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Brainstem, Fourth Ventricle, and Petrosal Cerebellar Surface: Stepwise Anterior Exposure

Surgical Correlation

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G, The left half of the medulla has been removed. The superior half of the roof is formed by the superior medullary velum, which has the lingula of the vermis layered on its outer surface. The lower half of the roof is formed by the inferior medullary velum, which arises on the surface of the nodule, and the tela choroidea in which the choroid plexus arises. The choroid plexus is composed of paired L-shaped fringes, which have medial and lateral segments. The lateral segments extend laterally through the foramen of Luschka and the medial segments extend longitudinally through the foramen of Magendie. H, The right half of the tela choroidea and choroid plexus have been removed to expose the upper pole of right tonsil. I, The right cerebellar tonsil has been removed. All of the surfaces of the tonsils are free surfaces except the superolateral margin, the site of the tonsillar peduncle, a bundle of white matter, which attaches the tonsil to the remainder of the cerebellum. The inferior medullary velum is a thin membranous layer of neural tissue that arises on the nodule and extends laterally above the rostral pole of the tonsil to blend into the flocculus and form the flocculonodular lobe of the cerebellum. The cranial loop of the PICA courses between the rostral pole of the tonsil and the inferior medullary velum. J, Both tonsils have been removed. The inferior medullary velum sweeps laterally from the surface of the nodule. (Images courtesy of AL Rhoton, Jr.)

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