Internal view of neurovasculature in the posterior fossa. The spinal cord has been transected at the level of the foramen magnum and brain contents of the posterior fossa removed. Ascending through this foramen are the spinal roots of the spinal accessory nerves and the vertebral arteries. The nerves lie posterior to the arteries. The spinal portion of the accessory nerve courses superolaterally toward the jugular foramen into which also course rootlets of the glossopharyngeal and vagus nerves. Rootlets of the hypoglossal nerves after leaving the medulla pass posterior to the vertebral arteries to enter the hypoglossal canals located about halfway between the foramen magnum and jugular foramen. The anterior spinal arteries arise from the vertebral arteries and usually join at the level of the foramen magnum into a single vessel which descends within the anterior median fissure of the spinal cord providing blood supply to the cord. The vertebral arteries join near the pontomedullary junction to form the basilar artery. The posterior inferior cerebellar arteries are the largest branches of the vertebral arteries and are one of three pairs of vessels supplying the cerebellum. In this specimen, the left posterior inferior cerebellar artery arises from the basilar artery, which occurs in about 10% of cases. (Image courtesy of PA Rubino)