Cornea

The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber and provides most of an eye's optical power http://www.eyeglossary.net/#C. Together with the lens, the cornea refracts light and consequently helps the eye to focus. The cornea gives a larger contribution to the total refraction than the lens, but whereas the curvature of the lens can be adjusted to "tune" the focus, the curvature of the cornea is fixed.

Layers of the cornea

The cornea consists of five layers. Here they are listed from the outside to the inside:

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• Corneal epithelium: a thin epithelial layer of fast-growing and easily regenerated cells. Tears keep this layer moist.
• Anterior limiting membrane (also Bowman's membrane): a tough layer that protects the corneal stroma. It consists of irregularly arranged collagen fibers.
• Corneal stroma (also substantia propria); a thick, transparent middle layer responsible for most of the focusing that the cornea performs. It consists of regularly arranged collagen fibers along with (few) fibroblasts. If the stroma is damaged, for example by injury or infection, it can lose its transparency, causing vision problems.
• Posterior limiting membrane (also Descemet's membrane): a thin acellular layer that serves as the modified basement membrane of the corneal endothelium.
• Corneal endothelium: a simple squamous or low cuboidal epithelium, an inner lining acting as a barrier to prevent water inside the eyeball from moving into and hydrating the cornea, which would lead to blurred vision. (The term endothelium is a misnomer here. The corneal endothelium is bathed by aqueous humour, not by blood or lymph, and has a very different origin, function and appearance from vascular endothelia.)

The cornea is composed mostly of dense connective tissue, similar to the surrounding sclera. However, the collagen fibers are arranged in a parallel pattern, allowing light waves to constructively interfere, allowing the light to pass through relatively uninhibited. The cornea is innervated by the long posterior ciliary nerves that branch from the trigeminal nerve's ophthalmic division.

Related Topics:
Sclera - Trigeminal nerve

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