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The skull
The skull is the bony skeleton of the head. It houses the brain, the organs of special sense, and the upper parts of the respiratory and digestive systems and provides attachments for many of the muscles of the head and neck. Movement is restricted and is found only in relation to the mandible at the temporomandibular joint and at the atlanto-occipital joint for movement of the head in relation to the neck.
The skull consists of the neurocranium, viscerocranium (facial skeleton) and mandible. The neurocranium may be subdivided into the calvaria (sometimes called the cranial vault) and the basicranium (also known as the cranial base). The neurocranium encloses the brain, intracranial portions of the cranial nerves, blood vessels, meninges and cerebrospinal fluid. Its walls are formed by parts of the frontal, ethmoid, parietal, sphenoid, temporal and occipital bones. The viscerocranium hangs down from the front of the neurocranium and houses the organs of sight, smell and taste, as well as the openings to the respiratory and digestive systems via the nose and mouth, respectively.
Internally, the cranial base can be divided into three regions corresponding to the floor of the anterior, middle and posterior cranial fossae. In clinical parlance, the terms skull base and cranial base are frequently used as though they are synonymous. Indeed, inspection of the inferior surface of a skull confirms that the middle and posterior cranial bases are the same as the middle and posterior skull bases. However, the anterior cranial base and the anterior skull base should not be used interchangeably because the anterior skull base includes the inferior surface of the facial skeleton, whereas the internal surface of the anterior cranial base is the floor of the anterior cranial fossa and may be further subdivided into a midline (central) and two lateral parts. During development, the parts of the skull where the bones are derived by endochondral ossification constitute the chondrocranium; the remaining bones are derived by intramembranous ossification.
The skull is the most complex bony structure in the body. The young adult skull is composed of an average of 28 separate bones, many of which are paired; some in the median plane are single, though symmetric. Most of the vault bones are flat and consist of two tables or plates of compact bone enclosing a narrow layer of relatively dense cancellous marrow (diploic bone). The marrow within the skull bones is a site of haemopoiesis, at least in the young individual. These bones form by intramembranous ossification of a highly vascular connective tissue membrane and have often been referred to as ‘dermal’ in deference to their alleged ancient phylogenetic origin. The inner table is thinner and more brittle while the outer tends to be thicker and more resilient; this is important to remember when examining fractures to the skull caused by either blunt or sharp trauma. This bone type is also of considerable value in cranial bone grafting in the young, where the two tables can be split to produce an autograft for use in maxillofacial reconstruction. The skull bones vary in thickness in different regions, with the bones of the viscerocranium being rather delicate and therefore particularly susceptible to fracture.
The majority of bones in the skull articulate via fibrous joints called sutures, which facilitate growth rather than movement in the developing skull. The three main sutural morphologies reflect the magnitude of strain placed on them. Thus, the margins of adjacent bones of a suture may be smooth and meet end to end, giving a simple (butt-end) suture (e.g. median palatine suture); may be bevelled, so that the border of one bone overlaps the other (e.g. parietotemporal suture); or may present numerous projections that interlock, giving a serrated jigsaw appearance (e.g. lambdoid suture). The complexity of serrated sutures increases from the inner to the outer surface. Fusion across sutures (synostosis) can start early in the third decade, although its variability precludes using this information to assess age with any degree of accuracy. The process of fusion starts on the internal surface of the cranium first and proceeds externally; the coronal and sagittal sutures are often the first to display synostosis. By middle age, many of the larger sutures will show evidence of synostosis, although there are some that rarely show fusion, e.g. the zygomaticofacial. Premature fusion of sutures during the early growth phase of the skull (craniosynostosis) results in various abnormalities.
The bones forming the base of the skull also play an important role in the overall growth of the face and the neurocranium. They develop mainly via endochondral ossification and the joints between most of them are primary cartilaginous joints. One of the most important is the spheno-occipital synchondrosis, between the body of the sphenoid anteriorly and the basilar part of the occipital bone posteriorly; fusion between them is generally completed between 13 and 18 years of age. Other joints include the occipital synchondroses and the jugular growth plate (or petro-exoccipital articulation).
