Posterior surface of right petrous bone. The petrous bone forms the anterior boundary of the posterior cranial fossa. On its posterior surface are openings for the internal auditory meatus and extraosseous portion of the endolymphatic sac. Along its inferior border is the jugular foramen, a space between the petrous bone and occipital bone. Between the petrous bone and clivus is the petroclival fissure for the inferior petrosal sinus. This drains the cavernous sinus into the internal jugular vein through the jugular foramen. Posteriorly, the sigmoid groove for the sigmoid sinus leads into this foramen.