4i8 



COMPARATIVE ELECTRO-PHYSIOLOGY 



is not the true resting-current, but rather an excitatory effect 

 consequent on preparation, is supported by the known fact 

 that it undergoes a decline more or less rapid. On the other 

 hand, from the fact just demonstrated that the nerve is the 

 more excitable, we should expect that, under natural or 

 primary conditions that is to say, in the absence of excitatory 

 disturbance the resting-current would be from the less to the 

 more excitable, or in other words, from cornea to nerve. This 

 conclusion I was able to verify by carefully dissecting away 



the socket of the frog's eye, and 

 making connections with the 

 longitudinal surface of the un- 

 detached nerve and the cornea. 

 Under these ideal conditions 

 the true resting-current was 

 detected, and was, as expected, 

 from the cornea to the nerve. 



In order, next, to determine 

 whether the current, observed to 

 flow, in a nerve-retina prepara- 

 tion from retina to nerve, is a 

 true resting-current, or merely 

 an excitatory after-effect, I pro- 

 ceeded to determine, in the 

 manner already described, which 

 of the two surfaces was the more 

 excitable. Under the excita- 

 tory effect of equi-alternating shocks, the responsive current 

 was found to flow from the retina to the nerve, thus 

 proving that the retina was the more excitable (fig. 250). 

 The first series of responses in fig. 251 gives a record of these 

 effects with a moderate intensity of stimulation, while the 

 second series shows the responses under an intensity nearly 

 twice as great. The so-called current of rest observed in the 

 nerve-retina preparation is thus to be taken as due to that 

 after-effect of preparation which is inseparable from the 

 isolation of such a highly excitable tissue. // will also be 



i' <? 



FiG. 250. Experimental Arrange- 

 ment for Demonstration of 

 Differential Excitability as be- 

 tween Retina and Optic Nerve 



c, so-called current of rest ; R, re- 

 sponsive current. 



