n. the part of the eye that regulates the amount of light that enters. It forms a coloured muscular diaphragm across the front of the lens; light enters through a central opening, the pupil. A ring of muscle round the margin contracts in bright light, causing the pupil to become smaller (see pupillary reflex). In dim light a set of radiating muscles contract and the constricting muscles relax, increasing the size of the pupil. The outer margin of the iris is attached to the ciliary body.
Subjects: Science and Mathematics — Medicine and Health.