The Respiratory Process in Muscle. 



465 



perhaps to a greater degree, upon the energy derived from the oxidation of 

 the lactic acid, residing in the physico-chemical system of the muscle, which 

 was produced during the previous contraction. 



If our picture of events is the true one, and if the machinery of 

 contraction is of the kind we have suggested, then carbohydrate metabolism 

 in muscle takes on an aspect of peculiar interest. 



We have already recalled the evidence gained from studies in general 

 metabolism, made without special reference to muscle, and have shown that 

 it points clearly to the conclusion that sugar does not suffer oxidation as 

 such, but only after it has first, at an early moment in its metabolic 

 progress, passed through the stage of lactic acid. 



But in the muscles, which after all form the chief seat of metabolism, the 

 acid intermediary product appears, if we are right, at such a stage and place 

 as to have more than a purely chemical significance. It marks, on the one 

 hand, an obligatory stage in a particular set of successive chemical reactions : 

 but, on the other hand, it has here its special role to play in connection with 

 the muscle machinery. In the evolution of muscle it would appear that 

 advantage, so to speak, has been taken of this acid phase in carbohydrate 

 degradation, and that by appropriate arrangement of the cell elements the 

 lactic acid, before it leaves the tissue in its final combustion, is assigned the 

 particular position in which it can induce those tension changes upon which 

 all the wonders of animal movement depend. 



In concluding, we would endeavour to convey in brief terms our reasons 

 for thinking that the particular standpoint thus taken is one which makes 

 for simplicity and clearness in our views concerning muscle, and perhaps in 

 more besides. 



Underlying all views concerning the source of contractile energy, there 

 has persisted till recently, almost as a tradition in physiology, the obstinate 

 assumption that this energy must necessarily be sought in an unstable 

 chemical substance of complex and unknown constitution — perhaps in 

 the protoplasmic molecule itself, perhaps in an "inogen" vaguely to be 

 distinguished from the protoplasm, perhaps only in some compound of a 

 more definite sort in which carbohydrate matter finds itself transformed and 

 endowed with a higher chemical potential. It must surely bring a gain to 

 the clearness and simplicity of our conceptions, and bring encouragement 

 also to the experimentalist, if such an assumption with its many attendant 

 difficulties, to some of which we have alluded, should prove unnecessary. 

 We believe it to be so. With an understanding that the relatively 

 permanent physico-chemical system of the muscle can, without itself 



