x ELECTROMOTIVE ACTION IN NERVE 283 



great to allow of a genuine positive variation, or (in other cases) 

 the latter alone may appear, when the initial negative effect is 

 often indicated by a distinct delay in the commencement of the 

 positive variation. The different mode of action of ascending 

 and descending currents also appears characteristically on 

 breaking the circuit. With the descending break there is rarely 

 any distinct augmentation of the negative deflection that is 

 present during closure (in consequence of the opening excitation 

 from the anode) ; for the most part there is no clear effect, or 

 merely a slight delay in the return swing of the magnet. With 

 ascending direction of current the negative " opening variation " 

 is much more frequent and regular. 



If these facts are compared with Bernstein's rheotome experi- 

 ments on medullated frog's nerve, as above, a certain analogy can 

 hardly be disputed. 



Setting aside, in the first instance, the phenomena relating to 

 " electrotonus," there is in the two cases a marked negative deflec- 

 tion immediately after closure of the descending polarising current, 

 which from its whole character must undoubtedly be regarded as 

 the galvanic expression of the make excitation. It is not surprising 

 that this should be a rapid variation in frog preparations, a persistent 

 modification in non-medullated molluscan nerve, seeing that in- 

 direct excitation of crayfish muscle by the constant current is, 

 as a rule, sufficient to produce closure tetanus (supra). The 

 closure of a not immoderate ascending current further produces in 

 both cases a weaker but equally negative variation, which must, 

 like the former, be referred to the closing excitation, and only 

 fails with strong currents (third stage of the law of contraction). 

 At a certain medium distance from the polarised tract there is in 

 this case in molluscan nerve a positive deflection, immediately 

 after the initial negative effect, which slowly increases during 

 closure, exactly as was determined by Bernstein under similar 

 conditions for frog's nerve. The graphic representation of this 

 reaction (Fig. 211) may, with slight modifications, serve to express 

 the consequences of ascending current in molluscan nerve on 

 leading off from transverse and longitudinal sections, at not too 

 great a distance from the polarising current. Owing, however, to 

 the time- differences in the respective excitatory manifestations, the 

 effects which in the former case require the application of the 

 repeating-method for their analysis, are here directly obvious with 



