On its inner surface the sclera is separated from the chorioid and ciliary body by the perichorioidal space. All the blood vessels and nerves supplying the uvea (that is chorioid, ciliary body and iris) pass through the sclera in canals — emissaria. The sclera is divided into three layers — episclera, containing blood vessels of its own and having more loosely arranged bundles; sclera proper, with its dense texture and the absence of vessels, and the lamina fusca, characterized by the appearance of chromatophores — branched connective tissue cells, containing pigment granules. (Fig. 7). In general, the scleral tissue is that of a tendon. In the sclera the bundles run in every direction, while in a tendon the bundles are parallel with one another. The fixed cells lie between the bundles. Posteriorly the sclera is continuous with the sheath of the optic nerve; anteriorly it goes over into the cornea. (Fig. 1). The narrow zone of transition between the cornea and sclera is spoken of as the limbus.
The cornea occupies about one-sixth of the circumference of the eyeball. (Fig. 1). It contains no blood vessels, but is nourished by lymph from loops of blood vessels adjoining it. The cornea is strongly curved, and has a weak refractive power because it is thinner in the centre than at the periphery. It has five layers (from without inward) — (1)Epithelium; (2) Bowman's Membrane; (3) Stroma; (4) Desce met's Membrane; (5) Endothelium.
The epithelium of the cornea is a stratified pavement epithelium of some five or six layers. The basal cells lying on Bowman's membrane are tall cylindrical cells. In the second layer the cells become rather cuboid in shape. From the third layer on they grow flatter, until those of the surface layer are exceedingly flat. The superficial flat cells present a perfectly smooth and even surface, giving the cornea its bril liancy and lustre. Between the epithelial cells are minute nerveendings,, endowing the cornea with exquisite sensibility. Bowman's membrane contains no nuclei. It has a smooth anterior surface, while posteriorly it merges with the stroma, of which it is considered a modified part.
The Stroma — substantia propria — com prises about 90 per cent of the cornea. It is made up of connective tissue lamella, running in all directions. Between the lamella are the fixed corneal cells. Leucocytes wander into the cornea, and into the epithelial layers, and are called wandering cells, in contradistinction to the fixed cells. The stroma is covered on its posterior surface by the homogeneous, elastic membrane of Descemet. This is a true mem brane having two sharply defined contours. It is of cuticular formation, being the product of the underlying endothelium. It increases in thick ness with age. A single layer of endothelial cells lines not only Descemet's membrane but extends over the trabecula at the angle of the anterior chamber onto the anterior surface of the iris.
Opposite the external scleral furrow, on the internal surface of the fibroti's coat of the eye, is another circular depression — sulcus sclera internus. This is partly filled by a loose mesh work, triangular in shape, going over from the cornea to the iris. To the scleral side of the meshwork one finds an irregular lumen— the canal of Schlemm. (Fig. 1). This lies .pro tected in the sclera on all sides, except its inner wall which comes in contact with the aqueous. The aqueous finds its way into the canal by a process of filtration. On the scleral side of Schlemm's canal tiny. vessels go off, conveying the aqueous directly into the neighboring vems.
The perichoricndal space is but a narrow cleft, traversed by delicate lamellm extending from the chorioid and cilliary body to the sclera. The chorioidea— vascular layer of the eye — is a thin brownish membrane, divided into three layers, (1) vessel layer, (2) capillary layer (choriocapillaris) and (3) lamina vitrea. (Fig. 7). The vessel layer comprises the greater part of the chorioid. In the connective tissue supporting the vessels are numerous chromatophores, giving the chorioid the tessel lated appearance seen with the ophthalmo scope. Most characteristic of the chorioid is the capillary layer, forming a stratum of c,a pillaries for the nourishment of the pigment epithelium and much of the retina. The thin, transparent lamina vitrea, like Descemet's mem brane, is of cuticular formation —the product of the overlying pigment epithelium. The pig ment epithelium is an expanse of one single layer of hexagonal cells, present everywhere from the optic nerve to the pupillary margin. Each hexagonal cell gives off a number of proc esses which project inward among the rods and cones.
The retina (Fig. 7), fourth of the principal layers of the eyeball, is a soft transparent mem brane, extending from the optic nerve to its jagged anterior margins — the ora serratea — opposite the insertion of the iecti muscles. At the anterior and posterior margins the union between the retina and adjoining structures is firm, but elsewhere the retina is held to the pig ment epithelium only by the above mentioned processes. Therefore, the retina is very prone to detachment. Some 3.5 mm. to the temporal side of the optic nerve and slightly below the horizontal meridan is the fovea centmlis. Anatomically this is a flattened funnel-like depression, caused by the absence of nearly all elements of the retina except the cones. (Fig. 1). The term macula lutea (yellow spot) refers to the orange color seen in anatomic study of this part of the retina.