ix ELECTRICAL EXCITATION OF NERVE 143 



experiments cannot be accepted in the original sense. Pfliiger 

 himself has made certain experiments with chemical excitation of 

 single points of the intrapolar tract of the nerve which show it 

 to be divided into two sections, separated by an "indifferent 

 point " in one of which excitability is depressed, while it appears 

 to be raised in the other the former occurring in the vicinity of 

 the anode, the latter in that of the kathode. With increasing 

 strength of current the indifferent point shifts from the region 

 of the anode to that of the kathode (independently of the direc- 

 tion of the current) in a higher degree in proportion with the 

 strength of the polarising current. The anodic depression of 

 excitability spreads in this way with increasing strength of 

 current over a constantly enlarging area of the tract of nerve 

 traversed. These facts, as to alterations of excitability in the 

 intrapolar tract, have been recently confirmed by Tigerstedt (20) 

 with mechanical single stimuli, since his observations coincide 

 entirely with all the other results of Pfliiger. 



The after-effects of the constant current upon the excitability of 

 nerve (among the immediate consequences of which must be 

 reckoned the break excitation itself) are no less important than 

 the alterations of excitability which appear during the closure of 

 the polarising current. Here again certain isolated researches 

 date from the first days of galvanism, and have been carefully 

 summarised by Pfliiger (Electrotonus, p. 72 ff.) : these refer 

 mainly to the conditions of appearance, and to the interpretation 

 of the opening excitation. But, as Pfliiger pointed out, the after- 

 effects of the passage of current are expressed not merely in visible 

 excitatory phenomena, but also in regular alterations in response 

 at all those parts of the nerve which manifested changes of 

 excitability during the passage of the polarising current. This 

 may be briefly expressed by saying that there is, generally 

 speaking, at all points where a rise of excitability is apparent 

 during the passage of the current, a diminution of response 

 immediately after the opening of the circuit, and vice versa. It 

 should be added that the positive modification (increase of excita- 

 bility) on both sides of the kathode of the polarising current is 

 only temporarily reversed at break, and finally terminates in a new 

 increment of excitability, while the negative modification (depres- 

 sion of excitability) in the region of the anode undergoes a 

 permanent positive modification, and subsides as such. The 



