A, The third ventricle sits in the center of the cranium below the corpus callosum, body of the lateral ventricles, and septum pellucidum, above the midbrain and interpeduncular fossa, anterior to the quadrigeminal cistern and vein of Galen, and posterior to the anterior cerebral arteries. The interhemispheric fissure, along the side of the falx, offers one avenue to the third ventricle. The posterior part of the third ventricle can also be approached along the junction of the falx and tentorium, adjacent the straight sinus. B, Enlarged view. The septum pellucidum separates the bodies and frontal horns of the lateral ventricles and is crossed by anterior and posterior septal veins. The anterior cerebral artery ascends along the front wall of the third ventricle, the basilar bifurcation is positioned below the floor, and the vein of Galen blocks access to the posterior wall. C, Enlarged view of the third ventricle. The anterior wall of the third ventricle is formed by the lamina terminalis and anterior commissure and blends above into the rostrum of the corpus callosum. The roof is formed by the body of the fornix and the velum interpositum through which the internal cerebral veins and medial posterior choroidal arteries course. The posterior wall, formed by the pineal and habenular and posterior commissures, is located anterior to the quadrigeminal cistern and the venous complex created by numerous veins converging on the vein of Galen. The floor is formed, from anterior to posterior, by the optic chiasm, tuber cinereum above the pituitary stalk, mamillary bodies, and upper midbrain. The section extends to the lateral side of the mamillary bodies. The velum interpositum is the space within the roof of the third ventricle along which the internal cerebral veins and medial posterior choroidal arteries pass. The body of the fornix is located above the velum interpositum. The upper wall of the velum interpositum is formed by the layer of tela choroidea attached to the lower margin of the fornix. The floor is formed by the layer of tela attached along the striae medullaris thalami. The internal cerebral veins and medial posterior choroidal arteries course between the two layers of tela. The choroid plexus in the roof of the third ventricle arises in the lower layer of tela. D, Another third ventricle. This section extends just to the left of the midline through the column and body of the fornix. The body of the fornix forms the roof of the third ventricle. The columns pass anterior to the foramen of Monro and descend behind the anterior commissure to reach the mamillary bodies. (Images courtesy of AL Rhoton, Jr.)