Anterior view of brainstem at tentorial notch. The midbrain has been sectioned transversely and presents the cerebral peduncles anteriorly and cerebral aqueduct and tectum posteriorly. The edges of the tentorial notch extend from the clinoid processes anteriorly and intersect with the falx cerebri posteriorly. The optic nerves emerge from the optic canals and their medial portions join at the optic chiasm. Between the optic nerves and anterior clinoid processes the internal carotid arteries emerge and shortly divide into their terminal anterior cerebral (not shown) and middle cerebral arteries. The superior cerebellar and terminal posterior cerebral arteries arise from the basilar artery within the interpeduncular cistern. The oculomotor nerves arise from the lateral walls of the interpeduncular fossa of the midbrain and pass forward between these two vessels to penetrate dura overlying the roof of the cavernous sinus. The trochlear nerve exits the dorsum of the midbrain, wraps around the cerebral peduncle and enters dura along the lateral wall of the cavernous sinus. A portion of the tentorium cerebelli has been removed near the petrous apex to reveal the right trigeminal nerve. The left posterior communicating artery arises from the internal carotid and connects to the left posterior cerebral artery (PCA). The P1 segment of the left PCA is short, but larger in diameter than the P1 segment of the right PCA. Reciprocally, the right posterior communicating artery is longer and larger than the left one. Numerous thalamoperforating arteries arise from the P1 segments and posterior communicating arteries and penetrate the posterior perforated substance within the interpeduncular fossa of the midbrain. The P2 segments pass around the midbrain and near the quadrigeminal cistern give rise to P3 segments that course above the tentorium to the medial parts of the temporal lobes. Peduncular veins drain the anterior midbrain and empty into the basal veins of Rosenthal, which course posteriorly and medially near the posterior cerebral arteries. These drain blood from the midbrain and medial parts of the temporal lobe and empty into the great cerebral vein of Galen. This latter vein is formed by the union of the internal cerebral veins draining deep internal areas and empties into the straight sinus. (Image courtesy of AL Rhoton, Jr.)