The pupil is a circular opening located in the center of the iris that controls the amount of light entering the eye. It constricts (miosis) and dilates (mydriasis) under autonomic nervous system influence. The iris contains two sets of muscles - the sphincter pupillae contracts the pupil in response to parasympathetic stimulation while the dilator pupillae dilates it with sympathetic stimulation. Abnormal pupils may be unequal in size (anisocoria), irregularly shaped, or have abnormal reactions to light. Various diseases and drugs can affect the pupils.