Cornea

At a glance

English

Cornea

Latin (TA2)

Cornea

System

Eye

Region

Head

Side

Paired, left and right

About the cornea

The cornea forms the transparent front sixth of the outer fibrous coat of the eye; the opaque sclera makes up the rest. It is more steeply curved than the sclera, so it bulges forward like a watch glass, and the two meet at the limbus, a ring marked on the outside by a shallow groove. In adults it measures roughly eleven and a half millimetres across and is thinnest at the centre, about half a millimetre, thickening towards the edge. Its front is bathed by the tear film and its back by the aqueous humour of the anterior chamber.

From front to back it has five classic layers. A non-keratinised stratified squamous epithelium, continuous with that of the conjunctiva, renews itself rapidly from its basal cells. Beneath it lies Bowman's layer, an acellular sheet of collagen. The stroma makes up about nine-tenths of the thickness; its collagen fibrils are of uniform size and packed in regular sheets, which is what makes the tissue clear. Descemet's membrane, a thick basement membrane, follows, and the inner surface is covered by a single layer of endothelial cells that pump fluid out of the stroma. A thin pre-Descemet (Dua's) layer has also been described.

Transparency depends on the absence of blood vessels. Oxygen reaches the cornea mostly from the air through the tears, and nutrients come from the tear film and the aqueous humour. The tissue is instead richly supplied with bare sensory nerve endings, which makes it one of the most sensitive surfaces in the body; touching it triggers the blink (corneal) reflex.

Optically, the air-tear interface over the cornea supplies about two-thirds of the eye's focusing power, around forty-three dioptres, but this focus is fixed; adjustment for distance is the job of the lens. Reshaping the cornea with lasers, as in LASIK, corrects refractive error. Scratches, ulcers, keratitis, swelling from endothelial failure and the cone-shaped thinning of keratoconus all blur vision, and a badly scarred cornea can be replaced by transplantation, which succeeds partly because the tissue is avascular and immunologically privileged.

Function and supply

Function

Acts as a protective, transparent window and the eye's main fixed refracting surface, bending incoming light towards the lens and retina.

Nerve supply

Sensory fibres from the trigeminal nerve's ophthalmic division, chiefly through the long ciliary nerves, forming a dense plexus beneath the epithelium.

Blood supply

Avascular; nourished by diffusion from the tear film, the aqueous humour and the capillary loops at the limbus derived from the anterior ciliary arteries.