Crural and ambient cisterns. The posterior cerebral artery is the major artery within this dissection and is divided into segments: P1 (precommunicating), P2 (postcommunicating), P3 (quadrigeminal) and P4 (calcarine). Within the crural cistern, the medial posterior choroidal artery courses along with the P2 segment posteriorly to enter the temporal horn of the lateral ventricle at the choroidal fissure between the fimbria of the fornix and the thalamus. The basal vein of Rosenthal lies within the ambient cistern along with the posterior P2 segment of the PCA and proceeds to join the precentral cerebellar vein and internal cerebral veins to form the great cerebral vein of Galen within the quadrigeminal cistern. (Image courtesy of AL Rhoton, Jr.)