A, Superior view of the posterior cranial fossa. The osseus walls of the posterior fossa are formed by the occipital, temporal, and sphenoid bones. The fossa is bounded in front by the dorsum sellae and posterior part of the sphenoid bone and the clival part of the occipital bone; behind by the lower portion of the squamosal part of the occipital bone; and on each side by the petrous and mastoid parts of the temporal bone, and the lateral part of the occipital bone. One small part above the temporal bone is formed by the inferior angle of the parietal bone. B, Nerves and arteries of the posterior fossa. Only 2 of the 12 pairs of cranial nerves course entirely outside the posterior fossa. The tentorium, which is attached along the petrous ridges, roofs the posterior fossa. (Images courtesy of AL Rhoton, Jr.)