Pupil

The pupil is the adjustable opening at the center of the iris through which light enters the eye. Its diameter changes to regulate how much light reaches the retina.

Pupillary light reflex: A reflex that constricts the pupils in response to increased light entering the eyes. This reflex adjusts pupil diameter to limit retinal exposure to bright light.
Iris: The colored, contractile tissue that surrounds the pupil and regulates its diameter. The pupil is the opening whose size the iris changes.
Pupillometry: The measurement of pupil size and responses to light or other stimuli. It quantifies the pupil's changes in clinical and experimental settings.
Miosis: Constriction of the pupil, whether as a normal response or a pathological finding. It names the narrowed state opposite to dilation.
Iris sphincter muscle: A circular smooth muscle in the iris that constricts the pupil when it contracts. Its contraction narrows the pupil during bright-light responses and near focusing.
Retina: The light-sensitive neural tissue lining the back of the eye that begins visual processing. Light passing through the pupil ultimately reaches this tissue.
Horner syndrome: A neurological condition involving impaired sympathetic supply to the face, often causing a small pupil and drooping eyelid. Its characteristic pupil constriction can reveal disruption of sympathetic pathways.
Depth of field: The range of object distances that appear acceptably sharp in an optical system. A smaller pupil increases depth of field, though it does not change the eye's focus.
Iris dilator muscle: A radial smooth muscle in the iris that enlarges the pupil when it contracts. Its contraction widens the pupil in dim light and during sympathetic arousal.
Aqueous humor: The clear fluid filling the front chambers of the eye between the cornea and lens. Light passes through this fluid-filled region after entering the pupil.

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