For the character, see Iris (patient). The iris is the part of the eye that controls the amount of light that enters the pupil. It contains pigment cells that determine eye color. The iris contains small muscles that control its movement and contract to let more light through the pupil - a sphincter surrounding the pupil and other muscles which draw it back. The muscles are controlled by the autonomous nervous system in reaction to the amount of light passing through the pupil. Many drugs, particularly narcotics, can paralyze the muscles, leading to dilated pupils. The pigment is actually behind the muscles. The muscles are fed by small blood vessels that can be seen in the sclera.
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