Chapter 6 
The Skull
1 Introduction
The skeleton of the head is a complex articulation of many bones and teeth, which are collectively referred to as the skull or cranium. On the basis of function the skull may be conveniently divided into two main areas: the (1) neurocranium and (2) the facial skeleton (Figure 6-1).
2 Views
FRONTAL VIEW
Bones
The bones evident in the anterior view (Figure 6-2, A) are the right and left maxillae, nasal bones, the right and left zygomatic bones, the right and left lacrimal bones, the right and left inferior conchae, the ethmoid bone, the vomer, the sphenoid bone, the frontal bone, and the mandible.
Features
Forehead Region
The forehead region is formed mainly by a portion of the frontal bone (Figure 6-2, B). The following features are found within the region of the forehead.
Orbital Area
The orbits contain the eyeballs and the extraocular muscles (see Figures 6-2 and 6-3). Several bones contribute to the margins (rims) and to the inner walls of the bony orbit.
Orbital Openings
1 The supraorbital notch or foramen (either may be present) is located directly above the medial third of the superior margin. It represents a communication between the orbit within and the forehead without and transmits the supraorbital vessels and nerve.
2 The infraorbital foramen is situated about 7 to 8 mm below the midpoint of the inferior margin. It communicates posteriorly with the infraorbital canal and groove in the floor of the orbit and transmits the infraorbital vessels and nerve.
3 The optic canal lies deep within the orbit at its posterior pole. It passes through the lesser wing of the sphenoid posteriorly to the middle cranial fossa within the skull. The optic canal transmits the optic nerve (cranial nerve II) and the ophthalmic artery.
4 The superior orbital fissure is a cleft between the lesser wing and the greater wing of the sphenoid. It is the superior limb of a sideways V-shaped opening and communicates posteriorly with the middle cranial fossa. It transmits the ophthalmic veins and cranial nerves III, IV, V-1, and VI.
5 The inferior orbital fissure is the lower limb of the sideways V. It is a cleft between the greater wing of the sphenoid and the maxilla and permits communication between the posterior infratemporal region and the orbit. The inferior orbital fissure transmits the infraorbital nerve and vessels via the infraorbital groove.
6 The posterior and anterior ethmoidal foramina lie on the medial wall between the frontal bone above and the ethmoid bone below. The foramina transmit the posterior and anterior ethmoidal nerves and vessels from the orbit to the ethmoidal air sinus.
Nasal Region
3 The midline bony nasal septum is formed by the vomer bone inferiorly and the vertical plate of the ethmoid bone superiorly. The septum is usually deviated to the left.
4 The nasal conchae are three pairs of scroll-like, delicate shelves that hang into the nasal cavity from the lateral wall. The superior and middle conchae are part of the ethmoid; the inferior concha is a separate bone.
The Jaws
1 The teeth are the obvious features of this region. There are 32 teeth in the complete adult dentition and 20 in children’s. The teeth are considered with the features of the oral cavity in Chapter 7.
2 The alveolar processes of the maxilla and mandible are the bony sockets that support the teeth. The facial aspect of the anterior maxillary alveolar process presents a very thin plate of bone over the roots of the anterior maxillary teeth. This is important clinically. Injected local anesthetic solution over the root apex area diffuses readily through the thin bone to the nerves at the root apex and results in localized anesthesia. In the fabrication of dentures the ridges of bone overlying the anterior maxillary teeth roots are simulated by “festooning.”
3 The canine ridge of the maxilla is a particularly prominent ridge of bone overlying the long and prominent maxillary canine tooth. If the maxillary canines are missing because of a previous extraction or if they fail to erupt because of a lack of space (impaction), the canine ridges are not present and the face appears flatter on the affected side. The canine ridges are important in forming the corners of the mouth.
4 The incisive fossa of the maxilla is the depression overlying the maxillary incisor roots. In this area, the maxillary teeth can be individually anesthetized.
