part_of part of part of develops from has muscle origin has muscle insertion has muscle antagonist system muscle of iris sphincter pupillae http://upload.wikimedia.org/wikipedia/commons/2/2f/Gray878.png constricts iris WP says: It is found in vertebrates and some cephalopods; this class refers to the vertebrate structure In humans, it functions to constrict the pupil in bright light or during accommodation. Its dimensions are about 0.75 mm wide by 0.15 mm thick. It is controlled by parasympathetic fibers that originate from the Edinger-Westphal nucleus, travel along the oculomotor nerve (CN III), synapse in the ciliary ganglion, and then enter the eye via the short ciliary nerves. Initially, all the myocytes are of the smooth muscle type, but much later in life, so that ultimately most cells are of the striated muscle type. [WP,unvetted] dilatator pupillae iris neurectoderm iris smooth muscle sphincter muscle smooth muscle sphincter