Skull

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Chapter 9 Skull

The skull is the bony skeleton of the head and is the most complex osseous structure in the body. It is protective, shielding the brain, the organs of special sense and the cranial parts of the respiratory and digestive systems. It also provides attachments for many of the muscles of the head and neck, thus allowing for movement. Of particular importance is movement of the lower jaw (mandible), which occurs at the temporomandibular joint. The marrow within the skull bones is a site of haemopoiesis, at least in the young skull.

The skull is composed of 28 separate bones, most of which are paired; however, some bones in the median plane are single. Many of the bones are flat bones, consisting of two thin plates of compact bone enclosing a narrow layer of cancellous bone containing bone marrow. In terms of shape, however, the bones are far from flat and can exhibit pronounced curvatures. The term diploë is used to describe the cancellous bone within the flat bones of the skull. The inner table is thinner and more brittle; the outer table is generally very resilient. Many bones are so thin that the tables are fused, for example, the vomer and pterygoid plates. The skull bones vary in thickness in different regions but tend to be thinner where they are covered by muscles, such as in the temporal and posterior cranial fossae. The skull is thicker in some races, but there is no relationship between skull thickness and cranial capacity, which averages approximately 1400 ml. In all races the bone is thinner in women and children than in adult males.

The majority of bones in the skull are held firmly together by fibrous joints termed sutures. In the developing skull, sutures allow for growth. There are three main arrangements: the margins of adjacent bones of a suture may be smooth and meet end to end, resulting in a simple (butt-end) suture (e.g. median palatine suture); the margins of adjacent bones may be bevelled, so that the border of one bone overlaps the other (e.g. zygomaticomaxillary suture); or the margins of adjacent bones may present numerous projections that interlock, giving a serrated appearance (e.g. sagittal suture). The complexity of serrated sutures increases from the inner to the outer surface. Fusion across sutures (synostosis) commences at approximately 30 years of age, but the variability of this process precludes its use to determine the age of skulls. Fusion commences on the internal surface of the cranium, and the sagittal suture is one of the first affected. At approximately 40 years of age the sphenofrontal, lambdoid and occipitomastoid sutures close. In the facial region the posterior part of the median palatine suture starts to close at about 30 years, followed by the sutures around the nose. The squamosal, zygomaticofrontal and anterior parts of the intermaxillary suture rarely exhibit synostosis. Premature fusion of sutures during the early growth phase of the skull results in various cranial abnormalities.

The bones forming the base of the skull develop endochondrally and play an important part in growth. In this region, therefore, primary cartilaginous joints are encountered during growth; one of the most important is the spheno-occipital synchondrosis, which disappears at approximately 14 to 16 years of age. The skull articulates with the first cervical vertebra at the synovial atlanto-occipital joints. These joints allow flexion and extension of the skull. Rotation of the skull does not directly involve any joints of the skull but occurs at the atlanto-axial joint between the first and second cervical vertebrae.

Many important nerves and vessels pass in and out of the skull via openings termed foramina. The skull is a prime site for fractures resulting from trauma, and these structures can be damaged as a result of head injury. Detailed clinical examination should reveal signs and symptoms that, together with radiological examination, provide information regarding the extent and seriousness of a traumatic incident. In addition to the main foramina, irregular emissary foramina allow veins situated externally on the face and scalp to communicate with those lying intracranially. Spread of infection along these routes can have serious clinical consequences.

For ease of navigation, the skull can be divided into the cranium and the mandible, based on the fact that the mandible is easily detached, whereas most of the bones of the skull articulate by relatively fixed joints. The cranium can then be subdivided into a number of regions: the cranial vault, which is the upper, dome-like part of the skull and includes the skullcap or calvaria; the cranial base, which consists of the inferior surface of the skull extracranially and the floor of the cranial cavity intracranially; the facial skeleton, which includes the orbital cavities and the nasal fossae; the tooth-bearing bones or jaws; the acoustic cavities, which contain the middle and inner ears; and the cranial cavity, which houses the brain. Alternatively, the skull can be divided into the neurocranium and viscerocranium. The neurocranium is defined as that part of the skull that houses and protects the brain and the organs of special sense, whereas the viscerocranium is associated with the cranial parts of the respiratory and digestive tracts.

