CHAPTER X 



RETINAL PHOTOPIGMENTS 



Both rod and cone vision depend upon the decomposition 

 by Ught of a photosensitive substance which occurs in the 

 outer segments of the visual cells. Since the substance is af- 

 fected by visible light, it obviously absorbs visible light and 

 is, therefore, a pigment. It is apparent also that the photo- 

 pigment when broken down must be resynthesized or vision 

 would cease soon after irradiation had begun. The stimulat- 

 ing product or products, as well, must be removed rapidly 

 or vision would continue long after irradiation has ceased. 



In connection with rod function the photopigment rho- 

 dopsin (visual purple) has long been known. This photo- 

 sensitive substance was discovered in the rods of frogs by 

 Franz Boll in 1876. It is a rose-colored substance and was 

 termed visual purple or rhodopsin by Kiihne (1878) who 

 with Ewald studied it in much detail, as did many subse- 

 quent investigators.^ Koenig (1894) first showed the simi- 

 larity of the visibility curve at low illuminations and the 

 curve representing the absorption spectrum of visual purple 

 (Figure 82). The obvious inference to be derived is that 

 visual purple is concerned with vision at low light intensities. 

 Many added physiological facts have completely supported 

 the early-formed impressions. As to the origin of this pig- 

 ment, many diverse opinions have been elaborated. Ayres 

 and Kiihne (1878) maintained that there is a relation between 

 epithelial pigment and the regeneration of visual purple. 

 They compared the epithelial pigment cell to a gland, the 

 secretory product of which is visual purple. This idea was 

 supported by experiments in which the use of pilocarpin 



1 For a recent discussion of the photochemistry of visual purple, v. Krause, A. C, 

 Biol. Symposia, v. 7, p. 23. 1942. 



120 



