A, Anterior view. The clivus and adjacent part of the occipital and temporal bones have been removed to expose the front of brainstem, vertebral and basilar arteries, facial and vestibulocochlear nerves in the right internal acoustic meatus, and the hypoglossal nerve in the right hypoglossal canal. The left AICA loops into the porus of the meatus. B, Enlarged view of the right cerebellopontine angle. The AICA passes between the facial and vestibulocochlear nerves. The hypoglossal nerves are stretched around the posterior surface of the vertebral artery. The vertebral artery kinks upward into the cerebellopontine angle where the PICA arises in close relationship to the root exit zone of the facial nerve, a common finding in hemifacial spasm. A labyrinthine artery arises from the AICA. C, Another enlarged view of the right cerebellopontine angle. The labyrinthine artery passes laterally with the facial nerve. The PICA loops upward and contacts the lower margin of the facial nerve. The vein of the cerebellopontine fissure ascends to empty into the superior petrosal sinus. D, The left AICA passes below the abducens, facial, and vestibulocochlear nerves and loops into the porus where it gives off two labyrinthine branches. Some of the hypo-glossal rootlets are stretched over the PICA. The posterior trigeminal nerve was divided behind Meckel’s cave. The proximal stump arises from the midpons and the distal portion enters Meckel’s cave. (Images courtesy of AL Rhoton, Jr.)