Superior View of the Perimesencephalic Cistern. A, Specimen showing superior view of the perimesencephalic cisterns. The perimesencephalic cisterns include the interpeduncular, crural, ambient, and quadrigeminal cisterns surrounding the midbrain. The interpeduncular cistern lies between the cerebral peduncles. The crural cistern lies between the posterior segment of the uncus and the cerebral peduncle. The ambient cistern extends from the posterior edge of the crural cistern to the lateral edge of the midbrain colliculi. The head, body, and tail of the hippocampus and the choroid plexus are exposed in the floor of the temporal horn. Note how the right P2p courses above the medial edge of the parahippocampal gyrus, whereas the left P2p courses medial to the gyrus. B, Specimen showing inferior view of the basal cisterns. The right P2a and P2p have been removed to expose the roof of the crural and ambient cisterns. The lower part of the temporal lobe has been removed to expose the temporal horn and basal cisterns. The anterior choroidal artery enters the choroid plexus in the temporal horn. The lateral geniculate body and optic tract sit in the roof of the cisterns. The P1, P2, and P3 segments; inferior temporal branches; and medial posterior and lateral posterior choroidal arteries are exposed. C, Specimen showing anterior inferior temporal lobe below the choroidal fissure removed to expose the relationship between the P2p and choroidal fissure. The choroidal fissure begins at the posterior edge of the posterior uncal segment at the site where the anterior choroidal artery enters the temporal horn to become the plexal portion. Note how the right P2 ascends and courses laterally to reach the upper surface of the medial edge of the parahippocampal gyrus in close proximity to the choroidal fissure. D, Axial magnetic resonance imaging scan of a cadaver using the Stealth workstation demonstrating the segments of the perimesencephalic cisterns. E, Coronal magnetic resonance imaging scan demonstrates the “C” shape of the ambient cistern. (Images courtesy of AL Rhoton, Jr.)