The “cornea” is a transparent membrane located at the surface of the “iris” of the eye.
Light coming in from outside passes through the cornea into the eye, then passes through the lens and is perceived by the retina. The cornea is not just a passage through which light passes; it also works together with the lens as a convex lens to focus light on the retina.

The cornea has a diameter of about 12 mm and is about 0.5 mm thick in the center and about 0.7 mm thick at the periphery.
The inside is divided into five layers, from the outside in: the corneal epithelium, Bowman layer, stroma, Descemet membrane, and endothelium.
The corneal epithelium is made up of many layers of transparent cells. It is the outermost layer of the eye and has a structure that makes it difficult for bacteria to invade. When epithelial cells are damaged, cells in the peripheral part of the cornea divide to regenerate the epithelium.
In contrast, corneal endothelial cells are composed of hexagonal cells and play an important role in maintaining the transparency of the cornea. Unlike epithelial cells, corneal endothelial cells cannot regenerate once damaged. The area of ​​the cells surrounding the damaged cells increases to compensate for the area of ​​the damaged cells. The normal value for corneal endothelial cells is said to be 2500 to 3000/mm2, with an abnormal value being 2000/mm2 or less. Furthermore, if it falls below 500/mm2, water accumulates in the cornea and the transparency of the cornea cannot be maintained, leading to bullous keratopathy and the need for a corneal transplant.

Body tissues receive oxygen and nutrients from blood, but the cornea is a transparent tissue and does not have blood vessels. Therefore, the cornea receives oxygen and other nutrients from tears and aqueous humor.
In addition, tears have the role of smoothing the surface of the cornea by covering it. The cornea also acts as a convex lens, so a constantly smooth surface provides stable vision.

In addition, when contact lenses are worn, the cornea is covered by the lens, making it difficult for tears to be exchanged on the surface of the cornea. A lack of oxygen increases the likelihood of cell damage, leading to blood vessels penetrating the cornea, scratches on the corneal epithelium making it more susceptible to bacterial infection, and in some cases, the corneal endothelium may decrease, causing the cornea to become cloudy (bullous keratopathy). Corneal transplants may ultimately be required for severe corneal damage or clouding, so it is important to keep the cornea healthy.