306 



ELECTRO-PHYSIOLOGY 



CHAP. 



nerve, extends not merely to equality of an- and katelectrotonic 

 deflections, but also to the more or less approximate proportion 

 which exists at a given position of leading-in and galvanometer 

 electrodes, between the magnitude of effect produced and intensity 

 of the polarising current (38). An essential characteristic of 

 " electrotonic " currents, as opposed to ordinary current escape, is 

 presented by the fact that the direction of the extrapolar 

 current led off is determined by the position of the galvanometer 



VJL/ 



FIG. 217. Schema of current diffusion in an ordinary conductor partially traversed by current. 



FIG. 218. Schema of current diffusion in a "core-model." (Griinhagen.) 



current. This is shown by the accompanying schema (Fig. 2 1 7), 

 where it is seen that the extrapolar branches of current led off 

 from opposite sides of the conductor must necessarily be hetero- 

 dromous. On the other hand, this does not occur either in the 

 nerve or in any of the models described above. Whatever the 

 position of the galvanometer electrodes, the current led off is invari- 

 ably homodromous with the polarising current. The sole essential 

 condition is that the axis of a moist conductor should contain a core of 

 better conductivity than the sheath. It is immaterial whether, as in 



