A, Third ventricle from above. The body of the fornix separates the body of the lateral ventricle from the roof of the third ventricle. The body of the fornix blends posteriorly into the crus of the fornix, which is situated above the posterior part of the third ventricle. The choroidal fissure, the site of attachment of the choroid plexus, is situated between the fornix and thalamus. B, The fornix was divided at the level of the columns, just behind the foramen of Monro, and reflected posteriorly to expose the posterior commissure, pineal, and adjacent part of the quadrigeminal cistern. C, The quadrigeminal cistern is located behind the pineal and the colliculi and between the pulvinars. It extends into the cerebellomesencephalic fissure. The trochlear nerves arise below the inferior colliculi. D, View similar to C, except that the vessels have been preserved. The internal cerebral and basal veins join the vein of Galen behind the pineal. The PCA and SCA exit the ambient cistern to enter the lateral part of the quadrigeminal cistern. Both the infratentorial supracerebellar and occipital transtentorial approaches are directed to this area. E, The venous complex emptying into the vein of Galen blocks access to the pineal region. This complex includes the internal occipital, basal and internal cerebral veins, and the vein of the cerebellomesencephalic fissure. A tentorial branch of the SCA crosses the exposure. F, The vein of Galen has been retracted to expose the splenium. The vein of the cerebellomesencephalic fissure has been retracted to expose the pineal. (Images courtesy of AL Rhoton, Jr.)