The Normal Eye - Cornea
The adult cornea is a highly sensitive avascular transparent structure, which covers approximately one-sixth of the eyeball' (the remaining five-sixths being covered by the sctera). The cornea consists of three cell layers: the outer corneal epithelium, the stroma and the innermost corneal endothelium. These offer a unique combination of rigidity and transparency while serving an important role in focusing light entering the eye (Figure 2). The epithelial layer rests on a basal lamina (Bowman's membrane) and is continuous with the conjunctival epithelium. The epithelial cells provide protection for the cornea, a role that is assisted by a renewable tear film which is secreted largely by the meibomian gland and is spread over the cell-layer by constant blinking. The tear film provides a smooth refractive surface on the cornea, acts as a lubricant, prevents dehydration and has inherent antibacterial properties. The corneal epithelium is highly dependent on the free supply of oxygen, which is mainly derived from atmospheric oxygen dissolving in the tear film.
A plexus of
unmyelinated nerve axons, which penetrate the anterior stroma and epithelium as
bare, highly sensitive, nerve endings, provides sensory innervation of the
cornea. These nerve endings, which are directly derived from the branching of
the ciliary nerve, are concentrated in the central cornea, tailing off towards
the limbus (Figure 3). Sensitivity can be variable between individuals and
generally reduces with ageing. The cornea is probably the most sensitive tissue
in the body with the nerve fibres showing exquisite sensitivity to mechanical
stimulation.
Damage to any of these three cell layers can result in scarring of the cornea with subsequent reduction of visual acuity. While the corneal epithelium can repair itself after insult. Bowman's membrane shows little regenerative capacity and may be subject to fibrous scar- formation if damaged. With age, a whitish ring may appear around the periphery of the cornea. This is known as the areas senilis and is due to phospholipid deposition in the peripheral cornea and is universal in those aged over 80 years. It is harmless and does not affect vision.