Inferior (Basal) Surface

The inferior surface of the skull, the base of the cranium, is complex and extends from the upper incisor teeth in front to the superior nuchal lines of the occipital bone behind (Fig. 9.1). The region contains many of the foramina through which structures enter and exit the cranial cavity. The inferior surface can be conveniently divided into anterior, middle, posterior and lateral parts. The anterior part contains the hard palate and the dentition of the upper jaw, and it lies at a lower level than the rest of the cranial base. The middle and posterior parts can be arbitrarily divided by a transverse plane passing through the anterior margin of the foramen magnum. The middle part is occupied mainly by the base of the sphenoid bone, the petrous processes of the temporal bones and the basilar part of the occipital bone. The lateral part contains the zygomatic arches and the mastoid and styloid processes. Whereas the middle and posterior parts are directly related to the cranial cavity (the middle and posterior cranial fossae), the anterior part (the palate) is some distance from the anterior cranial fossa, being separated from it by the nasal cavities.

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Fig. 9.1 Inferior view of the skull.

(By permission from Berkovitz, B.K.B., Moxham, B.J., 1994. Color Atlas of the Skull. Mosby-Wolfe, London.)

Anterior Part of the Cranial Base

The bony palate within the superior alveolar arch is formed by the palatine processes of the maxillae and the horizontal plates of the palatine bones, which meet at a cruciform system of sutures. The median palatine suture runs anteroposteriorly and divides the palate into right and left halves. This suture is continuous with the intermaxillary suture between the maxillary central incisor teeth. The transverse palatine (palatomaxillary) sutures run transversely across the palate between the maxillary and the palatine bones. The palate is arched sagittally and transversely; its depth and breadth are variable but are always greatest in the molar region, with the average width between the maxillary first molars being approximately 50 mm. The incisive fossa lies behind the central incisor teeth, and the lateral incisive foramina, through which incisive canals pass to the nasal cavity, lie in its lateral walls. Median incisive foramina, present in some skulls, open on the anterior and posterior walls of the fossa. The incisive fossa transmits the nasopalatine nerve and the termination of the greater palatine vessels. When median incisive foramina occur, the left nasopalatine nerve traverses the anterior foramen, and the right nerve traverses the posterior foramen. The greater palatine foramen lies near the lateral palatal border of the transverse palatine suture, and a vascular groove that is deep posteriorly leads forward from it. The lesser palatine foramina (usually two) lie behind the greater palatine foramen and pierce the pyramidal process of the palatine bone, which is wedged between the lower ends of the medial and lateral pterygoid plates. The palate is pierced by many other small foramina and is marked by pits for palatine glands. Variably prominent palatine crests extend medially from behind the greater palatine foramina. The posterior border projects back as a median posterior nasal spine. The alveolar arch has 16 sockets or alveoli for teeth, varying in size and depth; some are single, and some are divided by septa in adaptation to tooth roots.

The nasal fossae, separated in the midline by the nasal septum, lie above the hard palate. The two posterior nasal apertures (choanae) are located where the nasal fossae end. The posterior part of the septum is formed by the vomer. The upper border of the vomer is applied to the inferior aspect of the body of the sphenoid, where it expands into an ala on each side. The lateral border of each ala reaches a thin vaginal process that projects medially from the medial pterygoid plate. The two may touch, or the vaginal process may overlap the ala of the vomer inferiorly. The inferior surface of the vaginal process bears an anteroposterior groove, which is converted into a canal anteriorly by the superior aspect of the sphenoidal process of the palatine bone. This palatovaginal canal opens anteriorly into the pterygopalatine fossa and transmits a pharyngeal branch of the pterygopalatine ganglion and a pharyngeal branch from the third part of the maxillary artery. An inconstant vomerovaginal canal may lie between the ala of the vomer and the vaginal process of the sphenoid bone, medial to the palatovaginal canal, and lead into the anterior end of the palatovaginal canal. It transmits the pharyngeal branch of the third part of the maxillary artery.

Middle Part of the Cranial Base

The middle part of the cranial base is made up of the occipital, sphenoid and temporal bones. The body of the sphenoid bone lies anteriorly, and the basilar part of the occipital bone lies posteriorly, just in front of the foramen magnum. Where these two bones meet in the growing skull, the junction between them is a primary cartilaginous joint, the spheno-occipital synchondrosis. This joint is important for growth of the skull in an anteroposterior direction and ossifies at approximately 14 to 16 years of age. The basilar part of the occipital bone bears a small midline pharyngeal tubercle, which provides an attachment to the pharyngeal raphe and the highest attachment of the superior pharyngeal constrictor.

