41 6 COMPARATIVE ELECTRO-PHYSIOLOGY 



rest, which, as we have already seen, does not constitute a 

 very definite or unvarying standard. A still further source 

 of complication is introduced again when, under certain 

 internal changes in the retina itself, the normal response is 

 found to be reversed. 



The present inquiry, then, resolves itself into the follow- 

 ing questions : (i) What is the true current of rest, and do 

 the various currents which have been observed really fall 

 under this head or not? (2) What is the galvanometric 

 character, positive or negative, of the excitatory reaction of 

 the retina that is to say, is the excitatory phenomenon in 

 the eye similar to, or different from, that of other tissues ? 

 (3) May we not discover, in the case of the retina, those mul- 

 tiple excitatory reactions which we have already found to be 

 induced by light in vegetable tissues ? (4) And, lastly, what 

 are the after-effects of this particular stimulus in the retina? 



Of these we shall deal first with the question of the direction 

 of the current of rest. In an eyeball which is isolated entire 

 it has been found that the nerve is negative to the cornea, 

 and the current of rest is therefore believed to flow through 

 the eye from nerve to cornea. In an isolated nerve-retina 

 preparation, on the other hand, the rod-surface of the retina 

 is negative to the nerve, the current of rest being thus from 

 retina to nerve. These observed currents, however, may not 

 be true currents of rest, but rather excitatory after-effects, 

 due to injury in preparation. In such cases we have seen 

 that the naturally more excitable becomes persistently nega- 

 tive. We have also found that the true current of rest flows 

 from the less to the more excitable, the latter being thus 

 galvanometrically positive. In order, therefore, to determine 

 the true nature of the resting-current, we have first to deter- 

 mine which of each two surfaces, nerve and cornea, and 

 nerve and retina, is the more excitable. 



I have already described how, by means of equi-alter- 

 nating electrical shocks, the differential excitability of a 

 preparation can be determined. We saw also that under 

 such conditions the resulting responsive current flows from 



