A pterygium is an abnormal, noncancerous growth of the conjunctiva. The conjunctiva is a thin membrane lining the inside of the eyelid and part of the eyeball. It is located between the sclera, or the "white of the eye" which surrounds the eyeball, and the cornea, the dome-shaped window covering the front of the eye which is responsible for the refraction of light. If a pterygium continues to grow, it may spread onto the cornea.

The Conjunctiva
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Excessive growth of the conjunctiva leads to a pterygium. The exact cause of pterygium is unknown.