The chorioid, which is the most extensive part of the middle coat, reaching from the entrance of the optic nerve to about 3/16 inch from the corneal margin, consists of three layers. The outermost layer, or stratum vasculosum, is pigmented, and contains large branches of the short posterior ciliary arteries and, more superficially, large veins, which in converging form whorls, and open into the vena vorticosa. The middle coat, or lamina chorio-capillaris, is composed of capillaries, while the innermost layer, or lamina basalts (membrane of Bruch), is transparent and almost structureless.
Externally the chorioid is in contact with the sclerotic, to which it is firmly attached at the point of entrance of the optic nerve, while in front of this it is loosely attached by the vessels and the loose lamina fusca. Internally it is in contact with the pigmented layer of the retina. Extensive hamor-rhage from injury, or sudden decrease of intra-ocular tension as in cataract operations, not infrequently occurs between the coats. The chorioid may be ruptured by a blow, and melanotic sarcoma may arise from it.
Owing to its rich vascular supply, the uveal tract is particularly prone to inflammatory affections. Thus iritis is not infrequently seen resulting from trauma or late secondary syphilis. Such inflammations are apt to spread to the ciliary region (irido-cyclitis), or to the entire uveal tract, destroying the eyeball (phthisis bulbi), and, in the septic type, by extension along the lymphatics of the optic tract, not infrequently involves the other eye (sympathetic ophthalmia). Hence excision of the affected eyeball is frequently performed as a preventive measure, or evisceration of the globe, leaving the posterior sclerotic to act as a cup for the artificial eye, may be done (Mule's operation). Owing to the extensive destruction which is apt to follow septic infection of the ciliary region, it has been termed the dangerous area of the eye. In iritis the anterior ciliary vessels are engorged, producing the characteristic red circumcorneal zone ; the pupil becomes contracted from spasm ; the inflammatory exudate infiltrates the iris itself, causes adhesions (posterior synechia) to the lens, and turbidity of the aqueous humour.
The retina constitutes the innermost of the three coats of the eyeball. Commencing at the point of entrance of the optic nerve, which lies to the nasal side of the direct optic axis, it ends in a wavy line, the ora serrata, a short distance behind the ciliary body. A prolongation from the retina, consisting of an outer pigmented and an inner epithelial layer, is, however, continued from the retina beyond the ora serrata over the ciliary body and posterior surface of the iris, and these are termed the pars ciliaris retina? and pars iridica retina? respectively. The retina, in which the fibres of the optic nerve are spread out, consists of an outer pigmented layer, which is attached to the chorioid externally, and an inner nervous layer, which is in contact with the hyaloid membrane of the vitreous internally. The point of entrance of the optic nerve, or Úlind spot, is marked by the optic disc, which is slightly raised peripherally and cupped centrally. The optic nerve, like the other cranial nerves, receives a sheath both from the dura and the pia-arachnoid, and these sheaths extend to the optic disc. Thus intracranial inflammatory or congestive conditions may easily affect the optic disc. In optic neuritis the optic disc, which normally is well defined and of a faint pink tint, which contrasts sharply with the red reflex of the fundus generally, becomes swollen and blurred, while the vessels become engorged and tortuous. Optic neuritis occurs in cases of cerebral tumour, and sometimes of abscess, and is said to be due to intracranial pressure. Where optic neuritis has persisted it may be followed by optic atrophy, in which the disc is sharply defined and brilliantly white, while the vessels become diminished in calibre. In glaucoma the normal cupping of the disc becomes much more marked, forming the glaucomatous cup.
The central artery of the retina, which supplies the retina with blood, runs forward, accompanied by its vein, in the optic nerve, until it reaches the papilla, where it divides into an upper and lower branch, each of which again almost immediately divides. As the anastomosis of the central artery is almost negligible, complete thrombosis or embolism of the central artery is followed by blindness. Both upper and lower branches of the artery supply blood to the yellow spot. In embolism of the central artery both the disc and the vessels become white.
The macula lutea, or yellow spot of the retina, is situated in the direct optical axis, is somewhat oval in shape, raised at the margin, and presents a central depression, the fovea centralis. In the macula, which constitutes the region of distinct vision, the structure of the retina is much modified, till in the fovea only the layers of cones and of cone fibres are present.
The retina sometimes becomes detached from blows on the eyeball and idiopathically. The only tumour which affects it is glioma, which occurs in young children and is very malignant. It increases the intra-ocular tension (secondary glaucoma), and may cause detachment of the retina. It involves the whole eyeball, and causes rupture, with formation of a fungus haematodes, and also involves the optic nerve, by which it may extend to the brain. Secondary deposits may be found in the liver. Retinitis of many forms occurs, such as haemorrhagic, syphilitic, and albuminuric.
The space between cornea and lens, which is occupied by the aqueous humour, is divided into a large anterior and very small posterior chamber by the iris The aqueous humour is secreted by the ciliary processes, situated behind the iris in the posterior chamber, passes thence between lens and iris and through the pupil of the iris into the anterior chamber, where it is absorbed peripherally by the spaces of Fontana, which communicate with the canal of Schlemm, and are situated in the angle between cornea and iris (filtering angle).