Pupil

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Structure of the eyeball seen in a transverse section.

The pupil is the small black circle located in the center of the eyeball. Surrounding the pupil is the colored part of the eye, the iris. The pupil’s function is to allow light to pass through and enter the eye. This light then interacts with the cells of the retina, working as part of the visual pathway to provide the ability of sight.

The pupil changes size to allow more, or less, light to enter the eyeball. These movements are involuntary, under the control of the autonomic nervous system. Pupil dilation increases the size of the pupil, allowing more light to pass through. This occurs in low light situations by the action of the dilator pupillae muscle under sympathetic nervous system control. Pupil constriction occurs in bright light situations. The action is produced by the sphincter pupillae muscle via parasympathetic nervous system control (Cranial nerve III: Oculomotor).