Norma Basalis (external view)
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 temporo-mandibular 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.
The maxillae form the upper jaw and alveolar arch and the anterior two-thirds of the hard palate.
The paired palatine bones form the posterior one-third of the hard palate.
The unpaired vomer forms the posterior and lower part of the bony nasal septum.
The paired zygomatic bones form the prominence of the cheek and anterior half of the zygomatic arch on each side.
The unpaired sphenoid bone (Gr. spheno+wedge) contributes a strip across the entire base of the skull anterior to the occipital and temporal bones. In this external view, the sphenoid is seen to send a pterygoid process downward on each side, behind the end of the superior alveolar arch.
The unpaired occipital bone constitutes the central region of the posterior half of the skull base.
The paired temporal bones form the remaining, posterolateral portions of the skull base. Each sends a projection, the petrous pyramid (L. petrosus = hard, rock-like), toward the midline that intervenes between the sphenoid and occipital bones.
The intermaxillary suture lies on the midline of the hard palate between the two maxillae.
The median palatine suture lies on the midline of the hard palate, between the left and right palatine bones.
The transverse palatine suture extends horizontally across the hard palate, between the right and left maxillae and palatine bones.
The paired sphenovomerine sutures run anteroposteriorly on each side, between the vomer and the sphenoid bones.
The sphenosquamous suture extends anteroposteriorly between the sphenoid and temporal bones on each side of the skull base.
The petrosphenoidal suture lies between the petrous part of the temporal bone and the greater wing of the sphenoid. Its width varies greatly, and in some skulls it appears as a fissure rather than a suture.
The lower end of the lambdoidal suture can be seen on each side between the parietal and occipital bones. This fissure can be traced downward and medially into the occipitomastoid and petrooccipital fissures, between the occipital bone and the mastoid and petrous parts of the temporal bone, respectively.
NEUROVASCULAR EXIT PORTALS:
Foramen magnum (L. magnum=great) lies on the midline in the occipital bone. Through it the brain stem is continuous with the cervical spinal cord. The vertebral arteries pass upward through foramen magnum on their way toward the basilar artery and the circle of Willis, and the spinal accessory nerves also ascend from upper cervical cord segments into the cranial cavity through the foramen.
On each side, the cervical entrance to the carotid canal leads the internal carotid artery into the petrous portion of the temporal bone. The canal, which runs parallel to the long axis of the petrous pyramid, transmits the artery upwards into the cranial cavity towards the circle of Willis. The artery is accompanied by the (internal) carotid nerve, a bundle of postganglionic fibers originating from cells of the superior cervical sympathetic ganglion.
The jugular fossa lies on the petrooccipital suture, immediately posterior to the carotid canal. The fossa lodges the jugular bulb, the dilated upper end of the internal jugular vein, which communicates with the intracranial system of dural venous sinuses through the jugular foramen. The foramen can be seen from within the skull. It also transmits the glossopharyngeal, vagus, and spinal accessory nerves from the cranial cavity into the neck.
The stylomastoid foramen, the lower (outer) end of the facial canal in the temporal bone, lies lateral to the jugular fossa behind the base of the styloid process. It transmits the facial nerve and the stylomastoid branch of the external carotid arterial system.
The foramen spinosum lies at the base of the spinous process of the sphenoid bone, anterior to and across the petrosphenoid fissure from the carotid canal. The foramen transmits the middle meningeal branch of the external carotid arterial system into the cranial cavity. The artery is accompanied by a postganglionic plexus originating from the superior cervical sympathetic ganglion and by the nervus spinosus, a recurrent meningeal branch of the mandibular division of the trigeminal nerve (V3).
The foramen ovale lies anteromedial to the foramen spinosum, at the base of the lateral pterygoid plate of the sphenoid bone. The foramen transmits both the mandibular division of the trigeminal nerve (V3) and the lesser petrosal branch of the glossopharyngeal nerve from the cranial cavity into the infratemporal fossa. The accessory meningeal branch of the external carotid arterial system enters the cranial cavity through the foramen.
The greater palatine foramen, the exit of the greater palatine canal, lies at the posterior lateral angle of the hard palate, just medial to the last molar tooth. The foramen transmits the greater palatine nerve and vessels. The nerve is a branch of the maxillary division of the trigeminal (V2). The artery is derived from the maxillary branch of the external carotid system, and the vein drains to the pterygoid plexus, a tributary to the maxillary vein.
The lesser palatine foramen lies behind the greater palatine foramen. It transmits the lesser palatine nerve and vessels to the soft palate. The nerve is a branch of the maxillary division of the trigeminal (V2), the artery is from the maxillary branch of the external carotid, and the vein drains to the pterygoid plexus. The lesser palatine foramen may be small or large, single or multiple.
