668 COMPARATIVE ELECTRO-PHYSIOLOGY 



interchange of the one effect with the other, is that of 

 electrotonus. 



Under the normal condition of a medium intensity of 

 E.M.F., we know that it is the kathode which enhances excit- 

 ability, and the anode which depresses it. Hence an indifferent 

 sensation, however caused, will be rendered painful when the 

 excited point is made kathode, or pleasurable when anode. If, 

 again, the stimulus be of sufficient intensity to render the 

 resulting sensation moderately painful, kat- electrotonus will 

 make it intensely painful, and an-electronus convert it into 

 soothing, by taking away from it the negative element. 



The an- and kat-electrotonic effects referred to here are 

 those which fall under the generalisation made by Pfliiger. 

 I have shown, however, that the application of this law is 

 but limited. Under the action of a feeble E.M.F. these an- 

 and kat-electrotonic effects are exactly reversed, and it is 

 then the anode which renders the tissue excitable, the 

 kathode inducing relative depression. If, then, sensation- 

 changes are dependent, both qualitatively and quantitatively, 

 on antecedent physiological changes, we may expect corre- 

 sponding reversals to take place in sensation, according as 

 the anodic or kathodic applications are feeble or moderate. 



And, lastly, we might expect to meet with other charac- 

 teristic changes in responsive sensation, due to the peculiar 

 differences shown in figs. 320 and 401, between the positive 

 and negative effects. We there saw that the positive 

 response was short-lived, whereas the negative was more 

 persistent. Even the negative response itself further was, 

 under moderate stimulation, of briefer duration than under 

 strong. In accordance with these facts there ought to be a 

 difference in the fusion of sensation. Let us suppose that 

 the frequency of stimulus, for the induction of the indifferent 

 sensation, be adjusted in such a way as to cause a sensation 

 which is so fused as to be almost continuous. Under normal 

 kathode, this response, being converted into more persistent 

 negative, will now become completely fused and painful 

 Under normal anode, on the other hand, the individua 



