The Brainstem and Intradural Segment of the VA and PICA. Upper, The brainstem, the intradural segment of the VA and PICA, and the left dorsal quadrant of the cervical cord have been removed to show the relationship of the VA, dentate ligament, ventral rootlets of C-1, and the accessory rootlets to the extradural and initial intradural segment of the PICAs. The VA passes in front of the accessory nerve and the upper attachment of the dentate ligament. The initial intradural segment of the PICA passes behind the dentate ligament and in front of the accessory nerve. The posterior meningeal artery arises below the atlantooccipital joint, after the VA has ascended through the transverse foramen of C-1. Lower, Posterior view of the foramen magnum and VAs. A PICA with an extradural origin may arise just outside the dura (A), farther laterally above the transverse foramen of the atlas (B), or at the point at which the VA ascends between the transverse foramina of C-2 and C-1 (C). The typical origin is intradural (D). (Images courtesy of AL Rhoton, Jr.)