cornea
Meaning of cornea: Clarity is bought at a price: with no blood supply the tissue stays transparent, but it must take oxygen from tears and air.
Definition of cornea
The cornea is the clear, curved front window of the eye, lying over the colored iris and the pupil. It bends incoming light and, together with the lens behind it, brings that light to a focus on the retina. It supplies most of the eye's focusing power, and its transparency depends on carrying no blood vessels of its own.
What is it made of?
The cornea is layered, and each layer has a distinct job. Its transparency comes from the way the middle layer is built: collagen fibers of uniform thickness laid in sheets at regular spacing, an arrangement that lets light pass without scattering. Disturb that order through swelling, scarring or injury and the tissue clouds.
| Layer | Role |
|---|---|
| Epithelium | The surface skin of the eye; renews within days and carries dense nerve endings |
| Stroma | Most of the thickness; ordered collagen giving both strength and clarity |
| Endothelium | A single inner layer pumping water out to stop the stroma swelling; does not regenerate |
How does it survive without blood vessels?
No artery reaches it. Oxygen is absorbed straight from the air through the tear film, and nutrients arrive from the aqueous humor, the clear fluid filling the chamber behind it. That arrangement is exactly what preserves clarity, since blood vessels would scatter light. It also has consequences. Contact lenses worn too long starve the surface of oxygen, and the eye may respond by growing vessels inward, which reduces clarity for good. The absence of vessels also limits the immune system's access, so a corneal graft is less likely to be rejected than any other transplanted tissue and is usually performed without matching donor to recipient.
What goes wrong with it?
The cornea carries more nerve endings for its size than almost any other tissue, so even a tiny scratch causes intense pain, watering and a sensation of grit. A surface scratch is an abrasion, and it usually heals within a day or two leaving no mark. An ulcer is deeper, involves infection, and can leave a scar over the pupil that blocks light permanently. Bacteria, viruses such as herpes simplex, fungi and amoebae carried in contaminated water can all infect it. Shape matters as much as clarity: an unevenly curved cornea produces astigmatism, and in keratoconus the tissue thins and bulges into a cone, distorting vision in a way that neither the lens behind it nor ordinary spectacles can fully correct. Because the cornea sits in front of the iris, anything clouding it dims vision even when the rest of the eye is healthy.
Used in a sentence
Examination revealed a small corneal abrasion following the reported eye injury.
Good to know
A corneal abrasion, a superficial scratch of the corneal surface, is distinct from a corneal ulcer, which involves infection and carries a greater risk of scarring and vision loss.
Word origin
Where “cornea” comes from
From Latin corneus, meaning horny, derived from cornu, meaning horn, describing the tissue's tough, translucent quality.