The transparent cornea is the primary dioptric (refractive element) of the eye and is covered with nonkeratinized, stratified squamous epithelium. Its stroma consists of alternating lamellae of collagen fibrils and fibroblasts (keratocytes). The fibrils in each lamella are extremely uniform in diameter and uniformly spaced; fibrils in adjacent lamellae are arranged at approximately right angles to each other. This orthogonal array of highly regular fibrils is responsible for the transparency of the cornea. The posterior surface is covered with a single layer of low cuboidal cells, the corneaI endothelium, which rest on a thickened basal lamina called Descemet's membrane. Nearly all of the metabolic exchanges of the avascular cornea occur across the endothelium. Damage to this layer leads to corneal swelling and can produce temporary or permanent loss of transparency.
The lens is a transparent, avascular, biconvex epithelial structure suspended by the zonular fibers. Tension on these fibers keeps the lens flattened; reduced tension allows it to fatten or accommodate to bend light rays originating close to the eye to focus them on the retina.
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