The cornea is the projecting transparent part of the external tunic, and forms the anterior sixth of the surface of the bulb.
It is almost circular in outline, occasionally a little broader in the transverse than in the vertical direction. It is convex anteriorly and projects like a dome in front of the sclera. Its degree of curvature varies in different individuals, and in the same individual at different periods of life, being more pronounced in youth than in advanced life.
The cornea is dense and of uniform thickness throughout; its posterior surface is perfectly circular in outline, and exceeds the anterior surface slightly in diameter. Immediately in front of the sclero-corneal junction the cornea bulges inward as a thickened rim, and behind this there is a distinct furrow between the attachment of the iris and the sclero-corneal junction. The sulcus circularis corneæ is bounded externally by the trabecular tissue forming the inner wall of the sinus venosus scleræ.
Between this tissue and the anterior surface of the attached margin of the iris is an angular recess, named the iridial angle or filtration angle of the eye.
Immediately outside the filtration angle is a projecting rim of scleral tissue which appears in a meridional section as a small triangular area, termed the scleral spur. Its base is continuous with the inner surface of the sclera immediately to the outer side of the filtration angle and its apex is directed forward and inward. To the anterior sloping margin of this spur are attached the bundles of trabecular tissue just referred to; from its posterior margin the meridional fibers of the Ciliaris muscle arise.
Structure —The cornea consists from before backward of four layers, viz.: (1) the corneal epithelium, continuous with that of the conjunctiva; (2) the substantia propria (3) the posterior elastic lamina; and (4) the endothelium of the anterior chamber:
- The corneal epithelium (epithelium corneæ anterior layer) covers the front of the cornea and consists of several layers of cells. The cells of the deepest layer are columnar; then follow two or three layers of polyhedral cells, the majority of which are prickle cells similar to those found in the stratum mucosum of the cuticle. Lastly, there are three or four layers of squamous cells, with flattened nuclei.
- The substantia propria is fibrous, tough, unyielding, and perfectly transparent. It is composed of about sixty flattened lamellæ, superimposed one on another. These lamellæ are made up of bundles of modified connective tissue, the fibers of which are directly continuous with those of the sclera. The fibers of each lamella are for the most part parallel with one another, but at right angles to those of adjacent lamellæ. Fibers, however, frequently pass from one lamella to the next.
- The lamellæ are connected with each other by an interstitial cement substance, in which are spaces, the corneal spaces. These are stellate in shape and communicate with one another by numerous offsets. Each contains a cell, the corneal corpuscle, resembling in form the space in which it is lodged, but not entirely filling it.
- The layer immediately beneath the corneal epithelium presents certain characteristics which have led some anatomists to regard it as a distinct membrane, and it has been named the anterior elastic lamina (lamina elastica anterior; anterior limiting layer; Bowman's membrane). It differs, however, from the posterior elastic lamina, in presenting evidence of fibrillar structure, and in not having the same tendency to curl inward, or to undergo fracture, when detached from the other layers of the cornea. It consists of extremely closely interwoven fibrils, similar to those found in the substantia propria, but contains no corneal corpuscles. It may be regarded as a condensed part of the substantia propria.
- The posterior elastic lamina (lamina elastica posterior; membrane of Descemet; membrane of Demours) covers the posterior surface of the substantia propria, and is an elastic, transparent homogeneous membrane, of extreme thinness, which is not rendered opaque by either water, alcohol, or acids. When stripped from the substantia propria it curls up, or rolls upon itself with the attached surface innermost.
- At the margin of the cornea the posterior elastic lamina breaks up into fibers which form the trabecular tissue already described; the spaces between the trabeculæ are termed the spaces of the angle of the iris (spaces of Fontana); they communicate with the sinus venosus scleræ and with the anterior chamber at the filtration angle. Some of the fibers of this trabecular tissue are continued into the substance of the iris, forming the pectinate ligament of the iris; while others are connected with the forepart of the sclera and choroid.
- The endothelium of the anterior chamber (endothelium cameræ anterioris; posterior layer; corneal endothelium) covers the posterior surface of the elastic lamina, is reflected on to the front of the iris, and also lines the spaces of the angle of the iris; it consists of a single stratum of polygonal, flattened, nucleated cells.
Vessels and Nerves.—The cornea is a non-vascular structure; the capillary vessels ending in loops at its circumference are derived from the anterior ciliary arteries. Lymphatic vessels have not yet been demonstrated in it, but are represented by the channels in which the bundles of nerves run; these channels are lined by an endothelium. The nerves are numerous and are derived from the ciliary nerves. Around the periphery of the cornea they form an annular plexus, from which fibers enter the substantia propria. They lose their medullary sheaths and ramify throughout its substance in a delicate net-work, and their terminal filaments form a firm and closer plexus on the surface of the cornea proper, beneath the epithelium. This is termed the subepithelial plexus, and from it fibrils are given off which ramify between the epithelial cells, forming an intraepithelial plexus.
This definition incorporates text from a public domain edition of Gray's Anatomy (20th U.S. edition of Gray's Anatomy of the Human Body, published in 1918 – from http://www.bartleby.com/107/).