Posterior view of the medulla and spinal cord as it exits the foramen magnum. The cerebellum forms the inferior roof of the 4th ventricle and has been removed at the middle cerebellar peduncles. The vertebral arteries ascend through the transverse foramina of cervical vertebrae, pass around the lateral masses of C1 and then through the foramen magnum into the posterior fossa where they unite to form the basilar artery (removed). Facial and vestibulocochlear nerves can be seen passing through the internal acoustic (auditory) meatus. The spinal accessory nerve enters the cranium through the foramen magnum and exits with the glossopharyngeal and vagus nerves through the jugular foramen. In this view, the trochlear nerve is seen emerging from the posterior surface of the midbrain, the only cranial nerve that arises from the posterior brain stem.  (Image courtesy of AL Rhoton, Jr.)