Final answer:
The sympathetic nervous system, not a specific cranial nerve, is responsible for pupil dilation, while the oculomotor nerve (cranial nerve III) is involved in the pupillary light reflex that constricts the pupil. Mydriasis is facilitated through adrenergic receptors on the iris.
Step-by-step explanation:
The cranial nerve involved with pupil dilation is the sympathetic nervous system, which acts through the superior cervical ganglion. Although cranial nerves are not directly involved in pupil dilation, the oculomotor nerve (cranial nerve III) is responsible for the pupillary light reflex which constricts the pupil. The dilation of the pupil, or mydriasis, occurs when norepinephrine binds to adrenergic receptors on the radial fibers of the iris smooth muscle, as seen when phenylephrine is applied to the eye.