Osseous Relationships of the Exocranial Surface of the Clivus and the Adjacent Cranial Base. I, Enlarged view. J, Lateral view of the cranial base. The pterygopalatine fossa is the narrow space between the superior half of the posterior wall of the maxillary sinus and the pterygoid process of the sphenoid bone. The sphenopalatine foramen opens through the medial wall of the fossa into the nasal cavity. K, Right lateral nasal wall in a sagittally sectioned specimen. The superior, middle, and inferior nasal conchae have been removed. A thin, curved bar of bone, called the uncinate process, bridges the anterior part of the maxillary hiatus and divides the maxillary hiatus into anterior and posterior spaces. The posterior space, called the semilunar hiatus and situated between the uncinate process and ethmoid bulla, is characterized by its arc-like shape. The anterior space is closed by the mucosa in life. The posterior space is also closed by mucosa except at the ostia draining the maxillary, ethmoidal, and frontal sinus into the nasal cavity. The sphenopalatine foramen is located behind the ethmoid bulla. L, Anterior view of the sphenoid bone. The left half of the anterior wall of the sphenoid sinus has been opened. The anterior wall of the sphenoid sinus consists of 2 areas, an upper and lateral depressed area, which shares its wall with the posterior ethmoid cells, and is hidden behind the ethmoid air cells. In this specimen, the most posterior ethmoid cells extend into the sphenoid sinus and are called Onodi cells. A lower and medial smooth triangular area that can be viewed through the nasal cavity behind the superior turbinate corresponds to the sphenoid conchae. The ostia of the sphenoid sinus are located near the superior angles of the sphenoid concha. The sphenoidal crest forms a small part of the nasal septum and articulates with the perpendicular plate of the ethmoid bone. M, Inferior view of the sphenoid and palatine bones and the vomer. The upper border of the vomer is expanded into an ala on each side and attaches to the sphenoid bone. The lateral border of each ala articulates with a thin bony lamella, the vaginal process of the sphenoid bone, which projects medially from the root of the medial pterygoid plate to complete the palatovaginal canal. The pterygoid hamulus, around which the tensor veli palatini muscle turns medially to insert into the soft palate, projects laterally from the tip of the medial pterygoid plate. N, Inferior view of the occipital bone. The pharyngeal tubercle is located in the midline of the clivus approximately 1 cm anterior to the anterior edge of the foramen magnum. The occipital condyles project downward along the anterolateral margin of the foramen magnum. There is a small bony groove, the supracondylar groove, for the insertion of the rectus capitis anterior muscle in front of each occipital condyle. The extracranial end of the hypoglossal canal opens above the occipital condyle. O, Anterior view. The sphenoid and occipital bones are fitted together. The areas corresponding to the upper, middle, and lower clivus are shown on the exocranial aspect. The anterior end of the vidian canals sits at the junction of the upper and middle clivus. The pharyngeal tubercle can be used to estimate the junction of the middle and lower clivus. (Images courtesy of AL Rhoton, Jr.)