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 106), 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 fbrous 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 107. Just posterior to the lens is the large cavity of the eye, the vitreous cavity (V ), which is filled with a thick jellylike material, the vitreous humor or body. Anterior to the lens are two additional, fluid-filled chambers of the eye, the anterior chamber (AC ) and the posterior chamber (PC ), separated by the iris.
KEY AC, anterior chamber C, cornea CB, ciliary body Ch, choroid FC, fovea centralis I, iris L, lens ON, optic nerve OS, ora serrata PC, posterior chamber R, retina S, sclera V, vitreous cavity arrows, muscle insertions
PLATE 105 Eye II: Retina
The retina and optic nerve are projections of the forebrain. The fibrous cover of the optic nerve is an extension of the meninges of the brain. The neural retina is a multilayered structure consisting of photoreceptors (rods and cones), neurons, some of which are specialized as conducting and associating neurons, and supporting cells (Mler’s cells). External to the neural retina is a layer of simple columnar retinal pigment epithelium (RPE). The Mler’s cells are comparable to neuroglia in the rest of the central nervous system. Processes of Mler’s cells ramify virtually through the entire thickness of the retina. The internal limiting membrane is the basal lamina of these cells; the external limiting membrane is actually a line formed by the junctional complexes between processes of these cells and the photoreceptor cells. The neurons of the retina are arranged sequentially in three layers: (1) a deep layer of rods and cones; (2) an intermediate layer of bipolar, horizontal, and amacrine cells; and (3) a superficial layer of ganglion cells. Nerve impulses originating in the rods and cones are transmitted to the intermediate layer and then to the ganglion cells. Synaptic connections occur in the outer plexiform layer (between the rods and cones and the intermediate neuronal layer) and the inner plexiform layer (between the intermediate layer and the ganglion cells), resulting in summation and neuronal integration. Finally, the ganglion cells send their axons to the brain as components of the optic nerve.