648 COMPARATIVE ELECTRO-PHYSIOLOGY 



which we saw that the two nervous impulses were exhibited 

 by two opposite electro- motive variations those of galvano- 

 metric positivity and negativity respectively. This reaction 

 of expansion and galvanometric positivity, however, may 

 also occur as the expression of the increase of internal 

 energy, in whatever way produced. Indeed, the positive 

 form of response under moderate stimulation may be 

 regarded as a case falling within this definition. Thus we 

 sec that, beginning with very moderate stimulus, we obtain 

 in the tissue a purely positive effect ; and that, as the 

 stimulus is augmented, the true negative excitatory effect 

 also begins to make its appearance in increasing degree, the 

 positive component of the response being now more or less 

 masked. The energy that afterwards remains latent in the 

 tissue goes to enhance the tonic condition. The amount 

 thus held latent depends on the difference between income 

 and expenditure. As a general rule, it will be under 

 intense stimulation that the expenditure of energy will be 

 likely to exceed the income. Thus we have two extreme 

 cases, first, that in which moderate stimulus brings about 

 increase of energy ; secondly, that in which excessive stimulus 

 brings about run-down of energy ; and between the two a 

 large range of variation, within which either one condition or 

 the other may predominate. It must, of course, be under- 

 stood that anything which increases the tonic condition is for 

 the well-being or health of the organism, and is associated 

 with positivity. Similarly, any fall of the tonic condi- 

 tion below par makes for exhaustion and against healthy 

 tone. 



We have next to take a rapid survey of the changes 

 induced by stimulus in the conducting nerve itself, or any of 

 its attached indicators. Such variations may, for purposes of 

 convenience, be classified as motile, electrical, and sensory. 

 In the nerve itself we have found, as has already been 

 pointed out, by means of the Kunchangraph, that the motile 

 change induced by feeble stimulus was one of expansion, the 

 same change being shown electrically by galvanometric 



