The cornea is the transparent part of the eye that covers the front portion of the eye. It covers the pupil (the opening at the center of the eye), iris (the colored part of the eye), and anterior chamber (the fluid-filled inside of the eye). The cornea's main function is to refract, or bend, light. The cornea is responsible for focusing most of the light that enters the eye.
The cornea is composed of proteins and cells. It does not contain blood vessels, unlike most of the tissues in the human body. Blood vessels may cloud the cornea, which may prevent it from refracting light properly and may adversely affect vision.
Since there are no nutrient-supplying blood vessels in the cornea, tears and the aqueous humor (a watery fluid) in the anterior chamber provide the cornea with nutrients.
The cornea is comprised of five layers: the epithelium, Bowman's layer, the stroma, Descemet's membrane, and the endothelium. The first layer, the epithelium, is a layer of cells covering the cornea. It absorbs nutrients and oxygen from tears and conveys it to the rest of the cornea. It contains free nerve endings. It also prevents foreign matter from entering the eye.
The cornea tends to repair itself quickly from minor abrasions. However, deeper abrasions may cause scars to form on the cornea, which causes the cornea to lose its transparency, leading to visual impairment.