5 The canine fossa of the maxilla, paradoxically, is the depression overlying the premolar roots distal to the canine ridge. Injections in this area anesthetize the maxillary premolar teeth.
7 The incisive fossa of the mandible is the concavity just inferior to the mandibular incisors and above the mental protuberance.
8 The mental foramen of the mandible is an opening located at the level of the second mandibular premolar, midway between the alveolar crest and the inferior border of the mandible. It transmits the mental nerve and vessels. Note that the mouth of the foramen opens posteriorly and upward, rendering injections into the foramen difficult.
LATERAL VIEW
Bones
The bones seen from the lateral view are the frontal bone (single), parietal bones (paired), temporal bones (paired), occipital bone (single), greater wings of the sphenoid bone (paired processes of a single bone), zygomatic bones (paired), maxillae (paired), nasal bones (paired), lacrimal bones (paired), and the mandible (single). (Figure 6-4, A).
Features
The Cranial Vault Region
1 The vertex is the highest point on the superior margin of the cranial vault oriented in the Frankfort position.
2 Bregma is an anthropological landmark anterior to the vertex. It is the point where the right and left parietal bones meet the frontal bone.
3 The temporal lines (superior and inferior) are two curved lines that arch upward and backward from the zygomatic process of the frontal bone, travel across the frontal bone and parietal bone, and then recurve downward and forward on the parietal bone to end inferiorly on the temporal bone. They represent the superior limit of the curved origin of the temporalis muscle.
4 Pterion is an anthropological landmark named for the H-shaped junction of the frontal, parietal, greater wing of sphenoid, and temporal bones on the lateral aspect of the cranial vault.
5 The temporal fossa is the slightly depressed area on the anterolateral aspect of the skull inferior to the temporal lines. It is filled in by the temporalis muscle.
6 The external auditory meatus of the temporal bone forms the entrance to the middle ear within the temporal bone.
7 The mastoid process is a lump of bone immediately posterior and inferior to the external auditory meatus. It serves as part of the insertion of the sternomastoid muscle.
8 The mandibular, or glenoid, fossa is the depression anterior to the external auditory meatus. It houses the head, or condyle, of the mandible.
9 The postglenoid tubercle is a small wedge of bone that intervenes between the mandibular fossa and the external auditory meatus.
The Facial Region
4 The gonial angle is the junction of the posterior border of the ramus and the inferior border of the body of the mandible.
5 The condyle, or condylar head, of the mandible is a roller-shaped process that articulates with the mandibular fossa of the temporal bone above.
7 The coronoid process is the sharp, bladelike, and anterosuperior process of the ramus. With the teeth occluded, the tip of the coronoid process is obscured by the zygomatic arch.
9 The sharp anterior border of the ramus continues inferiorly onto the lateral aspect of the body of the mandible as the external oblique line. It continues obliquely downward and forward to end near the chin area on the inferior aspect of the body as the mental tubercle.
The Infratemporal Region
The infratemporal region is obscured by the ramus of the mandible, which serves as the lateral wall of the region (Figure 6-5). With the mandible removed, the limits of the infratemporal region can be further delineated. The infratemporal region is separated from the temporal fossa above by an indistinct infratemporal crest.
Figure 6-5 Lateral view of skull with mandible removed to expose infratemporal region. A, Bones. B, Features.
Bones and Walls
Features
1 The inferior orbital fissure is a cleft between the greater wing of the sphenoid above and the maxilla below. It communicates with the orbit anteriorly. The infraorbital groove continues anteriorly from the midpoint of the lower margin of the fissure.
2 The pterygomaxillary fissure is a cleft between the pterygoid process of the sphenoid bone behind and the maxilla in front. The cleft communicates with the deeper pterygopalatine fossa, which is discussed in detail in Chapter 7.
3 The posterior superior alveolar foramina are centered in a grouping on the convexity (tuberosity) of the posterior or infratemporal aspect of the maxilla. They transmit the posterior superior alveolar nerves and vessels.
