518 



Mr. K. Lucas. 



[June 6, 



a possible explanation of the observed facts of long and of slowly increasing 

 currents. But we must not overlook the consequence, that the original simple 

 hypothesis has become profoundly modified. Instead of the constant concen- 

 tration of ions which sufficed to explain the facts of excitation by short 

 currents, we now postulate a concentration which must be greater the more 

 slowly it is produced. And we are further burdened with a chemical change 

 produced by the ions, and constituting the necessary condition for the initia- 

 tion of a propagated disturbance. Also the distance between the membranes 

 now enters as a factor of importance, and this involves a fresh difficulty, to 

 which I shall return when I come to speak of the location of the membranes. 

 Several suggestions have been made in the attempt to escape from these 

 complications. Hill* proposed a possible partial permeability of the 

 membranes, which might serve to reduce the concentration of the ions by 

 leakage. This condition could not be treated mathematically, and is also open 

 to another objection, of which I am about to speak, that it offers no help 

 towards the understanding of the occurrence of excitation at the anode when 

 the exciting current is broken. Lapicquef looked for a solution in the 

 modification of jSTernst's fundamental hypothesis. He postulated as the 

 conditions of excitation first a constant difference, and later a constant ratio, 

 between the concentrations at two points unequally distant from the 

 membranes. It seems doubtful whether this hypothesis does really account 

 for the failure of slowly increasing currents. Hill found that it would predict 

 the failure of such currents only if of such strength that they could not have 

 excited in any case. Lapicque and Petetin are unable to confirm this 

 conclusion in their more recent work. But in any case it does not appear 

 that the hypothesis is a real advance on Nernst's, since it leaves the question 

 of the anodic excitation still untouched. If we must admit a complication of 

 the hypothesis in order to explain the failure of slowly increasing currents, 

 the advantage will obviously be on the side of such a postulate as will explain 

 also the anodic excitation. This, as we shall see, Nernst's suggestion does 

 achieve. 



The anodic excitation on opening an exciting current is one of the most 

 significant facts in the whole of our knowledge of excitation. It has received 

 very little attention from critics of Nernst's hypothesis, probably because it 

 is so old and well established as to have become less interesting than quanti- 

 tative relations more recently investigated. Yet it is a fact which we clearly 

 shall not explain if we postulate, as the condition which an exciting agent 



* Hill, loc. tit., p. 208. 



t Lapicque, 'Journ. de Physiol.; 1908, vol. 10, p. 601; 1909, vol. U, p. 1009; 

 Lapicque and Petetin, ibid., 1910, vol. 12, p. 696. 



