ix ELECTRICAL EXCITATION OF NERVE 117 



mediately after the closure of the battery current, from which 

 point it declines slowly, while anelectrotonus is relatively slow to 

 develop and to diffuse ; the maximum invariably occurs some time 

 after closure. We shall see that this agrees perfectly with the 

 galvanic alterations of the nerve in electrotonus. 



If conductivity is, in the words of Gad, only the expression 

 of " longitudinal excitability " in the nerve, i.e. capacity of trans- 

 mitting a local excitation longitudinally from section to section, 

 it seems a priori highly probable that it should undergo alterations 

 of conductivity coextensive with the electrotonic alterations of 

 excitability. The law of contraction indeed indicates directly 

 that the persistent anelectrotonus (with ascending direction of 

 current), as well as the diminishing katelectrotonus (with descend- 

 ing direction of current), do cause an inhibition of conductivity 

 as regards the excitation approaching in the first case from the 

 kathode, in the last from the anode. It must further be assumed, 

 in view of the first and second stages of the law of contraction, 

 that the depression of conductivity effects an active inhibition only 

 with relatively high strengths of polarising current. Von Bezold's 

 admirable investigation (19) of conductivity in electrotonised 

 nerve only corresponds partially with these presumptions. It 

 has been stated that every excitation discharged above a tract 

 of nerve traversed by an ascending or descending current 

 remains without effect at a certain strength of polarising 

 current, because the diminution of excitability (and conduc- 

 tivity) is presumably so considerable in the entire anelectrotonic 

 tract, that it offers an actual hindrance to the propagation 

 of the stimulus to the muscle. Before this point is reached, 

 however, this betrays itself in a more or less considerable 

 delay in the entrance of the muscle-twitch, which is greater in 

 proportion with the strength and duration of passage of the polaris- 

 ing current. In order to determine exactly the share taken by 

 the polarised tract, the two poles, and also the extrapolar tract of 

 nerve, v. Bezold in the first place stimulated the muscle directly, 

 and then the nerve at three different points (a, l>, c) of its course, by 

 a single induction shock (Fig. 186). It was then possible from 

 the differences of latent period to calculate the rate of conduc- 

 tivity of the excitation from a to the muscle, from & to , 

 and from c to 5. An ascending battery current was then led 

 uninterruptedly through the tract c, the secondary coil of an 



