Ragena

This is the transparent cover of the front part of the eye that refracts light rays. The central part of the cornea is 0.6-0.8 mm, and at the periphery – 1.0-1.2 mm thick. The cornea is transparent, containing about 75% water, composed of transparent fibers and corneal cells. The cornea consists of five layers:

multilayered epithelium;
anterior elastic membrane (Bowman’s membrane);
corneal stroma (substantia propria corneae);
posterior elastic membrane (Descemet’s membrane);
posterior corneal epithelium (epithelium posterius corneae).

The cornea does not have blood vessels, it is nourished by the anterior ciliary arteries; it is innervated by ciliary nerves. The cornea can react very sensitively to various stimuli. It helps maintain the shape of the eyeball, allows light rays to enter the eye, and refracts them with a power of about 43 diopters.

Corneal metabolism. There are no capillaries in the cornea, so the main energy substrates (glucose and oxygen) enter it by diffusion from the eye tissues with capillaries. In the cornea, glucose is oxidized, and the resulting ATP is used for active transport of substances and ions in the cornea and for transmitting nerve impulses. Glucose enters from the anterior chamber fluid, oxygen – from tears and anterior chamber fluid. About 50% of the glucose that enters is oxidized during glycolysis: about 88% of this amount is broken down into lactate (under anaerobic conditions) and about 12% – into pyruvate (under aerobic conditions). The remaining glucose is broken down in the pentose phosphate pathway, forming ribose-5-phosphate and NADPH.

Source | Glossary of Most Commonly Used Biomedical Terms and Concepts | Lithuanian University of Health Sciences | Academician Professor Antanas Praškevičius, Professor Laima Ivanovienė