A, View from below of the cisterns bordering the tentorial incisura. The middle incisural space opens upward into the crural cistern located between the uncus and peduncle and the ambient cistern located between the parahippocampal gyrus and the lateral surface of the brainstem. The PCAs course through the crural and ambient cisterns to reach the posterior incisural space, the site of the quadrigeminal cistern. The basal vein accompanies the PCA in the upper part of the crural and ambient cisterns and empties into the vein of Galen in the quadrigeminal cistern. The medial posterior choroidal arteries course around the brainstem on the medial side of the PCAs with the long circumflex perforating branches. B, The medial part of the right temporal lobe has been removed to expose the temporal horn. The fimbria of the fornix, which arises on the upper surface of the hippocampus and forms the lower margin of the choroidal fissure, has been preserved. The thalamus, geniculate bodies, and optic tract are in the roof of the crural and ambient cisterns. C, The right PCA has been removed. The basal vein passes backward above the PCA and empties into the vein of Galen with the internal cerebral and internal occipital veins. The lower surface of the thalamus, the geniculate bodies, and the optic tract form the roof of the crural and ambient cisterns. The anterior choroidal artery passes posteriorly above the uncus and through the choroidal fissure to supply the choroid plexus in the temporal horn. D, Both PCAs have been removed to expose the roof of the middle incisural space on both sides and the basal veins, which drain the neural structures in the region. (Images courtesy of AL Rhoton, Jr.)