ix ELECTRICAL EXCITATION OF NERVE 157 



direction. Similar effects may be observed in frog's nerve on 

 warming the cut end to 4060 C., or by freezing for an extent 

 of about 1 cm. By gradually moving the electrodes along a 

 distance of 12 cm. it is easy to find the place at which, begin- 

 ning with minimal currents, there is with ascending direction of 

 current a break twitch, with descending direction a make 

 twitch, only. This is obviously another proof of the law of polar 

 excitation. 



We have seen that the closure and opening of a current sent 

 through a regular parallel-fibred muscle will only excite normally 

 when the active electrode is at the uninjured end of the muscle. 

 The explanation of this phenomenon as given above leads us to 

 conjecture, from the wide-reaching conformity of reaction to the 

 electrical current in nerve and muscle, that analogous effects 

 would appear in the partially injured nerve also. But we have 

 seen that it is not sufficient to apply one electrode to the cross- 

 section of an otherwise normal nerve, while the other rests 

 upon any point of the longitudinal surface, in order to obtain 

 the effects corresponding with the third stage of Pfliiger's law : 

 it is necessary to destroy a considerable portion of the nerve from 

 the transverse section, while as far as possible preserving its finer 

 histological structure. This, as we shall presently see, is due to 

 the fact that there are in medullated nerve (in consequence of a 

 peculiar diffusion of the exciting current on either side of both 

 kathode and anode) innumerable points where the current leaves 

 or enters, so that the " physiological kathode " or anode extends 

 over a comparatively large portion of the nerve, according to the 

 strength of current at the moment. The intrapolaf tract of nerve 

 thus falls into two parts, differing according to its length and the 

 intensity of the current, and termed respectively anodic and 

 kathodic, while each of these also embraces a larger or smaller 

 portion of the extrapolar region. These two segments, in one of 

 which (the kathodic) activity is set up simultaneously at many 

 points at closure, in the other at opening of the current, are 

 separated by a spot known as the " indifferent point." 



If the nerve is destroyed as locally as possible, without 

 essential alteration of structure at the point corresponding 

 with the electrode distal to the muscle, the extrapolar interference 

 from the kathodic or (according to direction of current) anodic 

 section is eliminated ; but the excitatory action of the intrapolar 



