A, The bone and structures lateral to the orbit, infratemporal, and pterygopalatine fossa, and the parapharyngeal space and petrous part of the temporal bone have been removed to expose the structures below the anterior, middle, and posterior cranial base. The orbit and maxillary sinus are located below the anterior cranial base. The infratemporal and pterygopalatine fossae and the parapharyngeal space are located below the middle cranial base, and the suboccipital area is located below the temporal and occipital bones. The first trigeminal division is related to the upper part of the orbit. The second trigeminal branch is related to the lower part of the orbit and maxilla. The mandibular nerve exits the cranium through the foramen ovale and enters the infratemporal fossa. The pterygoid and levator and tensor veli palatini muscles have been removed to expose the eustachian tube and its opening into the nasal pharynx. The lateral part of the temporal bone has been removed to expose the cochlea, vestibule, and semicircular canals. The petrous carotid passes upward and turns medially below the cochlea. The sigmoid sinus turns downward under the semicircular canals and vestibule where the jugular bulb is located. The segment of the vertebral artery passing behind the atlanto-occipital joint is located below the posterior cranial base. B, The dura has been opened to show the relationships of the frontal and temporal lobes and the cerebellum to the cranial base. The orbit is exposed below the frontal lobe. The pterygopalatine and infratemporal fossae and the temporal bone are located below the temporal lobe. The jugular bulb and internal jugular vein have been removed to show Cranial Nerves IX through XII exiting the jugular foramen. (Images courtesy of AL Rhoton, Jr.)