308 ELECTRO-PHYSIOLOGY CHAP. 



and exit of the current are confined essentially to the actual 

 contacts and their immediate vicinity, the polar action of the 

 current finds localised expression 011 the one hand as excitation, 

 on the other as inhibition, only at the points at which it is 

 initiated. But where, as in medullated nerve, the physiological 

 anode or kathode (i.e. the region within which lines of current 

 pass in and out of the excitable substance of the axis-cylinder) 

 has any considerable extension, the same must of course hold 

 good of all the consequences of excitation and inhibition. The 

 spatial extension of physical electrotonus as the sum of all the 

 changes directly produced by the electrical current is, in other 

 words, masked by the spatial diffusion of anodic and kathodic 

 points in the tissue traversed by the current. If it is thus a law 

 for muscle as well as nerve that, within certain limits of current 

 intensity and passage at the physiological kathode (i.e. at every 

 point by which current leaves the excitable substance), there is 

 during closure a condition of augmented " expectancy " - the 

 contrary being the case at the physiological anode we have in 

 this a direct interpretation of the facts of intra- and extrapolar 



alterations of excitability, as diffused 

 antagonistically from the poles in a 

 polarised medullated nerve. This 

 further explains the striking rise of 



^~"^577"[ O t/ 



excitability in the vicinity of each 



FIG. 2i9.-Eiectrotonic diffusion of the artificial cross - section. As in the 



demarcation current along the nerve 



(weak longitudinal currents). (Her- case of an external current, the 



demarcation current of each medul- 

 lated nerve-fibre will not merely equalise itself in the immediate 

 proximity of the demarcation surface, but (again for the same 

 reasons) will give off lines of current to a long distance from 

 the cross -section, as indicated in the accompanying schema 

 (Fig. 219); and these, escaping in all directions from the 

 axis-cylinder, throw the latter into katelectrotonus with all its 

 sequela?, the intensity of the same of course declining rapidly 

 with distance from the cross-section. The so-called weak longi- 

 tudinal currents may therefore, as was first pointed out by 

 Hermann (cf. Fig. 219), be regarded simply as the electrotonic 

 spread of the demarcation current. 



Lastly, there is the fact already alluded to that, in electrical 

 excitation of a medullated nerve that has undergone local morti- 



