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Anatomy of the Skull: Norma basalis ( Anterior part , Middle part and Posterior part )

When the intact skull is viewed from below, the limits of the oral cavity can be seen. Its roof is formed by the hard palate; its walls by the upper and lower teeth embedded in the alveolar processes of the maxillae and the mandible. The posterior border of the mandible can be followed upward to the temporomandibular joint between the head of the mandible and the base of the skull, and the ramus of the mandible can be seen as the lateral wall of the infratemporal fossa.

With the mandible removed, norma basalis is seen to include the upper jaw and teeth, the hard palate, the posterior nasal apertures or posterior nares (also termed the choanae, from the Greek, meaning funnel), the bony nasal septum between the two nasal cavities, the prominences of the cheeks, the zygomatic arches, the mandibular fossae that lodge the heads of the mandible, the entrance and floor of the external auditory meati, and the foramen magnum, through which the brain stem becomes continuous with the spinal cord. The foramen magnum is flanked anterolaterally by the occipital condyles, which articulate with the first cervical vertebra, the atlas. In this view, the posterior extremity of the lateral nasal wall is formed by the pterygoid process of the sphenoid bone. The process bears the bony medial and lateral pterygoid plates, which project posteriorly, enclosing the pterygoid fossa between them. If the outline of the mandibular ramus is projected onto the base of the skull, the limits of the infratemporal fossa become obvious. The maxilla forms its anterior wall, the lateral pterygoid plate its medial wall, the ramus of the mandible its lateral wall, and the base of the skull (here the greater wing of the sphenoid bone, vide infra) its roof.

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