726 COMPARATIVE ELECTRO-PHYSIOLOGY 



shocks, the response shows a serration of the apical line, 

 proving that the individual responses are not completely 

 fused. On the abrupt cessation of tetanising shocks a 

 sudden enhancement of the contractile effect occurs, followed 

 by the usual recovery. This is analogous to the sudden 

 enhancement of response on the cessation of tetanisation, 

 seen in the retina, in magnetic response, and in the response 

 of certain sensitive inorganic preparations under similar 

 circumstances (pp. 536, 428, 383). 



The next subject to be surveyed is that of electrotonus, 

 and the variations in excitatory effects induced by it. It has 

 been shown that the polarising currents induce extra-polar 

 currents in the plant nerve, exactly as in that of the animal 

 (fig- 338). As regards the effect of electrotonic currents 

 generally on excitability, the results obtained by Bernstein 

 are described as polarisation-decrement, whereas those of 

 Hermann are known as polarisation-increment. That is 

 to say, with one experimental arrangement the induced 

 electrotonic current is seen to undergo a diminution under 

 excitation, and with a different arrangement an increase. 

 These and other electrotonic variations appear to be very 

 anomalous, and incapable of mutual reconciliation. 



I have, however, been able to show that all these effects 

 may be regarded as combining the variations of two distinct 

 factors, namely conductivity and excitability, under the 

 influence of an electrical current. One of the principal 

 difficulties in the correct explanation of these phenomena 

 has hitherto lain in the assumption that Pfliiger's Law, 

 relating to the polar effects of currents, was of universal 

 application. I have shown, however, to the contrary, that 

 it applies only to a certain middle range of electro-motive 

 intensity, the excitatory effect at anode and kathode being, 

 at a very high E.M.F., exactly reversed. Going, again, to the 

 other extreme, of a low electro-motive force, I have shown 

 that, in opposition to Pfliiger's generalisation, it is the anode 

 that enhances the excitability of a nerve, while the kathode 

 depresses it. This I have been able to demonstrate by 



