A, Lateral view with the right hemisphere removed. The internal cerebral veins course between the upper parts of the thalami. The basal vein courses posteriorly above the posterior cerebral artery. The nerves in the wall of the cavernous sinus have been exposed. B, Superolateral view of the quadrigeminal cistern. The section of the brainstem extends through the cerebral peduncle and lateral geniculate body. The basal vein passes posteriorly above the posterior cerebral artery to join the internal cerebral vein in the quadrigeminal cistern. A vein courses parallel and below the basal vein connecting the veins in the quadrigeminal cistern and cerebellomesencephalic fissure with the superior petrosal veins emptying into the superior petrosal sinus. The trochlear nerve arises below the inferior colliculus. C, The right hemisphere including the thalamus has been removed to expose the basal vein coursing through the crural, ambient, and quadrigeminal cisterns and the internal cerebral veins coursing in the roof of the third ventricle. The hippocampus and fimbria have been preserved. The internal cerebral and basal veins course in close relationship to the fornix. The internal cerebral vein courses below the body of the fornix. The basal vein courses medial to the fimbria and the basal and internal cerebral veins join to form the vein of Galen in the area medial to the crus of the fornix. A column of the fornix and the anterior commissure are at the anterior margin of the exposure. D, The right temporal lobe, including the hippocampus and the choroid plexus, has been removed to expose the right basal vein passing through the ambient and quadrigeminal cistern. The roof of the temporal horn formed by the thalamus and tapetum of the corpus callosum is drained by the inferior ventricular vein that joins the basal vein by passing through the choroidal fissure. This basal vein in this case does not empty into the vein of Galen, but passes laterally below the temporal lobe to empty into a tentorial sinus. E, Lateral view of another basal vein. The middle segment of this basal vein is hypoplastic. The posterior segment of the basal vein receives the inferior ventricular vein and passes around the midbrain to empty into the vein of Galen. The anterior part of the territory normally drained by the basal vein empties into the sylvian veins, leaving a hypoplastic midsegment lateral to the peduncle. F, Anterosuperior view of the left basal vein coursing through the crural, ambient, and quadrigeminal cisterns. The basal vein arises at the union of the sylvian and anterior cerebral veins and passes posteriorly above the posterior cerebral artery in the crural cistern, located between the peduncle and uncus. It exits the crural cistern to enter the ambient cistern, located between the midbrain and parahippocampal gyrus, and terminates in the quadrigeminal cistern. The third nerve passes below the posterior cerebral artery. Medial atrial veins cross the medial atrial wall and empty into the veins in the quadrigeminal cistern. The internal cerebral vein courses in the roof of the third ventricle. (Images courtesy of AL Rhoton, Jr.)