Osseous Relationships of the Exocranial Surface of the Clivus and the Adjacent Cranial Base. A, Inferior view. The exocranial surface of the clival part of the occipital bone is convex from side to side and has an irregular surface. It is wider above than below at the foramen magnum. B, Enlarged view. The petroclival fissure separates the lateral edge of the clivus from the petrous part of the temporal bone. This fissure extends from the foramen lacerum above to the jugular foramen below. There is an anteroposterior groove along the inferior surface of the vaginal process of the sphenoid bone. This groove joins with the upper surface of the sphenoidal process of the palatine bone to form the palatovaginal canal, which opens anteriorly into the pterygopalatine fossa. An inconsistent vomerovaginal canal may lie medial to the palatovaginal canal between the ala of the vomer and the vaginal process of the sphenoid. C, Anterior view. The middle and inferior conchae have been removed. The clival part of the occipital bone is seen through the lower part of the nasal cavity and the posterior nasal apertures (choanae), the opening between the nasal cavity and nasopharynx bounded below by the horizontal plate of the palatine bone, superiorly by the body of the sphenoid, and laterally by the medial pterygoid plates. The sphenoid sinus occupies the upper part of the view through the nasal cavity. The hard palate is positioned at the level of the foramen magnum posteriorly. The lateral part of the cranial base is hidden behind the maxillae. D, Anterior view. The sphenoid, ethmoid, and both palatine bones have been fitted together. The ethmoid bone forms the roof of the nasal cavity in front of the sphenoid bone. The middle nasal conchae are parts of the ethmoid bone, whereas the inferior nasal concha is a separate bone. The posterior end of the middle conchae articulates with the ethmoidal crest of the perpendicular plate of the palatine bone. E, The sphenoid, ethmoid, and maxillary bones and the vomer have been fitted together. The perpendicular plate of the ethmoid forms the upper part of nasal septum and the vomer forms the lower part. The frontal processes of the maxillae form the anterior part of the lateral wall of the nasal cavity. F, Anterior view. The sphenoid, occipital, and left palatine bones have been fitted together. The pharyngeal tubercle is exposed in the midline on the clival part of the occipital bone. The sphenopalatine notch of the palatine bone, located between the orbital and sphenoid processes, is closed superiorly by the inferior surface of the sphenoid bone to form the sphenopalatine foramen. The anterior end of the vidian canal is sometimes seen through the sphenopalatine foramen. G, Anterior view. The occipital, sphenoid, and left palatine bones and the axis and atlas have been fitted together. The clivus and atlas (C1) can be viewed through the nasal cavity between the body of the sphenoid above, the horizontal plate of the palatine bone below, and the vertical plate of the palatine bone and pterygoid process of the sphenoid bone laterally. H, Oblique view of the left lateral wall of the nasal cavity. The sphenoid and left maxillary and palatine bones have been fitted together. The lower half of the lateral nasal wall is composed of, from anterior to posterior, the maxilla, the perpendicular plate of the palatine bone, and the medial pterygoid plate. The sphenopalatine foramen is located just above the ethmoidal crest of the palatine bone. The maxillary hiatus, a large opening in the medial wall of the maxillary sinus, is formed by the maxilla anteriorly and the perpendicular plate of the palatine bone posteriorly. The hiatus is partially closed by the inferior nasal concha and the ethmoid bone. (Images courtesy of AL Rhoton, Jr.)