512 ELECTRO-PHYSIOLOGY CHAP. 



from another source, or other elements, in consequence of 

 excitation, but solely of alteration in the galvanic effects of the 

 same elements, which must be regarded during rest as the cause 

 of the differences in potential. 



From this point of view, explanation of the actual experi- 

 mental effects consequent on excitation, presents no serious 

 difficulties, even when complicated double, or multiple, variations 

 are exhibited. Taking first the simple case, where, as in the 

 frog's tongue, a strong ingoing current is present from the be- 

 ginning, augmentation of the same, i.e. a positive variation, is 

 only likely to appear when (with direct or indirect excitation) 

 the "negative process" is increased above the "positive," which in the 

 instance cited, where that is already so preponderant, is not very 

 probable ; it seems much more likely that the process which is 

 initially less developed should be increased by excitation than 

 the other. From this standpoint it would also be comprehensible 

 that a " negative variation " should follow upon excitation, the 

 more exclusively and distinctly in proportion as the original 

 current is stronger. That, further, a positive after-effect fre- 

 quently makes its appearance, is also intelligible, as soon as 

 it is realised (as proved by experiment) that the positive 

 effect which depends on augmentation of the " negative process " 

 invariably declines much more slowly than the opposite effect, 

 so that when the one has already returned to its normal, the 

 other, from its greater constancy, entails a positive increment of 

 the original current. 



The conditions of appearance of a positive variation during 

 excitation, either independently or as fore-swing to a subsequent 

 negative variation, are accordingly so much the more favourable 

 in proportion as the homodromous, incoming current is weaker, 

 i.e. as the " negative process " is less preponderant. For obviously 

 there is then greater opportunity of strengthening this latter 

 so much by excitation that it in turn becomes uppermost. 

 Strength of the tetanising current is also an important factor, 

 since it would appear that the process leading to development of 

 negative potential at the surface is, under equal conditions, more 

 easily excited than its opposite, so that, as more especially in the 

 mucosa of throat and cloaca, the positive effect appears with 

 weak, the diphasic or single negative effect with stronger, excita- 

 tion. In particular cases a diphasic effect may of course appear 



