RECAPITULATION 163 



the rod photopigment to be slightly different from rhodopsin, 

 in possessing an additional double bond in the carotenoid 

 chain. He has termed this substance porphyropsin, which 

 in its final bleaching in the eye is transformed into vita- 

 min A2. 



The properties of both rhodopsin and porphyropsin and 

 their relations to the vitamins A are discussed fully in 

 Chapter X wherein a description is also given of certain 

 histological entities in the fixed retina which are believed, 

 at least tentatively, by the author to be identified with 

 retinene — the intermediate decomposition product between 

 vitamin A and visual purple. 



The dependence of both rod and cone vision upon the 

 presence of vitamin A in the diet is now very generally 

 known. Whereas night blindness has long since been recog- 

 nized as due to dietary deficiencies, it has been in recent 

 years only that this problem has undergone vigorous ex- 

 perimental investigation in animals and man. The data 

 obtained from many of these experiments are presented in 

 Chapter XI. Here also are presented recent observations 

 which not only show that vitamin A deficiencies affect the 

 physiological thresholds of the visual elements, but that, 

 prolonged avitaminosis A may bring about structural break- 

 down of the rods. Evidence is presented also to show that 

 degenerated rods in rats are able to undergo repair follow- 

 ing prolonged feeding on an adequate diet. The theoretical 

 possibilities of such structural changes occurring in the 

 human retina are considered in relation to the varying 

 periods of time required by many persons to recover normal 

 vision after vitamin A depletion. 



