The external surface of the base of the skull, excluding the mandible, is bounded in front by the incisor teeth of the maxillae, behind by the superior nuchal lines of the occipital bone, and laterally by the remaining teeth, the zygomatic arch and its posterior root, and the base of the mastoid process.It is very irregular and, unlike the internal surface, shows no natural subdivision into regions suitable for descriptive purposes. It is, however, both usual and useful to divide the area into anterior, middle and posterior parts. The anterior part, which is formed by the hard palate and the alveolar arches, projects downwards to a lower level than the part behind. The remainder of the surface is divided, in an arbitrary manner, into a middle and a posterior part by a transverse line drawn through the anterior margin of the foramen magnum.
The bony palate (fig. 324.) lies within the arch formed by the teeth of the maxilla and the alveolar processes. The palatine processes of the maxilla and the horizontal plates of the palatine bones take part in its formation and are separated from one another by the intermaxillary, interpalatine and palatomaxillary sutures. These sutures form a cruciate figure, which can be identified without difficulty in the skull. Owing partly to the downward projection of the alveolar arches, the palate is arched both from before backwards and from side to side. The depth and the breadth of the palatine vault are open to considerable variation but are always greatest in the region of the molar teeth. A deep fossa, termed the incisive fossa, lies in the median plane anteriorly. The lateral incisive foramina, which lead into the incisive canals, are situated in its lateral walls; the median incisive foramina, which are present in some skulls, open on its anterior and posterior walls. The greater palatine foramen, which is the lower orifice of the canal of the same name, opens close to the lateral border of the palate just behind the palatomaxillary suture. A vascular groove, deep behind and becoming shallower in front, leads forwards away from the foramen. The lesser palatine foramina, usually two in number, lie behind the greater foramen. They pierce the tubercle (pyramidal process) of the palatine bone, which projects backwards and laterally from the posterolateral corner of the bony palate and becomes wedged into the notch between the lower ends of the two pterygoid plates. The vault of the bony palate is uneven, pierced by numerous small foramina and marked by depressions for the palatine glands. Near the posterior border it presents a slightly curved ridge of variable prominence, termed the palatine crest, which commences behind the greater palatine foramen and runs medially. The posterior border of the bony palate is free and projects backwards in the median plane to form the posterior nasal spine.
The alveolar arch provides sixteen sockets (alveoli) for the reception of the roots of the teeth. These sockets vary in size and depth and are single or subdivided by septa according to the teeth which they contain.
The middle part of the external surface of the base of the skull (fig. 322) extends from the posterior border of the bony palate to an arbitrary line drawn transversely through the anterior margin of the foramen magnum. In the median plane anteriorly the posterior border of the vomer separates the two posterior nasal apertures. Immediately behind the vomer the posterior part of the inferior surface of the body of the sphenoid is directly continuous with the inferior surface of the basilar part of the occipital bone, which forms a broad bar of bone extending backwards and downwards to the foramen magnum. It is convex from side to side and wider behind than in front. A short distance in front of the foramen magnum it is marked in the median plane by a small elevation, termed the pharyngeal tubercle, which gives attachment to the highest fibers of the superior constrictor muscle of the pharynx.
Behind the third molar tooth the pterygoid process descends from the junction of the greater wing with the body of the sphenoid bone. It consists of two laminae, termed the medial and lateral pterygoid plates, which are separated from each other by a V-shaped interval, directed backwards and somewhat laterally and named the pterygoid fossa. Inferiorly the two pterygoid plates are separated by a narrow fissure which is occupied by the tubercle (pyramidal process) of the palatine bone, and the suture lines can usually be identified. Above the pterygoid fissure the two plates are fused to each other. On the medial side they articulate with the posterior border of the perpendicular plate of the palatine bone, and form with it the flattened area of bone which lies in the lateral wall of the posterior nasal aperture and nasal part of the pharynx. On the lateral side the fused lamina: are separated from the posterior surface of the maxillae by the pterygomaxillary fissure. The medial pterygoid plate is the narrower of the two and is placed in a parasagittal plane. Its medial aspect is covered with mucous membrane in the recent state and forms the lateral boundary of the posterior nasal aperture and part of the lateral wall of the nasal part of the pharynx. The posterior border of the medial pterygoid plate is thin and sharp, and presents a small projection about its midpoint. Above this projection the border is curved and is attached to the pharyngeal end of the pharyngotympanic (auditory) tube. At its upper end the border divides to enclose the shallow scaphoid fossa (fig. 325); below, it projects beyond the rest of the plate as the slender pterygoid hamulus. This process curves downwards and laterally and is grooved anteriorly at its root by the tendon of the tensor palati (tensor veli palatini) muscle. The lateral pterygoid plate projects backwards and laterally and its lateral aspect forms the medial wall of the infratemporal fossa. Superiorly it is continuous with the infratemporal surface of the greater wing of the sphenoid, which forms the anterior part of the roof of the infratemporal fossa. This part of the greater wing is directed downwards and, sometimes, slightly to the lateral side. It is roughly pentagonal ; its anterior margin forms the posterolateral border of the inferior orbital fissure ; and in front and to the lateral side it is limited by the roughened infratemporal crest. Laterally it articulates with the squama of the temporal bone; medially it becomes continuous with the root of the pterygoid process and the side of the body of the sphenoid ; and posteriorly it articulates with the petrous part of the temporal bone.
