A, Inferior view of the basal surface of the frontal, temporal, and occipital lobes. The orbital surface of the frontal lobe is supplied by the ACA and MCA. The branches of the ACA overlap from the interhemispheric fissure onto the adjacent part of the orbital surface of the frontal lobe (blue arrows) and the MCA branches overlap onto the lateral part of the orbital surface (red arrows). Most of the lower sur-face of the temporal and occipital lobes is supplied by the PCA; however, branches of the MCA overlap onto the basal surface of the temporal pole and adjacent part of the temporal lobe (red arrows). Branches of the PCA (yellow arrows) extend around the occipital pole lower hemispheric margin to reach the lateral surface of the temporal and occipital lobe (yellow arrows). B, The temporal lobe has been removed to expose the M1 bifurcating into superior and inferior trunks below the anterior perforated substance and passing across the insula and the frontoparietal operculi. The superior trunk supplies most of the lateral surface of the frontal lobe and the inferior trunk supplies most of the lateral surface of the parietal and temporal lobe. The M1 courses below the anterior perforated substance, the M2 courses on the insula, the M3 passes around the opercular lips, and the M4 is formed by the cortical branches. C, The PCAs arise in the interpeduncular cistern in front of the brainstem and pass through the crural cistern, located between the uncus and cerebral peduncle, and the ambient cistern, located between the midbrain and parahippocampal gyrus, to reach the quadrigeminal cisterns. The P2 segment courses in the crural and ambient cisterns, the P3 in the quadrigeminal cistern, and the P4 is the cortical segment. The P2 is divided into a P2A that courses in the crural cistern and a P2P that courses in the ambient cistern. The floor of the right atrium and the lower lip of the calcarine sulcus have been removed to expose the calcarine branches of the PCA coursing in the depths of the calcarine sulcus adjacent to the medial atrial wall. The PCA branches in the depths of the calcarine sulcus are separated from the medial wall of the atrium by only the thin layer of cortex and white matter that form the calcar avis, the prominence in the medial atrial wall overlying the deep end of the calcarine sulcus. D, The floor of the left temporal horn, except for some of the head of the hippocampus and the fimbria, has been removed. The head of the hippocampus folds into and constitutes most of the posterior segment of the uncus, which faces the P2A. The amygdala is located in the anterior uncal segment, which faces the carotid and PComAs. The lower lip of the calcarine sulcus, formed by the lingula, has been removed to expose the upper lip, formed by the cuneus, and the calcarine arteries coursing just outside the medial wall of the atrium. The calcarine branch courses deeply into the calcarine sulcus, and the parieto-occipital branch ascends in the parieto-occipital sulcus. The fimbria of the fornix has been preserved. The LPChAs arise below the thalamus and pass through the choroidal fissure, located between the thalamus and fimbria, to reach the choroid plexus in the temporal horn and atrium. The thalamogeniculate branches arise from the P2P and enter the roof of the ambient cistern by passing through the lower thalamus in the region of the geniculate bodies. (Images courtesy of AL Rhoton, Jr.)