A, The left PICA is larger than the right. Both PICAs enter the cerebellomedullary fissure, pass around the tonsils, and exit the fissure to supply the suboccipital surface. The natural cleft between the right tonsil and the biventral lobule has been opened. The tonsil is attached to the remainder of the cerebellum by the tonsillar peduncle, a white matter bundle along its superolateral margin. All of the other margins of the tonsils are free margins. B, Enlarged view. The left biventral lobule has been elevated to expose the flocculus protruding from the margin of the lateral recess. C, The tonsils have been retracted laterally to expose the PICAs coursing in the cerebellomedullary fissure. The right PICA bifurcates into medial and lateral trunks before reaching the cerebellomedullary fissure. The left PICA bifurcates within the fissure. The medial trunks supply the vermis and adjacent part of the hemisphere and the lateral trunks supply the remainder of the hemisphere. D, The right tonsil has been removed to expose the lateral recess and bifurcation of the right PICA into medial and lateral trunks. E, Both tonsils and the tela have been removed to expose the ventricular floor and walls. The left PICA divides into its trunks within the cerebellomedullary fissure. The inferior medullary velum has been preserved, but is a thin layer that can be opened, if needed, to increase the exposure of the fourth ventricle. F, Enlarged view showing the relationship of the PICAs to the fourth ventricle. The PICAs, after passing between the rootlets of the accessory rootlets course along the caudolateral margin of the fourth ventricle on the inferior cerebellar peduncle before entering the cerebellomedullary fissure. The left PICA has been reflected laterally. The facial colliculus is in the upper and hypoglossal and vagal nuclei are in the lower part of the floor. (Images courtesy of AL Rhoton, Jr.)