CORNEA (Lat. "homy,'" ahornlikeo), the transparent concavo-convex disc which forms the anterior fifth of the globe of the eye, fitted accurately into the sclerotic or fibrous coat forming the posterior four-fifths of the organ. It is a segment of a smaller sphere than the sclerotic, and is from seven to seven and a half lines in diameter; the greatest diameter being the transverse. Its anterior convex surface is covered by a continuation of the conjunctival epithelium, and its posterior concave surface is lined also with delicate pavement epithelium, which is in contact with the aqueous humor, and supposed by some to be concerned in the secretion of this fluid. The degree of convexity varies, being usually greatest in children and near-sighted persons. Its circumference is de scribed as fitung into the sclerotic like a watch crystal into its frame. Its principal thicicness, which is nearly the same at all points, is made up of about 60 layers of soft indistinct fibres, continuous with and similar to those of the sclerotic, connected together by delicate areolar tissue; these may be separated by maceration. Behind the cornea proper is an elastic trans parent lamina called the membrane of De mours, which serves to maintain the corneal curvature. No vessels have been traced beyond the very edge of the cornea, which receives its nutriment in the form of lytnph. A superficial
and a deep series of vessels surround t'he cor nea, anastomosing freely around its margin; the superficial vessels are continuous with those of the conjunctiva, and the deep with the short ciliary arteries. In diseased conditions both sets of vessels may be prolonged into its sub stance. No nerves'have been traced into the cornea. Its diseases are many, frequent and dangerous to vision; from its exposed situation, it is liable to suffer from blows, cuts and the introduction of foreign substances. It is often inflamed in various ophthalmic diseases, result ing in opacity, ulceration, increased vascularity, softening and rupture from gangrene; these affections are tedious and difficult to cure, are often painful and generally leave the patient with more or less obstruction of the power of vision, if they do not destroy the eye itself. In old persons, the circumference of the cornea often presents a whitish zone, a line or two wide., the result of the deposition of fat, and not interfering with vision. The convexity of the cornea in aquatic and amphibious animals is slight, and sometimes almost lacking.