VIII. VITAMIN A DEFICIENCY IN HUMAN I5KINGS 141 



"keratinizing metaplasia," l'rc(iiicntly used to describe the reaction of the 

 epithehal surfaces. This may be looked upon as the easiest type of struc- 

 tural pattern for the basal cells to produce and maintain; a reaction to 

 adverse conditions comparable, in a sense, to encystment in lower forms 

 of life. 



Despite the striking alterations often observed, vitamin A therapy even- 

 tuall}'' results in complete restoration of the epithelium previously existing 

 at the involved surface, provided that repair is not complicated by tissue 

 damage resulting from localized infections and inflammatory reactions. Of 

 particular interest is the fact that cells of the stratum germinativum of the 

 metaplastic epithelium preserve, for an indefinite period, the potentiality 

 to differentiate and fulfill their destiny whenever an adequacy of vitamin A 

 is acquired by the organism. 



B. NIGHT BLINDNESS 



Recognized in Egyptian medicine as a disorder of the eyes curable by 

 ingestion of animal livers, night blindness represents one of the oldest dis- 

 eases known to man and the first nutritional disorder for which an effective 

 therapy was established. There are many records of its sporadic appearance 

 in populations existing for long periods on severely restricted diets, espe- 

 cially as the result of famine or Avars. It is regarded as one of the most 

 reliable signs of low vitamin A status in man. Since it is not easy to detect 

 in very young infants and appears so insidiously in older children and 

 adults, it has often been overlooked or ignored until the appearance of 

 xerophthalmia and keratomalacia. 



According to their etymology, the terms "nyctalopia" and "hemeralopia" 

 refer to impaired vision in dim light (scotopic or rod vision) and in bright 

 light (photopic or cone vision), respectively. However, from ancient times 

 to the present day each term has been used in either sense, and general 

 usage has favored the term hemeralopia (though, strictly-speaking, the 

 incorrect term) as synonymous with night blindness. It may also be perti- 

 nent to note here that there is growing evidence that vitamin A plays an 

 important role in cone vision as well as a predominant role in rod A'ision. 



1 . A'lTAMIX A IN THE ViSUAL PuRPLE CyCLE 



This impairment of vision in dim light manifests itself as sensitivity to 

 bright light, diflficultj^ in reading unless the light is brilliant, glittering 

 imagos and dancing specks before the eyes, tendency to stuml)lc or to 

 bump into objects in dim light, and prolonged delay in adaptation from 

 l)right to dim light. Those defects are much more marked in e\'ening twi- 

 light than in the dim light of daybreak. \'isual adaptation to dim light 

 depends upon the presence of adequate amounts of visual purple (rhodop- 



