Nerve: Short ciliary nerves
Action: Constricts pupil
Antagonist: Iris dilator muscle
The circular muscular fibers of the iris form the Sphincter pupillae; they are arranged in a narrow band about 1 mm. in width which surrounds the margin of the pupil toward the posterior surface of the iris; those near the free margin are closely aggregated; those near the periphery of the band are somewhat separated and form incomplete circles.