180 ELECTRO-PHYSIOLOGY CHAP. 



The curves in question were obtained by closing a strong 

 current for some seconds (after heightening the excitability of 

 the nerve in a marked degree by abstraction of water, treatment 

 with saline, or alcohol, which set up a tendency to secondary 

 opening excitation), to predispose the anodic points of fibres 

 for the discharge of primary opening twitches (by currents of 

 lower intensity). So long as the after-effect of the single closure 

 of a strong current persists, the double excitatory action may be 

 seen with the opening of the weaker currents, being indeed very 

 distinct, while with stronger currents the two twitches readily 

 fuse into one, on account of the shortened " latent - period " 

 of break twitch II as is also the case in Eitter's tetanus. 

 This last fact explains how it has been possible till now to over- 

 look the existence of two quite distinct opening effects of current. 



We are here met by the further question, whether the two 

 effects of the break excitation (as disclosed under certain experi- 

 mental conditions) may, notwithstanding their dissimilarity, be 

 referred to a common origin ; or if not, from what cause they are 

 derived. 



As regards the first point, a fair and unprejudiced con- 

 sideration of the facts ought to convince us of the im- 

 probability of a single origin for excitatory effects, as unlike in 

 conditions of appearance and general characteristics as the break 

 twitches I and II. While the appearance of the latter seems 

 to imply a considerable rise of excitability in the nerve, the 

 former, on the contrary, enters with depressed excitability ; and 

 while with weaker currents there is, as a rule, delayed entrance of 

 break twitch II and of the equivalent Eitter's tetanus (dependence 

 upon duration of current being also very evident), there is never 

 any perceptible interval between the moment of opening and the 

 appearance of break twitch I ; moreover, when the conditions of 

 its entrance are once present, the latter is almost independent of 

 the duration and intensity of the exciting current. 



The undoubted equivalence of break twitch II and Eitter's 

 tetanus points to a common origin. Pfliiger, who regarded 

 every opening twitch as a consequence of excitation of the nerve, 

 by the disappearance of anelectrotonus, gave the same explana- 

 tion of Eitter's tetanus, and actually demonstrated, by the well- 

 known experiment of cutting off a previously anelectrotonised 

 portion of the nerve, that the opening tetanus originates at that 