Two important foramina open on the infratemporal surface of the greater wing of the sphenoid. The foramen ovale, irregularly oval in outline, lies close to the posterior border and posterolateral to the upper end of the posterior margin of the lateral pterygoad plate. It transmits the mandibular division of the trigeminal nerve. Posterior and slightly lateral to the foramen ovale the foramen spinosum pierces the greater wing and transmits the middle meningeal artery to the middle cranial fossa. It is much smaller than the foramen ovale and is circular in outline. Immediately posterolateral to the foramen spinosum the posterolateral angle of this aspect of the greater wing forms an irregular downward projection, termed the spine of the sphenoid. The medial aspect of the spine is somewhat flattened, and together with the adjoining part of the posterior border of the greater wing forms the anterolateral border of a groove which is completed on the postero medial side by the petrous part of the temporal bone. This groove lodges the cartilaginous part of the pharyngotympanic (auditory) tube, and leads backwards into the canal for the tube in the petrous part of the temporal bone and forwards to the upper part of the posterior border of the medial pterygoad plate. In the roof of the groove the posterior border of the greater wing and the anterior border of the petrous temporal come into apposition with each other. It will be apparent that immediately below the skull the mandibular nerve and the middle meningeal artery must be related to the anterolateral aspect of the pharyngotympanic tube.
Behind and medial to the groove for the tube the inferior surface of the petrous temporal occupies the interval between the infratemporal surface of the greater wing of the sphenoid and the basilar part of the occipital bone. The anterior part of this surface is rough and uneven, and its apex is separated from the posterolateral part of the body of the sphenoid by an irregular bony canal, termed the foramen lacerum. Behind this rough area a large and approximately circular foramen leads upwards into the bone. It is the lower opening of the carotid canal, which is traversed by the internal carotid artery. Within the bone the canal turns forwards and medially and opens on the posterior wall of the foramen lacerum. After passing through the canal the artery turns upwards to gain the interior of the skull. The lower part of the foramen lacerum is occupied in the recent state by fibrocartilage, and no large structure enters the skull through this opening.
The spine of the sphenoid lies at the medial end of the squamotympanic fissure, which runs laterally and slightly backwards between the upper part of the tympanic plate and the articular fossa of the squamous part of the temporal bone. The fissure can usually be traced to the upper part of the anterior margin of the orifice of the external auditory meatus, but it is sometimes obliterated near its lateral end. The articular fossa is deeply concave from before backwards and gently concave from side to side, and is wider at its lateral than at its medial end. It lodges the condyle of the mandible when the mouth is closed. Anteriorly the articular surface passes on to a transverse rounded elevation, termed the articular eminence, which is continuous laterally with the anterior root of the zygoma. In front it becomes continuous with the part of the squamous temporal which lies in the roof of the infratemporal fossa. Behind the squamotympanic fissure the tympanic part of the temporal bone separates the articular fossa from the external auditory meatus. It is roughly triangular in outline, the apex being situated at the medial end of the squamotympanic fissure close to the root of the spine of the sphenoid. Its lower border is free and skirts the anterolateral margin of the lower opening of the carotid canal, extending backwards and laterally to reach the root of the styloid process. There it forms the sheath of the styloid process, which is deeper and more apparent on the lateral than on the medial side. At its lateral margin the tympanic part is fused with the mastoid part of the temporal bone below and is free above, where it forms the anterior border of the orifice of the external auditory meatus.
