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a muscle of the iris of the eye that dilates the pupil. It is composed of radial fibers, like spokes of a wheel, that converge from the circumference of the iris toward the center and blend with fibers of the sphincter pupillae near the margin of the pupil. The dilator pupillae is innervated primarily by nerve fibers from the sympathetic system (T2 sympathetic chain ganglia) through axons along the ciliary arteries. Compare sphincter pupillae.