The middle part of the cranial base is completed by the petrous processes of the two temporal bones, which pass from the lateral sides of the base of the skull toward the site of union of the sphenoid and occipital bones. Each petrous process meets the basilar part of the occipital bone at a petro-occipital suture, which is deficient posteriorly at the jugular foramen. The petrosphenoidal suture and the groove for the pharyngotympanic tube lie between the petrous process and the infratemporal surface of the greater wing of the sphenoid. The apex of the petrous process does not meet the spheno-occipital suture, and the deficit produced is called the foramen lacerum.

Each pterygoid process of the sphenoid bone bears medial and lateral pterygoid plates separated by a pterygoid fossa. Anteriorly, the plates are fused, except below, where they are separated by the pyramidal process of the palatine bone. Sutures are usually discernible at this site. Laterally, the pterygoid plates are separated from the posterior maxillary surface by the pterygomaxillary fissure, which leads into the pterygopalatine fossa. The posterior border of the medial pterygoid plate is sharp and bears a small projection near the midpoint, above which it is curved and attached to the pharyngeal end of the pharyngotympanic tube. Above, the medial pterygoid plate divides to enclose the scaphoid fossa; below, it projects as a slender pterygoid hamulus, which curves laterally and is grooved anteriorly by the tendon of tensor veli palatini. The pterygoid hamulus gives origin to the pterygomandibular raphe. The lateral pterygoid plate projects posterolaterally, and its lateral surface forms the medial wall of the infratemporal fossa. Superiorly and laterally, the pterygoid process is continuous with the infratemporal surface of the greater wing of the sphenoid bone, which forms part of the roof of the infratemporal fossa. This surface forms the posterolateral border of the inferior orbital fissure and bears an infratemporal crest associated with the origin of the upper part of the lateral pterygoid. The infraorbital and zygomatic branches of the maxillary nerve and accompanying vessels pass through the inferior orbital fissure. Laterally, the greater wing of the sphenoid bone articulates with the squamous part of the temporal bone. Features associated with the pterygoid plate region can be assessed radiographically (Fig. 9.2).

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Fig. 9.2 Horizontal computed tomography scan at the level of the upper part of the ramus of the mandible shows the relationships of the pterygoid plates.

(By permission from Berkovitz, B.K.B., Moxham, B.J., 1994. Color Atlas of the Skull. Mosby-Wolfe, London.)

A thin-walled depression in the temporal bone, the mandibular fossa, can be inspected when the mandible is removed; in front of this, the zygomatic arch extends laterally. A distinct ridge, the articular eminence, is anterior to the fossa, and three fissures can be distinguished behind it. The squamotympanic fissure extends from the spine of the sphenoid, between the mandibular fossa and the tympanic plate of the temporal bone, and curves up the anterior margin of the external acoustic meatus. A thin wedge of bone forming the inferior margin of the tegmen tympani lies within the fissure and divides the squamotympanic fissure into petrotympanic and petrosquamous fissures. The petrotympanic fissure transmits the chorda tympani branch of the facial nerve from the skull into the infratemporal fossa.

The foramen lacerum is bounded in front by the body and adjoining roots of the pterygoid process and greater wing of the sphenoid bone, posterolaterally by the apex of the petrous part of the temporal bone and medially by the basilar part of the occipital bone. Although it is nearly 1 cm long, no large structure completely traverses it. A large, almost circular foramen, the carotid canal, lies behind and posterolateral to the foramen lacerum in the petrous part of the temporal bone. The internal carotid artery enters the skull through this foramen, ascends in the carotid canal and turns anteromedially to reach the posterior wall of the foramen lacerum. It ascends through the upper end of the foramen lacerum with its venous and sympathetic nerve plexuses. Meningeal branches of the ascending pharyngeal artery and emissary veins from the cavernous sinus also traverse the foramen lacerum. In life, the lower part of the foramen lacerum is partially occluded by cartilaginous remnants of the developmental chondrocranium. The pterygoid canal can be seen on the base of the skull at the anterior margin of the foramen lacerum, above and between the pterygoid plates of the sphenoid bone. It leads into the pterygopalatine fossa and contains the nerve of the pterygoid canal and accompanying blood vessels.

