Pterygium usually refers
to a benign growth of the conjunctiva. A pterygium commonly
grows from the nasal side of the sclera. It is associated
with, and thought to be caused by ultraviolet-light
exposure (e.g. sunlight), low humidity, and dust. The
predominance of pterygia on the nasal side is possibly
a result of the sun's rays passing laterally through
the cornea where it undergoes refraction and becomes
focused on the limbic area. Sunlight passes unobstructed
from the lateral side of the eye, focusing on the medial
limbus after passing through the cornea. On the contralateral
side, however, the shadow of the nose medially reduces
the intensity of sunlight focused on the lateral/temporal
limbus.
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