1 64 RESPONSE IN THE LIVING AND NON-LIVING 



Abnormal effects. We have already treated of all 

 the normal effects of the stimulus of light on the retina, 

 and their counterparts in the sensitive cell. But the retina 

 undergoes molecular changes when injured, stale, or in a 

 dying condition, and under these circumstances various 

 complicated modifications are observed in the response. 



i. Preliminary negative twitch. When the light is 

 incident on the frog's retina, there is sometimes a 

 transitory negative variation, followed by the normal 



L. L. L. L. L, 



FIG. 106. TRANSIENT POSITIVE AUGMENTATION GIVEN BY THE FROG'S KETINA 

 ON THE CESSATION OF LIGHT L (WALLER) 



positive response. This is frequently observed in the 

 sensitive cell (see fig. 96, b). 



2. Reversal of response. Again, in a stale retina, 

 owing to molecular modification the response is apt to 

 undergo reversal (Waller). That is to say, it now 

 becomes negative. In working with the same sensitive 

 cell on different days I have found it occasionally 

 exhibiting this reversed response. 



