CHAPTER VIII 



THE NEUROMUSCULAR SYSTEM: NEUROMUS- 



CULAR FATIGUE 



NOTHING is more familiar than the fact of experience 

 that muscles used long and severely respond less and less 

 readily to our requisitions upon them. This progressive 

 flagging of efficiency, together with the subjective sense of 

 difficulty in commanding the muscles employed, we speak 

 of as fatigue. When we add the adjective "neuromuscu- 

 lar" we open the way to a discussion of the respective 

 share of muscular and nervous elements in the develop- 

 ment of this condition. In the laboratory we can devise 

 experiments such that fatigue of muscle is studied quite 

 by itself; in the living body muscles are use"d only when 

 their controlling nervous mechanisms are in action. 



Fatigue, whether in muscular fibers or in neurons, may 

 conceivably result from either of two causes. It will be 

 inevitable if the store of energy-producing substance 

 runs low. This is a type of fatigue which approaches ex- 

 haustion as a limit. The second possibility is that by- 

 products or end-products of the chemical process which 

 is going on may so accumulate as to become a hindrance 

 to the continuance of that process. This is probably the 

 ordinary happening: muscles which refuse to work are 

 not destitute of all fuel, but are, figuratively speaking, 

 clogged by ashes and half-burned clinkers which remain 

 as relics of the combustion which has recently taken 

 place. 



So far as muscular tissue is concerned, we know some- 

 thing of the products which are capable of depressing its 

 activity. Three compounds are usually mentioned as 

 bearing- a part carbon dioxid, lactic acid, and acid 



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