x ELECTROMOTIVE ACTION IN NERVE 247 



both ascending and descending currents, while the opening of 

 the exciting circuit has no visible effect. In the second the 

 consequences are exactly opposite with the two directions of 

 current, since a negative variation then appears only with closure 

 of the descending and opening of the ascending current, while 

 closure of ascending and opening of descending currents has no 

 effect. 



Further evidence of the causal relation between the galvanic 

 effects in question, and the excitation of the nerve by current, is 

 afforded in the fact that both are equally affected by killing the 

 terminal portion of the exciting tract. We have already seen 

 that in rnedullated and non-medullated nerve, as in striated and 

 smooth muscle, the excitatory process is hindered, or altogether 

 obstructed, when the current enters or leaves at a point that has 

 been injured. And in fact, after killing a portion of the exciting 

 tract (2-4 mm.), the negative variation at closure of the ascend- 

 ing current disappears almost as completely in inolluscan nerve 

 as does, in the other case, the closing excitation in the muscle. 

 On the other hand, the negative variation is as little affected by 

 the same injury at make of the descending current as by 

 break in the ascending direction, thus showing that the first 

 effect is caused by an alteration of the nerve deriving from the 

 kathode, the second by alteration from the anode. 



In medullated frog's nerve an analogous effect is apparently 

 hindered only by the suppressed inclination to persistent excita- 

 tion from the current flowing at constant density. Engelmann 

 (17) long ago observed on nerves that were in the peculiar state 

 in which every closure, or opening, of a battery current induces 

 more or less sustained excitation (expressed in the muscle by 

 closing or opening tetanus), that the galvanometer showed a corre- 

 sponding negative variation of the demarcation current, on 

 leading off from the transverse section. 



In order to obtain this effect in its integrity on medullated 

 nerve, i.e. undisturbed by other galvanic manifestations to be 

 discussed below, it is advisable to take very sensitive preparations 

 of cooled frogs, and to test at maximal distance between 

 leading-off and exciting tracts, with the weakest possible currents. 

 A distinct negative variation will then regularly appear below 

 the kathode of a descending current, reaching its maximum at a 

 low intensity of current, and being invariably more pronounced 



