Superior exposure of the right internal carotid artery, cavernous sinus, and basilar artery. The frontal and temporal lobes have been retracted to provide exposure to the terminal internal carotid and basilar arteries. The clinoidal segment of the internal carotid artery is a continuation of the cavernous segment, enclosed within the cavernous sinus. The clinoidal segment is closely related to the medial aspect of the anterior clinoid process of the lesser wing of sphenoid. This segment is just proximal to the origin of the ophthalmic artery (off the supraclinoid/ophthalmic or C6 segment of ICA) and is bounded by two dural rings, proximal and distal. The distal dural ring is shown here. It extends from the dura investing the superomedial aspect of the anterior clinoid process medially to dura overlying the optic nerve. The right optic nerve is shown entering the optic canal in company with the ophthalmic artery (not labeled). The internal carotid artery divides into the terminal anterior and middle cerebral arteries. The A1 and M1 segments, respectively, are shown for each here. The oculomotor and trochlear nerves exit, respectively, the anterior and posterior surfaces of the midbrain and course within the subarachnoid space. The oculomotor nerve enters the roof of the cavernous sinus. The trochlear nerve pierces the dura mater inferolateral to oculomotor to enter the lateral wall of the sinus. Between the brainstem and the tentorial notch one can see the basilar artery ascending on the anterior pons. Its superior cerebellar artery courses posteriorly in a subtentorial relationship to supply the superior aspect of the cerebellum. Its terminal posterior cerebral arteries (P1) course in a supratentorial direction to supply the occipital lobes and posteromedial temporal lobes. The right posterior cerebral artery receives, in this case, a posterior communicating artery off the M1 segment of the middle cerebral artery. (Image courtesy of PA Rubino)