Only an intact cornea is a guarantee for unclouded vision. With its enormous refractive power, it is of great importance for vision. The cornea requires special attention because it is directly exposed to the environment with its various hazards.
What is the cornea of the eye?
The cornea (Latin: cornea), along with the sclera, is a component of the outer skin of the eye. The eyeball is almost completely enveloped by the opaque sclera, except for the anterior part, which is occupied by the transparent, more convex cornea. Due to the curvature, the incident light rays are focused before they reach the lens. The diameter of the cornea is about 13 millimeters, the thickness in the middle is about half a millimeter. There are no blood vessels there to obstruct vision. The supply of nutrients takes place via the aqueous environment: via the aqueous humor and the lacrimal fluid. The region where the cornea and sclera meet is called the limbus (Latin for: edge). Behind the cornea are the pupil and the iris (Latin for: iris).
Anatomy and structure
The cornea is composed of five layers. On the surface is a multilayer squamous epithelium: a layer of cells with flat, interconnected cells that lie close together like paving stones. The thickness is one-tenth that of the cornea. The epithelium is capable of renewing itself about every seven days. The last layer of the epithelium is adjacent to the basement membrane, which merges into the so-called Bowman’s membrane. Bowman’s membrane is a solid and cellless layer that provides stability. It cannot renew itself. The stroma directly adjoins the Bowman membrane. The stroma is a connective tissue-like structure and accounts for 90 percent of the total thickness of the cornea. Structural proteins (collagens) are responsible for its strength and shape. The 78 percent water content and the special arrangement of collagen units ensure the transparency of the cornea. Collagen fibers of different composition than in the stroma are part of the adjacent basal membrane. It is called Descemet’s membrane and is very resistant despite its small thickness. Toward the anterior chamber of the eye, the single-layer corneal endothelium follows internally, representing the fifth layer.
Function and tasks
Because of its transparency, the cornea can perform an important task: the unobstructed passage of light rays to the retina. At the same time, it has a protective function. It serves as a kind of windshield for the eye and is thus a barrier against harmful external influences such as foreign bodies and germs. In the case of minor defects, the upper layers are able to repair them again by rapidly regrowing cells, thus preventing infection in the eye. With regard to the dangerous UV radiation in sunlight, the cornea acts as a filter. The most important property within the visual process is the ability to precisely refract the incident light so that it reaches the retina bundled through the lens. Due to its strong curvature, the cornea contributes two-thirds of the total refractive power of the visual system. This corresponds to about 40 out of a total of 65 diopters. The unit of measurement diopter is used to indicate the refractive power (also: refractive index) of optical systems. The refractive effect is supported by the aqueous humor, which is located between the cornea and the lens. The functioning of the eye is comparable to that of a camera. The cornea and lens function as a refractive medium like the lens system in a camera, the iris like the diaphragm, and the retina corresponds to film.
Diseases and disorders
One of the most common visual disorders affecting the cornea is corneal astigmatism, also known as astigmatism. In those affected, the cornea is irregularly shaped or curved to varying degrees. As a result, the incident light rays are not focused to a point, so that images appear distorted. This visual disorder is often congenital and frequently occurs together with nearsightedness or farsightedness. Diseases of the cornea can be inflammatory and non-inflammatory in nature or caused by injury. The rarely occurring non-inflammatory disorders are based on changes in shape that lead to functional limitations. In keratoconus, a cone-shaped deformation forms in the center of the cornea, causing it to thin and tear.Causes of corneal inflammation (Latin: keratitis) can be infections with bacteria or viruses, drying out of the cornea (for example, too infrequent blinking) or foreign bodies. A cornea damaged by pathogens can develop into a corneal ulcer (Latin: Ulcus corneae). Usually only the uppermost layers are affected by this ulcer. If pointed bodies pierce the cornea, they can cause infections in addition to the injury. Injuries with chemicals such as alkalis and acids are particularly dangerous because of their serious effects. Connective tissue scars form in the affected areas and vessels sprout into the cornea, impairing vision. Corneal opacities may result. Another cause of corneal haze is swelling of the cornea, which results in water retention. They can occur as complications of inflammation or ulceration of the cornea.