The two sites of synovial articulation associated with the exterior of the skull are the temporomandibular joint (between the glenoid fossa of the temporal bone and the condylar process of the mandible) and the atlanto-occipital joint (between the condyles of the occipital bone and the superior articular facets of the atlas). Movement at these joints does not facilitate rotation but participates in flexion and extension in the anteroposterior plane and bilateral flexion in the transverse plane.
Many important neurological and vascular structures enter and exit the skull via foramina. The skull is often fractured following direct trauma. 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 may have serious clinical consequences (e.g. cavernous sinus thrombosis).
In the account of the skull that follows, only generalized standard views will be considered. A more detailed account of each individual bone will be found associated with the relevant regional text ( ).
Frontal (Anterior) View
Viewed from the front, the skull is generally ovoid in shape and is wider above than below ( Fig. 34.1 ). The upper part is formed by the frontal bone, which underlies the forehead region above the orbits. Superomedial to each orbit is a rounded superciliary arch (more pronounced in males), between which there may be a median elevation, the glabella. The glabella may show the remains of the interfrontal (metopic) suture, which usually closes in the first postnatal year ( ) but persists in a small percentage of adult skulls in various ethnic groups. A retained interfrontal suture is usually present in the inferior portion of the suture, a feature known as metopism. The frontal bone articulates with the two nasal bones at the frontonasal sutures, which is marked by a depression at the root of the nose; the point at which the frontonasal and internasal sutures meet is the anthropometric landmark known as the nasion.
The upper part of the face is occupied by the orbits and the bridge of the nose. Each orbital opening is roughly quadrangular in shape, although this can differ depending on ancestral origin ( Ch. 44 ). The upper, supraorbital, margin is formed entirely by the frontal bone, interrupted at the junction of its sharp lateral two-thirds and rounded medial third by the supraorbital notch or foramen, which transmits the supraorbital vessels and nerve. The lateral margin of the orbit is formed largely by the frontal process of the zygomatic bone and is completed above by the zygomatic process of the frontal bone; the frontozygomatic suture between them lies in a palpable depression. The infraorbital margin is formed by the zygomatic bone laterally and the maxilla medially. Both lateral and infraorbital margins are sharp and palpable. The medial margin of the orbit is formed above by the frontal bone and below by the lacrimal crest of the frontal process of the maxilla.
The central part of the face is occupied mainly by the paired maxillae, separated by the anterior nasal aperture. Each maxilla contributes to the upper jaw, the floor and medial wall of the orbital cavity, the lateral wall of the nose, the floor of the nasal aperture and the bone of the cheek. The anterior nasal spine marks the upper limit of the intermaxillary suture at the lower margin of the anterior nasal aperture and is palpable in the nasal septum. The infraorbital foramen transmits the infraorbital vessels and nerve and lies about 1 cm below the middle of the infraorbital margin. The maxillary alveolar process bears the upper teeth. The short, thick zygomatic process of the maxilla has an oblique upper surface that articulates with the zygomatic bone at the zygomaticomaxillary suture. The frontal process of the maxilla ascends posterolateral to the nasal bone to articulate with it medially, the frontal bone superiorly and the lacrimal bone laterally.
The anterior nasal aperture is piriform in shape, wider below than above and bounded by the paired nasal bones and maxillae. The upper boundary of the aperture is formed by the nasal bones while the remainder is formed by the maxillae. In life, several cartilages (septal, lateral nasal, major and minor alar) help to delineate two nasal cavities. The shape of these bones can be used quite successfully to predict the shape of the cartilaginous nose in forensic facial reconstructions ( ).
The lower part of the face, below the external nose, is formed from the alveolar arch of the maxillae and the upper dentition, and the body and alveolar process of the mandible and the lower dentition. In the midline, the mental protuberance produces the characteristic prominence of the chin. The mental foramen, which transmits the mental nerve and accompanying vessels, lies in the same vertical plane as the supraorbital and infraorbital foramina.
Anteroposterior radiographs of the skull clearly show the central location of the paranasal air sinuses in the frontal bone, maxilla and ethmoid. The sinuses exhibit wide anatomical variations which can be useful indicators of identity when postmortem images are compared with antemortem clinical films.
