Trochlear nerve: The nerve that controls the superior oblique muscle of the eye, one of the muscles that move the eye (extraocular muscles). Paralysis of the trochlear nerve results in rotation of the eyeball upward and outward (and, therefore, in double vision). The trochlear nerve is the fourth cranial nerve, and it is the only cranial nerve that arises from the back of the brain stem. It follows the longest course within the skull of any of the cranial nerves.
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