ELECTROTONUS 563 



the extra-polar circuit, by reversal of polarising current, 

 the an-electrotonic current, opposed in direction to the former 

 kat-electrotonic, also undergoes diminution on excitation 

 of the nerve (fig. 340). It has been suggested that this 

 diminution of electrotonic current was due to a supposed 

 diminution, during excitation, of the susceptibility of the nerve 

 to polarisation. 



But this explanation is negatived by an experiment of 

 Hermann, showing the occurrence of polarisation increment 

 during excitation. In figs. 341 and 342 we have a polarising 

 and exciting circuit in series, excitation being caused by the 



gwam 



M iji 



K 



FIG. 342. 



Figs. 341, 342. Diagrams representing Hermann's Polarisation- 

 increment under Tetanising Shocks 



Inside thin arrow indicates the direction of polarising current ; the 

 outside thick arrow, the direction of excitatory current. 



secondary coil of an inductiorium. With such an arrange- 

 ment, the polarising current, whether from left to right or from 

 right to left, is found, during excitation, to undergo an 

 augmentation. Hermann refers these facts to alterations 

 of intensity in the negative wave of excitation, during its 

 passage through the nerve, when the latter is polarised. ' It is, 

 indeed, more pronounced at any point of the nerve, the more 

 strongly positive and weakly negative the polarisation of the 

 latter, i.e. it increases when it is becoming algebraically 

 more positive, and diminishes when it advances upon more 

 negative points ' (Hermann's Law of the ' Polarisation 

 Increment' of excitation). 1 



It would thus appear that the observations hitherto made, 

 as to the effects of electrotonus on excitatory response, are of 



1 Biedermann, Electro-physiology (Engl. transL). vol. ii. p. 315. 



002 



