E, Enlarged view. The left AICA arises from the basilar artery and passes laterally toward the porus of the internal acoustic meatus before turning medially between the facial and vestibulocochlear nerves. The tortuous PICA loops upward between the AICA and the facial nerve before turning downward. F, The AICA and the nerves entering the internal acoustic meatus have been divided. The PICA loops upward before turning caudally and passing between the rootlets of the vagus and accessory nerves. The hypoglossal nerve arises from the brainstem in front of the olive. One of the rootlets of the hypoglossal nerve loops upward around the origin of the PICA before descending to join the other rootlets at the hypoglossal canal. A bridging vein passes from the medulla to the jugular bulb. G, The section has been extended downward to the level of the medulla to show the perforating branches of the vertebral and basilar arteries entering the medullary pyramids and the lateral medulla. The glossopharyngeal, vagus, and accessory nerves arise dorsal to the olives. The hypoglossal nerve arises ventral to the olives and passes behind the vertebral arteries. H, The medullary section has been extended caudally. The level of the PICA origins from the vertebral arteries are asymmetric. The right PICA intermingles with multiple rootlets of the hypoglossal nerve, while the left PICA, which arises at a higher level, has only the upper hypoglossal rootlet stretched around it. The PICAs encircle the medulla and appear on the dorsal surface behind the fourth ventricle. The left is larger than the right vertebral artery. (Images courtesy of AL Rhoton, Jr.)