The innermost layer is the retina (R), which consists of several layers of cells. Among these are receptor cells (rods and cones), neurons (e.g., bipolar and ganglion cells), supporting cells, and a pigment epithelium (see Plate 101). The receptor components of the retina are situated in the posterior three fifths of the eyeball. At the anterior boundary of the receptor layer, the ora serrata (OS), the retina becomes reduced in thickness, ancl nonreceptor components of the retina continue forward to cover the posterior or inner surface of the ciliary body (CB) and the iris (I). This anterior nonreceptor extension of the inner layer is highly pigmented, and the pigment (melanin) is evident as the black inner border of these structures.
The uvea, the middle layer of the eyeball, consists of the choroid, the ciliary body, and the iris. The choroid is a vascular layer; it is relatively thin and difficult to distinguish in the accompanying figure except by location. On this basis, the choroid (Ch) is identified as being just external to the pigmented layer of the retina. It is also highly pigmented; the choroidal pigment is evident as a discrete layer in several parts of the section.
Anterior to the ora serrata, the uvea is thickened; here, it is called the ciliary body (CB). This contains the ciliary muscle (see Plate 102), which brings about adjustments of the lens to focus light. The ciliary body also contains processes to which the zonular fibers are attached. These fibers function as suspensory ligaments of the lens (L). The iris (I) is the most anterior component of the uvea and contains a central opening, the pupil.
The outermost layer of the eyeball, the fibrous layer, consists of the sclera (S) and the cornea (C). Both of these contain collagenous fibers as their main structural element; however, the cornea is transparent, and the sclera is opaque. The extrinsic muscles of the eye insert into the sclera and effect movements of the eyeball. These are not included in the preparation except for two small pieces of a muscle insertion (arrows) in the lower left and top center of the illustration. Posteriorly, the sclera is pierced by the emerging optic nerve (ON). A deep depression in the neural retina lateral to the optic nerve (above the ON in this figure) is the fovea centralis (FC), the thinnest and most sensitive portion of the neural retina.
The lens is considered in Plate 103. Just posterior to the lens is the large cavity of the eye, the vitreous cavity (V), which is filled with a thick jelly-like material, the vitreous humor or body. Anterior to the lens are two additional, fluid-filled chambers of the eye, the anterior (AC) and posterior chambers (PC), separated by the iris.
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