The pupil is essentially an optical element that controls the amount of light striking the retina by acting as an aperture stop: the most light-restrictive element of an optical system. Pupillary diameter, or more precisely iris size, is controlled by two muscles: the sphincter pupillae and dilator pupillae. Contraction of the sphincter, accompanied by relaxation of the dilator, produces pupil constriction (miosis); while contraction of the dilator, accompanied by relaxation of the sphincter, produces pupil dilation (mydriasis). These muscles are innervated by the autonomic nervous system and controlled by central neural pathways that are influenced by a variety of factors such as retinal illuminance, viewing distance, alertness, and cognitive load. © 2008 Elsevier Inc. All rights reserved.