568 COMPARATIVE ELECTRO-PHYSIOLOGY 



with the normal responses in a. The application of higher 

 polarising E.M.F. of '5, I, and 1*5 volts respectively, now 

 induced appropriate increments of conductivity, as seen in c, 

 d, and e. I was unable to use an E.M.F. of higher than 

 1*5 volts because the galvanometer spot of light became 

 unsteady. It is to be borne in mind that the specimens in 

 these experiments were 20 cm. long, and the maximum 

 potential gradient employed was only '07 volt per cm. In 

 experimenting on electrotonic effects, it must be remembered 

 that the E.M.F. employed is, generally speaking, feeble. 



FIG. 347. Photographic Record showing. Enhanced Conduction from 

 Kathodic to Anodic Region 



a, Normal responses ; b t c, d, e, Responses gradually enhancing under 

 increasing polarising current. 



From the results described, then, we arrive at the follow- 

 ing law of the effect of a moderate or feeble constant electric 

 current on conductivity. 



A moderate polarising E.M.F. induces variations of conduc- 

 tivity. The conductivity is increased in the direction from the 

 kathodic to the anodic region, and depressed in the opposite. 



Having thus demonstrated the pure effect of a constant 

 electric current on conductivity, we have next to study the 

 unmixed effect of polarisation on excitability. We have 

 seen that when two equally excitable points in the same 

 circuit are simultaneously excited by an identical stimulus, 

 there is no resultant response, since the two excitatory 



