x ELECTROMOTIVE ACTION IN NERVE 313 



(iii.) When the polarising current is opened, the polarisation 

 rapidly reaches its climax, and then sinks again continuously ; 

 this fall occurs quickly at first, and afterwards more slowly, so 

 that polarisation lasts for a long time after the opening of the 

 polarising current, and only reaches its zero asymptotically. 



The opening twitch agrees in all three points with the nega- 

 tive polarisation current. We have already referred to the fact 

 that motor frogs' nerves are so altered by the action of dilute 

 solutions of alkaline salts, or alcoholic salt solution, that at a 

 given stage the weakest constant currents will discharge opening 

 twitches, after quite brief closures, of the same character as the 

 break twitch from a transverse section, which alteration may be 

 completely neutralised by washing out the foreign substances. 



Tigerstedt finds that " the (negative) polarisability of the 

 nerve also rises on treatment with alcoholic saline to 1'5 times 

 its original height," and sees in this fact a further confirmation 

 of the view that the opening twitch is the closure twitch of the 

 negative polarisation current. Finally, Tigerstedt refers the 

 earlier appearance of the break twitch 011 exciting the divided 

 sciatic plexus, as compared with the excitation of peripheral 

 points of the nerve (Biedermann and Grutzner), to a more ready 

 polarisability of those sections of the nerve. The demarcation 

 current must, however, play the principal part. 



In summing up these facts, it can hardly be doubtful 

 that certain forms of opening twitch are to be interpreted as closure 

 twitches from the negative polarisation current. Such sweeping 

 generalisations as those formulated by Tigerstedt, and more 

 recently by Hoorweg (49), which " refer the opening excitation 

 and all phenomena that occur on opening the polarising current " 

 to " the (negative) polarisation current, and in certain exceptional 

 cases to the nerve-(rnuscle) current," are, however, quite unjustifi- 

 able. They are more especially contradicted by the fact that, as 

 was pointed out by Hermann, break twitches also appear on 

 merely diminishing the current (in negative variations of intensity), 

 in which case there is usually 110 polarisation current, since the 

 anode can never become kathode if the diminution is less than 

 half. 



There is yet another point of view from which it appears 

 possible to approach the question of whether the electrotonic 

 incremental current is due solely to physical current escape, or to 



