1912.] The Process of Excitation in Nerve and Muscle. 



519 



must fulfil, merely the production of a certain concentration of cations. 

 When a current ceases to flow the concentration of cations does, of course, 

 rise at the anode by diffusion until their normal concentration is regained, but 

 there is no rise of the concentration of cations at the anode above its normal 

 level. The one feature which is common to the cathode when the current is 

 first made, and to the anode when it has just ceased to flow, is an increase 

 of the concentration of cations* above the value which occurred at each 

 point immediately before. It would seem, therefore, on this showing that 

 the condition to be fulfilled by an exciting agent is either an increase in 

 the concentration of cations above its recent level or some change which 

 such an increase would produce. Nernstf pointed out that the conception 

 which he had introduced to account for the failure of slowly increasing 

 currents supplied just this need. Hillf dealt with the same point, perhaps 

 even more explicitly, but it seems that the explanations which they offered 

 are fundamentally the same. In both cases the diminished concentration of 

 cations produced at the anode during passage of the current leads the 

 reversible chemical combination between the ions and the postulated 

 substance to take up a new equilibrium. When the current ceases to flow 

 the compound is suddenly faced with a concentration of cations greater than 

 that with which it is at the moment in equilibrium. That is, of course, also the 

 condition with which it is faced at the cathode when the current begins to flow. 

 On this supposition, therefore, the conditions at the anode on break of the 

 current are brought into line with those which normally occur at the cathode 

 on make of the current, and if excitation is to occur at the one it should be 

 expected at the other also. 



As far as I am aware, this supposition is the only one which has been 

 offered up to now as a common solution of these two fundamental facts of 

 excitation, the failure of progressive currents and the excitation at the anode. 

 The postulate that the concentrated ions are concerned in a reversible 

 chemical change may appear, as I have suggested, an additional burden on 

 the hypothesis, but if their concentration is to be the condition which an 

 exciting current has to fulfil, we must suppose that they initiate some change 

 dependent on their concentration, and it may be necessary that we should 

 at this stage include the nature of that change as part of the whole hypothesis. 

 To say that the supposition now before us is or is not satisfactory is at 

 present impossible. Such a decision must rest with quantitative experi- 

 mental work which lias not yet been done ; the mere qualitative explanation 



* Or, of course, decrease in the concentration of anions. 



t Loc. cii, p. 281. 



I Hill, loc. cit, p. 222. 



2 o 2 



