The bones making up the calvaria (Fig. 8.22) are unique in their structure, consisting of dense internal and external tables of compact bone separated by a layer of spongy bone (the diploë).
The base of the skull is seen in the inferior view and extends anteriorly from the middle incisor teeth posteriorly to the superior nuchal lines and laterally to the mastoid processes and zygomatic arches (Fig. 8.23).
For descriptive purposes the base of the skull is often divided into: an anterior part, which includes the teeth and the hard palate, a middle part, which extends from behind the hard palate to the anterior margin of the foramen magnum, and a posterior part, which extends from the anterior edge of the foramen magnum to the superior nuchal lines.
The main features of the anterior part of the base of the skull are the teeth and the hard palate.
The teeth project from the alveolar processes of the two maxillae. These processes are together arranged in a U-shaped alveolar arch that borders the hard palate on three sides (Fig. 8.23).
The hard palate is composed of the palatine processes of each maxilla anteriorly and the horizontal plates of each palatine bone posteriorly.
The paired palatine processes of each maxilla meet in the midline at the intermaxillary suture, the paired maxillae and the paired palatine bones meet at the palatomaxillary suture, and the paired horizontal plates of each palatine bone meet in the midline at the interpalatine suture.
Several additional features are also visible when the hard palate is examined: the incisive fossa in the anterior midline immediately posterior to the teeth, the walls of which contain incisive foramina (the openings of the incisive canals, which are passageways between the hard palate and nasal cavity); the greater palatine foramina near the posterolateral border of the hard palate on each side, which lead to greater palatine canals; just posterior to the greater palatine foramina, the lesser palatine foramina in the pyramidal process of each palatine bone, which lead to lesser palatine canals; a midline pointed projection (the posterior nasal spine) in the free posterior border of the hard palate.
The middle part of the base of the skull is complex:
Forming the anterior half are the vomer and sphenoid bones.
Forming the posterior half are the occipital and paired temporal bones.
Anteriorly, the small vomer is in the midline, resting on the sphenoid bone (Fig. 8.23). It contributes to the formation of the bony nasal septum separating the two choanae.
Most of the anterior part of the middle part of the base of the skull consists of the sphenoid bone.
The sphenoid bone is made up of a centrally placed body, paired greater and lesser wings projecting laterally from the body, and two downward projecting pterygoid processes immediately lateral to each choana.
Three parts of the sphenoid bone, the body, greater wings, and pterygoid processes, are seen in the inferior view of the skull (Fig. 8.23). The lesser wing of the sphenoid is not seen in the inferior view.
The body of the sphenoid is a centrally placed cube of bone containing two large air sinuses separated by a septum.
It articulates anteriorly with the vomer, ethmoid, and palatine bones, posterolaterally with the temporal bones, and posteriorly with the occipital bone.
Extending downward from the junction of the body and the greater wings are the pterygoid processes (Fig. 8.23). Each of these processes consists of a narrow medial plate and broader lateral plate separated by the pterygoid fossa.
Each medial plate of the pterygoid process ends inferiorly with a hook-like projection, the pterygoid hamulus, and divides superiorly to form the small, shallow scaphoid fossa.