80 VERTEBRATE PHOTORECEPTORS 



Earlier workers apparently paid little attention to the mam- 

 malian rod, and the data are sparse. In my own investiga- 

 tions I have been unable to find any responses of the 

 rod to light in the eyes of mice, rats, and the bat (Detwiler, 

 1924). 



It can be concluded, then, that when photomechanical 

 responses do occur in response to light stimulation, the pig- 

 ment migrates forward, the cones contract and the rods 

 elongate or fail to respond (e.g. urodeles and mammals). In 

 dim light or darkness the typical response is the reverse, 

 viz., the pigment contracts, the cones elongate, and the rods 

 contract (Figures 59 and 60). 



These changes have been shown to be affected by condi- 

 tions other than light and darkness. For example, tempera- 

 ture has been found to play a significant role. Arey (1916a) 

 showed in fishes that both in light and darkness, the retinal 

 pigment exhibits greater expansion at low than at high 

 temperatures. He showed also that in darkness, the retinal 

 pigment of the frog undergoes striking expansion between 

 0° and 14° C and between 19° and 33° C. At the intermediate 

 temperatures of 14° to 19° C it is highly contracted. Tem- 

 perature was found to have no effect upon light-adapted 

 eyes. Essentially similar results were obtained on the frog 

 by Detwiler and Lewis (1926). In these experiments, how- 

 ever, we found that in previously light-adapted eyes, the 

 retinal pigment will contract after one hour's exposure to 

 darkness between the temperatures of 18° and 23° C, thus 

 showing a wider temperature range for contraction than 

 found by Herzog (1905) and Arey (op. cit.). 



Arey found that the cone myoids of fishes in the dark 

 shorten at low temperatures and elongate at high tem- 

 peratures. Temperature was found to be ineffectual in the 

 light. He showed also that the rods of fishes elongate at 

 high temperatures and shorten at low temperatures, but 

 the extent of change is much less than in the cones. 



Carbon dioxide and ether were found to block completely 

 the movements of all retinal elements in the intact and 



