H, The septum pellucidum has been opened to expose the frontal horns and bodies of both lateral ventricles. The columns of the fornix arch anterior and superior to the openings of the foramen of Monro into both lateral ventricles. The body of the fornix forms part of the roof of the third ventricle. I, The body of the fornix has been split in the midline to expose the third ventricle. The internal cerebral veins and medial posterior choroidal arteries course in the roof of the third ventricle. This transforniceal approach is suitable for exposing lesions located in the third ventricle behind the foramen of Monro. (Images courtesy of AL Rhoton, Jr.)