A, The PICAs pass around the posterior medulla to reach the lower margin of the cerebellomedullary fissure. The left PICA courses around the lower pole of the tonsil. The right PICA descends well below the tonsil to the level of the foramen magnum before ascending along the medial tonsillar surface. B, The PICAs ascend between the tonsils and medulla to reach the interval between the tonsil and uvula and to supply the suboccipital surface. C, The posterior medullary segment of the right PICA divides into a medial trunk supplying the vermis and paravermian area and a lateral trunk supplying the hemisphere. D, The cerebellum has been sectioned in an oblique coronal plane to show the relationship of the rostral pole of the tonsil to the inferior medullary velum and dentate nucleus. The dentate nucleus is located above the posterolateral part of the ventricular roof, near the fastigium, where it wraps around, and is separated from, the rostral pole of the tonsil by the inferior medullary velum. The left tonsil has been removed while preserving the left half of the inferior medullary velum. The SCAs course in the cerebellomesencephalic fissure. The PICA passes between the walls of the cerebellomedullary fissure formed above by the inferior medullary velum and below by the upper pole of the tonsil. E, Both tonsils have been removed. The PICAs ascend through the cleft between the inferior medullary velum and rostral pole of the tonsil. F, The superior part of the ventricular roof has been removed and the nodule and the inferior medullary velum has been folded downward to expose the floor. (Images courtesy of AL Rhoton, Jr.)