These nerve fibers surround the internal carotid artery as a plexus in the cavernous sinus from where they emerge to enter the orbit with the nasociliary branch of the ophthalmic nerve, a division of the trigeminal nerve. The dilator muscle is innervated by third-order, postganglionic sympathetics originating from the superior cervical ganglion. Cholinergic discharges from their muscarinic end plates result in miosis and accommodation. They course anteriorly on the inner surface of the sclera to innervate the ciliary muscle and sphincter Several postganglionic short ciliary nerves pierce the posterior sclera around the optic nerve. Preganglionic parasympathetic pupillomotor fibers travel with the inferior division of the oculomotor nerve and synapse in the ciliary ganglion. Pupillomotor nerves enter the orbit through the superior orbital fissure and annulus of Zinn. Therefore, with prudence, pupillary reactions may be evaluated even in the presence of iris atrophy, traumatic rupture of the sphincter, and congenital or surgical coloboma. The sphincter can be seen in light or atrophic irides and with infrared transillumination. Thus, rather than retracting toward one quadrant when severed or ruptured, the sphincter continues to function except in the altered segment. Although connected by gap and tight junctions, each group is innervated and functions separately. The band measures 1 mm in width and occupies an area about 2 to 4 mm from the pupil. ![]() The sphincter muscle consists of circumferential smooth muscle fibers that lie in bundles parallel to the pupillary margin within the superficial iris stroma. Mechanically, the diameter of the pupil is determined by the antagonistic actions of the iris sphincter and dilator muscles, with the dilator fibers playing the minor role. ![]() The iris consists of four parts: (1) anterior condensation of fibroblasts, melanocytes, and collagen fibrils (2) loose fibrocollagenous stroma containing the muscles, blood vessels, nerves, fibroblasts, macrophages, and melanocytes (3) dilator muscle and (4) pigmentary epithelium. Lying slightly inferonasal to the central iris, the pupil is surrounded by a thick frill or collarette of deeply pigmented tissue that rests on the anterior lens capsule. The pupil is a mobile opening within the iris diaphragm, the most anterior projection of the uveal tract.
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