The trochlear nerve emerges from the dorsal aspect of the brainstem at the level of the caudal mesencephalon, just below the inferior colliculus. It circles anteriorly around the brainstem and runs forward toward the eye in the subarachnoid space. It passes between the posterior cerebral artery and the superior cerebellar artery, and then pierces the dura just under free margin of the tentorium cerebelli, close to the crossing of the attached margin of the tentorium and within millimeters of the posterior clinoid process.[2] It runs on the lateral wall of the cavernous sinus, where it is joined by the other two extraocular nerves (oculomotor—cranial nerve III and abducens—cranial nerve VI) and the first two branches of the trigeminal nerve (V), Ophthalmic (V1) and Maxillary (V2).