Complex sentence: These fibers exit the midbrain with CN III and synapse with postganglionic parasympathetic neurons of the ciliary ganglion, which innervates the sphincter muscle of the iris.
Split into simpler sentences: These fibers exit the midbrain with CN III and synapse with postganglionic parasympathetic neurons in the ciliary ganglion. These postganglionic nerve fibers innervates the sphincter muscle of the iris.