The skull (cranium), which is rigid in adults but pliable in newborn infants, surrounds the brain and meninges completely and forms a strong mechanical protection. In adults, the volume of the brain can increase beyond the capacity of the intact cranium as a result of swelling after injury, and this can further compress the already injured brain and cause herniation. Increased cranial pressure in infants may cause the fontanelles to bulge or the head to begin to enlarge abnormally (see Fig 11–12).
Essential structures (eg, cranial nerves, blood vessels) travel to and from the brain through various openings (fissures, canals, foramens) in the skull and are especially subject to compression as they traverse these small passageways. Thus, a good knowledge of their anatomy is important for the clinician.
The anterior portion of the base of the skull, the hard palate, projects below the level of the remainder of the inferior skull surface. The choanae, or posterior nasal apertures, are behind and above the hard palate. The pterygoid plates lie lateral to the choanae (Fig 11–16).
Basal view of the skull, external aspect. Major bones are labeled in blue.
At the base of the lateral pterygoid plate is the foramen ovale, which transmits the third branch of the trigeminal nerve, the accessory meningeal artery, and (occasionally) the superficial petrosal nerve. Posterior to the foramen ovale is the foramen spinosum, which transmits the middle meningeal vessels. At the base of the styloid process is the stylomastoid foramen, through which the facial nerve exits.
The foramen lacerum is a large irregular aperture at the base of the medial pterygoid plate. Within its superior aspect is the carotid canal. The internal carotid artery, which emerges from this aperture, crosses only the superior part of the foramen lacerum.
Lateral to the foramen lacerum is a groove, the sulcus tubae auditivae, that contains the cartilaginous part of the auditory (eustachian) tube. It is continuous posteriorly with the canal in the temporal bone that forms the bony part of the auditory tube. Lateral to the groove is the lower orifice of the carotid canal, which transmits the internal carotid artery and the carotid plexus of the sympathetic nerves.
Behind the carotid canal is the large jugular foramen, which is formed by the petrous portion of the temporal and occipital bones and can be divided into three compartments. The anterior compartment contains the inferior petrosal sinus; the intermediate compartment contains the glossopharyngeal, vagus, and spinal accessory nerves; and the posterior compartment contains the sigmoid sinus and meningeal branches from the occipital and ascending pharyngeal arteries.
Posterior to the basilar portion of the occipital bone is the foramen magnum, which transmits the medulla and its membranes, the spinal accessory nerves, the vertebral arteries, and the anterior and posterior spinal arteries. The foramen magnum is bounded laterally by the occipital condyles.
Behind each condyle is the condyloid fossa, perforated on one or both sides by the posterior condyloid canal (which may transmit an emissary vein from the transverse sinus). Farther forward is the anterior condylar canal, or hypoglossal canal, which transmits the hypoglossal nerve and a meningeal artery.
The inner surface of the calvaria (skull cap) is concave, with depressions for the convolutions of the cerebrum and furrows for the branches of the meningeal vessels. Along the midline is a longitudinal groove, narrow anteriorly and posteriorly wide, that contains the superior sagittal sinus. The margins of the groove provide attachment for the falx cerebri. At the rear are the openings of the parietal emissary foramens (when these are present). The sutures of the calvaria (sagittal, coronal, lambdoid, and others) are meshed lines of union between adjacent skull bones.
Floor of the Cranial Cavity
The internal, or superior, surface of the skull base forms the floor of the cranial cavity (Fig 11–17 and Table 11–3). It is divided into three fossae: anterior, middle, and posterior. The floor of the anterior fossa lies higher than the floor of the middle fossa, which in turn lies higher than the floor of the posterior fossa. A number of openings (many of them termed foramens) provide entrance and exit routes, through the floor of the cranial cavity, for vascular structures, cranial nerves, and the medulla.
Floor of the cranial cavity, internal aspect.
Table 11–3 Structures Passing through Openings in the Cranial Floor. |Favorite Table|Download (.pdf)
Table 11–3 Structures Passing through Openings in the Cranial Floor.
Cribriform plate of ethmoid
Optic nerve, ophthalmic artery, meninges
Superior orbital fissure
Oculomotor, trochlear, and abducens nerves; ophthalmic division of trigeminal nerve; superior ophthalmic vein
Maxillary division of trigeminal nerve, small artery and vein
Mandibular division of trigeminal nerve, vein
Internal carotid artery, sympathetic plexus
Middle meningeal artery and vein
Internal acoustic meatus
Facial and vestibulocochlear nerves, internal auditory artery
Glossopharyngeal, vagus, and spinal accessory nerves; sigmoid sinus
Medulla and meninges, spinal accessory nerve, vertebral arteries, anterior and posterior spinal arteries
The floor of this is formed by the orbital plates of the frontal bone, the cribriform plates of the ethmoid, and the lesser wings and anterior part of the sphenoid. It is limited at the rear by the posterior borders of the lesser wings of the sphenoid and by the anterior margins of the chiasmatic groove.
The lateral segments of the anterior cranial fossa are the roofs of the orbital cavities, which support the frontal lobes of the cerebrum. The medial segments form the roof of the nasal cavity. The medial segments lie alongside the crista galli, which, together with the frontal crest, afford attachment to the falx cerebri.
The cribriform plate of the ethmoid bone lies on either side of the crista galli and supports the olfactory bulb. This plate is perforated by foramens for the olfactory nerves. The cranial openings of the optic canals lie just behind the flat portion of the sphenoid bone (planum sphenoidal).