The foramen ovale and foramen spinosum lie lateral to the foramen lacerum on the infratemporal surface of the greater wing of the sphenoid bone. The foramen ovale, near the posterior margin of the lateral pterygoid plate, transmits the mandibular nerve as well as the lesser petrosal nerve, the accessory meningeal branch of the maxillary artery and an emissary vein that connects the cavernous venous sinus to the pterygoid venous plexus in the infratemporal fossa. Posterolaterally, the smaller and rounder foramen spinosum transmits the middle meningeal artery and a meningeal branch of the mandibular nerve. The irregular spine of the sphenoid projects posterolateral to the foramen spinosum. The medial surface of the spine is flat and forms, with the adjoining posterior border of the greater wing of the sphenoid, the anterolateral wall of a groove that is completed posteromedially by the petrous part of the temporal bone. This groove contains the cartilaginous pharyngotympanic (auditory) tube and leads posterolaterally into the bony portion of the tube lying within the petrous part of the temporal bone. Occasionally, the foramen ovale and foramen spinosum are confluent. The posterior edge of the foramen spinosum may be defective. A small foramen, the sphenoidal emissary foramen (of Vesalius), is sometimes found between the foramen ovale and scaphoid fossa. When present, it contains an emissary vein linking the pterygoid venous plexus in the infratemporal fossa with the cavernous sinus in the middle cranial fossa.

The zygomaticotemporal foramen passes up and backward from the posterior surface of the zygomatic bone in the anterior wall of the infratemporal fossa. It transmits the zygomaticotemporal nerve and a small accompanying artery.

Posterior Part of the Cranial Base

The posterior part of the cranial base is formed by the occipital and temporal bones. Prominent features are the foramen magnum and associated occipital condyles, jugular foramen, mastoid and styloid processes of the temporal bone, stylomastoid foramen, mastoid notch and squamous part of the occipital bone up to the external occipital protuberance and the superior nuchal lines, hypoglossal canals (anterior condylar canals) and condylar canals (posterior condylar canals).

The foramen magnum lies in an anteromedian position. It is oval and wider behind, with its greatest diameter being anteroposterior. It contains the lower end of the medulla oblongata, the vertebral arteries and the spinal accessory nerve. Anteriorly, the margin of the foramen magnum is slightly overlapped by the occipital condyles, which project down to articulate with the superior articular facets on the lateral masses of the atlas. Each occipital condyle is oval in outline and oriented obliquely so that its anterior end lies nearer the midline. It is markedly convex anteroposteriorly and less so transversely; its medial aspect is roughened by ligamentous attachments. The hypoglossal canal, directed laterally and slightly forward, traverses each condyle and transmits the hypoglossal nerve, a meningeal branch of the ascending pharyngeal artery and an emissary vein from the basilar plexus. A depression, the condylar fossa, lies immediately posterior to the condyle and sometimes contains a (posterior) condylar canal for an emissary vein from the sigmoid sinus. A jugular process joins the petrous part of the temporal bone lateral to each condyle, and its anterior border forms the posterior boundary of the jugular foramen.

Laterally, the occipital bone joins the petrous part of the temporal bone anteriorly, at the petro-occipital suture, and the mastoid process of the temporal bone more posteriorly, at the petromastoid suture. The jugular foramen, a large irregular hiatus, lies at the posterior end of the petro-occipital suture between the jugular process of the occipital bone and the jugular fossa of the petrous part of the temporal bone. A number of important structures pass through this foramen: inferior petrosal sinus (anterior); glossopharyngeal, vagus and accessory nerves (midway); internal jugular vein (posterior). A mastoid canaliculus runs through the lateral wall of the jugular fossa and transmits the auricular branch of the vagus nerve. The canaliculus for the tympanic nerve—a branch of the glossopharyngeal nerve in the cavity of the middle ear—lies on the ridge between the jugular fossa and the opening of the carotid canal. A small notch, related to the inferior glossopharyngeal ganglion, may be found medially, on the upper boundary of the jugular foramen (it is more easily identified internally). The orifice of the cochlear canaliculus may be found at the apex of the notch.

The stylomastoid foramen lies between the mastoid and styloid processes of the temporal bone on the lateral aspect. It transmits the facial nerve and the stylomastoid artery. A groove, the mastoid notch, lies medial to the mastoid process and gives origin to the posterior belly of the digastric. A groove related to the occipital artery often lies medial to the mastoid notch. A mastoid foramen may be present near or in the occipitomastoid suture; when present, it transmits an emissary vein from the sigmoid sinus. The external acoustic meatus lies in front of the mastoid process. It is surrounded inferiorly by the tympanic plate, which partly ensheathes the base of the styloid process.

