164 RESPONSE IN THE LIVING AND NON-LIVING 
Abnormal effects—We have already treated of all 
the normal effects of the stimulus of heht 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. 
1. Preliminary negative twitch.—When the light is 
incident on the froe’s retina, there is sometimes a 
transitory negative variation, followed by the normal 
: } e ae 
IE. L. DL. 
Fic. 106.—TRanstent Positive AUGMENTATION GIVEN BY THE F'ROG’s RETINA 
ON THE Cessation oF Licur L (WALLER) 









































positive response. This is frequently observed in the 
sensitive cell (see fig. 96, 0). 
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. 