Particular features.-The upper border of the vomer, which is applied to the inferior surface of the body of the sphenoid, is expanded into an ala on each side (fig. 322), and the groove between the alae receives the rostrum of the sphenoid. The lateral border of each alae comes into relation with a thin lamella, which projects medially from the root of the medial pterygoid plate. The two may come merely into contact or the edge of the ala may extend into the narrow interval between the body of the sphenoid above and the vaginal process below. The inferior surface of the vaginal process is marled by an anteroposterior groove, which is converted into a canal anteriorly by the upper surface of the sphenoidal process of the palatine bone. This canal is termed the palatinovaginal (pharyngeal) canal and opens anteriorly through the posterior wall of the pterygopalatine fossa. It transmits the pharyngeal branch of the sphenopalatine ganglion and a minute pharyngeal branch from the third part of the maxillary (internal maxillary) artery. A second canal may be present in this situation on the medial side of the palatinovaginal canal. It lies between the ala of the vomer and the upper surface of the vaginal process and is termed the vomerovaginal canal. When present it leads forwards into the anterior end of the palatinovaginal canal.
In front of the pharyngeal tubercle the inferior surface of the basilar part of the occipital bone is intimately related to the roof of the nasal pharynx and the nasopharyngeal tonsil. Lateral to the tubercle the bone receives the insertion of the longus capitis muscle, and the area extends forwards on each side beyond the tubercle. Behind the longus capitis the rectus capitis anterior is inserted into the bone in front of the occipital condyle and medial to the outer opening of the anterior condylar (hypoglossal) canal.
The medial surface of the medial pterygoid plate (fig. 331) is smooth and covered with the mucous membrane of the nasal pharynx in the recent state. At the upper part of its posterior border the scaphoid fossa gives origin to the anterior fibers of the tensor palati (tensor veli palatini) muscle, which descends along the lateral aspect and posterior border of the plate to reach the hamulus. The tendon of the muscle twists medially round the lateral and anterior aspects of the process to gain the soft palate. The posterior border of the medial pterygoid plate, notched above by the pharyngotympanic (auditory) tube (p. 257), gives attachment to the pharyngobasilar fascia. Its lower part and the posterior aspect of the hamulus give origin to the highest fibers of the superior constrict-or muscle of the pharynx. which curve upwards and medially to be inserted into the pharyngeal tubercle. The tip of the hamulus gives attachment to the pterygomandibular ligament (raphe). At its upper end the posterior border of the medial pterygoid plate is marked by a small tubercle, which lies on the medial side of the scaphoid fossa. This tubercle projects backwards below, and conceals the posterior opening of the pterygoid canal, which leads forwards to open on the posterior wall of the pterygopalatine fossa. It transmits the nerve and vessels of the pterygoid canal and lies in the line of fusion of the. pterygoid process and greater wing with the body of the sphenoid bone.
The pterygoid fossa lies between the opposed surfaces of the two pterygoid plates and is completed below and in front by the tubercle (pyramidal process) of the palatine bone. The lateral wall of the fossa gives origin to the deep head of the medial pterygoid muscle. The lateral pterygoid plate (fig. 320) is a wider lamina than the medial plate, and its irregular posterior border may present a backward projection, termed the pterygospinous process which is connected by a: ligament, sometimes ossified, to the spine of the sphenoid. The lateral surface, which is the rougher of the two, gives origin to the lower head of the lateral pterygoid muscle; the medial surface gives origin to the deep head of the medial pterygoid muscle. The lateral aspect of the tubercle (pyramidal process) of the palatine bone, which appears between the tuberosity of the maxilla and the lower part of the lateral pterygoid plate, gives origin to some fibers of the superficial head of the medial pterygoid muscle.
