Pupil - Definition

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The pupil is the black hole that appears in the center of the eye, in the middle of the iris. The light enters through it before being picked up by the tissues, mainly by the retina. It undergoes involuntary movements of contraction (miosis) and relaxation (mydriasis). This dual mechanism is called pupillary reflex and keeps a sharp image depending on the brightness. An emotion or substance (drug) may also alter the diameter of the pupil.