The circular pigmented membrane behind the cornea, perforated by the pupil; the most anterior portion of the vascular tunic of the eye, it is made up of a flat bar of circular muscular fibers surrounding the pupil, a thin layer of smooth muscle fibers by which the pupil is dilated, thus regulating the amount of light entering the eye, and posteriorly two layers of pigmented epithelial cells [Brenda].
Hierarchy: 3
Parent: visual apparatus
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