Superior portion of right cavernous sinus wall. The right oculomotor (CNIII) nerve can be seen leaving the ventral midbrain toward the lateral wall of the cavernous sinus. The nerve penetrates the dura between the anterior and posterior petroclinoid ligaments (these attach between the anterior and posterior clinoid processes, respectively, and the petrous apex) lateral to and in front of the posterior clinoid process. The trochlear nerve (CNIV) emerges from the dorsal aspect of the midbrain, wraps around the midbrain and penetrates the dura just below the free border of the tentorium cerebelli. Here, it courses within the lateral wall of the cavernous sinus inferior to the oculomotor nerve and superior to the ophthalmic nerve. All three nerves enter the orbit through the superior orbital fissure. The superior ophthalmic vein is shown passing through this fissure draining blood from the orbit into the cavernous sinus. In this view, the cavernous segment of the internal carotid artery continues as the transitional clinoid segment bounded by the proximal and distal dural rings. This segment lies immediately inferior to the anterior clinoid process. The artery becomes the supraclinoid or ophthalmic segment once it enters the subarachnoid space distal to the distal dural ring. Medial to the anterior clinoid process is the optic nerve (CNII) emerging from the optic canal. The internal carotid artery terminates in the anterior and middle cerebral arteries. (Image courtesy of AL Rhoton, Jr.)