Right lateral view of middle cranial fossa with exposure of the cavernous sinus and brainstem. Dura has been removed on the floor of the middle fossa and the apex of the petrous bone drilled away to expose the petrous and cavernous parts of the internal carotid artery. The cisternal part of the trigeminal nerve exits the mid-pons anteriorly and crosses the apex of the petrous bone to enter a split of dura (Meckel's cave) where its fibers form the trigeminal (Gasserian) ganglion. In this dissection, the greater superficial petrosal nerve is seen coursing beneath the ganglion towards the foramen lacerum. From the anterior margin of the ganglion the ophthalmic nerve courses forward to the orbit through the superior orbital fissure. Its maxillary division can be seen entering the foramen rotundum and its mandibular division enters the foramen ovale. Posterolateral to this foramen is the foramen spinosum through which passes the middle meningeal artery. Along the posterior border of the petrous bone the facial nerve, nervus intermedius, and vestibulocochlear nerves can be seen converging on the internal auditory meatus. They are accompanied by a loop of the anterior inferior cerebellar artery. The abducens nerve ascends along the clivus before entering dura near the petrous apex. Here it passes beneath the petroclinoid ligament, a fold of dura that connects the petrous apex to the posterior clinoid process, to enter the cavernous sinus along the lateral surface of the internal carotid artery. The oculomotor nerve is observed leaving the midbrain-pontine junction and penetrating dura to enter the lateral wall of the cavernous sinus enroute to the orbit via the superior orbital fissure. Inferior to the oculomotor nerve, the trochlear nerve penetrates the dura near the posterior clinoid process and enters the lateral wall of the cavernous sinus before passing through the superior orbital fissure to the orbit where it supplies motor innervation to the superior oblique muscle. (Image courtesy of AL Rhoton, Jr.)