Third ventricle and right lateral ventricle anatomy. The third ventricle has been opened from the right lateral ventricle. Anteriorly, the arching fibers of the fornix form the roof and anterior wall of the foramen of Monro, a short, tunnel-like passageway through which cerebrospinal fluid flows from the lateral ventricle into the third ventricle.  The vein of the septum pellucidum courses posterior to the foramen of Monro to join the thalamostriate vein and form the internal cerebral vein. The medial posterior choroidal arteries, branches of the P2 segment of the posterior cerebral artery, course dorsal to the midbrain before ascending into the roof of the third ventricle paired with the internal cerebral veins. (Image courtesy of AL Rhoton, Jr.)