A, The basal veins are formed below the anterior perforated substance by the union of the posterior orbitofrontal, superficial and deep sylvian, and uncal veins and course posteriorly across the optic tracts. Only the anterior and posterior segments of the basal vein are exposed because the middle part is hidden above the uncus and parahippocampal gyrus. B, The uncus has an anterior and posterior segment. The lower part of the posterior segment of the right uncus and adjacent part of the parahippocampal gyrus has been removed, while preserving the fimbria of the fornix, to expose the inferior ventricular and lateral atrial veins. The segment of the right basal veins coursing lateral to the cerebral peduncle is very small. The inferior ventricular and lateral atrial veins pass through the choroidal fissure, situated between the thalamus and fimbria, to empty into the basal vein. The longitudinal hippocampal veins course along the fimbria. The peduncular veins, in this case, are quite small. The lateral atrial veins, which drain the lateral atrial wall and the posterior part of the roof of the temporal horn, pass below the pulvinar to reach the basal vein. C, Enlarged view after removal of the fimbria. The large veins draining the roof of the temporal horn and lateral atrial wall and crossing the lower and posterior surface of the thalamus, are analogous to the thalamostriate vein that crosses the upper surface of the thalamus. All three veins drain a portion of the central core of the hemisphere and pass through the choroidal fissure between the thalamus and choroid plexus. D, The choroid plexus has been removed. (Images courtesy of AL Rhoton, Jr.)