THE RETINA AND VISUAL SENSATIONS. 1045 



THE RETINA AND VISUAL SENSATIONS. 



Physiological Retinal Processes. 



Visual purple. — The most obvious change which occurs when light 

 acts upon the retina is the bleaching of the visual purple or rhodopsin 

 contained in the outer limbs of the rods. The existence of this sub- 

 stance was first described by Boll in 1876, and its properties have been 

 most fully investigated by Kuhne. 1 In the process of bleaching an 

 intermediate substance, visual yellow is formed. Visual purple varies in 

 colour to some extent in different animals, inclining more to violet in 

 some, including man. The red colour in some cases is ascribed by 

 Kuhne to mixture with visual yellow. Green rods occur amongst the 

 others in amphibia. The bleaching is limited to the part of the retina 

 on which light falls, and produces an optogram, or image of an external 

 object, both in the living and the recently killed animal. The eye 

 should be kept from the light for ten minutes, then exposed for about 

 three minutes in a room lighted by one window, so that an image of the 

 window falls on the retina ; the eyeball should then be extirpated by the 

 light of a sodium flame, and the retina laid in a 4 per cent, solution of 

 alum. The eye of a recently killed animal will then show a well- 

 defined colourless optogram, while in an eye exposed during life it will 

 be still pink and less sharply defined. 



The bleaching of visual purple during life cannot be brought about 

 in any other way than by the direct action of light. Exposure of one eye 

 only to light does not lead to any change in the purple colour of the other 

 eye, nor is any change produced by exposure of the body to light. 



Regeneration of visual purple takes place in both the living and 

 extirpated eye, when kept in the dark ; it also occurs in red light. Boll 2 

 supposed that red light acts as a stimulus to regeneration, but from ex- 

 periments with optograms Kuhne found that, with moderate intensities, 

 regeneration takes place at the same rate in red light as in the dark, and 

 with brighter red light more slowly. Regeneration seems to depend upon 

 some substance derived from the pigment epithelium, to which Kuhne 

 has given the name rhodophyllin. If a piece of the retina be separated 

 from the underlying pigment layer, bleached by light, and then replaced 

 on the pigment, regeneration takes place, and even without replacement 

 regeneration may occur, probably owing to the particles of pigment 

 retained between the rods. In some cases this takes place when no 

 pigment seems to be present, especially in the lower half of a retina 

 placed vertically, and Kuhne supposes that in this auto-regeneration, 

 which takes place very slowly, rhodophyllin is present in the rods. 

 Regeneration is uninfluenced by light applied to the other eye, or to 

 another part of the same eye. Both bleaching and regeneration are thus 

 independent of the nervous system, and visual purple has been found to 

 be present a year after division of the optic nerves. Regeneration is 

 hastened by pilocarpin and muscarin, probable owing to increase of the 

 secretory activity of the pigment epithelium. 



Rhodophyllin is apparently, like visual purple itself, soluble in bile 

 salts, for a solution of visual purple in bile salts is capable of regeneration after 



1 r>rfrrsn,ch. a. d. phyainl. Inst. d. Univ. Heidelberg, 1878-82. 

 "Arch./. Physiol., Leipzig, 1877, S. 4. 