4 The pyramidal process of the palatine bone is the only portion of the palatine bone visible externally. It intervenes inferiorly between the converging pterygoid process and the maxilla.
5 The maxillary tubercle is the rounded elevation of bone distal to the last maxillary molar. Occasionally, the maxillary tuberosity is invaded by the maxillary air sinus from within and presents a hazard during third molar removal. Fracture of the tubercle could cause a communication between the oral cavity and the maxillary sinus (oroantral fistula).
6 The zygomatic process of the maxilla is the anterior continuation of the inferior border of the zygomatic arch that curves downward to end as a buttress over the roots of the maxillary first molar. It is the contribution of the maxilla to the zygomatic arch, and it considerably thickens the buccal bone overlying the roots of the maxillary first molar. This bony thickness may present a problem in achieving profound local anesthesia (see Chapter 11).
SUPERIOR VIEW
Bones
The superior aspect of the skull presents a somewhat egg-shaped outline with the small end anteriorly (Figure 6-6). Only four bones are seen from this view: the frontal bone anteriorly, the right and left parietal bones laterally, and the occipital bone posteriorly.
Features
3 The lambdoidal suture separates the parietal and temporal bones from the occipital bone (Figure 6-7).
4 Bregma is an anthropological landmark marked by the intersection of the sagittal and coronal sutures.
5 Lambda is an anthropological landmark located at the intersection of the lambdoid and sagittal sutures.
7 The parietal eminences are the more prominent, posterolateral bulges. Both sets of eminences represent initial sites of ossification of the frontal and parietal bones.
POSTERIOR VIEW
The most prominent feature of the posterior view is the rounded posterior pole of the skull, called the occiput (see Figure 6-7). Hence, the area is often referred to as the occipital area.
Features
1 The inion, or external occipital protuberance, is a lump of bone of varying size located in the midline between the lambda and the foramen magnum.
2 The superior nuchal line radiates laterally on either side of the inion in a curved arc ending at the mastoid process on either side.
4 The highest nuchal line is an even less distinct parallel line just above the superior nuchal line.
5 The external occipital crest runs down in the median plane from the inion to the foramen magnum. It provides a bony attachment for the ligamentum nuchae.
6 The mastoid process of the temporal bone is a palpable bony lump that provides a site of insertion for the sternomastoid muscle.
8 The occipital groove is created by the occipital artery making its way onto the occiput of the skull.
BASAL OR INFERIOR VIEW
Bones
The bones seen from the basal aspect (Figure 6-8, A) are the right and left maxillae (palatal processes), the right and left palatine bones (palatal processes), the sphenoid bone (body, pterygoid processes, and greater wings), the vomer, the right and left temporal bones, and the occipital bone.
Features
Anterior Region
1 The maxillary teeth and alveolar process lie in a U-shaped arrangement, which ends posteriorly as the maxillary tubercles (Figure 6-8, B).
2 The bony palate lies within the alveolar process and consists of two bones. The palatal processes of the maxillae form the anterior two thirds of the palate; the palatal processes of the palatine bones form the posterior third, ending as a double crescent-shaped, posterior-free border.
3 The posterior nasal spine is a midline posterior projection from the posterior border of the bony palate.
4 The incisive canal opens via the incisive foramen onto the palate distal to the central incisors. It transmits the nasopalatine nerve and vessels.
5 The greater palatine canal opens as the greater palatine foramen onto the palatal process of the palatine bone in line with the last maxillary molar. It transmits the greater palatine nerve and vessels.
6 The lesser palatine canal opens as the lesser palatine foramen posterior to the greater palatine foramen. It transmits the lesser palatine nerve and vessels.
7 The posterior choanae, or posterior nasal apertures, represent the posterior limits of the nasal cavity. The free edge of the bony nasal septum formed by the vomer divides the nasal cavity into right and left chambers.
8 The palatinovaginal canal runs between the body and the vaginal process of the sphenoid bone. It transmits the pharyngeal branch of the pterygopalatine ganglion posteriorly to the superior portion of the pharynx.
