sphincter pupillae, a muscle that contracts the iris, narrowing the diameter of the pupil of the eye. It is composed of circular fibers arranged in a narrow band, about 1 mm wide, surrounding the margin of the pupil toward the posterior surface of the iris. The circular fibers near the free margin of the iris are closely packed; those that are near the periphery of the band are more separated and form incomplete circles. The fibers of the sphincter pupillae blend with the fibers of the dilatator pupillae near the margin of the pupil and are innervated by a motor root of the ciliary ganglion from the oculomotor nerve. Compare dilator pupillae.