The squamous part of the occipital bone exhibits the external occipital protuberance; supreme, superior and inferior nuchal lines; and the external occipital crest, all of which lie in the midline, posterior to the foramen magnum. The region is roughened for the attachment of muscles whose primary function is extension of the skull.

Cranial Fossae

The base of the cranial cavity is divided into three distinct fossae: anterior, middle and posterior (Fig. 9.3). The floor of the anterior cranial fossa is at the highest level and the floor of the posterior fossa is at the lowest.

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Fig. 9.3 Floor of the cranial cavity showing the cranial fossae.

(By permission from Berkovitz, B.K.B., Moxham, B.J., 1994. Color Atlas of the Skull. Mosby-Wolfe, London.)

Anterior Cranial Fossa

The anterior cranial fossa is formed at the front and sides by the frontal bone. Its floor contains the orbital plate of the frontal bone, the cribriform plate and crista galli of the ethmoid bone and the lesser wings and anterior part of the body of the sphenoid. Unlike the other cranial fossae, it does not directly communicate with the inferior surface of the cranium; instead, it is related to the roofs of the orbits and the nasal fossae.

A perforated plate of bone, the cribriform plate of the ethmoid bone, spreads across the midline between the orbital plates of the frontal bone and is depressed below them, forming part of the roof of the nasal cavity. Olfactory nerves pass from the nasal mucosa to the olfactory bulb of the brain through numerous small foramina in the cribriform plate. Anteriorly, a spur of bone, the crista galli, projects upward between the cerebral hemispheres. A depression between the crista galli and the crest of the frontal bone is crossed by the frontoethmoidal suture and bears the foramen caecum, which is usually a small blind-ended depression but occasionally accommodates a vein draining from the nasal mucosa to the superior sagittal sinus. The anterior ethmoidal nerve enters the cranial cavity where the cribriform plate meets the orbital part of the frontal bone and then passes into the roof of the nose via a small foramen by the side of the crista galli; the nerve grooves the crista galli. The anterior ethmoidal vessels accompany the nerve. The posterior ethmoidal canal, which transmits the posterior ethmoidal nerve and vessels, opens at the posterolateral corner of the cribriform plate and is overhung by the sphenoid bone.

The convex cranial surface of the frontal bone separates the brain from the orbit and bears impressions of cerebral gyri and small grooves for meningeal vessels. Posteriorly, it joins the anterior border of the lesser wing of the sphenoid bone, which forms the posterior boundary of the anterior cranial fossa. The medial end of the lesser wing constitutes the anterior clinoid process. The lesser wing joins the body of the sphenoid body by two roots that are separated by the optic canal. The anterior root, broad and flat, is continuous with the jugum sphenoidale; the smaller and thicker posterior root joins the body of the sphenoid bone near the posterior bank of the sulcus chiasmatis. The frontosphenoid and sphenoethmoidal sutures divide the sphenoid from the adjacent bones.

The posterior border of each lesser wing fits the stem of the lateral cerebral sulcus and may be grooved by the sphenoparietal sinus. Above is the inferior surface of the frontal lobe of the cerebral hemisphere, and medial is the anterior perforated substance. Inferiorly, the lesser wing bounds the superior orbital fissure and completes the orbital roof. Each anterior clinoid process gives attachment to the free margin of the tentorium cerebelli and is grooved medially by the internal carotid artery as it leaves the cavernous sinus. It may be connected to the middle clinoid process by a thin osseous bar, completing a caroticoclinoid foramen around the artery.

CASE 1 Foster Kennedy Syndrome

A 49-year-old woman undergoes surgery for a subfrontal tumour, which is found to be a meningioma arising in the olfactory groove. Her clinical abnormalities include a mild contralateral hemiparesis, behavioural change, ipsilateral optic atrophy with contralateral papilloedema and ipsilateral anosmia. Similar symptoms may also appear with other tumours, such as meningioma involving the sphenoid wing.

Discussion: Owing to the subfrontal site of this woman’s tumour, she developed anosmia due to primary involvement of the olfactory nerve in the olfactory groove by the tumour, along with mild frontal signs such as behavioural change. Reduced visual acuity with optic atrophy is due to direct compression of the ipsilateral optic nerve or optic chiasma, whereas papilloedema involving the contralateral optic nerve clearly indicates the presence of increased intracranial pressure, a combination that is uncommon today in light of early diagnostic studies. See Figure 9.4.

