The transparent thin-walled dome that forms the front of the eyeball. The cornea is joined at its circumference to the sclera (white of the eye); the black pupil and the coloured iris are visible beneath it. The main functions of the cornea are to help focus light-rays on to the retina at the back of the eye and to protect the front of the eye. It is kept moist by tears produced by the lacrimal gland and the mucus- and fluid-secreting cells in the eyelids and conjunctiva. cornea, disorders of Injuries or diseases affecting the cornea, the outer shell of the eyeball. Injuries include corneal abrasions, which sometimes become infected and progress to a corneal ulcer. Penetrating corneal injuries can cause scarring, which may lead to impairment of vision. Chemical injuries can result from contact with a corrosive substance and require immediate flushing of the eye with water.
In actinic keratopathy, the outer layer of the cornea is damaged by ultraviolet light. In exposure keratopathy, damage is due to reduced protection by the tear film and blink reflex. The cornea can also be infected by viruses, bacteria, and fungi, the herpes simplex virus being especially dangerous. True inflammation of the cornea (called keratitis) is uncommon as the cornea contains no blood vessels.
Other disorders include: keratomalacia as a result of vitamin A deficiency; keratoconjunctivitis sicca (dry eye); corneal dystrophies such as keratoconus; and oedema, in which fluid builds up in the cornea and impairs vision.
Rare congenital defects include microcornea (smaller cornea than normal) or megalocornea (bigger than normal) and buphthalmos, or “ox-eye’’, in which the entire eyeball is distended as a result of glaucoma.
Degenerative conditions of the cornea such as calcium deposition, thinning, and spontaneous ulceration occur mainly in the elderly, and are more common in previously damaged eyes.
n. the transparent circular part of the front of the eyeball. It refracts the light entering the eye onto the lens, which then focuses it onto the retina. The cornea contains no blood vessels and it is extremely sensitive to pain. The innermost layer (corneal endothelium) consists of a single layer of cells that cannot regenerate. —corneal adj.
a white or greyish line in the periphery of the cornea, concentric with but separated from the edge by a clear zone. It begins above and below but may become a continuous ring. It consists of an infiltration of fatty material and is common in the elderly (arcus senilis). When it occurs in younger people (arcus juvenilis) it may indicate *hyperlipidaemia. It does not affect vision.... corneal arcus
reflex blinking of both eyes normally elicited by lightly touching the cornea of one eye. This reflex is lost in deep coma, during general anaesthesia, and in death; it is therefore one of the tests used to confirm brainstem death.... corneal reflex
a ring designed to be inserted into the peripheral tissue of the cornea in order to alter the curvature of the corneal surface. It is undergoing trials to assess its ability to correct errors of refraction. In myopia (short-sightedness), for example, the ring would be required to stretch the corneal tissue peripherally and thus flatten the central corneal curvature in order to correct the myopia.... corneal ring
Health Encyclopedia