Cerebellomedullary Fissure and Nearby Anatomy. A, The cerebellomedullary fissure extends upward between the tonsils posteriorly and the medulla anteriorly. The upper pole of the tonsils face the uvula. The vallecula opens between the tonsils into the fourth ventricle. The PICAs pass around the tonsil, often forming a caudal loop, which may approximate the level of the lower pole of the tonsil, and a cranial loop, which approximates the position of the rostral pole of the tonsil. The left PICA is larger than the right PICA. B, Both tonsils have been retracted laterally to expose the inferior medullary velum and tela choroidea, which form the lower portion of the roof of the fourth ventricle. The nodule of the vermis, on which the inferior medullary arises, is hidden deep with respect to the uvula. C, Enlarged view of the left half of the cerebellomedullary fissure. The choroidal arteries and veins course along the tela choroidea from which the choroid plexus projects into the roof of the fourth ventricle. The vein of the cerebellomedullary fissure is the largest vein in the cerebellomedullary fissure. The dashed line shows the site of the incision into the tela to provide the exposure seen in the next step (D). D, The tela choroidea has been opened, extending upward from the foramen of Magendie to the junction of the tela with the inferior medullary velum. The uvula has been displaced to the right side to provide this view of the fourth ventricle from the aqueduct to the obex. (Images courtesy of AL Rhoton, Jr.)