iv GENERAL PHYSIOLOGY OF NERVOUS SYSTEM 247 



pheral regions. The anodic effect is more pronounced than the 

 kathodic. 



Conductivity is also affected by the passage of the polarising 

 current. When the central portion of a nerve is excited by an 

 electrical stimulus of minimal intensity, and the galvanic current 

 then passed through its peripheral part, the muscular reaction 

 diminishes or fails altogether. This effect persists for a short time 

 after opening the current. 



The electrotonic decrease of conductivity is greater in propor- 

 tion to the strength and duration of the polarising current. It 

 appears to be associated with the fall of excitability at the anode 

 on closing the circuit, which is not compensated by the rise of 

 excitability and conductivity at the kathode. This can be 

 demonstrated as follows : A polarising current is sent through the 



s.c. 



FIG. 159. Diagram of tripolar application of polarising current to nerve. (Danilewsky.) 

 s.c., exciting current ; m, muscle ; a, k, lateral electrodes joined together, connected with 

 kathode ; c, central electrode connected with anode. 



frog's nerve-muscle preparation by means of three electrodes (as 

 shown in Fig. 159), the two side electrodes being connected with 

 the kathode of the cell and the middle electrode with the anode. 

 In this case the katelectrotonic effect prevails over the auelectro- 

 tonic, because the kathodic region is more extended than the anodic. 

 If a point of the nerve remote from the muscle be now excited the 

 response of the muscle is greater than usual, owing to the kat- 

 electrotonic rise of excitability and conductivity. If the experi- 

 ment is reversed by putting two positive electrodes at the sides 

 and one negative in the middle, the opposite result appears, i.e. 

 the response of the muscle is less than normal, owing to the 

 preponderance of anelectrotonus over katelectrotonus. 



The polar electrotonic changes affect not only the amplitude 

 of the reaction, but also the velocity of conduction. On closing 

 the polarising circuit there is acceleration at the kathode and 

 delay at the anode, except where the effects at the two poles are 

 in perfect equilibrium, when the rate of conduction remains 



