ELECTROTONUS 565 



reduced to nil. The excitation from the stimulator is 

 transmitted across the intervening conducting region, either 

 along the slope of a falling electrical potential, that is to 

 say, from a galvanometrically positive to a galvanometrically 

 negative point, or against that direction, namely, in an 

 electrically uphill manner, from the galvanometrically 

 negative to the galvanometrically positive. Now, if the 

 direction of an electrical current have an effect on the 

 conduction of excitation, this fact will be detected by 

 the modification induced in the normal response during the 

 passage of the current. 



The results of the present experiment will be found to 

 determine this question. Excitation was induced by means 



A K 



FIG. 343. FIG. 344. 



Figs. 343, 344. Experiment with Petiole of Fern demonstrating Variation 

 of Conductivity by Polarising Current 



of the thermal stimulator, and the normal responses taken, 

 shown in fig. 345 a, as ' up.' The polarising current was 

 now sent from left to right ; hence excitation will now be 

 transmitted through the intervening conducting region in an 

 electrically downhill manner, or in the direction of the falling 

 potential that is to say, from the region of the anode to that 

 of the kathode. It will be seen presently that conduction is 

 retarded or abolished when excitation is made to travel elec- 

 trically downhill from the anode to the kathode. 1 If this be 

 so, we shall expect to detect the fact by the diminution of 

 the amplitude of the normal response, or even by its actual 

 reversal. For we have seen that when the excitatory re- 

 action of galvanometric negativity is sufficiently retarded, its 

 opposite, the positive effect, often makes its appearance alone. 



1 These remarks apply to a feeble or moderate rate of fall of potential. 



