464 Dr. W. M. Fletcher and Prof. F. G. Hopkins. 



particularly if this, as Parnas's experiments seem to show, amounts to not 

 less than half the oxidative energy of lactic acid. In our opinion the con- 

 ception of a chemical " inogen " of any kind is false, and fated to disappear. 



The high potential energy required for the rapid act of contraction may be 

 stored, however, in some other form. The potential, which is lost upon 

 contraction and restored by subsequent oxidation, may reside, not in an 

 unstable chemical substance, but in a particular condition of a ph)'sico- 

 chemical system. Here we may return to the image we previously 

 depicted of the muscle machinery in our provisional hypothesis. 



In a system of colloidal fibrils, or of longitudinal surfaces, into relation 

 with which H-ions of lactic acid lie ready to be brought, we have a 

 potential of energy which may be discharged as work, with or without heat, 

 on the development of a new state of tension in the fibrils, whether tension 

 due to inhibition or to added surface tension along the longitudinal surfaces. 

 The observed heat production of anaerobic contraction may be in part due 

 to the exothermic molecular change which yields the free acid from its 

 precursor, and in part due to the resultant change in colloidal surfaces or 

 substances upon the delivery to them of the acid ions. 



Upon recovery by oxidative removal of the lactic acid, the energy of 

 combustion is discharged in part as heat and in part (and what fractional 

 part that is we have seen to be at present uncertain) returned to the muscle 

 in the restoration of the initial potential. In this restoration will be 

 involved the separation of the acid ions from the colloidal fibrils, by which 

 the condition will be given for the return of the fibrils to their former 

 tension — the tension, that is to say, of the muscle in the state of relaxation 

 and rest, and possessed of the potential inherent in them. 



We have been speaking so far of changes of potential in connection with 

 the contractile act. As regards the actual main reservoir of energy, it is 

 clear that this must be contained within the muscle itself, because most of 

 our data have been obtained from excised muscle. That this main reservoir 

 of energy is to be sought in the carbobydrate stores is, we believe, quite 

 certain. The question arises, Are we to assume that carbohydrate must 

 first be converted into a substance of higher chemical potential before it 

 can serve as a contributory source of contractile energy by its breakdown to 

 lactic acid ? The small energy change which that breakdown .involves has 

 been thought by some to make this assumption necessary. It is just this 

 assumption, however, that the conception of a change in the physico- 

 chemical system of colloid fibrils, as the vehicle of a rise of potential, 

 makes unnecessary. The contractile act may call, not only upon the 

 chemical energy liberated when sugar becomes lactic acid, but also, and 