Posterior View
The parietal, temporal and occipital bones form the entirety of the posterior view ( Fig. 34.2 ). The superolateral region is occupied by the parietal bones, the mastoid region of the temporal bones makes up the inferolateral regions, and the central portion is occupied by the squamous portion of the occipital bone, which is the reason why this aspect is also referred to as the occipital view. The parietal bones articulate with the occipital bone at the lambdoid suture, which extends inferiorly into the occipitomastoid and the parietomastoid sutures behind and above the mastoid processes, respectively. The lambda is the anthropometric point where the occipital bone meets the two parietal bones. Accessory sutural bones (or wormian bones) are islands of bone that may be found within a suture. They may arise from separate centres of ossification and they appear to have no clinical significance, being of genetic rather than pathological aetiology; their initial formation is thought to be caused by a degree of dural strain and increased sutural width ( ). They are most frequent in the lambdoid suture, which is the most tortuous of the sutures. However, they are also often observed within the sagittal and coronal sutures. A large, central interparietal bone is not uncommon and is sometimes referred to as an Inca bone (see Fig. 34.2 ). Wormian bones are considered to be markers for various congenital diseases, including osteogenesis imperfecta and cleidocranial dysostosis.
The external occipital protuberance is a midline elevation on the occipital bone that can become particularly well developed and palpable in males and is a marker for sex estimation. The location of the protuberance coincides with the anthropometric inion. Superior nuchal lines extend laterally from the protuberance and represent the boundary between the scalp and the neck. Inferior nuchal lines run parallel to, and below, the superior nuchal lines; a set of highest nuchal lines may sometimes develop above the superior lines. The external occipital protuberance, nuchal lines and roughened external surface of the occipital bone between the nuchal lines afford attachment to the muscles of the neck.
Superior View
Seen from above, the contour of the calvaria varies greatly but is usually ellipsoid, or more strictly, a modified ovoid with its greatest width lying nearer to the occipital pole ( Fig. 34.3 ). Four bones constitute this view and articulate via three well-defined sutures. The squamous part of the frontal bone is anterior, the squamous part of the occipital bone is posterior, and the two parietal bones meet in the midline separating the frontal from the occipital bone. The maximal parietal convexity on each site is palpable at the parietal tuber or eminence; it is most conspicuous in the female (retention of a paedomorphic appearance). The superior and inferior temporal lines run close to the parietal eminence but are best seen in a lateral view.
The coronal suture marks the articulation between the posterior margin of the frontal bone and the anterior margins of the two parietal bones. It descends across the calvaria and projects inferiorly until it meets the junction between the greater wing of the sphenoid and the squamous temporal bone at the pterion. The sagittal suture runs in the midline between the two parietal bones and extends from the bregma anteriorly to the lambda posteriorly. The lambdoid suture delineates the articulation between the posterior borders of the right and left parietal bones and the superior border of the occipital bone.
The bregma represents the position of the anterior fontanelle in the young child. This diamond-shaped, membrane-filled space located between the two frontal and two parietal bones of the developing fetal skull persists until approximately 18 months after birth. A depression can sometimes be found posterior to the bregma (post-bregmatic depression) and is often used as an indicator of sub-Saharan African ancestral origin. The lambda, at the junction of the sagittal and lambdoid sutures, represents the site of the posterior fontanelle, which persists for the first 2–3 months after birth.
A parietal foramen may pierce either or both parietal bones near the sagittal suture about 3.5 cm anterior to the lambda. It transmits a small emissary vein from the superior sagittal sinus. The vertex is the highest point on the skull and it usually occupies a position in the middle third of the sagittal suture.
Lateral View
The skull, viewed from the side, can be subdivided into three zones: face (anterior), temporal region (middle) and occipital region (posterior) ( Fig. 34.4 ). The face has been considered in the section on the anterior view of the skull.
The temporal region can be divided into an upper temporal fossa and a lower infratemporal fossa, separated by the position of the zygomatic arch. The upper temporal fossa is bounded inferiorly by the zygomatic arch, superiorly and posteriorly by the temporal lines, and anteriorly by the frontal process of the zygomatic bone. It is continuous inferiorly with the infratemporal fossa deep to the zygomatic arch. The temporal lines often present anteriorly as distinct ridges but become much less prominent as they arch posteriorly across the parietal bone. The inferior temporal line becomes more prominent as it curves down the posterior part of the squamous temporal bone, forming a supramastoid crest at the base of the mastoid process. The superior temporal line gives attachment to the temporal fascia while the inferior temporal line provides attachment for temporalis.
