MOVEMENTS IX THE VISUAL CELLS 187 



means of 'retino-motor' nerve fibers, as maintained by Engle- 

 mann, is not substantiated. 



16. Within the experimental limits at which fishes can be 

 kept alive, the retinal pigment and visual cells are not affected 

 by an increased or diminished oxygen supply. 



17. Both in darkness and in light, and in excised as well as 

 in normal eyes, carbon dioxide and ether completely check the 

 movements of all the rethial elements of fishes. Chloretone and 

 urethane, on the contrary, are inefficient hi this respect. The 

 action of carbon dioxide suggests that this may be the catabolic 

 ])roduct that in many cases restrains the movements of the 

 retinal elements when the circulation of the blood is interrujited. 



IS. Although the movements of the visual cells and rethial 

 pigment, when ])resent, may have a certain unknown significance 

 in connection with the mechanism of light perception, such 

 movements can be inteipreted at present only in terms of ]iroto- 

 plasmic responses to definite stimulating agents. 



(\nfibri(l(/c, Mass., April 10, 1915. 



I'OSTSrUII'T 



A study of the inllucncc of Hght on the in()\enients of the 

 frog's rod has just been completed by the writer. Careful meas- 

 urements prove that these elements are extended in the light and 

 are retracted in darkness. Hence the results of the older workers 

 (p. 123), who believed that the jihotic responses of the frog's rod- 

 myoid are the re\-erse of those occurring in fishes and birds, are 

 not subslaiitiated. 



