Dilator pupillae m

The dilator pupillae m. is a smooth muscle that expands the puppilary aperture, causing the pupil to dilate. Its innervation is sympathetic, from post-ganglionic fibers that originate at the superior cervical ganglion and wind their way to eye by "hitchiking" along arteries and other nerves. The muscle sits within the iris and fibers radiate outwards from the pupil.

It may be useful to remember that in a "fight-or-flight" response gathering as much light to the retina  seems adaptive, thus the diator puppilae muscle recieves sympathetic innervation.