The infratemporal surface of the greater wing of the sphenoid gives origin to the upper head of the lateral pterygoid muscle, and is crossed by the deep temporal and. the masseteric nerves, which run between the muscle and the bone. In addition to the mandibular nerve the foramen ovate transmits the accessory meningeal artery. Its posterior border is thin and sharp and gives origin to fibers of the tensor palati muscle, which intervenes between the mandibular nerve and the pharyngotynnpanic (auditory) tube. The foramen spinosum transmits the small recurrent meningeal branch of the mandibular nerve in addition to the middle meningeal artery. In the interval between the foramen ovale and the scaphoid fossa the bone is sometimes pierced by a small foramen, termed the emissary sphenoidal foramen (foramen Vesalii), which transmits an emissary vein from the cavernous sinus. The spine of the sphenoid which varies greatly in size and may be sharply pointed or blunt at its extremity, gives attachment to the sphenomandibular ligament. It is related laterally to the auriculotemporal nerve and medially to the chorda tympani-by which its medial aspect is sometimes grooved-and to the pharyngotyrnpanic (auditory) tube. Anteriorly it gives origin to the most posterior fibers of the tensor pa:lati muscle. The groove for the tube varies in width and depth and its roof is occasionally completed by membrane. The lateral or sphenoidal wall of the groove gives origin, posteriorly, to fibers of the tensor tympani muscle.
The floor of the articular fossa is very thin and corresponds to the most lateral part of the floor of the middle cranial fossa. It is covered in the recent state by white ftbro-cartilage. The tubercle of the root of the zygoma gives attachment to the ternporoinandibniar ligament. A thin edge of bone may be visible in the depths of the medial end of the squamotympanic fissure. It is the lower border of the down-turned lateral portion of the tegmen tympani and therefore is a part of the petrous temporal.
It divides the upper part of the squamotympanic fissure into a petrotympanic and a petrosquamous fissure. Through the former the chorda tympani travels in its anterior canaliculus, as it passes downwards and forwards from the tympanic cavity. The anterior tympanic artery from the maxillary (internal maxillary) artery also traverses the petrotympanic fissure.
The tympanic part of the temporal bone (fig. 325) is separated from the capsule of the temporomandibular joint by a portion of the parotid gland, which is pierced by the auriculotemporal nerve. It is thinnest near the centre of this surface and is occasionally deficient in this situation (p. 300). The grooved upper aspect of the plate forms the anterior wall, the floor and the lower part of the posterior wall of the external auditory meatus. Except where it ensheathes the styloid process its posterior aspect is fused with the petrous part of the bone.
The median portion of the posterior subdivision of the external surface of the base of the skull is occupied in front by the foramen magnum of the occipital bone, which leads into the floor of the posterior cranial fossa. The foramen is somewhat oval in shape and its anteroposterior exceeds its transverse measurement. The curve of its margin is wider behind than in front. It transmits a large number of structures of which the most important is the lower end of the brain-stem. Anteriorly the margin of the foramen magnum is overlapped slightly on each side by the occipital condyle, which projects downwards to articulate with the superior articular facet on the lateral mass of the atlas. Oval in outline, the condyle is placed obliquely so that its anterior end is nearer to the median plane than its posterior end. It shows a pronounced convexity from before backwards and a gentle convexity from side to side. The medial aspect is roughened for ligamentous attachments. Above the anterior part of the condyle the occipital bone is pierced by the anterior condylar (hypoglossal) canal, which runs laterally and slightly forwards from the posterior cranial fossa and transmits the hypoglossal nerve.
A depression of variable depth marks the occipital bone behind the condyle. It is termed the condylar fossa, and its floor may be pierced by the posterior condylar canal, which, when present, transmits an emissary vein from the sigmoid sinus. Lateral to the condyle the jugular process of the occipital bone articulates with the petrous temporal. The anterior border of the process is free and forms the posterior boundary of the jugular foramen. This foramen lies between the occipital bone and the jugular fossa of the petrous temporal and is placed at the posterior end of the petro-occipital suture. In front it is separated from the lower orifice of the carotid canal by a raised ridge of bone, and on its lateral side it is related to the medial aspect of the sheath of the styloid process. Medially it is separated from the anterior condylar (hypoglossal) canal by a thin bar of bone. The foramen is usually larger on the right side of the skull and its long axis is directed forwards and medially. The anterior part of the foramen transmits the inferior petrosal sinus ; its intermediate part, the glossopharyngeal, vagus and accessory nerves ; and its posterior part the internal jugular vein. When the superior bulb of the internal jugular vein is well developed the jugular fossa of the temporal bone is hollowed out in an upward and lateral direction to accommodate it.