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Fig. 9.4

Middle Cranial Fossa

The middle cranial fossa is deeper and more extensive than the anterior cranial fossa, particularly laterally. It is bounded in front by the lesser wings and part of the body of the sphenoid, behind by the superior borders of the petrous part of the temporal bone and the dorsum sellae of the sphenoid and laterally by the squamous parts of the temporal bone, parietal bone and greater wings of the sphenoid. This region corresponds with the middle part of the cranial base.

Centrally, the floor is narrower and formed by the body of the sphenoid bone. The hollowed-out area is the site of the hypophysial (pituitary) gland and is therefore termed the hypophysial (pituitary) fossa. The area has the shape of a Turkish saddle and is also known as the sella turcica. The anterior edge of the hypophysial fossa is completed laterally by a middle clinoid process. The floor forms the roof of the sphenoidal air sinuses, and the posterior boundary presents a vertical pillar of bone, the dorsum sellae. The superolateral angles of the dorsum sellae are expanded as the posterior clinoid processes. A fold of dura, the diaphragma sella, is attached to the anterior and posterior clinoid processes and roofs the hypophysial fossa. The smooth upper part of the anterior wall of the fossa is the jugum sphenoidale, which is bounded behind by the anterior border of the grooved sulcus chiasmatis, leading laterally to the optic canals. The optic nerve and ophthalmic artery pass through the optic canal, and the optic chiasma usually lies posterosuperior to the sulcus chiasmatis. Below the sulcus chiasmatis is the tuberculum sellae. The cavernous sinus lies lateral to the hypophysial fossa, and the lateral wall of the body of the sphenoid contains a shallow carotid groove related to the internal carotid artery as it ascends from the carotid canal and runs through the cavernous sinus. Posterolaterally, the groove may be deepened by a small projecting lingula.

Laterally, the middle cranial fossa is deep and supports the temporal lobes of the cerebral hemispheres. Anteriorly are the orbits, laterally the temporal fossae and inferiorly the infratemporal fossae. The middle cranial fossa communicates with the orbits by the superior orbital fissures, each bounded above by a lesser wing, below by a greater wing and medially by the body of the sphenoid bone. Each fissure is wider medially and has a long axis sloping inferomedially and forward. Many nerves and vessels pass through it: the oculomotor, trochlear and abducens nerves and the lacrimal, frontal and nasociliary branches of the ophthalmic division of the trigeminal nerve, together with filaments from the internal carotid plexus (sympathetic), the ophthalmic veins, the orbital branch of the middle meningeal artery and the recurrent branch of the lacrimal artery.

Three foramina can be identified in the greater wing of the sphenoid bone. The foramen rotundum is situated just below and behind the medial end of the superior orbital fissure and leads forward into the pterygopalatine fossa, to which it conducts the maxillary nerve. Behind the foramen rotundum is the foramen ovale, which transmits the mandibular nerve. The foramen spinosum is posterolateral to the foramen ovale and transmits the middle meningeal artery. The latter, with companion veins, ascends lateral to the squamous part of the temporal bone and turns anterolaterally across the sphenosquamosal suture to the greater wing of the sphenoid bone, where it divides into frontal and parietal branches. The frontal branch ascends across the pterion to the anterior part of the parietal bone; at or near the pterion it is often in a bony canal. The parietal branch runs back and up onto the squamous part of the temporal bone, crossing the squamosal suture to gain the parietal bone. These arteries and veins groove the floor and lateral wall of the middle cranial fossa. The foramen ovale and foramen spinosum connect with the underlying infratemporal fossa.

The foramen lacerum is situated at the posterior end of the carotid groove, posteromedial to the foramen ovale. Its boundaries and contents were already described in the section on the intermediate part of the cranial base. A small foramen may occur at the root of the greater wing of the sphenoid medial to the foramen lacerum; when present, this emissary sphenoidal foramen transmits a vein from the cavernous sinus.

