The sclerotic is a dense, unyielding, nonvascular structure, which does not readily become inflamed nor yield to pressure. This latter characteristic accounts for the pain from pressure on the nerves experienced in increased intra-ocular tension, as in glaucoma. The weakest part of the sclerotic is at the optic disc, which yields to increased tension, producing the glaucomatous cup, while, with a further increase of pressure, the eyeball expands laterally rather than vertically. When rupture of the globe occurs from violence, it is generally the sclerotic which gives way, the rent occurring through the thinnest part, near the corneal junction.
The cornea, which is transparent, and convex in shape, converges parallel rays of light upon the lens. Not infrequently it is irregularly curved, the curvature being generally greatest in the vertical meridian, thus giving rise to astigmatism. Occasionally it has a conical shape (keratoconus). The posterior elastic lamina of the cornea, or membrane of Descemet, is of importance, as it shuts off the cornea from the aqueous humour, and so preserves its translucency. It also prevents the invasion of leucocytes from the anterior chamber in inflammatory processes, which therefore collect in the chamber, causing hypopyon. It must, therefore, not be broken through in operating for opacity by transplanting rabbit's cornea.
This posterior elastic lamina sends peripherally radiating fibres into the iris-ligamentum pectinatum iridis-and in this ligament are a number of spaces of Fontana, which afford communication between the anterior chamber and the canal of Schlemm. The canal of Schlemm is a circular venous sinus situated at the junction of cornea and sclerotic, and embedded in the tissue of the junction. The cornea after birth has no bloodvessels, except near the margin, where they form loops, but is nourished by abundant lymph streams derived from these marginal vessels, and returned to the circular venous channel QV canal of Schlemm.
Despite the absence of bloodvessels, the cornea has a marked vitality, and wounds heal readily, sometimes leaving no subsequent opacity. Ulcers, on the other hand, frequently cause opacity, owing to their causing considerable destruction of tissue, which is replaced by connective tissue. Corneal ulcers may lead to perforation, with escape of the aqueous humour and prolapse of the iris, which may become adherent (anterior synechia). Vascularization of the cornea, however, does occur, a superficial form called pannus being frequently associated with conjunctivitis, while a deep vascularization, generally associated with keratitis, may also occur, and is called a salmon patch when the vessels form close leashes.
The cornea is richly supplied with nerves, derived from a plexus round its periphery, supplied by the ciliary nerves. The branches from the plexus ramifying in the cornea have no medullary sheath. In consequence of the rich sensory nervous supply, inflammation of the cornea, keratitis, is painful, and accompanied by photophobia or intolerance of light. The painfulness gives rise to excessive lachrymation, and the photophobia to blepharospasm, or spasm of the orbicularis muscle, both conditions being reflex. In glaucoma, on the other hand, the pressure on the nerves produces anaesthesia of the cornea. Herpes, or zona ophthalmica, also occurs occasionally on the cornea.
Ar cus senilis is a degeneration of the corneal tissues, confined to the periphery, generally appearing as a crescent, first in the upper segment, and followed by a second in the lower segment, the two fusing later. It is frequently associated with general atheroma of the vessels, but does not interfere with the healing of corneal wounds.
The middle coat of the eyeball, or uveal tract, consists of iris, ciliary body, and chorioid, and possesses a rich vascular supply.
The iris, or diaphragm, attached peripherally to the ciliary processes, and to the cornea by the ligamentum pectinatum iridis, and perforated a little to the nasal side by the pupil, hangs in front of the lens, the pupillary margin resting upon, but not attached to, the anterior surface of the lens capsule. It is covered by epithelium on both surfaces, is furnished with sphincter and dilator muscular fibres, and is pigmented. In albinos there is no pigment, and the red colour of the iris is due to the rich vascular supply. In blue eyes the pigment is confined to the posterior layers, while in brown eyes it is scattered throughout. In some cases the pupil may be displaced peripherally and present an oval outline (corectopia), or more than one pupil may be present (polycoria). The peripheral attachment of the iris is not a very strong one, and may be detached by injury without damage to the other structures. It is also easily detached in the operation of iridectomy, where a radiating portion of iris is removed, producing a coloboma, bleeding being arrested by muscular contraction. Congenital coloboma, due to persistence of the chorioidal fissure, generally occurs at the lower and inner quadrant. After removal of the lens, the iris loses its chief support, and becomes ' tremulous.' Congenital absence of the iris, irideremia, may occur.
The ciliary body connects the periphery of the ids with the chorioid. In structure it resembles the chorioid. Immediately behind the iris the ciliary body is thrown into a series of some seventy radiating folds-the ciliary processes-and is further rendered prominent by the development of the ciliary muscle in its middle coat. The ciliary processes contain large tortuous vessels, are covered with two layers of epithelium, continuous with that of the retina (pars ciliaris retinae), and are supposed to secrete the aqueous humour. The ciliary muscle consists chiefly of radial fibres springing from the junction of cornea and sclerotic, and from the ligamentum pectinatum, to be inserted into the ciliary processes and orbiculus ciliaris (q.v.). Besides giving attachment to the iris anteriorly, the ciliary processes give attachment to the suspensory ligament of the lens posteriorly. The lens has an inherent tendency to become more convex, but is prevented by the tension kept upon it by the capsule. When, however, by a contraction of the ciliary muscle the ciliary processes are pulled forwards and inwards, the capsule is relaxed, and the lens becomes more convex. This contraction of the ciliary muscle occurs in accommodation for near objects, and is associated with contraction of the pupil. The portion of the ciliary body nearest the chorioid is called the orbiculus ciliaris, and here the ciliary processes fade into a large number of faintly marked radiating ridges, which interdigitate with others in the zonule of Zinn, which invests the vitreous body at this part.