330 ELECTRO-PHYSIOLOGY CHAP. 



physiological current is thus developed in the fibre, in the same 

 direction as the opened foreign current. This physiological 

 current may be termed homodromous, in contradistinction to 

 that previously defined as lieterodromous. It appears the more 

 certainly in proportion as the substance is more energetic ; and the 

 less the vital processes are affected by the foreign current, the 

 more rapidly will the allonomous alterations, induced by the latter 

 (after-effect of excitation), disappear, and the opposite autonomous 

 changes develop, when it is broken. The homodromous physio- 

 logical current is more or less likely to be disturbed by complica- 

 tion with physical polarisation currents, heterodromous to the 

 foreign current. 



" If a foreign current is led through the central portion of a 

 medullated nerve, the points by which it enters and leaves the 

 excitable matter spread far beyond the contacts of the physical 

 electrodes. So far as these points of entrance and exit extend, 

 there is correlatively with the distribution of the lines of current 

 a purely physical ' an- and katelectrotonus,' as may be de- 

 monstrated, e.g. on a dry hollow stalk of grass without internodes, 

 or on a bundle of the same stalks that have been lying for some 

 time in distilled water, or weak alcohol, and are then moistened 

 externally, and saturated internally, with salt solution. From 

 this wide distribution in the excitable substance (axis-cylinder) of 

 the nerve, of the collective points at which the foreign current 

 enters and leaves it i.e. the physiological anode and kathode 

 proper those ' up ' and ' down ' changes develop respectively in 

 the. nerve, which are fundamental to physiological electrotonus 

 (Pfliiger). Both down and up change may, after closure of the 

 foreign current, be transmitted along the nerve beyond the tracts 

 altered in a kathodic (negative) or anodic (positive) sense by the 

 direct action of the current, so that fugitive alterations may occur 

 even at very remote parts of the fibre, as expressed in its electro- 

 motive reactions. On breaking the foreign current, an opposite 

 alteration takes place at the points of entrance and exit, with 

 corresponding changes in the living matter, 'i.e. autonomous 

 descending or ascending alterations respectively. The two points 

 have interchanged their parts ; the ascending alteration, character- 

 istic of physiological anelectrotonus, now appears at the former 

 kathode, the descending alteration, significant of physiological 

 katelectrotonus, at the former anode. 



