POLARISATION PHENOMENA. 585 



In the case of polarising currents which traverse the discs, the most 

 appropriate nomenclature for indicating their direction is that first 

 employed by du Bois-Keymond, who termed all currents traversing the 

 discs in the same direction as those produced by the functional organ 

 response, homodromous, whilst those of opposite direction were described 

 as heterodromous. 1 



In Torpedo organ it is found that homodromous currents excite a 

 response, on the whole more effectually than heterodromous ones ; with 

 the former the trunks of the nerve fibres entering the discs, to be dis- 

 tributed on its ventral aspect, are evidently in the region of the anodic 

 polarisation, whilst the terminations of such nerves in the disc would 

 lie in regions of cathodic polarisation. 2 The reverse will be the case 

 with heterodromous currents, the nerve-endings being now anodic rather 

 than cathodic. Any increased exciting efficiency of homodromous cur- 

 rents may be thus reasonably supposed to be related to cathodic polar 

 changes in the nerve-endings contained in these. It appears, however, 

 from recent experiments by Schonlein, that there is considerable varia- 

 tion as regards exciting efficiency, and it is extremely doubtful if any 

 definite augmentation of excitability can be produced by homodromous 

 currents. 



In Malapterurus organ the present writer has always observed that 

 the heterodromous current excites better than the homodromous. It 

 must be remembered that the discs in this organ are peculiar, since the 

 nerve fibres end in the caudal stalk and do not ramify in the expansion 

 of the disc. The current due to functional activity is directed through 

 the discs from the cephalic to the caudal aspect ; this would therefore 

 be the direction of homodromous exciting currents. On the passage of 

 such homodromous currents, the nerve-endings would, owing to their 

 position, be in an anodic region, whilst, with heterodromous currents, 

 the nerve-ending layer would, for similar reasons, be within the cathodic 

 region. We should therefore expect, provided these regions are the 

 seat of polar changes in excitability during the flow of a current, that 

 the heterodromous would be more effectual for evoking the response than 

 the homodromous. Such results as are definite both in Malapterurus 

 and Torpedo are thus consistent with the view that polar changes of 

 excitability may occur in the nerve-endings of electrical organs. 



This is confirmed by observations upon the exciting efficiency of 

 currents applied to the nerve trunk outside the organ. It has been 

 found by Schonlein that an ascending galvanic current (directed up 

 the electrical nerve) evokes a response in Torpedo of much less marked 

 character than one directed down the nerve. 3 In the former case, 

 the peripheral nerve branches are in the region extrapolar to the 

 anode (anelectrotonic), in the latter catelectrotonic, and the nerve- 

 endings undoubtedly share in the extrapolar state. The augmented 

 response to the descending current, and the diminished response to 

 the ascending one, appear to indicate that the excitability of the 

 nerve-endings in the supplied discs is augmented in the one case, 

 diminished in the other. The result might be due to polar changes in 

 the nerve outside the organ, interfering with the adequate propagation 

 of the excitatory state along the nerve trunk ; but such interference is 

 insufficient to account for the difference in the organ responses, for the 

 following reason. The response to the nerve excitation by the descend- 



1 du Bois-Reymond, loc. tit. 2 Gotcli, loc. cit. 3 Schonlein, loc. cit. 



