Superior view of right intradural internal carotid artery, its terminal branches, and terminal branches of the basilar artery. The supraclinoid or ophthalmic segment of the internal carotid artery is shown here with its ophthalmic branch coursing with the optic nerve into the optic canal and orbit. 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 (unlabeled) 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  (Image courtesy of PA Rubino)