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Introduction The eye is the receptor organ of the visual system. It can react to various intensities and qualitites of light and enables the organism to distinguish between differences in the form and size of objects and changes in their positions. The inner, nervous layer of the eye, the retina, functions in photoreception. The retina is protected and nourished by two layers: a tough, outer fibrous coat and a vascular layer positioned between the fibrous layer and photoreceptive layer. The protective layer is white and opaque over the posterior 5/6 of the eye and is known as the sclera. The anterior 1/6 is transparent and is called the cornea. The vascular layer is the uvea and consists of three zones: the choroid, the ciliary body and the iris. Anteriorly between the transparent cornea and the iris is an expanded portion, the anterior chamber. Between the iris and the lens lies the posterior chamber. These two chambers contain an aqueous fluid (aqueous humor) and communicate through the pupil. Posterior to the lens the entire eye cavity is filled with gel-like vitreous humor. Cornea. The cornea is clear, avascular, and transparent. Histologically this structure consists of five layers. The outer epithelium is non-keratinized stratified squamous which lies on a thin basal lamina. A modified layer of the corneal stroma which is composed of arranged fibrils exists beneath the basal lamina of the epithelium and is known as Bowman's membane. Some 90% of the cornea consists of a stroma of parallel lamellae of collagen fibers. Fibroblasts are present and have been called keratocytes by some authors. The fourth layer of the cornea is Descemet's membrane and is the basement membrane of the endothelium. The corneal endothelium is composed of a single layer of cuboidal cells which function to keeps the cornea dehydrated. Sclera. The sclera is the white of the eye and contains a more haphazard arrangement of collagen fibers as compared to the cornea. This layer contains a few blood vessels, no lymphatics, and a few nerve fibers. Just below the epithelium small blood vessels are present. At the base of the sclera are pigmented melanocytes. Uvea. The vascular coat is divided into three regions from posterior to anterior: the choroid, the ciliary body, and the iris. The characteristics of the choroid include an extreme degree of vascularity and the presence of pigment. The layer of capillaries within the choroid is the choriocapillaris, and the choroid is separated from the pigmented cells of the retina by a glossy, elastic membrane called Bruch's membrane. The ciliary body encircles the eye anterior to the ora serrata which is the anterior end of the retina. The bulk of the ciliary body is composed of smooth muscle bundles between which is a network of elastic fibers and melanocytes. This muscle functions in accomodation and is responsible for alterations in the shape of the lens in changes from near to distant vision. Ciliary processes extend from the anterior part of the ciliary body into the posterior chamber. The ciliary body is covered with an inner non-pigmented layer and an outer pigmented layer. (Note: inner refers to the center of the eyeball). A vascular layer overlies its smooth muscle. The iris contains a pigmented epithelium on its posterior surface a delicae connective tissue stroma containing blood vessels and pigment cells, and anteriorly a discontinuous layer of melanocytes and fibroblasts. Myoepithelial cells are arranged to serve as sphincter (constrictor pupillae) and dilator msucles of the pupil. Chambers of the Eye. The anterior chamber is bounded by the posterior surface of the cornea, the central lens area, the iris, and the anterior surface of the ciliary body. At the angle of the anterior chamber is a mass of sponge-like tissue comprising a meshwork of collagenous trabeculae. Here the aqueous humor drains into the Canal of Schlemm which encircles the eye. The fluid passes from the canal into the veins of the deep scleral plexus. The posterior chamber is bounded anteriorly by the iris and posteriorly by the anterior lens surface. The epithelium of the ciliary process produces aqueous humor which bathes the structures bounding the anterior and posterior chambers. If this fluid is not drained and formation of aqueous humor continues, there will be an increase in intraocular pressure, glaucoma, which, if untreated, may result in retinal damage and blindness. Lens. The lens is a biconvex, crystalline structure which is elastic in the young individual and becomes larger with age. An elastic capsule of collagenous fibers is attached to the zonule or suspensory ligament which extends from the ciliary body to the lens equator. A single layer of subcapsular epithelium (cuboidal) lines the anterior region of the capsule. There is no epithelium on the posterior pole of the lens. The bulk of the lens substance is composed of fibers arranged in the shape of 6-sided prisms. Retina. The retina is the innermost coat of the eye and is the light sensitive layer. In a histological cross section through the retina ten distinct layers can be observed. There are three neurons located within this layer and the retina is arranged from innermost (the region bordering on the choroid) to outermost (the region bathed by vitreous humor). |