Sagittal view of pons, medulla, fourth ventricle, and cerebellum. The fourth ventricle is located between the posterior surfaces of the pons (pontine tegmentum) and medulla anteriorly and the vermis of the cerebellum posteriorly. The primary fissure of the cerebellum separates the anterior and posterior lobes, while the posterolateral fissure separates the posterior and flocculonodular lobes (nodule portion seen in this section). The cerebellar tonsils, parts of the posterior lobes, are ovoid structures on the inferomedial surfaces of each hemisphere. The right vertebral artery is shown wrapping around the medulla to join the left artery (not present in image) at the pontomedullary junction to form the basilar artery. It previously had given off the posterior inferior cerebellar artery, which has curved around to the posterior surface of the medulla where it ascends to the fourth ventricle, and the right anterior spinal artery arises from its medial wall to descend in the anterior median fissure. A portion of the basilar artery is missing due to plane of section, but shows its large anterior inferior cerebellar branch and a transverse directed pontine branch. (Image courtesy of AL Rhoton, Jr.)