AFTER-EFFECT OF STIMULUS ON CONDUCTIVITY 503 



section by internal short-circuiting of the nerve-current. 

 That this explanation, however, does not meet all the 

 requirements of the case will appear from certain experi- 

 ments which I shall describe, where, under exactly similar 

 electrotonic changes due to section, a result directly the 

 opposite of this, that is to say, of depression, is seen to be 

 induced. 



All these various facts will be found fully reconcilable, 

 however, on the basis of a proposition which I shall establish, 

 namely that in a nerve, moderate stimulation enhances ex- 

 citability and conductivity, while excessive stimulation has the 

 opposite effect of bringing about the depression of both. It is 

 indeed natural to expect that while moderate stimulation, by 

 increasing molecular mobility, would bring about one effect, 

 excessive stimulus, by inducing overstrain, would result in 

 exactly the opposite. Before proceeding to give an experi- 

 mental demonstration of this hypothesis, we shall first 

 consider the explanation which it affords of the peculiar 

 excitatory changes observed in the case of cross-sectioned 

 nerve. In the first place we know that a cut acts as, a 

 stimulus. And since we found that the effect of stimulus 

 decreases with the distance from the point of stimulation, it 

 would appear that at the section itself the stimulation would 

 be excessive ; moderately strong at a certain distance from 

 it ; and practically negligible when very far away. In 

 complete accordance with this is the resulting increase of 

 excitability which has been observed near the point of 

 section, while at the point itself the nerve is relatively 

 inexcitable. 



The fact that stimulation, when not excessive, increases 

 the conductivity and excitability, we found illustrated in the 

 staircase increase of electrical response, and in the enhance- 

 ment of amplitude after tetanisation, in vegetable and animal 

 nerves (figs. 275 and 286). The same fact will be demonstrated 

 later by means of the mechanical response of nerve. I shall 

 now describe certain experiments which demonstrate it once 

 more in a new and interesting manner. 



