Cornea, the transparent concavo-convex disk 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 7 to 7 1/2 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 hu: mor, 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 generally described as fitting into the sclerotic like a watch crystal into its frame. Its principal thickness, which is nearly the same at all points, is made up of six to eight layers of soft indistinct fibres, continuous with and similar to those of the sclerotic; these may be separated by maceration. Behind the cornea proper is described an elastic transparent lamina, called the membrane of Demours. Though no vessels have been traced into the cornea, the phenomena of inflammation, adhesion, and ulceration indicate their existence.
A superficial and a deep series of vessels surround the cornea, anastomosing freely around its margin; the former 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 substance. It is supplied with delicate filaments from the ciliary nerves. 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, resulting 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. In old persons, the circumference of the cornea often presents a whitish zone, a line or two wide, the result of physiological causes," and not interfering with vision. The convexity of the cornea in aquatic and amphibious animals is slight, the membrane being sometimes nearly flat.