78 THE VISUAL PROCESS 



The substance whose lack is the usual cause of nyctalopia was shown 

 in 1925 to be vitamin A, a colorless material manufactured in the liver 

 from carotene, a reddish plant pigment. Although there are types of 

 nyctalopia which are hereditary, and the condition also occurs as a symp- 

 tom of degenerative retinal diseases, in its various degrees it is usually 

 the first detectible sign of vitamin A deficiency. Nutritionists and pedia- 

 tricians are consequently much interested in attempts to devise clinical 

 tests— by which they mean quick and easy ones — for nyctalopia; but 

 for various reasons a reliable test which is really simple seems hardly 

 possible, and the literature of the subject reveals more and more pessi- 

 mistic statements. 



Soon after 1925, the obvious conclusion was drawn that vitamin A 

 is the precursor of rhodopsin, that it is actually converted into that 

 substance, and may be formed again when rhodopsin is disrupted by 

 light. Elaborate diagrams of this closed circuit, with the supposed 

 intermediate compounds, are commonly seen in print. But the most 

 recent and careful chemical studies of rhodopsin itself (r. s) have great- 

 ly weakened our faith in a direct genetic relationship between it and 

 vitamin A. All that can be safely said at the moment is that the vitamin 

 is essential for the synthesis of rhodopsin, probably as a minor contrib- 

 utor rather than as a principal raw material. 



Rhodopsin may be the essential, the one and only photochemical sub- 

 stance that is ever present in rods, but there is no proof that this is 

 so. There are rods which contain none, though perhaps in all of these 

 {e.g., in Sphenodon, Xantusia, Phyllorhynchus) the lack of rhodopsin 

 is owing to these rods' having had relatively recent origin from cones. 

 They presumably get along perfectly well with the photochemical system 

 inherited from their cone ancestors — for all anyone knows at present, 

 the complete color-vision mechanism may still be functioning in them. 

 The photochemical substance or substances in cones may indeed have 

 chemical kinship with rhodopsin, for it has recently been reported that 

 the dark-adaptibility of the cones (which in terms of intensity-limit ratios 

 is actually about equal to that of the rods) is influenced by the dietary 

 intake of vitamin A. 



Just a few years ago, it was being claimed by the Finnish retinal 

 physiologists associated with Ragnar Granit that when a rat retina has 

 been so brightly illuminated that all of the rhodopsin is bleached, the 

 optic nerve no longer carries the electrical discharges which can normallv 

 be detected in it during photic stimulation of the retina. This was hailed 



