A, Posterolateral view. The SCA passes around the midbrain to enter the cerebellomesencephalic fissure, where it sends perforating branches into the posterior midbrain below a line between the superior and inferior colliculi, and down the superior peduncle to the dentate nucleus. The AICA loops around the flocculus and the facial and vestibulocochlear nerves. The left PICA passes between the rootlets of the nerves entering the jugular foramen and turns caudally around the lower pole of the left tonsil, which has been removed, and then ascends to form a cranial loop at the upper pole of the tonsil bordering the inferior half of the ventricular roof. B, Another specimen. The left half of the cerebellum has been removed. The SCA passes around the midbrain below the PCA in the lower part of the ambient and quadrigeminal cisterns, enters the cerebellomesencephalic fissure, and loops over the posterior lip of the fissure to supply the tentorial surface. The PICA arises from the vertebral artery, passes around the medulla, crosses the inferior cerebellar peduncle, and enters the cerebellomedullary fissure, where it passes along the inferior half of the ventricular roof, and exits the fissure to supply the suboccipital surface. The AICA passes laterally around the pons and above the flocculus. C, Enlarged oblique view. The right PICA loops around the caudal and rostral poles of the tonsil. The left PICA dips below the level of the foramen magnum. D, Posterior view after removing all of the cerebellum except for the right tonsil and dentate nucleus. (Images courtesy of AL Rhoton, Jr.)