The cornea

Abstract

It would be perhaps unreasonable to rank various elements or segments of the human eye according to a criterion based on their level of importance. The eye works as an eminently complex system where each part plays a definite role in the process of maintaining normal vision. Yet, a tendency toward emphasizing the functional prominence of the cornea can be noticed in the ophthalmic literature. A possible reason may be the fact that the cornea is the first layer of the eye, in direct contact with the external environment, which results in functional tasks different from most of the other ocular elements. Indeed, the cornea acts as the essential barrier that protects the eye against physical and chemical injuries. Other functions of the cornea include the following: (a) it assures almost 100 % transmission of visible light and about 70 % of the total dioptric power of the human eye; (b) it protects the posterior ocular elements against damage from UV radiation by absorbing the rays between the wavelengths 200 and 300 nm; (c) it withstands the intraocular pressure (IOP) exerted from within the eye; (d) it functions as an impervious barrier against microbes and other pathogens. The intricate architecture of the cornea, comprising five discrete layers (epithelium, Bowman’s layer, stroma, Descemet’s membrane, and endothelium, as counted from outside toward inside), all within 500-600 µm of tissue, may be both a consequence and an indication of the diversity of the functional requirements the cornea must meet. As it is now in the vertebrate eye-a transparent, clear, smooth, elastic, tough, and relatively thick multilayered tissue-the cornea appears as an ideal result of evolution and the epitome of an expedient structure-to-function relationship.