The cornea is the eye’s outermost layer. It is the clear, dome-shaped surface that covers the front of the eye.
Although the cornea is clear and seems to lack substance, it is actually a highly organized group of cells and proteins. Unlike most tissues in the body, the cornea contains no blood vessels to nourish or protect it against infection. Instead, the cornea receives its nourishment from the tears and aqueous humor that fills the chamber behind it. The cornea must remain transparent to refract light properly, and the presence of even the tiniest blood vessels can interfere with this process. To see well, all layers of the cornea must be free of any cloudy or opaque areas.
Because the cornea is as smooth and clear as glass but is strong and durable, it helps the eye in two ways:
- It helps to shield the rest of the eye from germs, dust, and other harmful matter. The cornea shares this protective task with the eyelids, the eye socket, tears, and the sclera, or white part of the eye.
- The cornea acts as the eye’s outermost lens. It functions like a window that controls and focuses the entry of light into the eye. The cornea contributes between 65-75 percent of the eye’s total focusing power.
When light strikes the cornea, it bends–or refracts–the incoming light onto the lens. The lens further refocuses that light onto the retina, a layer oflight sensing cells lining the back of the eye that starts the translation of light into vision. For you to see clearly, light rays must be focused bythe cornea and lens to fall precisely on the retina. The retina converts the light rays into impulses that are sent through the optic nerve to the brain,which interprets them as images.
The refractive process is similar to the way a camera takes a picture. The cornea and lens in the eye act as the camera lens. The retina is similar to thefilm. If the image is not focused properly, the film (or retina) receives a blurry image.
The cornea also serves as a filter, screening out some of the most damaging ultraviolet (UV) wavelengths in sunlight. Without this protection, the lensand the retina would be highly susceptible to injury from UV radiation.
National Eye Institute
2020 Vision Place
Bethesda, MD 20892-3655