SCLERA The sclera accounts for approximately 93% of the outer coat of the eye. Anteriorly, it is continuous with the cornea at the corneoscleral junction (see Fig. 42.1). It is punctured by a number of foramina containing nerves and blood vessels, most notably the optic foramen, which lies 3 mm medial to the midline and 1 mm below the horizontal, and houses the optic nerve. Smaller openings contain anterior ciliary arteries that penetrate anteriorly, vortex veins that cross the sclera equatorially, and the long and short ciliary nerves and arteries that enter posteriorly. There is considerable individual variation in scleral dimensions. The sclera is thickest at the posterior pole (approximately 1 mm) and decreases anteriorly, reaching a minimum equatorially at about half this thickness. It also thins approaching the optic nerve. The sclera is thinner when the eye is elongated in myopia. The external surface of the sclera is covered by a delicate episcleral lamina of loose fibrovascular tissue, which contains sparse blood vessels and is in contact with the inner surface of the fascial sheath of the eyeball. Anteriorly, the external scleral surface is covered by conjunctiva, which is reflected on to it from the posterior surfaces of the eyelids. The scleral internal surface adjacent to the choroid is attached to it by a delicate fibrous layer, the suprachoroid lamina, which contains numerous fibroblasts and melanocytes. Anteriorly, the inner sclera is attached to the ciliary body by the lamina supraciliaris. Posteriorly, the sclera is pierced by the optic nerve. Here, the outer half of the sclera turns back to become continuous with the dura mater, while the inner half is modified to form a perforated plate, the lamina cribrosa sclerae. The optic nerve fascicles pass through these minute orifices, while the central retinal artery and vein pass through a larger, central aperture. The lamina cribrosa sclerae is the weakest part of the sclera and bulges outwards (a cupped disc) when intraocular pressure is raised chronically, as in glaucoma. Like the cornea, the scleral stroma is composed mainly of densely packed collagen embedded in a matrix of proteoglycans, which are mixed with occasional elastic fibres and fibroblasts. However, in contrast to the cornea, scleral collagen fibrils show a large variation in diameter and spacing, and the lamellae branch and interlace extensively. This arrangement of fibres results in increased light scatter, which is responsible for the opaque, dull-white appearance of the sclera, and also imparts a high tensile strength to the sclera to resist the pull of the extraocular muscles and contain the intraocular pressure. Collagen fibre bundles are arranged circumferentially around the optic disc and the orifices of the lamina cribrosa. The fibres of the tendons of the recti intersect scleral fibres at right angles at their attachments, and then interlace deeper in the sclera. Collagen fibres of the scleral spur are orientated in a circular fashion, and there is an increased incidence of elastic fibres here (Figs 42.2–42.3A). Although the sclera acts as a conduit for blood vessels, scleral vessels are few and mainly disposed in the episcleral lamina, especially close to the limbus. Its nerve supply is surprisingly rich, accounting for the intense pain associated with scleral inflammation (Watson and Young 2004). Scleral development is under active regulation to ensure an eye of the correct axial length to produce a focused image (Wallman and Winawer 2004). Filtration angle and aqueous drainage Aqueous humour is produced by the ciliary epithelium; it passes through the pupil and circulates within the anterior chamber, supplying the avascular cornea and lens with nutrients and removing metabolic waste products. It drains from the eye mainly through the trabecular meshwork into the canal of Schlemm.