A, The cerebellomedullary fissure extends upward between the tonsils posteriorly and the medulla anteriorly. The vallecula opens between the tonsils into the fourth ventricle. B, Both tonsils have been retracted laterally to expose the inferior medullary velum and tela choroidea that form the lower part of the ventricular roof. The nodule of the vermis, on which the inferior medullary arises, is hidden deep to the uvula. C, Enlarged view of the left half of the cerebellomedullary fissure. The choroidal arteries course along the tela choroidea from which the choroid plexus projects into the roof of the fourth ventricle. The vein of the cerebellomedullary fissure, which crosses the inferior medullary velum, is the largest vein in the cerebellomedullary fissure. The interrupted line shows the site of the incision in the tela to provide the exposure seen in the next step. The telovelar junction is the line of attachment of the tela to the velum. D, The tela choroidea has been opened extending from the foramen of Magendie to the junction with the inferior medullary velum. The uvula has been displaced to the right side to provide this view extending from the aqueduct to the obex. (Images courtesy of AL Rhoton, Jr.)