Surgical Approaches to the Posterior Fossa. Presigmoid and retrosigmoid craniotomies have been performed here, demonstrating posterior fossa anatomic landmarks. The auricle has been removed to reveal the external auditory meatus. The petrous temporal bone has been drilled to reveal the posterior, superior, and lateral semicircular canals. The sigmoid sinus drains into the jugular bulb in the jugular foramen, formed anteriorly by the petrous portion of the temporal bone and posteriorly by the occipital bone. The styloid process is visible just deep to the mastoid tip. The V3 segment of the vertebral artery is visible coursing along the superior edge of C1 (the atlas). The posterior atlantooccipital membrane is visible in the midline between the atlas and the foramen magnum. (Image courtesy of AL Rhoton, Jr.)