The incisive fossa lies on the intermaxillary suture, just behind the medial upper incisors. It receives the left and right incisive canals descending through the hard palate from the nasal cavities. On each side, the canal transmits the nasopalatine nerve from above and the greater palatine vessels from below. The nerve is a branch of the maxillary division of the trigeminal (V2), the artery arises from the maxillary branch of the external carotid, and the vein drains to the external jugular via the pterygoid plexus.
PROMINENT LANDMARKS AND REFERENCE POINTS:
A number of important landmarks can be appreciated only with the mandible in place.
The mental spines (genial tubercles) project posteriorly from the inner surface of the mandibular body close to the midline. The spines anchor muscles of the floor of the mouth and the tongue.
Lateral to each spine, the mylohyoid line, a narrow ridge of bone, runs posteriorly along the inner surface of the body of the mandible, ending below the rear molar tooth. This ridge anchors a thin transversely-directed sheet of muscle, the mylohyoid, that forms the floor of the mouth and supports the tongue.
The angle of the mandible marks the union of the horizontal body and the vertical ramus of the bone.
The lingula (L. diminutive of lingua=little tongue) is a small tongue of bone projecting upward from the midpoint of the inner surface of the mandibular ramus. It anchors the lower end of the sphenomandibular ligament.
The spine of the sphenoid, a downward projection from the base of the skull, anchors the upper end of the sphenomandibular ligament.
The spine of the sphenoid and many other landmarks can be seen more clearly once the mandible has been removed.
The pterygoid process projects downward from the inferior surface of the body of the sphenoid bone, just behind the alveolar arch of the maxilla. The process bears the bony medial and lateral pterygoid plates, which project posteriorly, enclosing the pterygoid fossa between them.
The medial pterygoid plate forms the posterior extremity of the lateral nasal wall.
The pterygoid hamulus (L.=little hook) curves downward from the medial pterygoid plate to anchor the upper end of the pterygomandibular raphe, a cord of connective tissue in which the muscles of the cheek and nasopharynx meet each other.
The boundaries of the infratemporal fossa become obvious if the outline of the mandibular ramus is projected onto the base of the skull. 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) its roof.
The cartilage of the auditory tube (not visible in a prepared skull) is lodged in and fills the groove between the petrous temporal bone and the greater wing of the sphenoid. The roof of this groove is the petrosphenoidal suture (fissure) referred to earlier. The cartilaginous and bony parts of the tube are continuous at the posterolateral end of the groove.
The foramen lacerum (L. lacerare=to tear) lies at the junction of petrous temporal, sphenoid, and occipital bones. This jagged aperture does not exist in life. It is formed after death when the cartilaginous tip of the petrous temporal bone disappears and varies greatly in size and shape from skull to skull. The only structures to pass through the thickness of the skull floor here are a small emissary vein and very small branches of the ascending pharyngeal artery.
The mandibular fossa, the temporal contribution to the temporomandibular joint, lies behind the posterior end of the zygomatic arch and in front of the external auditory meatus.
The styloid process (L. Stilus, Gr. stylos=pillar), projects downward and forward from the temporal bone behind the external auditory meatus and the temporomandibular joint. The process anchors the stylohyoid ligament and extrinsic muscles of the tongue and pharynx.
The mastoid process of the temporal bone lies immediately behind the external auditory meatus, at the lateral margin of norma basalis.
The foramen magnum (L. magnum=great) lies on the midline. It provides communication between the cranial cavity and the vertebral canal. by the occipital condyles, which articulate with the first cervical vertebra, the atlas.
The occipital condyles flank the anterolateral margins of foramen magnum. They articulate with the first cervical vertebra, the atlas.
Basion (Gr. basis=a base) is a craniometric landmark on the midline at the anterior margin of foramen magnum.
Opisthion (Gr. opithios=posterior) is another craniometric landmark, opposite basion on the midline at the posterior margin of foramen magnum.
The external occipital crest is a ridge (often faint) extending along the midline from opisthion to inion, the external occipital protuberance. The crest and the protuberance anchor the ligamentum nuchae to the skull. Inferiorly, the ligament is attached to the spinous processes of the cervical vertebrae.
The inferior nuchal line is a transverse ridge extending laterally from the midpoint of the external occipital crest. Rectus capitis posterior major and minor muscles, ascending from the spinous processes of axis and atlas, respectively, attach to the skull between foramen magnum and the inferior nuchal line.
The superior nuchal line extends transversely from the external occipital protuberance toward the mastoid process. The semispinalis capitis and superior oblique muscles ascend from cervical and upper thoracic vertebrae to attach between superior and inferior nuchal lines. Trapezius, sternocleidomastoid, and the deeper splenius capitis muscles all attach to the superior nuchal line itself.