The Respiratory Process in Muscle. 



461 



chemical reaction which is inhibited by acid, and is therefore self-controlled. 

 It was suggested, therefore, that the maximum production in heat rigor does 

 not represent the total exhaustion of the lactic acid precursor, but "rather the 

 normal cessation of a reaction at a critical concentration of hydrogen ions. 

 So long as a sufficiency of the precursor is present, therefore, at the end of 

 an experiment such as that described above, there is no difficulty about the 

 attainment of a similar " maximum " by muscles restored after fatigue, and 

 by fresh muscles, respectively. If this be so, our experiments gave no proof 

 of a reconstruction of the lactic acid into something else. The experiments 

 of Embden were not wholly satisfactory, because in muscle-juice lactic acid 

 has so nearly reached a maximum, as the result of the expression of the 

 juice, that the amount of change to be observed is but small. But, at our 

 suggestion, Mr. Winfield has recently carried out some experiments at 

 Cambridge upon intact muscles placed in Singer's solution which fully 

 confirm the fact that acid production in muscle depends upon a self-limiting 

 reaction which ceases when a certain grade of acidity is reached. We are 

 inclined, therefore, to doubt if there be any evidence on these direct lines 

 that the lactic acid in recovery processes is rebuilt into a precursor in such a 

 way as to restore the former level of the source of acid supply. 



On this question of the possible restoration of lactic acid to its former 

 molecular position, Hill has discussed some' indirect evidence derived from 

 his own thermodynamic studies. Peters found at Cambridge (14) , by an 

 adaptation of Hill's thermo-electric methods, that the heat production of 

 chloroform rigor was equal to the sum of the two stages of heat production 

 in similar muscles, in the first stage stimulated to give a partial lactic acid 

 yield, and in the second stage killed with chloroform to give full rigor and 

 the acid maximum. This observation in itself is interestina; as an added 

 proof that the heat production, like the lactic acid production, is derived 

 from the same source whether in contraction or in rigor. Taking this total 

 heat production found by Peters, together with our own lactic acid estima- 

 tions for the same conditions, Hill argues that there is a heat production 

 of 450 caloi-ies for each gramme of lactic acid formed. But his own results 

 showed that the heat of oxidative removal of the acid was approximately 

 equal to the heat of production, so that the oxidative removal of 1 grm. 

 should yield about 450 calories. But the combustion of 1 grm. of lactic 

 acid yields about 3700 calories. Hill urges accordingly that the indication 

 of our experiment just quoted (fig. 8) should be accepted, and the lactic 

 acid regarded as being replaced in its former position in the muscle during 

 the oxidative recovery, the energy for that restoration of potential being 

 derived t from the combustion of some other constituent (e.g. carbohydrate) 



