ix ELECTRICAL EXCITATION OF NERVE 139 



make twitch must obviously be much longer (if the excitation is 

 discharged at closure of the current from the kathode, distal to 

 the muscle, thus having a longer course than the anodic opening 

 excitation) than the latent period of the break twitch under 

 otherwise similar conditions. The contrary must occur with a 

 descending current. The difference corresponds in either case 

 with the time required by the excitation to propagate itself 

 through the intrapolar portion. These presumptions are con- 

 firmed by the results of v. Bezold. The interval between the 

 moment of excitation and commencement of the muscle-twitch is 

 greater at closure of the ascending, and opening of the descending, 

 current than it is conversely. 



Further evidence, at least for the localisation of the changes 

 in the nerve that underlie the break excitation, was brought forward 

 by Pfltiger himself, when he showed that a Eitter's opening tetanus 

 manifested under favourable circumstances with descending direc- 

 tion of current disappeared as soon as the nerve was divided in 

 the middle of the intrapolar tract, the muscle being thus removed 

 from the influence of the anode. This experiment does not of 

 course come off in a Eitter's tetanus with ascending direction of 

 current. 



In order to explain the phenomena comprised under Pfliiger's 

 law of contraction, we are experimentally forced to assume that 

 the electrical current, along with the excitatory action proceed- 

 ing at make from the kathode, at break from the anode, elicits 

 simultaneous inhibitory action, the localisation of which can only 

 be conjectured. If by analogy with the excitatory process we 

 regard the inhibitory effect also as polar, we should a priori, 

 on the analogy of the muscle, presume that changes take place 

 in the substance of the nerve at the anode at closure, at the 

 kathode on opening the current --as expressed in a more or 

 less pronounced depression of excitability and also of conduc- 

 tivity. All the phenomena of the law of contraction are 

 satisfactorily explained on this hypothesis, with the aid of the 

 further postulate that the development of excitation arid of 

 inhibition are not perfectly parallel, since weaker currents suffice, 

 as a rule, to produce the former, than are required to elicit the 

 latter. We can thus understand why currents of medium 

 strength should evoke both closing and opening twitches, with 

 either direction of current. The inhibition which they discharge 



