M, Oblique anterior view. The cross section of the right temporal lobe crosses the posterior uncal segment. The floor of the third ventricle has been removed back to the level of the aqueduct to expose the interpeduncular fossa located between the cerebral peduncles and above the pons. The posterior part of the floor of the third ventricle is formed by the midbrain. N, Enlarged view of the medial part of the posterior segment of the uncus. The posterior uncal segment is divided by an uncal notch into upper and lower parts. The lower part is formed by the parahippocampal gyrus, which is the site of the subicular zones, and the upper part is formed predominantly by the hippocampal head. The inferior choroidal point, the lower end of the choroid plexus and choroidal fissure and the point where the anterior choroidal artery enters the temporal horn is located just behind the head of the hippocampus. The pyramidal and granule cell layers are organized to give the hippocampal formation its characteristic appearance. O, The cross section of the right temporal lobe has been extended back to the level of the midportion of the temporal horn. The ambient cistern is limited medially by the midbrain and laterally by the parahippocampal and dentate gyri. P, Enlarged view. The collateral sulcus cuts deeply into the hemisphere and forms a prominence, the collateral eminence in the floor of the temporal horn on the lateral side of the hippocampus. (Images courtesy of AL Rhoton, Jr.)