The floor of the temporal fossa is formed by the frontal and parietal bones superiorly and the greater wing of the sphenoid and squamous part of the temporal bone inferiorly. All four bones of one side meet at a roughly H-shaped sutural junction called the pterion. This is an important anthropometric landmark because it commonly overlies both the anterior branch of the middle meningeal artery and the lateral fissure of the cerebral hemisphere ( ). The pterion corresponds to the site of the anterolateral (sphenoidal) fontanelle of the neonatal skull, which closes in the third month after birth.
The vertical suture between the sphenoid and temporal bones, the sphenosquamosal suture, is formed by articulation between the posterior border of the greater wing of the sphenoid and the anterior border of the squamous part of the temporal bone.
The lateral surface of the ramus of the mandible will be described briefly here because it lies within the middle region of this view of the skull. The ramus is a plate of bone projecting upwards from the body of the mandible; its lateral surface gives attachment to masseter. The ramus bears two prominent processes superiorly, the coronoid process anteriorly and the condylar process posteriorly, separated by the mandibular notch. The coronoid process is the site of insertion of temporalis; the condylar process articulates with the mandibular (glenoid) fossa of the temporal bone at the temporomandibular joint. The inferior and posterior borders of the mandible meet at the angle; it is more commonly splayed in the male, reflecting the larger site of attachment for medial pterygoid on the internal surface.
The zygomatic arch stands proud of the rest of the skull, and the temporal and infratemporal fossae communicate via the gap thus created. In life, this space is largely filled by temporalis. The zygomatic bone is the principal bone of the cheek together with the zygomatic processes of the maxilla and temporal bones. The term ‘zygomatic arch’ is generally restricted to the temporal process of the zygomatic bone and the zygomatic process of the temporal bone, which articulate at the zygomaticotemporal suture. The suture between the zygomatic process of the frontal bone and the frontal process of the zygomatic bone is the frontozygomatic suture; the suture between the maxillary margin of the zygomatic bone and the zygomatic process of the maxilla is the zygomaticomaxillary suture; and the suture between the sphenoid and zygomatic bones is the sphenozygomatic suture. As the zygomatic process of the temporal bone passes posteriorly, it widens to form the articular eminence (tubercle) of the mandibular fossa anteriorly.
The temporal bone is a prominent structure on the lateral aspect of the skull. Its squamous part lies in the floor of the upper temporal fossa and its zygomatic process contributes to the structure of the cheek. Additional components visible in the lateral view of the skull are the mandibular fossa and its articular eminence, the tympanic plate, the external acoustic meatus (external auditory meatus), and the mastoid and styloid processes. The mandibular fossa is bounded in front by the articular eminence and behind by the tympanic plate. The articular eminence provides a surface over which the mandibular condyle glides during mandibular movements and acts to prevent dislocation of the temporomandibular joint. The tympanic plate of the temporal bone contributes most of the margin of the external acoustic meatus; the squamous part forms the posterosuperior region. The external margin is roughened to provide an attachment for the cartilaginous part of the meatus. A small depression, the suprameatal triangle, lies above and behind the meatus and is related to the lateral wall of the mastoid antrum. The mastoid process is an inferior projection of the temporal bone. It lies posteroinferior to the external acoustic meatus and is the site of attachment of sternocleidomastoid. It is in contact behind with the posteroinferior angle of the parietal bone at the parietomastoid suture and with the squamous part of the occipital bone at the occipitomastoid suture. These two sutures meet the lateral end of the lambdoid suture at the asterion. This coincides with the site of the posterolateral fontanelle in the neonatal skull, which closes during the second year. A mastoid foramen may be found near, or in, the occipitomastoid suture; it transmits an emissary vein from the sigmoid sinus. Sutural bones may appear in the parietomastoid suture.
