64 ELECTRO-PHYSIOLOGY CHAP. 



transmission of excitation from particle to particle, and might 

 thus be designated as the propagation of the excitatory process. 



On the other hand, there are several indications which make 

 it probable that excitability (expectancy) and conductivity are dis- 

 tinct properties in nerve, and not in causal inter-relation. The first 

 exact physiological observation of this was made by Mimk (23), 

 who noticed, in following out the changes in excitability associated 

 with the dying of the nerve in a frog's nerve-muscle preparation, 

 that the principal bifurcating points of the sciatic nerve could be 

 insensitive to the strongest electrical stimuli, at a time when the 

 muscle still responded by vigorous twitches to a much weaker 

 excitation applied to more central parts of the nerve. A better 

 known instance is that of Erb (24), who found that when, after 

 crushing the sciatic nerve of frog or rabbit, regeneration had set 

 in, and the lamed extremities were again moved normally by the 

 animal, the part of the nerve that had been crushed, and was now 

 regenerated, was still insensitive to electrical stimuli. Here, too, 

 we must include the more complete experiments on the spinal 

 cord (infra) by which Schiff was led to his doctrine of " aesthe- 

 sodic " and " kinesodic " nerve-substance, capable, i.e., of con- 

 ducting, but not directly excitable. Above all, however, the 

 experiments already quoted of Griinhagen, Efron, Gad-Sawyer, 

 Goldscheider, and Piotrowsky, on the effect of local narcosis on 

 motor nerve, have contributed to bring forward the view that the 

 two processes of response to stimulus and conduction of stimulus 

 are distinct from one another. Indeed, the fact that a peripheral 

 tract of nerve under C0 2 narcosis is inexcitable, and yet transmits 

 an excitation coming from a more central point, hardly admits of 

 any other interpretation, save that excitability and conductivity 

 may alter independently of each other. 



If we are justified in regarding the process concomitant with 

 the excitatory condition of a nerve element, as a stimulus by 

 which the element longitudinally next to it is excited, conduc- 

 tivity must be a permeability of the nerve to certain in- 

 fluences which affect it in the longitudinal direction. We may, 



\j ' 



with Gad, denote this sensitiveness as " longitudinal lability." It 

 is conceivable, and even probable, that the stimulus which one 

 nerve-molecule exerts upon the next molecule may, although closely 

 related to an external stimulus, or identical with it, find even 

 more favourable conditions than the latter. This assumption (cf: 



