[TA] intrinsic so-called muscle of eyeball; radially disposed layer of muscular processes of the myoepithelial cells that form the epithelium of the posterior surface of the iris, which extends from the pupillary to the ciliary margin; sympathetic stimulation causes contraction, which slowly dilates the pupil to allow more light to reach the retina. SYN: musculus dilatator pupillae [TA], dilator iridis, dilator of pupil, musculus dilator iridis.
