Axial view of the right temporal horn, crural and ambient cistern region at the level of the choroidal fissure. For perspective, the bottom of the image is anterior. The internal carotid artery bifurcates into the middle cerebral artery M1 segment and the anterior cerebral artery A1 segment.  The MCA continues into posteriorly-coursing M2 segments within the sylvian fissure. At this axial level, the temporal horn of the lateral ventricle has been opened to reveal the choroidal fissure containing the basal vein of Rosenthal and the anterior choroidal artery. The posterior cerebral artery is visible just superior to the right oculomotor nerve coursing around the crus within the crural cistern (P2a segment) into the ambient cistern (P2p segment) just superior to the tentorium. (Image courtesy of AL Rhoton, Jr.)