CHAP. XVII.] THE CHOROIDAL EPITHELIUM. 23 



The pigmentary matter within these cells is of a sepia colour, 

 and occurs in the form of oblong or oval grains, less than Yoio-o f an 

 inch long (fig. 114, b). These grains exhibit molecular motion when 

 removed from the cells, and sometimes even within the cells (vol. i. 

 p. 60). They are insoluble in hot or cold water, in the dilute mineral 

 acids, and in strong acetic acid, in oil, alcohol, and ether; but are 

 dissolved, after long digestion, by diluted liquor potassse. The 

 ash consists of common salt, lime, phosphate of lime, and oxide 

 of iron (Gmelin and Berzelius). 



In albinos the colouring matter is deficient, not only in the eyes, 

 but in other organs in which it usually exists. The eyes have con- 

 sequently a pink appearance, derived chiefly from the blood in the 

 choroid and iris. 



Of the Choroidal Epithelium. On the inner surface of the 

 choroid, within the capillary network, and adhering slightly to it, 

 is an epithelium, consisting of a single layer of nucleated particles, 

 of a pentagonal or hexagonal shape, filled with pigment. This was 

 first particularly described by Mr. Wharton Jones, who termed it 

 the membrane of the black pigment. In using this name, it must be 

 remembered that the colouring matter is not peculiar to this epithe- 

 lium; and that this epithelium exists without pigment in front of 

 the tapetum lucidum of animals; and also that it is present, without 

 pigment, in albinos, as was first pointed out by Mr. Jones. Hence 

 the presence of pigment in its cells is a secondary character. The 

 nuclei of the cells project on the inner surface of the membrane. 

 They are concealed by the pigment if it is very abundant, but in 

 general they are visible. Both conditions are seen in fig. 1 14, A. 



In many quadrupeds and fishes the inner surface of the choroid, in its pos- 

 terior part, has a brilliant lustre owing to the presence of a thick layer of wavy 

 fibrous tissue, peculiarly arranged, outside the choroidal epithelium (here 

 colourless). This tapetum lucidum must act as a concave reflector, causing 

 the rays of light to traverse the retina a second time, and thus, probably, in- 

 creasing the visual power, particularly when the quantity of light admitted 

 into the eye is small. 



In the osseous fishes there is a singular vascular organ of a horse-shoe shape 

 appended to the outer surface of the choroid, and covered by a silvery mem- 

 brane. Its structure is imperfectly made out, and its office is quite unknown. 

 It is called the choroid gland. 



In birds there is a remarkable plicated, comb-like process of the choroid, 

 projected into the vitreous humor, and termed the pecten. It is a vascular 

 membrane, and covered with pigment ; its base commencing at the entrance 

 of the optic nerve, and its apex reaching more or less nearly to the crystal- 

 line lens. The retina does not extend over it. No satisfactory use has yet 

 been assigned to it. Its size and shape are subject to considerable variety. 



