Superior view of right internal carotid artery, its terminal branches, and branches of the basilar artery. The clinoidal segment of the internal carotid artery 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 optic nerves are shown entering the optic canals where they are accompanied by the ophthalmic branches of the internal carotid arteries. 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 arteries course posteriorly in a subtentorial relationship and its terminal posterior cerebral arteries (P1) course in a supratentorial direction to supply the occipital lobes and posteromedial temporal lobes. (Image courtesy of PA Rubino.)