The styloid process has been described above. Posterior to its root the stylomastoid foramen, which represents the lower opening of the facial canal, transmits the facial nerve. Behind and lateral to the foramen the tip of the mastoid process projects downwards and forwards, and its projecting part forms the lateral wall of the mastoid (digastric) notch, from which the posterior belly of the digastric muscle takes origin. Medial to the notch this part of the temporal bone may be grooved by the occipital artery.
In the median plane posteriorly the squamous part of the occipital bone presents the external occipital crest, which gives attachment to the upper end of the ligamentum nuchae. It terminates behind at the external occipital protuberance. Near its midpoint the inferior nuchal line curves backwards and laterally, roughly parallel to the superior nuchal line, which extends in the same direction from the external occipital protuberance and may be raised into a distinct crest in its medial part.
Particular features.-The foramen magnum provides a wide communication between the posterior cranial fossa and the vertebral canal. Anteriorly it transmits the apical ligament of the odontoid process (dens) and the membrana tectoria, both of which gain attachment to the upper or cerebral surface of the basilar part of the occipital bone. Its wider posterior part transmits the lower end of the medulla oblongata and the meninges. In the subarachnoid space the spinal root; of the accessory nerves, and the vertebral arteries, with their plexuses of sympathetic nerves, ascend to gain the interior of the cranium, the posterior spinal arteries descend, one on each posterolateral aspect of the brain stem, and the anterior spinal artery- descends on the front. of the brain stem in the median plane. In addition, the lower parts of the tonsils of the cerebellum may project into the foramen on each side of the medulla oblongata. The. anterior margin of the foramen gives attachment to the anterior atlanto-occipital membrane, which is continuous on each side with the capsular ligament of the atlanto-occipital joint. The posterior margin gives attachment to the posterior atlanto-occipital membnme, and the roughened medial aspect of the condyle to the alar ligament. In addition to the hypoglossal nerve the anterior condylar (hypoglossal) canal transmits a meningeal branch of the ascending pharyngeal artery and a small emissary vein from the basilar plexus. Not uncommonly the canal is divided into two parts by a spicule of bone, a variation wbich is in keeping with the composite origin of the hypoglossal nerve. The inferior surface of the jugular process of the occipital bone provides insertion for the rectus capitis lateralis muscle.
The jugular foramen (fig. 322) is directed upwards, medially and backwards, and on .the external surface of the base of the skull its apparent size is increased owing to the presence of the jtzaular fossa of the temporal bone on its lateral side. The floor of the fossa .separates the superior bulb of the internal jugular vein from the tvmpanic cavity, and its lateral wall is pierced by a minute canal, termed the mastoid canaliculus, which transmits the auricular branch of the vagus nerve. Passing laterally through the bone this nerve comes into intimate relationship with the facial canal and finally emerges in the line of the fympanomastoid suture. It is extra-cranial at birth but becomes surrounded by bone as the tympanic plate and the mastoid process develop. On or near the ridge which intervenes between the jugular fossa and the orifice of the carotid canal, the canaliculus, for the tympanic nerve pierces the bone to transmit. the tympanic nerve from the glossopharyngeal nerve to the middle ea.r. On the upper boundary of the jugular foramen near its medial end, there is a small notch-more easily identified on the internal surface-which lodges the inferior (petrous) ganglion of the glossopharyngeal nerve. The orifice of the cochlear canaliculus (p. 297) lies at the apex of the notch, the projecting edges of which may reach the occipital bone and divide the foramen into three parts.
The stylomastoid foramen lies behind the base of the styloid process and at the anterior end of the mastoid notch. As the facial nerve emerges from the foramen it is in close proximity to the posterior belly of the digastric, which it supplies before piercing the parotid kind. In addition to the facial nerve the foramen transmits the stylornastoid branch of the posterior auricular artery. A vascular groove crosses the inferior aspect of the mastoid part of the temporal bone medial to the mastoid notch. It is caused by the occipital artery and its absence indicates that the vessel lay at a lower level than usual and between the splenius capitis and longissimus capitis instead of deep to both muscles.
The area below the inferior nuchal line gives insertion medially to the rectus capitis posterior minor, and laterally to the rectus capitis posterior major (fig. 345). The interval between the inferior and the superior nuchal lines provides insertion medially for the semispinalis capitis and laterally for the obliquus superior. In its medial part the superior nuchal line gives origin to the highest fibers of the trapezius muscle; in its lateral part it gives insertion to fibers of the sternomastoid and, more anteriorly, splenius capitis.
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