CORNEA

curved transparent membrane about 12 millimetres in diameter that covers the front part of the eye. Except at its margins the cornea contains no blood vessels, but it contains many nerves and is very sensitive to pain or touch. It protects the pupil, the iris, and the inside of the eye from penetration by dust particles and is the first element in the eye's focusing system. As light passes through the cornea, it is partially refracted before reaching the lens. The curve of the cornea, which is spherical in infancy but changes with age, gives it its focusing power; when the curve becomes irregular, it causes a focusing defect called astigmatism, in which points appear elongated or distorted. The collagen fibres that make up the cornea are of a uniform size and are arranged in a strictly regular, geometric fashion. This regular arrangement has been shown to be the essential factor resulting in the cornea's transparency. When the cornea is damaged by infection or trauma, the collagen laid down in the repair processes is not regularly arranged, with the result that an opaque patch called a leukoma may occur. If the clouded cornea is removed and replaced by a healthy one, usually taken from a deceased donor, normal vision can result. Because the cornea has few blood vessels, and because most rejection factors are carried in the bloodstream, transplantation does not require extensive tissue matching.