The cornea is the clear layer that protects the front of the eye from foreign bodies and germs and helps filter out ultraviolet light. It serves a vital role in vision, helping the eye to focus on both near and distant objects. The cornea completely covers the colored region of the eye -- the iris -- and the pupil. A scratched cornea or corneal abrasion occurs when the cornea becomes damaged due to a trauma to the eye.
An accidental poke to the eye with a fingernail or other object is usually the cause of a scratched cornea. Foreign objects entering the eye, such as sand or sawdust, can also leave abrasions, as can rubbing the eye too hard or splashing chemicals. Some eye infections can also damage the cornea.
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