The clear layer of tissue on the white part of the eye is the conjunctiva. If the conjunctiva grows across the cornea it is known as a pterygium. The conjunctiva is thought to be stimulated to grow by sunlight and irritants, since pterygiums are common in persons who spend much of their lives outside in sunny, dusty, sandy or wind blown surroundings. If the pterygium is enlarging and encroaches on the pupillary area, it should be removed surgically.