The styloid process lies anterior and medial to the mastoid process and gives attachment to several muscles and ligaments. Its base is partly ensheathed by the tympanic plate and it descends anteromedially, its tip usually reaching a point medial to the posterior margin of the mandibular ramus. The styloid process is very variably developed. Its length ranges from a few millimetres to a few centimetres and increases with age ( ).
The infratemporal fossa is an irregular, postmaxillary space deep to the ramus of the mandible. It communicates with the upper temporal fossa deep to the zygomatic arch. It is best visualized when the mandible is removed but, for completeness, is considered here. Its roof is the infratemporal surface of the greater wing of the sphenoid, the lateral pterygoid plate lies medially, and the ramus of the mandible and styloid process lie laterally and posteriorly, respectively. The infratemporal fossa has no anatomical floor. Its anterior and medial walls are separated above by the pterygomaxillary fissure lying between the lateral pterygoid plate and the posterior wall of the maxilla. The infratemporal fossa communicates with the pterygopalatine fossa through the pterygomaxillary fissure.
Inferior View
With the mandible removed, the inferior surface of the skull extends from the upper incisor teeth anteriorly to the superior nuchal lines of the occipital bone posteriorly ( Fig. 34.5 ). The region contains many of the foramina through which structures enter and exit the cranial cavity. The inferior surface may be conveniently subdivided into anterior, middle, posterior and lateral parts. The anterior part contains the hard palate and the dentition of the maxillary arch and lies at a lower level than the rest of the cranial base. The middle and posterior parts may be arbitrarily divided by a transverse plane passing through the anterior margin of the foramen magnum. The middle part is occupied mainly by the sphenoid bone, the apices of the petrous processes of the temporal bones, and the basilar part of the occipital bone. The lateral part contains the zygomatic arches, mandibular fossae, tympanic plates, the styloid and mastoid processes. The posterior part lies in the midline and is formed almost exclusively from the occipital bone. Whereas the middle and posterior parts are directly related to the cranial cavity (the middle and posterior cranial fossae), the anterior part is some distance from the anterior cranial fossa, being separated from it by the nasal cavities.
Anterior part of inferior surface
The bony palate within the superior alveolar arch is formed by the palatine processes of the maxillae anteriorly and the horizontal plates of the palatine bones posteriorly, all meeting at a cruciform system of sutures (see Fig. 34.5 ). 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 maxillae 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. 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 are present in some skulls and open on to 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 are present, the left nasopalatine nerve usually 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; a vascular groove, deeper behind and shallower in front, leads forwards from the foramen. 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 the posterior nasal spine. In the adult, the alveolar arch normally bears a maximum of 16 sockets or alveoli for the teeth; the sockets vary in size and depth. Some are single and some are subdivided by septa, according to the morphology of the dental roots.
The nasal fossae lie above the hard palate and are separated by the nasal septum in the midline. The bony nasal septum is formed from the perpendicular plate of the ethmoid superiorly and the vomer inferiorly. The upper border of the vomer approximates to the inferior aspect of the body of the sphenoid, where it expands into an ala on each side. The two posterior nasal apertures (choanae) are located at the end of the nasal fossae; they are separated by the free posterior border of the vomer and bounded below by the posterior border of the horizontal plates of the palatine bones, above by the sphenoid and laterally by the medial pterygoid plates.
Middle part of inferior surface
The middle part of the inferior surface is made up from the sphenoid, petrous parts of the temporal bones and the basiocciput (see Fig. 34.5 ). It extends from the posterior nares anteriorly to an artificial line drawn transversely through the anterior margin of the foramen magnum posteriorly. In the adult, the body of the sphenoid fuses with the basiocciput, at the spheno-occipital synchondrosis, to form a midline bar of bone that extends posteriorly to the foramen magnum (internally, this is known as the clivus). The basiocciput bears a small midline pharyngeal tubercle, which gives attachment to the pharyngeal raphe, the highest attachment of the superior pharyngeal constrictor.
The petrous processes of the two temporal bones pass from the lateral sides of the base of the skull and fill the triangular space between the greater wing of the sphenoid anteriorly and the lateral margins of the basiocciput posteriorly. Each petrous process meets the basilar part of the occipital bone at a petro-occipital junction, which is deficient posteriorly at the jugular foramen. The petrosphenoidal junction 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 synchondrosis; the deficit so produced is the foramen lacerum.
