Fourth cranial nerve: The fourth cranial nerve, the trochlear nerve, is the nerve supply to the superior oblique muscle of the eye, one of the muscles that moves the eye. Paralysis of the trochlear nerve results in rotation of the eyeball upward and outward (and, therefore, double vision).

The twelve cranial nerves, the trochlear nerve included, emerge from or enter the skull (the cranium), as opposed to the spinal nerves which emerge from the vertebral column.

The trochlear nerve is the only cranial nerve that arises from the back of the brain stem and it follows the longest course within the skull of any of the cranial nerves.

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