Intermediate Area
From lateral to medial, the features encountered in the intermediate area are as follows (see Figures 6-8 and 6-9):
3 The tympanic plate, which forms the anterior wall of the external auditory meatus and the posterior, nonfunctioning wall of the mandibular fossa
4 The squamotympanic fissure, which separates the mandibular fossa of the squamous portion of the temporal bone from the tympanic plate of the temporal bone. (A complicating feature within this suture is an intervening small bit of petrous temporal bone giving rise to the petrotympanic fissure, which transmits the chorda tympani nerve, a branch of cranial nerve VII.)
5 The spine of the sphenoid, a pointed projection medial to the mandibular fossa; a ligament joins the spine to the lingula of the mandible (sphenomandibular ligament)
6 The foramen spinosum, a small opening just anterior to the spine of the sphenoid, which transmits the middle meningeal artery from the base of the skull to the interior of the skull
7 The gutter for the auditory (pharyngotympanic) tube, which lies medial to the spine of the sphenoid. (The membranous portion of the auditory tube lies within this gutter. It runs from the middle ear deep within the temporal bone to the area of the nasopharynx.)
Structures Straddling the Posterior Line
From lateral to medial the features crossed by the posterior transverse line are as follows:
1 The mastoid process of the temporal bone, which provides the site of insertion for the sternomastoid muscle
2 The styloid process, a long, slender, needlelike process that points downward, forward, and medially that is joined to the lesser horn of the hyoid bone by the stylohyoid ligament; also provides attachment for several muscles
3 The stylomastoid foramen, which lies between the styloid and mastoid processes, allowing passage of the facial nerve (cranial nerve VII) from within the temporal bone
4 The jugular foramen, a large opening through which the internal jugular vein and cranial nerves IX, X, and XI pass
5 The entrance to the carotid canal, which is immediately anterior to the jugular foramen and separated from the jugular foramen only by a small wedge of bone. (Passing through the canal is the internal carotid artery on its way to the brain within the skull.)
INTERNAL ASPECT OF THE BASE
To expose the internal features of the skull, the skullcap (calvaria) is sawn off and removed.
Bones
The bones seen in this view are the frontal bone, the ethmoid bone, the sphenoid bone (including the body, lesser wings, and greater wings), the right and left temporal bones, and the occipital bone (Figure 6-10, A).
Regions
The internal configuration of the base of the skull is conveniently tiered and named as three cranial fossae (Figure 6-10, B):
1 The anterior cranial fossa is bounded by the upward sweep of the frontal bone anteriorly and the sharp free borders of the lesser wings of the sphenoid bone posteriorly. It contains the frontal lobes of the cerebrum.
2 The middle cranial fossa is bounded posteriorly by the paramedian dorsum sellae and by two oblique petrous temporal ridges, which sweep back and laterally from the dorsum sellae. The middle cranial fossa accommodates the hypophysis cerebri in the median plane and the temporal lobes of the brain laterally.
Features (Figures 6-10,C and 6-11)
1 The frontal crest of the frontal bone is a midline bony extension into the anterior cranial fossa anteriorly. It serves as the bony attachment for the falx cerebri.
2 The crista galli (“crest of the rooster”) of the ethmoid bone projects upward in the midline as a posterior extension of the frontal crest. It also provides attachment for the falx cerebri.
3 The cribriform plates of the ethmoid bone are perforated areas on either side of the crista galli. The small perforations communicate with the roof of the nasal cavity below and transmit the olfactory nerve (cranial nerve I).
4 The foramen cecum is a blind-ended opening between the crista galli and the frontal crest. In the fetal period this opening is patent and transmits an emissary vein.
