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Iris and Pupil

The iris, which is the colored part of your eye, controls the amount of light that enters the eye. The iris is a ring shaped tissue with a central opening, which is called the pupil.
 
The iris has a ring of muscle fibers around the pupil, which, when contracted, causes the pupil to constrict (become smaller) in bright light. Another set of muscle fibers radiate outward from the pupil, which causes the pupil to dilate (become larger) in dim light or darkness.