Pterygoid processes descend from the junction between the greater wing and body of the sphenoid, separated by a pterygoid fossa. Anteriorly, the plates are fused, except inferiorly, where they are separated by the pyramidal process of the palatine bone. Sutures are usually discernible at this site in young skulls. 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 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 articulates with the squamous part of the temporal bone.
The medial aspect of the greater wing of the sphenoid presents a crescent of foramina of which only the most posterior two, foramen ovale and foramen spinosum, can be viewed on the basal aspect. The foramen ovale lies medial to the foramen spinosum and lateral to the foramen lacerum on the infratemporal surface of the greater wing of the sphenoid bone. It transmits the mandibular division of the trigeminal nerve, 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 recurrent 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, with the adjoining posterior border of the greater wing of the sphenoid, forms the anterolateral wall of a groove that is completed posteromedially by the petrous part of the temporal bone. This groove, the auditory sulcus, contains the cartilaginous pharyngotympanic tube, which leads posterolaterally into the bony portion of the tube that lies within the petrous part of the temporal bone. Occasionally, the foramen ovale and foramen spinosum are confluent or 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 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 nearly 1 cm long, it is not traversed by any major structure. The almost circular 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, accompanied by 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 embryological 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 transmits the nerve of the pterygoid canal and accompanying blood vessels.
Posterior part of inferior surface
The posterior part of the inferior surface of the skull is predominantly formed by the occipital bone (see Fig. 34.5 ). Prominent features are the foramen magnum and associated occipital condyles, jugular foramen, mastoid notch and the 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 and leads into the posterior cranial fossa. It is oval and wider behind, with its greatest diameter being anteroposterior. It contains the lower end of the medulla oblongata, meninges, cerebrospinal fluid, vertebral arteries and veins, and the accessory nerves; the apical ligament of the dens and the tectorial membrane pass through it to attach to the internal basiocciput. 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 orientated obliquely so that its anterior end lies nearer the midline than its posterior end. It is markedly convex anteroposteriorly, less so transversely, and its medial aspect is roughened by ligamentous attachments. The hypoglossal canal is directed laterally and slightly forwards, traverses deep to 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 may contain a (posterior) condylar canal for an emissary vein from the sigmoid sinus. This fossa accommodates the posterior margin of the atlas when the head is fully extended. A jugular process articulates with the petrous part of the temporal bone lateral to each condyle and its anterior free border forms the posterior boundary of the jugular foramen.
Laterally, the occipital bone approximates with the petrous part of the temporal bone anteriorly at the petro-occipital junction, and the mastoid process of the temporal bone more posteriorly at the occipitomastoid suture. The jugular foramen, a large, asymmetric and irregular hiatus, lies at the posterior end of the petro-occipital junction between the jugular process of the occipital bone and the jugular fossa of the petrous part of the temporal bone (see Fig. 42.1C ) ( ). A number of important structures pass through this foramen: inferior petrosal sinus (anterior); glossopharyngeal, vagus and accessory cranial nerves (middle); and 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 branch of the glossopharyngeal nerve 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 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 at the neck.
Lateral part of inferior surface
The lateral part of the inferior surface consists of the zygomatic arch and infratemporal fossa anteriorly and the mandibular fossa, tympanic plate and styloid and mastoid processes posteriorly (see Fig. 34.5 ). The anterior structures have been considered earlier in this chapter.
The mandibular fossa is a thin-walled, smooth, concave depression in the temporal bone and is most easily inspected when the mandible is removed. The zygomatic arch extends laterally in front of the fossa and a distinct ridge, the articular eminence, lies anterior to the fossa. Three fissures can be distinguished behind the mandibular fossa. 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 intracranial cavity into the infratemporal fossa. The tympanic plate forms the floor of the external acoustic meatus.
The stylomastoid foramen lies between the mastoid and styloid processes on the lateral aspect of the temporal bone. It transmits the facial nerve and the stylomastoid artery. The distance from the styloid process to the stylomastoid foramen decreases with age ( ). A groove, the mastoid notch, lies medial to the mastoid process and gives origin to the posterior belly of 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 as the vaginal process.
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