ORGANS OF THE SENSES. 513 



the veins: thus, then, its disposition is a counterpart of 

 that of the venas vorticose, the dense rows of cells exhibit 

 the same curves, the same mode of branching, and viewed 

 altogether with a low object-glass in the eyes of some 

 animals, as the sheep, nothing can exceed the beauty and 

 elegance of the object thus presented to our examination. 

 (See Plate LXVIII. fig. 1.) 



With regard to structure, it is remarkable that each radius 

 or arm of the cells is prolonged into a colourless fibre, in the 

 course of which several other cells may be included. (See 

 Plate LXVIII. fig. 13.) 



This structure is best seen in the lamina fusca, the fibres 

 of which are all of this nature, and are exceedingly dia- 

 phanous, often membranous, much disposed to curl up, and 

 unaffected by distilled vinegar, beyond undergoing a degree 

 of contraction. All the fibres met with in the choroid, ex- 

 cept those entering into the constitution of the blood-vessels, 

 are of this peculiar nature. 



The inner surface of the choroid is so smooth as to convey 

 the impression of the existence of a distinct membrane : of 

 this, however, no satisfactory evidence has yet been obtained, 

 although portions of membrane apparently devoid of struc- 

 ture have been seen on the margins of torn portions of 

 the choroid. If a membrane does really exist in this situ- 

 ation, it is possible that it is nothing more than the vessels 

 of the tunica Ruyschiana united into a membrane by the 

 fibres above described. 



Hexagonal choroidal epithelium. On the inner surface of 

 the choroid a layer of cells of a regularly pentagonal or 

 hexagonal form, filled with pigmentary granules, exists : 

 these cells are so coherent that they form a distinct layer 

 much more evident in the eyes of some animals, as the sheep 

 and pig, than in those of man. (See Plate LXVIII. Jig. 12.) 



This layer extends over that peculiar structure common 

 to the eyes of many quadrupeds and fishes, the tapetum luci- 

 dum ; but in that situation its component cells are of smaller 

 size, and almost entirely deprived of colouring matter. 



In albinoes the colouring matter is deficient, not only in 



8 S 4 



