156 CHANGES IN MUSCLE. [OH. XIII. 



broken up into carbonic acid, sarco-lactic acid, and a proteid 

 residue of myosin. 



There are other chemical changes in the muscle when it 

 contracts namely, a change of glycogen into sugar, and an 

 increase of nitrogenous waste. The question whether urea is in- 

 creased during muscular activity is, however, a much debated one, 

 and we shall return to it when we are studying the urine. What 

 is certain is that the increased consumption of carbon (possibly in 

 large measure derived from the carbohydrate stored in the muscle) 

 is a much more marked and immediate feature than an increase 

 in the consumption of nitrogen. 



Fatigue. 



If the nerve of a nerve-muscle preparation is continually 

 stimulated, the muscular contractions become more prolonged 

 (see p. 124), smaller in extent, and finally cease altogether. 



The muscle is said to be fatigued : this is due to the consump- 

 tion of the substances available for the supply of energy in the 

 muscle, but more particularly to the accumulation of waste 

 products of contraction ; of these, sarco-lactic acid is probably the 

 principal one. Fatigue may be artificially induced in a muscle by 

 feeding it on a weak solution of lactic acid, and then removed by 

 washing out the muscle with salt solution containing a minute 

 trace of an alkali. If the muscle is left to itself in the body, the 

 blood stream washes away the accumulation of acid products, and 

 fatigue passes off. 



The question next presents itself, where is the seat of fatigue ? 

 Is it in the nerve, the muscle, or the end-plates ? If, after fatigue 

 has ensued and excitation of the nerve of the preparation produces 

 no more contractions, the muscle is itself stimulated, it contracts ; 

 this shows it is still irritable, and, therefore, not to any great 

 extent the seat of fatigue. 



If an animal is poisoned with curare, and it is kept alive by 

 artificial respiration, excitation of a motor nerve produces no con- 

 traction of the muscles it supplies. If one goes on stimulating the 

 nerve for many hours, until the effect of the curare has disappeared, 

 the block at the end-plates is removed and the muscles contract : 

 the seat of exhaustion is therefore not in the nerves. 



By a process of exclusion it has thus been localised in the 

 nerve-endings. 



When the muscle is fatigued in the intact body, there is, how- 

 ever, another factor to be considered beyond the mere local 

 poisoning of the end-plates. This is the effect of the products of 



