Posterior inferior view of neurovasculature at level of foramen magnum. The posterior surface of the medulla is in view. The vertebral arteries and spinal roots of the spinal accessory nerves ascend through the foramen magnum into the posterior cranial fossa. The right cerebellar tonsil has been retracted to reveal the roof (opened) of the fourth ventricle. The posterior inferior cerebellar arteries are the principal blood supply to the dorsolateral rostral medulla and posteroinferior cerebellum. They are the largest branches of the vertebral arteries and have a tortuous course to their sites of distribution. They arise anterior to the olive of the medulla and course between it and the emerging roots of cranial nerves IX-XI. From here they descend to the lower pole of the cerebellar tonsil (caudal loop) and then loop upward along the medial aspect of the tonsil to the roof of the fourth ventricle where they provide branches to the choroid plexus and roof of the fourth ventricle. They reverse course again (cranial loop) and turn downward and posteriorly to the tonsillobiventral fissure. Here they emerge and divide into a medial or vermian branch and lateral branch supplying tonsillar and hemispheric surfaces. (Image courtesy of AL Rhoton, Jr.)