,^\^^L 



CHAPTER VII 



ON THE SIGNIFICANCE OF PHOTOMECHANICAL 

 MOVEMENTS 



Much has been written concerning the functional signifi- 

 cance or adaptiveness of the photomechanical changes in 

 the vertebrate retina. With respect to the various theories 

 Arey (1915) made an apt statement when he said ''. . . that 

 many such explanations reveal the resourcefulness of the 

 human mind rather than the ingenuity of nature." But as 

 he pointed out further, suggestive and stimulating hypotheses 

 serve their purpose by thus bringing the possible dynamic 

 interrelations to light. 



The various explanations held to account for these photo- 

 mechanical changes have never fully satisfied all the condi- 

 tions, mainly due to the lack of uniformity of the light 

 response throughout the vertebrate groups. 



Kiihne (1878), who is named as one of the discoverers of 

 pigment migration, thought that the chemical products 

 resulting from the decomposition of retinal pigment through 

 the action of light, stimulated the visual cells sufficiently to 

 initiate the sensation of vision. In cases of albinism where 

 pigment is absent, other light stimulating substances were 

 postulated as present, in addition to visual purple and the 

 brown pigment, fuscin. Essentially similar views were 

 postulated by Boll (1881) and by Gad (1894). 



Ayres and Kiihne (1878) argued for a relationship between 

 retinal pigment and the regeneration of visual purple. They 

 compared the pigment cell to a gland and supposed that the 

 epithelial pigment cells secreted visual purple. The main 

 support for their theory was based upon their observation 

 that the use of pilocarpin was found to reduce considerably 

 the time required for regeneration of visual purple in the 



