The Tentorium and Regional Anatomy. A, Superolateral view of the falx and tentorium after removing the cerebrum. The tentorial edges sweep along the lateral margin of the cerebral peduncle. The tentorial apex is located at the junction of the vein of Galen and the straight sinus. The tentorial edges slope downward from the apex. B, Superoposterior view of the cerebellomesencephalic fissure. The central lobule and culmen of the cerebellar vermis have been retracted posteriorly to expose the cerebellomesencephalic fissure, which extends forward between the cerebellum and midbrain. The anterior wall of the fissure is formed in the midline by the collicular plate and lingual, and laterally by the superior cerebellar peduncles. The trochlear nerve arises below the inferior colliculi. The initial part of the cisternal segment of the trochlear nerve is followed laterally in the cerebellomesencephalic fissure. C, Superolateral view of the right tentorial surface of the cerebellum. The tentorial edge sweeps along the lateral margin of the cerebral peduncle. The tentorium has been removed while the tentorial venous sinuses and tentorial edge have been preserved. The posterior cerebral artery runs posteriorly along the lateral margin of the midbrain above the tentorial edge. D, Subtemporal view. The right temporal lobe has been elevated to expose the PCA, SCA, and trochlear nerve in the anterior and middle incisural spaces. The PCA passes above and the SCA below the oculomotor nerve. The trochlear nerve courses around the side of the brainstem. It becomes subtentorial at the anterolateral part of the cerebral peduncle. (Images courtesy of AL Rhoton, Jr.)