A shallow trigeminal impression, adapted to the trigeminal ganglion, is situated posterior to the foramen lacerum on the anterior surface of the petrous part of the temporal bone, near its apex. Posterolateral to this impression is a shallow pit, limited posteriorly by a rounded arcuate eminence produced by the anterior semicircular canal. Lateral to the trigeminal impression, a narrow groove passes posterolaterally into the hiatus for the greater petrosal nerve, and even farther laterally is the hiatus for the lesser petrosal nerve. The anterior surface of the petrous part of the temporal bone is formed by the tegmen tympani, a thin osseous lamina in the roof of the tympanic cavity, which extends anteromedially above the auditory tube, anterolateral to the arcuate eminence. The posterior part of the tegmen tympani roofs the mastoid antrum, lateral to the eminence. The superior border of the petrous part of the temporal bone separates the middle and posterior cranial fossae and is grooved by the superior petrosal sinus. In young skulls, a petrosquamous suture may be visible at the lateral limit of the tegmen tympani, but it is obliterated in adults. The tegmen tympani then turns down as the lateral wall of the osseous auditory tube, and its lower border may appear in the squamotympanic fissure. Lateral to the anterior part of the tegmen tympani, the squamous part of the temporal bone is thin over a small area that coincides with the deepest part of the mandibular fossa.

A smooth trigeminal notch leads into the trigeminal impression and lies on the upper border of the petrous temporal bone, anteromedial to the groove for the superior petrosal sinus. At this point, the trigeminal nerve separates the sinus from bone. The petrosphenoidal ligament is attached to a tiny bony spicule, directed anteromedially at the anterior end of the trigeminal notch. The abducens nerve bends sharply across the upper petrous border, passing between the ligament and the dorsum sellae anterior to the petrosphenoidal ligament.

CASE 2 Pituitary Tumour with Bitemporal Visual Field Defect

A 56-year-old woman presents with a 6-month history of worsening bifrontal headache and difficulty reading. She can read single words but has difficulty tracking a sentence across the page. Evaluation was prompted when she struck her husband’s car on the right side as she drove into the garage; she stated she did not see his parked car.

Neuro-ophthalmological examination shows 20/25 vision bilaterally, bitemporal visual field defects, normal-appearing optic discs, intact pupillary function and normal extraocular motility. Magnetic resonance imaging demonstrates a large intrasellar and suprasellar mass. There are no endocrine abnormalities.

Discussion: Bitemporal hemianopsia localizes the lesion to the optic chiasma, with involvement primarily of the decussating fibres originating in the nasal portion of the retina. Tumours in this area can compress both the optic nerves and optic chiasma, sometimes producing a mixed defect consisting of bitemporal visual field disturbances (as in this woman) and a superimposed monocular visual field disturbance—a so-called junctional defect. Such a combination of monocular and bitemporal visual field defects can be diagnostically challenging. See lesion 3, Figure 12.11. As this case demonstrates, patients with bitemporal visual field defects may be unaware of the vision loss.

Posterior Cranial Fossa

The posterior cranial fossa is the largest and deepest of the cranial fossae. It is bounded in front by the dorsum sellae, posterior aspects of the sphenoidal body and basilar part of occipital bone; behind by the squamous part of the occipital bone; laterally by the petrous and mastoid parts of the temporal bone and by the lateral parts of the occipital bone; and above and behind by the mastoid angles of the parietal bones. The posterior cranial fossa contains the cerebellum, pons and medulla oblongata. The region corresponds extracranially with the posterior part of the cranial base.

The most prominent feature in the floor of the posterior cranial fossa is the foramen magnum in the occipital bone. A sloping surface called the clivus—formed successively by the basilar part of the occipital bone, the posterior part of the body and the dorsum sellae of the sphenoid bone—lies anterior to the foramen magnum. The clivus is gently concave from side to side. On each side it is separated from the petrous part of the temporal bone by a petro-occipital fissure, filled by a thin plate of cartilage and limited behind by the jugular foramen. Its margins are grooved by the inferior petrosal sinus. The spheno-occipital synchondrosis is evident on the clivus of a growing child.

