468 RETINAL EPITHELIUM. FUSCIN. [BOOK I. 



was somewhat roseate in hue and less sharply defined than that in 

 the second, which was perfectly white. The size of the images was 

 somewhat greater than one square millimeter. 



To the images obtained by following such a method as that 

 described Kiihne gives the name of Optograms. The process may be 

 modified by taking the retinae from the alum solution and then 

 drying them in vacuo in the dark. They are in this way rendered 

 very resistant to the action of light. 



Chemical facts relating to the Retinal Epithelium. 



The retinal epithelium cells (formerly termed hexagonal pigment 

 cells of the choroid) present most externally a covering of neuro- 

 keratin; more internally they present a protoplasm wherein are 

 found imbedded one or more nuclei, and still more internally that 

 protoplasm presents large numbers of pigment granules. From this 

 part of the cell proceed processes which make their way between 

 the outer segments of the rods. 



The protoplasm of these epithelium cells presents deposits which 

 are described by Kiihne as consisting of Myeloidin, besides a fat tinged 

 with yellow colouring matter termed Lipochrin ; in the more internal 

 part of the cell and in the processes is found the dark pigment now 

 termed Fuscin. 



idin " term Kiihne indicates the fact that the 



retinal epithelium cells contain deposits of a substance 

 closely resembling, if not identical with, that forming the medullary 

 sheaths of nerve fibres. 



Fat is not a constant ingredient of retinal epithelium, 



being absent in that of man, the ox and the pig. In the 



frog it always is tinged of a deep golden or citron colour. It remains 



fluid at low temperatures, and is readily extracted by ether, benzol and 



carbon disulphide. 



_. . . This is a yellow colouring matter extracted by ether 



Lipochrin. r J P ,. ^ > \ J i 



from the eyes ol irogs, irom which the retinae have 



previously been removed. 



Lipochrin presents two absorption bands, the position of which 

 differ according to the nature of the solvent. "When dissolved in 

 ether these bands a'e situated between F and G. This colouring- 

 matter seems to resemble somewhat a yellow colouring matter 

 which has been' named lutein, in consequence of its being readily 

 extracted from the corpora lutea of cows. Lipochrin is slowly bleached 

 in sunlight. 



Fuscin. 



The brown pigment of the fetinal epithelium has usually, like 

 other pigments of the same colour and appearance, been termed 

 Melanin; of late Ktihne has proposed to term it Fuscin. This 



