A, The tentorial cerebellar surface faces the tentorium and slopes downward from its apex located below the tentorial apex. The cerebellomesencephalic fissure extends forward between the cerebellum and midbrain. This surface, in which the vermis is the highest part, differs from the suboccipital surface in which the vermis is folded into a deep cleft, the incisura, between the cerebellar hemispheres. The straight sinus and tentorial edge have been preserved. The SCA exits the cerebellomesencephalic fissure and supplies the tentorial surface. B, The right half of the posterior lip of the cerebellomesencephalic fissure has been removed. The anterior wall of the fissure is formed in the midline by the collicular plate and lingula, and laterally by the superior cerebellar peduncles. The middle cerebellar peduncle wraps around the lateral surface of the superior peduncle. The trochlear nerve arises below the inferior colliculi. C, The right half of the lingula and superior medullary velum have been removed to expose the fourth ventricle. Additional white matter has been removed below the right superior peduncle to expose the dentate nucleus in which the superior peduncular fibers arise. D, Enlarged view. The dentate nucleus appears to wrap around the rostral pole of the tonsil. E, Oblique view into the fourth ventricle. Additional cerebellum has been removed to expose the nodule and rostral pole of the tonsil. The dentate nucleus wraps around the rostral pole of the tonsil. The upper half of the roof is formed by the superior medullary velum, which has the lingula layered on its outer surface. The upper part of the lower half of the roof is formed by the nodule in the midline and by the inferior medullary velum laterally. The inferior medullary velum, an almost transparent membrane, stretches laterally across the upper pole of the tonsil. F, The left half of the upper part of the roof has been removed. The velum arises on the nodule and sweeps laterally above both tonsils. The SCA courses within the cerebellomesencephalic fissure. (Images courtesy of AL Rhoton, Jr.)