Sclera and Cornea

The outer case of the eye is known as the fibrous tunic. It serves, when inflated by the internal fluid pressure, to give shape to the eye and to protect the delicate internal structures. The outermost layer of the tunic is Tenon’s capsule, a thin dense connective tissue layer overlying the episclera. The episclera can be distinguished from the sclera itself by being more loosely woven and rich in blood vessels. Below the episclera is the sclera. The outer layers of the sclera are joined to a sheath, the dura mater, which is a continuation of the membrane covering the brain.

The protein fibers are arranged into ribbons or lamellae in the sclera, and these in turn are interwoven to give strength and inelasticity. The lamellae have two main directions: in the anterior portion they run parallel to the limbus, but at the equator the lamellae begin to run meridionally and then proceed to cross each other at right angles in the posterior half of the globe. The thickness of the sclera varies at different points on the eye. In the human eye the sclera is about 0.6 to 0.8 mm at the limbus, thinning to 0.4 to 0.5 mm at the equator and thickening again to 1.0 mm in the posterior pole; under the tendons of the extraocular muscles the sclera may be as thin as 0.3 mm (Hogan et al. 1971).

The sclera is pierced by numerous nerves and blood vessels, but neither travel along its interior. The blood vessels observed on a “blood-shot” eye are in the outer, loose conjunctiva and episcleral tissue covering the sclera. A few living but nonreproducing cells, known as fibroblasts, are found in the sclera. Pigment cells similar to those found in the choroid are also present in large numbers in the sclera, particularly on the inner surface.

When the scleral lamellae cross the limbus into the cornea they undergo a change in fiber organization. This change, which is described in detail in Chapter 6, leads to the transparency of the cornea contrasted to the opacity of the sclera.

The cornea is a thin, convex—concave lens bathed on the posterior surface by liquid aqueous humor and separated from the air on the anterior surface by a microscopically thin layer of tears. The in vivo thickness of the human cornea in its central region is about 0.52 mm, thickening to about 0.67 mm at the limbus. The central radius of corneal curvature is about 7.8 mm (with some variation), and topographically it flattens peripherally to become more elliptical than spherical toward the limbus; its chord diameter is about 11.7 mm (Hogan et al. 1971).

Corneal structure is complex when studied in detail but can be divided into five recognizable layers in humans. From anterior to posterior, these layers are the epithelium with its basement membrane, Bowman’s layer, stroma, Descemet’s membrane, and the endothelium. The epithelium and endothelium are purely cellular layers, about 50 μm and 5 μm thick, respectively. The stroma is the major structural element, amounting to some 90% of the total thickness, but it has only 2% cells by volume, the keratocytes, the remainder being water, collagen fibers, and a jelly-like ground substance. Bowman’s membrane is a modification of the stroma, and...