A large jugular foramen, sited at the posterior end of the petro-occipital fissure, lies above and lateral to the foramen magnum. Its upper border is sharp and irregular and contains a notch for the glossopharyngeal nerve. The cochlear canaliculus, which contains the perilymphatic ‘duct,’ is sited in the deepest part of the notch. The lower border of the jugular foramen is smooth. Posteriorly, it is grooved by the sigmoid sinus, which continues into the foramen as the internal jugular vein. The accessory, vagus and glossopharyngeal nerves pass forward through the anterior part of the jugular foramen from behind, and they may groove the jugular tubercle as they enter the foramen. The hypoglossal (anterior condylar) canal lies medial to and below the lower border of the jugular foramen at the junction of the basilar and lateral parts of the occipital bone. This canal transmits the hypoglossal nerve (and its recurrent branch), the meningeal branch of the ascending pharyngeal artery and an emissary vein linking the basilar plexus intracranially with the internal jugular vein extracranially. If a posterior condylar canal is present behind the occipital condyle, its internal orifice is posterolateral to that of the hypoglossal canal and contains a sigmoid emissary vein (associated with the occipital veins) and a meningeal branch of the occipital artery. The occipital condyles lie within the anterior aspect of the foramen magnum; their medial aspects are roughened for the attachments of the alar ligaments associated with the atlanto-axial joints.

The posterior surface of the petrous part of the temporal bone forms much of the anterolateral wall of the posterior cranial fossa. It contains the internal acoustic meatus, which lies anterosuperior to the jugular foramen, and transmits the facial and vestibulocochlear nerves, nervus intermedius and labyrinthine vessels.

The mastoid part of the temporal bone lies behind the petrous part of the temporal bone in the lateral wall of the posterior cranial fossa. Anteriorly, it is grooved by a wide sigmoid sulcus (groove) running forward and downward, then downward and medially and finally forward to the jugular foramen. It contains the sigmoid sinus. Superiorly, where the groove touches the mastoid angle of the parietal bone, it is continuous with a groove transmitting the transverse sinus; it next crosses the parietomastoid suture and then descends behind the mastoid antrum. A mastoid foramen for an emissary vein from the sigmoid sinus and a meningeal branch of the occipital artery, sometimes large enough to groove the squamous part of the occipital bone, may be sited there. The lowest part of the sigmoid sulcus crosses the occipitomastoid suture and grooves the jugular process of the occipital bone. The right sigmoid sulcus is usually larger than the left.

A thin plate with an irregularly curved margin projects back behind the internal acoustic meatus and bounds a slit containing the opening of the vestibular aqueduct (which contains the saccus and ductus endolymphaticus and a small artery and vein). A small subarcuate fossa lies between the internal acoustic meatus and the aqueductal opening. It contains dura mater. Near the superior border of the petrous part of the temporal bone, it is pierced by a small vein. In infants, the fossa is a relatively large blind tunnel under the anterior semicircular canal.

The squamous part of the occipital bone displays a median internal occipital crest, which runs posteriorly from the foramen magnum to an internal occipital protuberance and gives attachment to the falx cerebelli. The internal occipital crest may be grooved by the occipital sinus. The internal occipital protuberance is close to the confluence of the sinuses and is grooved bilaterally by the transverse sinuses. The latter curve laterally, with an upward convexity, to the mastoid angles of the parietal bones. The groove for the transverse sinus is usually deeper on the right, where it is generally a continuation of the superior sagittal sinus; on the left, it is frequently a continuation of the straight sinus. On both sides, the transverse sulcus is continuous with the sigmoid sulcus. Below the transverse sulcus, the internal occipital crest separates two shallow fossae, adapted to the cerebellar hemispheres. The margins of the grooves for the transverse sinus and superior petrosal sinus, together with the posterior clinoid process, all provide anchorage for the attached margin of the tentorium cerebelli.

References

Berkovitz B.K.B., Moxham B.J. Color Atlas of the Skull. London: Mosby-Wolfe; 1994.

Howells W.W. Cranial Variation in Man. vol. 67. 1973. Papers of the Peabody Museum of Archaeology and Ethnography. Cambridge, MA.

Lieberman D.E., McCarthy R.C. The ontogeny of cranial base angulation in humans and chimpanzees and its implications for reconstructing pharyngeal dimensions. J. Hum. E.. 1999;36:487-517.

Lieberman D.E., Pearson O.M., Mowbray K.M. Basicranial influence on overall cranial shape. J. Hum. E.. 2000;38(2):291-315.

Moos, Baker, 1998Moos K.F. Baker A.W. Craniofacial surgery: Assessment and techniques, Langdon J.D., Patel M.F. Operative Maxillofacial Surgery. 1998. Chapman & Hall. London. 407-436.

A detailed description of the various craniosynostoses.

Relethford J.H. Craniometric variation among modern human populations. Am. J. Phys. Anthropol.. 1994;95:53-62.

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