242 PHYSIOLOGY CHAP. 



current through it must be accompanied by these polarisation 

 phenomena. 



When metal electrodes are applied to a nerve, the principal 

 seat of polarisation is the surface of contact of the electrodes 

 with the fluids of the nerve, which is therefore called external 

 polarisation. The intensity of this polarisation can be reduced 

 by employing currents of very brief duration (induced currents), 

 alternating in direction, and of approximately equal strength 

 (sinusoidal currents). It can be practically abolished by using 

 unpolarisable electrodes (see Fig. 45, p. 71). 



When a current is passed through a nerve by means of 

 unpolarisable electrodes, so that external polarisation is abolished, 

 internal polarisation, so-called, will still be manifested; it is 

 specially conspicuous in nerves with rnedullated fibres, and arises 

 from their peculiar structure. In this case, too, the electrolytic 



Anelectrotonic. Polarising Katelectrotonic 



current. current. current. 



FIG. 155. Diagram of electrotonic currents, to show polarising current thrown into median 

 portion of an exposed nerve ; anelectrotonic current led off to galvanometer from anodal 

 portion ; katelectrotonic current led off to galvanometer from kathodal portion of the 

 nerve. (Waller.) 



effects of the passage of current are more pronounced at the 

 poles, i.e. at the points of entrance (anode) and exit (kathode) of 

 the current, whence they spread with diminishing intensity, not 

 only in the intrapolar, but also in the extrapolar parts of the 

 nerve. The displacement of the electrolytic products or ions in 

 the direction of the poles during closure of the current is shown 

 in the intrapolar tract by a rise of electrical resistance (diminished 

 current intensity) and in the extrapolar tracts by electrical 

 currents which are in the same direction as the polarising current 

 when led off to the galvanometer (Fig. 155). These currents are 

 known as anelectrotonic currents in the extrapolar tract corre- 

 sponding to the anode, and katelectrotonic currents in the part 

 corresponding to the kathode. 



The strength of the electrotonic currents increases with the 

 strength of the polarising current, with diminished distance 

 between the galvanometer electrodes and those of the cell, lastly, 

 with increased length of the intrapolar tract. They do not 

 appear when the polarising current is passed transversely through 



