178 ELECTRO-PHYSIOLOGY 



the first minutes remains unaltered, as can easily be seen from 

 its graphic record. Later on, there is nearly always a difference 

 in the height of the make twitch, according as the current is 

 ascending or descending, and always in favour of that direction 

 of current with which the kathode is at the normal point of 

 the nerve. Sometimes, however, after local treatment of the 

 nerve for ten minutes or more with a 1 % solution of KN0 3 , 

 the excitability to closure stimuli of the point affected is hardly 

 diminished perceptibly. On the other hand, there is invariably 

 during this period a much increased susceptibility in the part of 

 the nerve treated with KN0 3 to opening stimuli, however weak, 

 whether the nerve be separated from the central organ or still 

 connected with it. 



According as the cotton-wool pad on the electrode is proximal 

 or distal to the muscle, the opening twitch appears, with either 

 ascending or descending direction of current, in addition to the 

 already existing closure twitch, and usually reaches the same 

 magnitude. 



Obviously these local alterations of excitability in nerve to 

 opening stimuli also, may be neutralised by washing out with 

 0-6 % saline. 



" Primary opening twitches " do not appear merely in con- 

 sequence of certain artificial, chemical alterations in the sub- 

 stance of the nerve, which may imply a considerable depression 

 of excitability, and are notably produced by the action of potas- 

 sium salts : the same alterations are apparently brought about 

 by the electrical current, at its points of entrance into the nerve. 



The dictum that electrical excitation of normal and un- 

 injured nerve gives rise to closure twitches alone, independent 

 of direction of current, applies, as we have said, in general to 

 weak and medium currents only. Biedermann has almost with- 

 out exception obtained effective break excitation with a Daniell 

 cell, after introducing resistance from a du Bois' rheochord, both 

 before and after separation from the central organ (in the latter 

 case at a point of nerve sufficiently removed from the cross- 

 section). 



Then, however, we are met by the remarkable fact that 

 immediately after the expiration of an opening twitch dis- 

 charged by a strong current, the fall of other weaker currents, 

 that had previously acted at closure only, will produce excita- 



