A, Anterior view. A1s of nearly equal size cross the front of the lamina terminalis. The right A2 enters the interhemispheric fissure in front of the left A2. The left recurrent artery arises near the level of the anterior communicating artery (AComA) and passes laterally below the anterior perforated substance. A perforating artery arises from the AComA. B, The view has shifted laterally above the carotid bifurcation. The recurrent artery passes laterally above the A1 and intermingles with the lenticulostriate branches of the M1. The posterior communicating artery (PComA) is directed medially and is seen through the opticocarotid triangle located between the carotid artery, optic nerve, and the A1. C, Anterolateral view. The PComA is seen through the opticocarotid triangle. The M1 bifurcates into superior and inferior trunks at the limen insula. D, The basal cisterns have been opened and the temporal pole retracted to expose the oculomotor nerve. The PComA is directed backward above and medial to the oculomotor nerve. The superior cerebellar artery courses below the oculomotor nerve. E, The temporal lobe has been elevated. The anterior choroidal artery (AChA) ascends on the medial side of the uncus. The PComA and the P1 join to form the P2, which continues backward on the medial side of the posterior part of the uncus. A medial posterior choroidal artery (MPChA) passes backward around the brainstem. The superior cerebellar artery passes below the oculomotor and trochlear nerves. The branches forming the P3 course through the quadrigeminal cistern. The P2 courses through the ambient and crural cisterns. A MPChA encircles the brainstem. F, The tentorium has been divided to expose the upper part of the basilar artery. The trigeminal nerve is exposed in the lateral margin of the tentorial opening. The posterior cerebral artery (PCA) courses above and the superior cerebellar artery courses below the oculomotor nerve. (Images courtesy of AL Rhoton, Jr.)