 INTRODUCTION The iris is the only tissue in which simultaneous interactions among smooth muscles stimulated by sympathetic and parasympathetic nerves can be observed with the naked eye. The iris contains the neural crest, which is comprised of the autonomous nervous system ANS and dorsal root ganglia . Fibers from the hypothalamus join the parasympathetic system through the dorsal longitudinal fasciculus and the medial forebrain bundle and unite with the Edinger-Westphal nucleus or the lateral horns of the thoracic spinal cord. A portion of these parasympathetic fibers innervate the constrictor muscle fibers of a circular structure underneath the area up to the collarette and limit the iris pupil via the oculomotor nerve. The hypothalamus is the major control and integration center of the ANS. The sympathetic nerves emerging from the hypothalamus extend along the length of the carotid artery synapse with the superior cervical ganglion and subsequently innervate the mydriasis muscle fibers beneath the iris collarette and iris root. 