Neurovascular Relationships of the Medial Temporal Region. E, Cortical anatomy of the posterior segment of the MTR. The anterior end of the calcarine sulcus extends into and divides the posterior portion of the parahippocampal gyrus into the isthmus of the cingulate gyrus above and the lingual gyrus below. The posterior segment of the MTR has 3 surfaces: inferior, medial, and anterior. The inferior surface is formed by the anterior part of the lingual gyrus, which is separated laterally from the occipitotemporal gyrus by the collateral sulcus. The medial surface is composed of the parasplenial region, which contains the medial aspect of the isthmus and the inferior extension of the precuneus. The anterior surface is composed of the anterior aspect of the isthmus, the extraventricular part of the hippocampal tail, and the crus of the fornix. The extraventricular hippocampal tail is formed by the fasciolar gyrus and the gyri of Andreas Retzius. The subsplenial gyrus, a prolongation of the fasciolar gyrus, forms the terminal segment of the hippocampal tail, surrounds the splenium, and is continuous with the indusium griseum above the splenium. The crus of the fornix wraps around the posterior surface of the pulvinar and arches superomedially toward the lower surface of the splenium. F, The P2 segment of the PCA is divided into a P2a, which passes through the crural cistern located between the posterior segment of the uncus and the cerebral peduncle, and a P2p, which courses through the ambient cistern, located between the lateral midbrain and parahippocampal gyrus. The P3 passes through the quadrigeminal cistern, where it gives rise to the P4 formed by the cortical branches, including the parieto-occipital and calcarine arteries. The lips of the parieto-occipital and calcarine sulci have been retracted to expose the parieto-occipital and calcarine arteries, which are commonly hidden between the sulcal lips. A medial posterior choroidal artery encircles the brainstem to reach the velum interpositum in the third ventricular roof. The cuneus forms the upper lip and the lingula forms the lower lip of the calcarine sulcus. The precuneus forms the upper lip and the cuneus forms the lower lip of the parieto-occipital sulcus. G, Laterobasal surface of the left temporal lobe. The temporal lobe has been elevated and the dura of the middle skull base fossa has been removed to examine the relationship between the basal surface of the MTR and the structures located in the floor of the middle fossa. The entorhinal area (anterior part of the parahippocampal gyrus) faces the dural covering of the cavernous sinus. Meckel’s cave is located deep to the posterior margin of the entorhinal area. The foramen ovale is located deep to the anterolateral margin of the entorhinal area. The foramen rotundum (V2) is located just anterior to the anterior end of the rhinal sulcus. The inferior surface of the posterior parahippocampal gyrus faces the tentorium, and the anterior margin of the superior petrosal sinus lies below the junction of the inferior surface of the anterior and middle segments of the MTR. The foramen spinosum and the semicircular canals lie lateral to the middle segment of the MTR and below on the occipitotemporal gyrus. H, Superior view of the ventricular surface of the left MTR. The central core of the hemisphere has been removed after opening the choroidal fissure and performing a horizontal cut at the level of the amygdala and limen insula. A vertical incision at the collateral sulcus has been made to remove the lateral part of the temporal lobe. The amygdala forms the anterior part of the anterior segment of the MTR, and is related medially to the carotid cistern and carotid bifurcation, and laterally to the proximal sylvian cistern and M1. The hippocampal head forms the posterior part of the anterior segment of the MTR and is related medially to the crural cistern and AChA. The middle segment of the medial temporal lobe is formed predominantly by the hippocampal gyrus and is related medially to the ambient cistern. The posterior segment of the MTR, where the hippocampal tail is located, is related medially to the quadrigeminal cistern. (Images courtesy of AL Rhoton, Jr.)