The cornea is the curved, transparent layer that covers the front part of the eye and protects its inner structures. 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.
The cornea has sensitive nerve endings; touch of the cornea causes an involuntary reflex to close the eyelid. Because transparency is of prime importance, the cornea does not have blood vessels; it receives nutrients via diffusion from the tear fluid at the outside and the aqueous humour at the inside. In humans, the cornea has a diameter of about 12 mm and a thickness of 0.5 - 0.7 mm in the center and 1.0 - 1.2 mm at the periphery.
Medical terms related to the cornea often start with "kerat-".
Layers of the cornea
The cornea consists of five layers. Here they are listed from the outside to the inside:
Corneal epithelium: a thin epithelial layer of fast-growing and easily regenerated cells. Tears keep this layer moist.
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.
Surgical procedures involving the cornea
Various refractive eye surgery techniques change the shape of the cornea in order to reduce the need for glasses or otherwise improve the refractive state of the eye. In the techniques used today, parts of the cornea are removed with lasers.
If the corneal stroma has developed opaque patches known as leukomas, a cornea of a deceased donor can be transplanted. Because there are few blood vessels in the cornea, there are also few problems with rejection of the new cornea.
There are also synthetic corneas in development. Most are merely plastic inserts, but there are also some made of plastics that encourage the eye tissue to grow into the synthetic cornea making it a full replacement.