Sclera

At a glance

English

Sclera

Latin (TA2)

Sclera

System

Eye

Region

Head

Side

Paired, left and right

About the sclera

The sclera is the fibrous outer coat of the globe, the part people call the eye's white. Together with the cornea it forms the fibrous tunic: the sclera covers about five-sixths of the surface and the clear cornea the front sixth, the two blending at the limbus. Behind, it is pierced by the optic nerve slightly to the nasal side of the posterior pole. There its outer layers merge with the dural sheath of the nerve, while its inner part bridges the opening as a sieve-like plate, the lamina cribrosa, through which the nerve fibres leave.

It is thickest at the back, around one millimetre, and thinnest, about a third of a millimetre, just behind the attachments of the rectus muscles. All six extraocular muscles insert into it. Its outer surface is covered by a loose vascular layer, the episclera, and separated from orbital fat by the fascial sheath of the eyeball; in front it is overlaid by the conjunctiva. Its inner surface, the brown lamina fusca, is loosely attached to the choroid. Besides the optic nerve, it is pierced by the ciliary arteries and nerves around the disc, by four or so vorticose veins behind the equator, and by the anterior ciliary vessels near the limbus.

The substance of the sclera is dense connective tissue of type I collagen with some elastic fibres. In the cornea the fibrils are evenly sized and regularly stacked, which makes it clear; in the sclera they vary in size and interlace irregularly, which makes it opaque and white. It is derived largely from neural crest. In young children it is thin enough for the pigment beneath to give a bluish tint, and in older people fat deposits may make it yellowish.

Mechanically the sclera keeps the eye's shape against the intraocular pressure and gives the muscles a firm anchor. Its own vessels are few, so inflammation of the overlying episclera or of the sclera itself (scleritis) shows as redness, the latter often deep and painful and linked to rheumatoid disease. A yellow sclera is a sign of jaundice. Blunt or penetrating injury can rupture it, usually near the muscle insertions where it is thin, with a serious threat to vision.

Function and supply

Function

Protects the internal structures of the eye, maintains its shape under intraocular pressure and provides attachment for the extraocular muscles.

Nerve supply

Sensory fibres from the long and short ciliary nerves, branches of the ophthalmic division of the trigeminal nerve.

Blood supply

Sparse; branches of the short and long posterior ciliary arteries and the anterior ciliary arteries, mainly supplying the episclera.