RESPONSE OF RETINA TO STIMULUS OF LIGHT 423 



response which is positive, followed by a second which is 

 negative (expansion followed by contraction). 



The second condition to induce a reversal of the normal 

 response occurs in consequence of the fatigue due to previous 

 over-stimulation. 



(3) A reversed positive response (expansion) in conse- 

 quence of fatigue. 



The various anomalies which occur in the response of the 

 eye will all be found resolvable into one or other of these 

 cases. I shall first describe certain experiments which will 

 demonstrate the reversal that is brought about by induced 

 sub-tonicity. We have seen that the normal response of an 

 eyeball, with two contacts at nerve and cornea, consists 

 of a current from the nerve to the cornea. I have already 

 given records of such normal responses, obtained by sub- 

 jecting the preparation to equi-alternating electric shocks 

 (fig. 249). In certain cases, however, in which the isolated 

 eyeball of the frog had fallen into a sub-tonic condition, the 

 response was found to be reversed, the nerve, under excita- 

 tion, becoming relatively positive instead of normally nega- 

 tive. It has already been said that such a reversal is due to 

 great depression in the condition of the nerve. In dealing 

 with these cases, therefore, it occurred to me that it ought to 

 be possible to restore the normal response by the application 

 of some exciting reagent say, dilute Na 2 CO 3 to the nerve 

 in its depressed condition. As the result of this application 

 I found the reversed response to be restored to the normal. 



I obtained results precisely similar to these with isolated 

 retina of the frog. The normal responses a current from 

 retina to nerve (fig. 238) under equi-alternating shocks, 

 were here found, in a depressed specimen, to be reversed. 

 But the application of Na 2 CO 3 solution on the retinal surface 

 brought the responses back to the normal. These abnormal 

 responses, then, rectifiable to normal, are those due to sub- 

 tonicity. And under fatigue finally, induced by long- 

 continued, or over stimulation, I find the normal response 

 of the eye to be reversed. 



