iv ELECTROMOTIVE ACTION IN MUSCLE 389 



current and rest current must alike be referred to the same 

 cause, " since both are to be regarded as the external symptom of 

 a different ratio of descending change in the two parts of the 

 muscle brought together in the circuit." Accordingly, there is 

 as little essential difference between action current and rest- 

 current, as between excited and dying muscle-substance. From 

 this point of view it is meaningless to ask whether the " potassium 

 current " in muscle (as above) is, or is not, to be regarded as an 

 action current. The circumstance that it appears in etherised 

 muscle proves as little against the former assumption as the 

 presence of the normal demarcation current, under the same con- 

 ditions, against the latter. 



From the standpoint of the molecular theory, the electro- 

 motive response of uninjured, currentless muscle encounters great 

 difficulties of interpretation, which can only be met by supple- 

 mentary hypothesis. It is superfluous to enter on the detailed 

 discussion of these, since they are based on the parelectronomic 

 theory, the invalidity of which can hardly be disputed. A brief 

 exposition of the fundamental canon by which du Bois-Reymond 

 interpreted the negative variation of the demarcation current 

 is all that is required. He derives it essentially from a diminu- 

 tion of E.M.F. in the "molecules," or from their rearrangement 

 in a form less centrifugally active. Bernstein's new " electro- 

 chemical theory " also postulates a " decrement of charge in the 

 molecules," from which he explains the negativity of each point 

 excited. "If the excitatory wave is propagated to the cross-section, 

 the charges of the molecules also decrease pari passu. When the 

 wave reaches the cross-section it fails to produce any current in 

 the opposite direction, i.e. second positive phase of variation, 

 because the charges of the molecules are always the same on 

 the side towards the cross-section." In order to explain the 

 electromotive action of currentless muscle, du Bois-Reymond is 

 forced into the hypothesis that the parelectronomic layer, or tract, 

 at the natural cross-section takes little or no part in the negative 

 variation, while, according to Bernstein, the uninjured ends of 

 fibres react like any other longitudinal points. Du Bois-Reymond 

 believed that the breaking of the excitatory wave upon the 

 natural cross-section was the direct cause of parelectronomy, 

 since he held that this was favourable to the development of the 

 parelectronomic molecules. 



