced in the frog by depriving the spinal cord of oxygen, 

 recovery ensuing when the cord is perfused, still in the 

 absence of the gas. Fatigue, then, would seem to be the 

 same process essentially, whether in the central nervous 

 system or in the muscles. 



