Sagittal section of the third ventricle. For orientation, the right side of the image is anterior. This midline view demonstrates the roof of the third ventricle formed by the fornices, just above the medial superior choroidal arteries (branches of the posterior cerebral arteries).  In the midline, the lamina terminalis, optic chiasm and pituitary infundibulum border the third ventricle anteriorly. The floor of the third ventricle is composed of the tuber cinereum of the hypothalamus, the mamillary bodies, and the midbrain tegmen. Above the fornices are the lateral ventricles, which are bordered laterally by the caudate nuclei. The corpus callosum forms the roof of the lateral ventricles; pericallosal arteries (branches of the anterior cerebral arteries) wrap around the corpus callosum to irrigate the medial frontal and parietal lobes.  (Image courtesy of AL Rhoton, Jr.)