The trochlear nerve is the fourth and thinnest cranial nerve. Each of the two trochlear nerves controls just one of the small muscles (ocular muscles) which moves each eyeball, and is purely motor. Like the oculomotor nerve, the trochlear nerve arises in the midbrain, issues through the back of it, curves forward round the side of it, and appears at the front of the brain between the posterior cerebral and superior cerebral arteries. It enters the dura mater through a small aperture hidden under the free margin of the tentorium at or immediately behind the point where the two margins cross. The nerve then runs forward in the lateral wall of the cavernous sinus below the oculomotor nerve and above the ophthalmic nerve. It next runs obliquely upward across the lateral side of the oculomotor nerve, and passes through the superior orbital fissure into the orbit, where it enters the superior oblique muscle of the eyeball almost at once.