Anatomy of the Eye (cont.)
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Iris and Pupil
The iris, which is the colored part of the 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 they contract, causes the pupil to constrict (become smaller). This occurs in bright light. A second set of muscle fibers radiate outward from the pupil. When these muscles contract, the pupil dilates (becomes larger). This occurs under reduced illumination or in darkness.
Medically Reviewed by a Doctor on 8/14/2012
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