SECTION VIII 

 POLARISATION PHENOMENA IN NERVE 



ELECTROTONIC CURRENT. If a constant current be passed 

 through a nerve fibre through the electrodes x and y x being 

 the anode and y the cathode and the extrapolar portions of the 

 nerve ab, cd be connected with galvanometers, the needles of both 

 are deflected, and the direction of the deflection shows the existence 

 of a current in the extrapolar portions of the nerve a to &/ and 

 from c to d. 



' G2 



FIG. 125. Diagram showing electrotonic currents, p, polarising circuit ; 

 G 1 , G 2 , galvanometers. 



The galvanometers will indicate, before the passage of the polarising current, 

 the ordinary demarcation current of the nerve resulting from the cross-section 

 at the upper end. This current flows, in the outer circuit, from equator to cut 

 end, and therefore in the nerve-fibre from a to b, and from d to c. The 

 effect of closing the polarising current will be to increase the current of rest 

 between a and b, and to diminish that between c and d. 



We thus see that the passage of a current through a part of a 

 nerve gives rise to a current flowing through a considerable portion 

 of the nerve fibre on each side of the polarising current and in the 

 same direction. This current is called the electrotonic current. It 

 must not be confounded with the current of action, which originates 

 at one of the poles, only at make or break of the current, and is trans- 

 mitted thence in the form of a wave with a measurable velocity (in the 

 frog) of about 30 metres per second. The electrotonic current is 



315 



