RESPONSE BY POSITIVE VARIATION 



177 



preparation. Under normal conditions the primary or natural 

 current C is from the less excitable A to the more excitable B, 

 represented by A -> B (fig. 1 19, a). The action-current R, being 

 in the opposite direction B -> A, constitutes a negative variation. 



But owing to the after-effect of excitation, such as may 

 occur in isolating the specimen by section, the normal resting 

 current C is reversed to B - A (fig. 1 19, b}. Here the end B may 

 still be the more excitable of the two, hence the action-current 

 B -> A will constitute a positive variation of the current of rest. 



But when B becomes fatigued, its excitability is reduced 

 below that of A; hence the action-current is from the 

 relatively more to less excitable, i.e. A->B. In this case, 



(a) 



FIG. 119. Typical Cases of Variation of Current of Rest and Action- 

 Current. Anisotropic organ, B end originally more excitable than A 



(a) Current of rest A->B ; action- current, R, in opposite direction ; response 

 by negative variation, (b) Owing to excitatory after-effect, current of 

 rest reversed to B-A ; B nevertheless more excitable than A ; action- 

 current, R, is B->A ; response by positive variation, (c ) Current of 

 rest reversed B->A ; action-current also reversed A->B, by depression 

 of excitability of B, owing to fatigue ; response by negative variation. 



on account of the reversal of both the current of rest and 

 action-current, the latter appears to constitute a negative 

 variation of the former (fig. 1 19, c). 



It will thus be seen how intricate and diverse are the 

 responsive variations of the resting current, induced by 

 stimulus. Sometimes negative, sometimes positive, it would 

 appear as if there were no guiding principle to regulate these 

 phenomena. The so-called explanations hitherto attempted 

 have consisted in assigning the positive variation to a 

 hypothetical process of assimilation, and the negative to 

 dissimilation. Such explanatory phrases reach the climax 

 of absurdity when we find ourselves compelled to ascribe 

 one identical excitatory reaction now to assimilation and 

 then to dissimilation. 



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