A, On the endocranial surface, the anterior and middle cranial base corresponds to the anterior and middle fossae. The anterior part of the cranial base is separated from the middle fossa by the sphenoid ridge and the chiasmatic sulcus. The middle cranial base is separated from the posterior cranial base by the dorsum sellae and the petrous ridges. The upper surface of the anterior cranial base is formed by the frontal bone, which roofs the orbit; the ethmoid bone, which is interposed between the frontal bones and is the site of the cribriform plate; and the lesser wing and anterior part of the body of the sphenoid, which forms the posterior part of the floor of the anterior fossa. The upper surface of the middle cranial base floor is formed by the greater sphenoid wing and posterior two-thirds of the sphenoid body anteriorly and the upper surface of the temporal bone posteriorly. The posterior part of the cranial base is formed by the temporal and occipital bones. The cribriform plate, sella, and clivus are located in the medial part of the cranial base. The lateral part of the cranial base is located above the orbits, pterygopalatine and infratemporal fossae, and the subtemporal and lateral part of the suboccipital areas. B, Exocranial surface of the cranial base. This surface is more complicated than the endocranial surface. It is not demarcated into three well-defined fossae as is the endocranial surface. The exocranial surface is formed by the maxilla, the zygomatic, palatine, sphenoid, temporal, and occipital bones, and the vomer. The maxilla, orbits, and nasal cavity are located below the anterior fossa. The anterior part of the hard pal-ate is formed by the maxilla and the posterior part is formed by the palatine bone. The anterior part of the zygomatic arch is formed by the zygoma and the posterior part by the squamosal part of the temporal bone. The mandibular fossa on the lower surface of the temporal squama is located below the posterior part of the middle fossa. The vomer attaches to the lower part of the body of the sphenoid and forms the posterior part of the nasal septum. C, Anterior view. The orbital rim is formed by the frontal bone, zygoma, and maxilla. The roof of the orbit is formed by the frontal and sphenoid bones; the lateral wall by the greater sphenoid wing and the zygomatic bone; the floor by the maxilla, except for a small part of the posterior floor formed by the palatine bone; and the medial wall of the orbit by the maxilla and lacrimal and ethmoid bones. The nasal bone is interposed above the anterior nasal aperture between the maxillae. The nasal cavity is located between the ethmoid bones above and the maxillae, palatine bones, and sphenoid pterygoid process below. It is roofed by the frontal and ethmoid bones and the floor is formed by the maxillae and palatal bones. The osseous nasal septum is formed by the perpendicular ethmoid plate and the vomer. The inferior concha is a separate bone, and the middle and superior conchae are appendages of the ethmoid bone. The orbit opens through the superior orbital fissure into the middle fossa and through the inferior orbital fissure into the pterygopalatine and infratemporal fossae. D, Anteroinferior view of the cranial base. The anterior part of the hard palate is formed by the maxillae and the posterior part is formed by the horizontal plate of the palatine bone. The vomer forms the posterior part of the nasal septum and divides the posterior nasal aperture in the midline. The infratemporal fossa is located below the greater sphenoid wing. The clivus is formed above by the body of the sphenoid bone and below by the basal part of the occipital bone. The petrous apex is interposed between the greater sphenoid wing and the clival part of the occipital bone. The mandibular condyles are set in the mandibular fossa, located below the posterior part of the middle fossa on the inferior surface of the squamosal part of the temporal bone. (Images courtesy of AL Rhoton, Jr.)