150 VERTEBRATE PHOTORECEPTORS 



were followed by some drop in threshold, it was only tem- 

 porary. In one case the threshold, on the day following 

 administration, had returned to its previous high position. 

 There was no evidence from their work that single large 

 doses of vitamin A rendered a ' cure ' of night blindness. The 

 most significant findings in the work of Hecht and Mandel- 

 baum (1940) are that visual recovery from an A-deficient 

 diet is not a matter of hours and days, but of weeks and 

 months. The fastest recovery which they recorded was 

 made in six weeks; the slowest was not complete after three 

 months, despite the fact that the individual received supple- 

 mentary daily doses of vitamin A as high as 100,000 units 

 plus other vitamins. This subject, however, had recovered 

 completely after a year. 



It is seen from these experiments that special dietary 

 supplements have little or no effect upon recovery rate. It 

 is true, also, that individuals vary greatly in their rate of 

 depletion as well as in their rate of recovery. The reported 

 dramatic rapid recoveries are apparently exceptions rather 

 than the rule. The bulk of the observations emphasize the 

 important fact that recovery from vitamin A-deficiency, as 

 measured by dark-adaptation, is a very slow process. 



Retinal Degeneration. The relation of vitamin A to the 

 production of various ocular disturbances is a subject about 

 which much has been written (Wolbach and Howe, 1925, 

 1933 ; Smith, Yudkin, Kriss, and Zimmermann, 1931 ; Yudkin, 

 Kriss, and Smith, 1931; Yudkin, 1933; Yudkin, Orten, and 

 Smith, 1937, and many others). The eye has been found 

 to exhibit a number of pathological conditions resulting 

 from an A-deficient diet, both in humans and in experimental 

 animals, e.g. xerosis conjunctivae, keratomalacia, inflamma- 

 tion of the lacrimal gland, and others. It is not within the 

 province of this monograph to enter into a discussion of 

 these clinical and pathological situations, but there is one 

 condition which merits description as a part of this treatise 

 and this pertains to retinal degeneration. Despite the great 

 interest in dietary night blindness and its known relationship 



