ix ELECTRICAL EXCITATION OF NERVE 173 



proves, not merely that there is no correspondence of fundamental 

 characteristics (as would necessarily occur if the same cause 

 underlay the opening excitation in both instances), but that 

 as becomes more and more evident on investigating the 

 phenomena there are here two perfectly different effects of 

 current, distinct not merely in regard to appearance, but also in 

 their mode of expression in the muscle. 



The characteristics of the opening twitch discharged by the 

 action of weak currents, upon nerves of which the excitability 

 is considerably heightened, are, in first degree (according to 

 the above experiments), its delayed entrance, as also its dependence 

 upon duration of closure ; and these distinguish it fundamentally 

 from the opening twitches which appear (under otherwise uniform 

 conditions of experiment) at the transverse section of an otherwise 

 normal nerve. In the curve of the latter there is never, unless 

 finer methods of time-measurement are resorted to, any perceptible 

 interval between the moment of opening the current and the 

 commencement of muscular contraction ; the curve is also much 

 steeper, and invariably exhibits a pointed apex, while it never 

 reaches the height of the opening twitches discharged in conse- 

 quence of artificially raised excitability in the nerve. It is, 

 however, remarkable that the duration of the exciting current 

 affects the magnitude only within a very narrow range, and in 

 no case the character, of the opening twitches from the transverse 

 section ; for the curve of the latter never acquires a more extended 

 form, or becomes tetanic, even when a tolerably strong current 

 traverses the cut end of a nerve protected from evaporation for 

 a considerable time, in the descending direction. These facts 

 alone would justify the assumption of a double opening excitation, 

 distinct in origin and in mode of manifestation ; there are, how- 

 ever, further and still more convincing proofs. 



In the first place, the uninjured nerve may, under certain 

 conditions, with even weak currents, exhibit opening twitches of 

 precisely the same character as those deriving from the transverse 

 section independent of action from any incision. Here, how- 

 ever, it is not so much a raised as a considerably diminished 

 excitability of the nerve which seems to favour their appearance. 



If the sciatic nerve of a frog is treated, as described, with a 

 moderately strong solution of alcoholic saline (about 10 vols. / Q \ 

 and excited once a minute by an ascending or descending battery 



