Which nerve innervates the iris?

The iris is primarily innervated by the autonomic nervous system, specifically through the oculomotor nerve (cranial nerve III). The parasympathetic fibers from this nerve control the sphincter pupillae muscle, responsible for constricting the pupil. Conversely, the sympathetic fibers originating from the superior cervical ganglion innervate the dilator pupillae muscle, allowing for pupil dilation.