CHAPTER XI 

 VITAMIN A-DEFICIENCY AND THE RETINA 



Visual Thresholds. Night blindness as a functional malady 

 has been known for centuries and its relation to poor nutri- 

 tion recognized. Only in recent years, however, has this 

 relation been traced specifically to vitamin A in the diet, 

 and become understandable in terms of the chemical relation 

 between vitamin A and the light-sensitive pigments of the 

 retina. Fridericia and Holm (1925) and Tansley (1931) 

 showed that animals deprived of vitamin A synthesize visual 

 purple more slowly than normal animals, and, in cases of 

 extreme deprivation, may be unable to form any visual 

 purple at all (Tansley, 1933a). In human beings night blind- 

 ness has been experimentally produced by an A-deficient 

 diet by Jeghers (1937), Hecht and Mandelbaum (1939), and 

 Wald, Jeghers, and Arminio (1938), and the condition has 

 also been remedied by the resumption of a normal diet. The 

 investigations of Wald (1935a, 19356, 1936) have shown that 

 vitamin A is an essential ingredient of the chemical make-up 

 of the rod visual system by demonstrating that vitamin A 

 is not only the precursor of rhodopsin (or of porphyropsin), 

 but, as well, the product of its decomposition. These rela- 

 tions have been discussed in Chapter X where both the 

 rhodopsin and porphyropsin rod visual cycles, as studied 

 spectrophotometrically by Wald, have been given. 



Not only has vitamin A-deficiency been found to raise 

 the rod visual thresholds, but it has been shown to affect 

 cone vision in like manner. This indicates that vitamin A 

 enters into the chemical cycle of cone vision and that the 

 cone photosensitive substance, iodopsin or visual violet 

 (Wald, 19376) is very likely a conjugated carotenoid protein 

 as is rhodopsin and porphyropsin. 



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