Middle Cranial Fossa
Features of the middle cranial fossa include the following:
2 The hypophyseal (pituitary) fossa, which is a concavity that resembles a rider’s seat in the saddle and accommodates the hypophysis cerebri, or pituitary gland
3 The optic canals, which pass obliquely laterally and forward to the orbits and transmit the optic nerves (cranial nerve II) and the ophthalmic arteries
5 The anterior clinoid processes, which extend posteriorly from the lesser wing of the sphenoid bone and provide attachments for dura
6 The posterior clinoid processes, which extend posteriorly from either end of the dorsum sellae and likewise provide dural attachments
7 The foramen lacerum, which provides an opening on either side of the hypophyseal fossa for the entrance of the internal carotid artery. (If a pipe cleaner is introduced into the carotid canal, it can be seen that the carotid canal communicates with the foramen lacerum as a “T-junction.” The internal carotid artery passes through the canal, continues superiorly through the upper portion of the foramen lacerum, and enters the middle cranial fossa. The artery marks a groove on the lateral aspect of the sella turcica as it travels anteriorly within the skull.)
8 The depression for the trigeminal ganglion, which sits near the apex of the petrous portion of the temporal bone, where it butts against the body of the sphenoid bone
9 The superior orbital fissure, is an inverted, comma-shaped gap between the greater and lesser wings of the sphenoid bone found under the cover of the lesser wing that communicates with the orbit anteriorly and through which pass the ophthalmic veins and cranial nerves III, IV, VI, and V-1
10 The foramen rotundum, a round opening posterior to the inferior orbital fissure that opens obliquely forward and inferiorly to lead to the pterygopalatine fossa; transmits the maxillary nerve (cranial nerve V-2)
11 The foramen ovale, an oval-shaped opening posterior to the foramen rotundum that leads to the infratemporal region below the skull and transmits the mandibular nerve (cranial nerve V-3)
12 The foramen spinosum, which lies immediately behind and lateral to the foramen ovale, is a small, round opening leading from the infratemporal region below that transmits the middle meningeal artery. (The grooves of the artery may be followed laterally and superiorly within the skull to the point where the artery divides into an anterior and a posterior branch.)
13 The hiatus for the greater petrosal nerve, which exits from the anterior slope of the petrous temporal ridge (A groove can be followed to the foramen lacerum. The hiatus and groove transmit the greater petrosal nerve from the depths of the petrous temporal bone to the foramen lacerum of the middle cranial fossa.)
Posterior Cranial Fossa
1 The superior petrosal sulcus is a narrow, shallow groove that runs along the crest of the petrous temporal ridge and is occupied by the superior petrosal venous sinus.
2 The inferior petrosal sulcus runs from the lateral aspect of the dorsum sellae downward and backward between the petrous temporal bone and the basiocciput. It carries the inferior petrosal venous sinus toward the jugular foramen.
3 The clivus is the body of the sphenoid bone behind the dorsum sellae, which fuses to the basiocciput at the spheno-occipital synchondrosis during adolescence. The clivus slopes posteriorly and inferiorly to end as the anterior margin of the foramen magnum.
4 The foramen magnum is a large, oval-shaped opening through which the spinal cord is continuous with the brainstem above.
5 The anterior condylar canal (hypoglossal canal) is found on the lateral margin of the foramen magnum anterior to the occipital condyle. It runs obliquely anteriorly and laterally, carrying the hypoglossal nerve (cranial nerve XII) to the base of the skull below.
6 The jugular foramen is a large opening lateral to the foramen magnum that leads to the base of the skull below. It transmits the internal jugular vein and cranial nerves IX, X, and XI.
7 The internal auditory meatus lies on the posterior slope of the petrous temporal ridge just above the jugular foramen. Passing through this opening to the middle ear region within the temporal bone are cranial nerves VII and VIII.
8 The internal occipital protuberance is an internal projection of bone at the posterior pole of the internal aspect of the skull.
9 The area of the confluence is just above the internal occipital protuberance, and it represents the junction of the lateral venous sinuses.
10 The transverse sulcus runs laterally from the confluence, curves anteriorly, and on encountering the petrous temporal ridge, curves medially as the sigmoid sulcus. The transverse sulcus contains the lateral venous sinus.
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