

CHAP. XVII.] CHOROID COAT OF THE EYE. 21 



the exact maintenance of the curvature of the cornea, so ne- 

 cessary for correct vision. This lamina extends only to the 

 circumference of the cornea, where it becomes thinnei; and it 

 ceases at the border of the iris, in a manner hereafter to be 

 described. 



Of the Epithelium of the Aqueous Humor. The elastic lamina is 

 itself lined by an exceedingly delicate epithelium, which exactly 

 resembles that existing on serous membranes, (fig. Ill, e, o, p; see 

 also p. 129, vol. i.) . This epithelium is probably concerned in the 

 secretion of the aqueous humor, but it does not extend over the 

 whole surface with which that fluid is in contact. It is probably 

 limited to the cornea. 



OftheChoroid. On turning aside the sclerotic and cornea (fig. 112), 

 the choroid, with its process the iris, is exposed. The choroid is of 

 course perforated behind by the optic nerve. Around this it adheres 

 pretty firmly to the sclerotic, but in the rest of its extent very slightly, 

 and only by the medium of a slender web(lamina fusca), and of those 

 vessels and nerves which pass from the one coat to the other. The 

 rupture of these adhesions occasions a flocculent appearance of the 

 choroid, and sets free some of the brown colouring matter with 

 which its structure is loaded. There is no serous cavity between 

 the sclerotic and choroid, as some have imagined, for a true epithe- 

 lium is wanting, though the lamina fusca contains nuclei. 



The choroid, on coming up to the cornea, gives off its process 

 the iris, and it there adheres intimately to the sclerotic by a very 

 narrow ring of white tissue the ciliary ligament. For an eighth 

 of an inch behind this, however, it is coated by a semi-transparent 

 band, which we shall distinguish as the ciliary muscle, and the 

 fibres of which radiate from the cornea. 



The choroid contains some fibrous tissue, resembling that of the 

 sclerotic; but it is composed principally of blood-vessels and pig- 

 ment-cells. It has been usual to describe it as having two layers, 

 an arterial and a venous; an incorrect view. It is in fact essentially 

 a thin lamina of capillaries, disposed in a close network, the meshes 

 of which are rather smaller behind than in front. This plexus forms 

 the inner surface of the choroid, and has been known as the tunica 

 Ruyschiana. The arteries supplying it, and the veins carrying off 

 its blood, come to it and leave it at very numerous points, but on 

 its outer surface only, where they are so thickly arranged, side by 

 side, as to appear to form the whole of that surface. The veins in 

 particular are large and numerous, and disposed in beautiful curves, 

 converging to four or five trunks, before quitting the choroid, and 



VV-/A4. V \^/ 



