Right posterolateral view of the brainstem and associated neurovasculature. The midbrain, pons, and medulla are in view in this specimen. The cerebellum has been removed and cut surfaces of the superior, middle, and inferior cerebellar peduncles can be seen. The right vertebral artery gives rise to its posterior inferior cerebellar artery (PICA) before uniting with the left vertebral near the pontomedullary junction to form the basilar artery. The left PICA (unlabeled) can be seen approaching the posterior surface of the medulla before ascending in a loop toward the inferior cerebellar peduncle. The proximal portion of the basilar artery gives rise to the anterior inferior cerebellar artery anteroinferior to the abducens nerve, which leaves the pons anteriorly at the pontomedullary junction. This artery supplies the inferior cerebellum, including the inferior and middle cerebellar peduncles. The labyrinthine artery to the inner ear and pontine branches also can be seen arising from the basilar. Near the pontomesencephalic junction the basilar gives rise to the superior cerebellar arteries and then bifurcates into the posterior cerebral arteries. The oculomotor nerve emerges from the anterior midbrain between these two vessels. The quadrigeminal artery, a small branch off the posterior cerebral, courses posteriorly across the cerebral peduncle toward the midbrain tectum where it supplies the superior and inferior colliculi. Here, the trochlear nerve emerges from the midbrain and travels forward around the cerebral peduncle. The large trigeminal nerve is seen emerging from the anterolateral pons. The facial and vestibulocochlear nerves (acoustic facial bundle) leave the pons at the lateral pontomedullary junction (cerebellopontine angle) in relationship with the anterior inferior cerebellar and labyrinthine arteries. (Image courtesy of AL Rhoton, Jr.)