This is deeper than the anterior cranial fossa and is narrow centrally and wide peripherally. It is bounded at the front by the posterior margins of the lesser wings of the sphenoid and the anterior clinoid processes. It is bounded posteriorly by the superior angles of the petrous portion of the temporal bones and by the dorsum sellae. It is bounded laterally by the temporal squamae and the greater wings of the sphenoid (Figs 11–17 and 11–18).
Basal view of the skull, internal aspect. Major openings are highlighted in color.
The narrow medial portion of the fossa presents the chiasmatic groove and the tuberculum sellae anteriorly; the chiasmatic groove ends on either side at the optic canal, which transmits the optic nerve and ophthalmic artery. Behind the optic canal, the anterior clinoid process is directed posteriorly and medially and provides attachment for the tentorium cerebelli. In back of the tuberculum sellae is a deep depression, the sella turcica; this structure, whose name means “Turkish saddle” (which it resembles), is especially important because it contains the hypophyseal fossa in which the hypophysis (pituitary) lies. The sella turcica is bounded posteriorly by a quadrilateral plate of bone, the dorsum sellae, whose sides project anteriorly as the posterior clinoid processes. These attach to slips of the tentorium cerebelli. Below each posterior clinoid process is a notch for the abducens nerve.
On either side of the sella turcica is the broad and shallow carotid groove, curving upward from the foramen lacerum to the medial side of the anterior clinoid process. This groove contains the internal carotid artery, surrounded by a plexus of sympathetic nerves.
The lateral segments of the middle fossa are deeper than its middle portion; they support the temporal lobes of the brain and show depressions that mark the convolutions of the brain. These segments are traversed by furrows for the anterior and posterior branches of the middle meningeal vessels, which pass through the foramen spinosum.
The superior orbital fissure is situated in the anterior portion of the middle cranial fossa. It is bounded above by the lesser wing, below by the greater wing, and in the middle by the body of the sphenoid. The superior orbital fissure transmits into the orbital cavity the oculomotor nerve, the trochlear nerve, the ophthalmic division of the trigeminal nerve, the abducens nerve, some filaments from the cavernous plexus of the sympathetic nerves, the ophthalmic veins, and the orbital branch of the middle meningeal artery.
The maxillary division of the trigeminal nerve passes through the foramen rotundum, which is located behind the medial wall of the superior orbital fissure. Behind the foramen rotundum is the foramen of Vesalius, which transmits an emissary vein or a cluster of small venules; it can be large, small, multiple, or absent in different skulls. The foramen ovale, which transmits the mandibular division of the trigeminal nerve, the accessory meningeal artery, and the lesser superficial petrosal nerve, is posterior and lateral to the foramen rotundum.
The foramen lacerum is medial to the foramen ovale. Its inferior segment is filled by fibrocartilage. Its superior segment transmits the internal carotid artery, which is surrounded by a plexus of sympathetic nerves. The anterior wall of the foramen lacerum is pierced by the pterygoid canal.
This fossa is larger and deeper than the middle and anterior cranial fossae. It is formed by the occipital bone, the dorsum sellae and clivus of the sphenoid bone, and portions of the temporal and parietal bones (see Fig 11–17).
The posterior fossa, or infratentorial compartment, contains the cerebellum, pons, medulla, and part of the midbrain. It is separated from the middle cranial fossa in and near the midline by the dorsum sellae of the sphenoid bone and on either side by the superior angle of the petrous portion of the temporal bone (petrous pyramid).
The foramen magnum lies in the center of the fossa. Just above the tubercle is the anterior condylar canal, or hypoglossal canal, which transmits the hypoglossal nerve and a meningeal branch for the ascending pharyngeal artery.
The jugular foramen lies between the lateral part of the occipital bone and the petrous portion of the temporal bone. The anterior portion of the foramen transmits the inferior petrosal sinus, the posterior portion transmits the transverse sinus and some meningeal branches from the occipital and ascending pharyngeal arteries, and the intermediate portion transmits the glossopharyngeal, vagus, and spinal accessory nerves.
Above the jugular foramen lies the internal acoustic meatus for the facial and acoustic nerves and the internal auditory artery. The inferior occipital fossae, which support the hemispheres of the cerebellum, are separated by the internal occipital crest, which serves for attachment of the falx cerebelli and contains the occipital sinus. The posterior fossae are surrounded by deep grooves for the transverse sinuses.
Tumors, inflammatory lesions, and other mass lesions can invade, and occlude, the foramens in the base of the skull. When they do so, they can compress and injure the cranial nerves and vessels running through these foramens. An example is shown in Figure 11–19.
An 81-year-old woman was admitted with shortness of breath and fever. She was found to have a right middle lobe pneumonia, the third pneumonia in three months. Neurologic examination revealed a vocal cord paralysis on the right side; the gag reflex was absent and there was loss of bulk of the trapezius and sternocleidomastoid muscles on the right side; the tongue appeared slightly atrophic on the right and deviated to the right upon protrusion; there was asymmetric elevation of the soft palate (deviation to the left due to paralysis on the right side). The patient aspirated during a swallowing evaluation. Magnetic resonance imaging of the brain demonstrated a mass lesion within the jugular foramen and the petrous bone on the right side [left image, arrow heads]. Computed tomography of the base of the skull showed osteolytic changes within the right petrous temporal and occipital bone [right: arrow heads; asterisk: jugular foramen]. A biopsy confirmed the clinically suspected diagnosis of glomus jugulare tumor which had impaired function of the ninth, tenth, eleventh and twelfth cranial nerves. The patient was treated with radiation.
(Courtesy of Dr. Joachim Baehring.)