ELECTRICAL EXCITATION OF NERVE 163 



is considerably increased towards* the electrical stimulus. This is 

 caused by the fact that both ascending and descending currents, 

 which produce only minimal effects of excitation upon closure, 

 will, when led in by unpolarisable electrodes to the lower portion 

 of the nerve, produce almost maximal closure twitches, provided 

 the nerve is divided at not too great a distance from the central 

 electrode. The appearance of the break twitch seems, at first 

 sight, to indicate the same thing, since it is hardly perceptible 

 with low intensity of current, if the anode falls within the region 

 which is obviously the most strongly affected by the cross-section. 



Although it is indisputable that the opening excitation 

 takes effect at a very low intensity of current if the anode is 

 placed in the immediate vicinity of a section (mechanical, 

 chemical, or thermal) applied to the nerve, the preceding, 

 and till now generally accepted, interpretation ought not to 

 pass muster. We need not dwell on the fact (as frequently con- 

 firmed by experiment) that the opening twitch, in those very 

 nerve-muscle preparations which are taken from animals brought 

 fresh from a cold chamber into the laboratory, and in which 

 excitability is very high, appears plainly for the first time with 

 relatively strong currents, while in preparations from less excitable 

 frogs there is sometimes (if rarely) an opening excitation without 

 the application of any transverse section, at a low strength of 

 current because the inevitable closure tetanus masks the weaker 

 opening effects, which are thus detected only by a delay in the 

 muscular relaxation. 



On the other hand, decisive evidence against the exclusive 

 agency of rise of excitability in the production of the opening 

 twitch is afforded by the fact that division of a nerve in the 

 vicinity of the anode is at once followed by the appearance of 

 the opening twitch ; even when excitability has from any reason 

 been much reduced during the experiment, so that (as far as can 

 be judged from the height of the closure twitches then discharged) 

 it is not, even near a fresh cross-section, as great as it was at the 

 same point in the uninjured nerve. Nevertheless the opening 

 twitch fails completely with the same strength of current at the 

 commencement of the experiment, while it appears immediately 

 after making the cross-section, in spite of the absolutely lower 

 excitability. 



In this connection there are certain very instructive experi- 



