[TA] supplies the superior oblique muscle of the eye; its origin is in the midbrain below the cerebral aqueduct, and its fibers decussate in the superior medullary velum and emerge from the brain at the side of the frenulum immediately caudal to the inferior colliculus. The trochlear nerve is the only cranial nerve to arise from the dorsal aspect of the brainstem; it therefore has the longest intracranial course, entering the dura in the free edge of the tentorium, close to the posterior clinoid process, and passing in the lateral wall of the cavernous sinus to enter the orbit through the superior orbital fissure. SYN: nervus trochlearis [CN IV] [TA], fourth cranial nerve [CN IV], pathetic nerve.