A, The tentorial surface faces the tentorium, which has been removed. The surface slopes downward from the apex to the posterior and lateral margins. The upper part of the tentorial surface surrounds the posterior half of the midbrain and forms the posterior lip of the cerebellomesencephalic fissure. The anterior cerebellar incisura, the notch where the brainstem fits into the anterior part of the tentorial surface, is located anteriorly and the posterior cerebellar incisura, the notch where the falx cerebelli fits into the cerebellum, is located posteriorly. B, Enlarged view of the cerebellomesencephalic fissure, which extends downward between the midbrain and the cerebellum. The superficial part of the posterior lip is formed by the culmen in the midline and the quadrangular lobule laterally. The quadrigeminal cistern extends caudally from the pineal into the cerebellomesencephalic fissure. C, The culmen has been removed to expose the central lobule and its wings, which form part of the posterior lip of the cerebellomesencephalic fissure. D, The central lobule and its wings, the lingula, the superior medullary velum, and medial part of the superior cerebellar peduncles have been removed to expose the fourth ventricle. The lower half of the roof is formed in the midline by the nodule and laterally by the inferior medullary velum, which passes laterally above, but is separated from the rostral pole of the tonsils by the cerebellomedullary fissure. E, Some of the middle peduncle has been removed to expose the choroid plexus extending through the lateral recess into the cerebellopontine angle below the facial and vestibulocochlear nerves. F, Oblique view of the lower half of the roof formed by the inferior medullary velum and the tela choroidea in which the choroid plexus arises. The inferior medullary velum arises on the surface of the nodule and extends laterally to blend into the flocculus and, with the flocculus and nodule, forms the flocculonodular lobe of the cerebellum. (Images courtesy of AL Rhoton, Jr.)