The Respiratory Process in Muscle. 



463 



the narrower standpoint of muscular activity alone, but in connection with 

 the animal body as a whole, there has been almost complete unanimity 

 in believing that lactic acid is an intermediary product on the main lines of 

 carbohydrate metabolism. There is cogent evidence for this view, though 

 it would, of course, be out of place for us to discuss it here. But it is the 

 muscle in which by far the greater part of the total metabolism of the body 

 takes place, and if, in muscle, lactic acid must be supposed in normal circum- 

 stances to appear only momentarily, and then, instead of following further 

 steps towards the end-products of metabolism, to suffer instead a return to 

 its source, it would be difficult to reconcile its history in muscle with what 

 is believed concerning its importance in general metabolism. 



It is significant in this connection to find, as we do (23), that the pancreas 

 which exerts so important an influence upon the processes of general 

 carbohydrate metabolism, exercises a direct control over the formation of 

 lactic acid in muscle. 



If it be not the lactic acid which is burnt, we must seek alternative fuel 

 for the undoubted combustion which occurs. Hill suggested that carbo- 

 hydrate as such was the fuel, giving energy for the restoration of the lactic 

 acid to its former position. But Parnas and Wagner (24) have supplied 

 definite evidence that carbohydrate, while it disappears from the muscle 

 during the anaerobic processes in which lactic acid appears, remains 

 unchanged in amount during the oxidative recovery. 



Winfield at Cambridge has shown that fats, moreover, are not oxidised in 

 the excised muscle (19), and we have good reasons, finally, to believe that in 

 normal circumstances protein material is not burnt. But if neither protein, 

 fat nor carbohydrate is the fuel we seek, what then is the material which 

 undergoes oxidation ? The only justification for doubting that it is lactic 

 acid, the one substance which obviously accumulates in the absence of 

 oxygen, and disappears in its presence, was the suggestion derived partly 

 by tradition from the teaching of Hermann, and partly from the supposed 

 evidence, already criticised, that the lactic acid disappears because from it 

 is reconstituted the unstable substance, the " inogen," imagined to be the 

 immediate source of the contractile energy. Apart from the absence of 

 direct evidence in its favour, there are grave difficulties associated with 

 the conception of an " inogen " capable of rapid breakdown and reconstruc- 

 tion, if we now abandon, as we have shown earlier that we must, the idea of 

 the inclusion within it of oxygen. It seems almost impossible to conceive of 

 an organic substance derived from lactic acid, and not containing " intra- 

 molecular" oxygen with unstable attachments, which could, by a non- 

 oxidative rupture of its molecule, yield the energy required for contraction, 



