Immediately internal to the anterior margin of the sclera (S) is the ciliary body (CB). The inner surface of this forms radially arranged, ridge-shaped elevations, the ciliary processes (CP), to which the zonular fibers (ZF ) are anchored. From the outside in, the components of the ciliary body are the ciliary muscle (CM), the connective tissue (vascular) layer (VL) representing the choroid coat in the ciliary body, the lamina vitrea (LV, inset), and the ciliary epithelium (CiEp, inset). The ciliary epithelium consists of two layers (inset), the pigmented layer (PE ) and the nonpigmented layer (npE ). The lamina vitrea is a continuation of the same layer of the choroid; it is the basement membrane of the pigmented ciliary epithelial cells.
The ciliary muscle is arranged in three patterns. The outer layer is immediately deep to the sclera. These are the meridionally arranged fibers of Brke. The outermost of these continues more posteriorly into the choroid and is referred to as the tensor muscle of the choroid. The middle layer is the radial group. It radiates from the region of the sclerocorneal junction into the ciliary body. The innermost layer of muscle cells is circularly arranged. These are seen in cross section. The circular artery (CA; barely discernible) and vein (CV) for the iris, also cut in cross section, are just anterior to the circular group of muscle cells.
KEY A, artery AC, anterior chamber C, cornea CA, circular artery CB, ciliary body CEp, corneal epithelium Ch, choroid CiEp, ciliary epithelium Cj, conjunctiva CjEp, conjunctival epithelium CM, ciliary muscle CP, ciliary processes CS, canal of Schlemm CV, circular vein I, iris IEp, iridial epithelium L, lens LV, lamina vitrea M, iridial constrictor muscle npE, nonpigmented layer of ciliary epithelium PC, posterior chamber PE, pigmented layer of ciliary epithelium S, sclera V, vein VL, vascular layer (of ciliary body) ZF, zonular fibers arrows, junction between cornea and sclera
PLATE 107 Eye IV: Sclera, Cornea, and Lens
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 angels 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 corneal 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.
PLATE 107 • EYE IV: SCLERA, CORNEA, AND LENS
Corneoscleral junction, eye, human, H&E 130. in thickness and rests on a loose vascular connective tissue. Together, this epithelium and underlying connective tissue represents the conjunctiva