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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.
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