Chapter 42: The skull and hyoid bone
The skeleton of the head and neck consists of the skull and hyoid bone and the cervical vertebrae (which have already been described). The skull is complex and learning the skull is facilitated by having one available while studying. Click here to watch a quicktime streaming video presentation on the skull.
Skull
The skull, or cranium, protects the brain and the organs of special sensation, allows the passage of air and food, and supports the teeth. It consists of a series of bones, mostly united at immovable joints. The mandible, however, can move freely at a synovial articulation, the temporomandibular joint. Some bones of the skull are paired, whereas others are not. Each consists of external and internal tables of compact bone and a middle spongy layer, the diploe. The skull is covered by periosteum (pericranium) and lined by dura (endocranium). The top part (skull cap) is termed the calvaria (calvarium is incorrect) (fig. 42-1). The various bones are shown in figures 42-1, 42-2, 42-3 and 42-8.
The fibrous joints between the bones are termed sutures, and they allow growth at the calvaria. With increasing age, many sutures disappear by osseous fusion. Bony areas termed sutural bones are frequent in some sutures.
When a subject is in the anatomical position, the skull is oriented so that the lower margins of the orbits and the upper margins of the external acoustic meatuses are horizontal, i.e., in the orbitomeatal plane.
The most frequently used views in radiography of the head are the right and left lateral (fig. 42-4), postero-anterior (fig. 42-5), and anteroposterior (fig. 42-6). Calcified areas may be found normally in the pineal body and in the choroid plexuses of the lateral ventricles.
Superior, posterior, and anterior aspects of skull
Superior Aspect (fig. 42-1).
The two parietal bones are separated by the sagittal suture. The frontal and parietal bones are separated by the coronal suture. The occipital and parietal bones are separated by the lambdoid suture. An emissary opening, the parietal foramen, may be found on one or both sides of the sagittal suture posteriorly.
Posterior Aspect (fig. 42-2).
The parietal and occipital bones meet the mastoid part of the temporal bone laterally. The external occipital protuberance is a median projection that is palpable in vivo. Its center is termed the inion. Superior nuchal lines extend laterally from the protuberance and mark the upper limit of the neck.
Anterior Aspect (see figs. 42-3 and 42-5).
This aspect presents the forehead, the orbits, the prominence of the cheek, the bony external nose, and the upper and lower jaws.
The skeleton of the forehead is formed by the frontal bone, which articulates below with the nasal bones medially and with the zygomatic bones laterally. The bony area between the eyebrows is the glabella, from which an elevation, the superciliary arch, extends laterally on each side. The two halves of the frontal bone are separated until the age of about six years by the frontal suture, which sometimes persists into adulthood as the metopic suture.
The orbits are the bony cavities that contain the eyes, and they are described in a later chapter. At its junction with the face, the rim of the orbit is described as superior, lateral, inferior, and medial margins. The superior margin presents either a supra-orbital notch or foramen medially for the corresponding nerve and vessels. Laterally, the superior margin ends in the zygomatic process of the frontal bone, which is readily palpable in vivo. The lateral margin is formed by the frontal and zygomatic bones, and the inferior margin by the zygomatic bone and the maxilla. Below the inferior margin, the maxilla presents the infraorbital foramen, which transmits the corresponding nerve and artery (fig. 42-7). The medial margin, less clearly defined, is formed by the maxilla, the lacrimal and frontal bones.
The prominence of the cheek is formed by the zygomatic (formerly malar) bone (fig. 42-8), which contributes to the face, the orbit, and the temporal fossa. It presents two processes, which articulate with the zygomatic processes of the frontal and temporal bones, respectively.
The bony part of the external nose is formed by the nasal bones and the maxillae. Its opening, to which the cartilaginous part of the nose is anchored in the intact state, is the piriform aperture. Through this the nasal cavity can be seen, divided by the nasal septum into right and left portions. The septum is composed of cartilage in front and bone (ethmoid and vomer) behind (fig. 52-2B). Each lateral wall of the nasal cavity presents three curled bony plates termed conchae (formerly turbinates).
The upper jaw is composed of the two maxillae. The growth of the maxillae is responsible for the vertical elongation of the face between the ages of six and twelve years. Each maxilla (see fig. 42-7) consists of a body, which contains the maxillary sinus; a zygomatic process, which extends laterally to articulate with the zygomatic bone; a frontal process, which projects upward to articulate with the frontal bone; a palatine process, which extends horizontally to meet its fellow of the other side and contribute to the palate; and an alveolar process, which carries the upper teeth.
The body of the maxilla contributes to the lateral wall of the nasal cavity, the floor of the orbit, the front wall of the infratemporal fossa, and the face. About 1 cm below the lower margin of the orbit, the infra-orbital foramen transmits the infra-orbital nerve and artery. The two maxillae are united at the intermaxillary suture.
The lower jaw, or mandible, which carries the lower teeth in its alveolar part, is described later.
Lateral aspect of skull (figs. 42-4 and 42-8)
The lateral aspect of the skull includes portions of the temporal bone and the temporal and infratemporal fossae.
Temporal Bone.
The temporal bone comprises squamous, tympanic, styloid, mastoid, and petrous parts (fig. 42-9).
Squamous part.
The parietal bone articulates below with the squamous part ofthe temporal bone (see fig. 42-8). From the squamous portion, the zygomatic process (or zygoma) projects forward to meet the zygomatic bone and form the zygomatic arch, which is readily palpable in vivo. The upper border of the zygomatic arch corresponds to the lower limit of the cerebral hemisphere (see fig. 43-8). The lower border of the zygomatic arch, which gives origin to the masseter muscle, can be traced backward to the tubercle of the root of the zygoma, behind which the head of the mandible is lodged in the mandibular fossa. The external acoustic meatus, situated behind the head of the mandible, leads toward the middle ear, from which it is separated in the intact state by the tympanic membrane. The roof and adjacent part of the posterior wall of the meatus are formed by the squamous part of the temporal bone, whereas the other walls are formed by the tympanic part. A small depression, the suprameatal triangle, lies immediately above and behind the meatus. The mastoid antrum, a cavity in the temporal bone, lies about 1 cm medial to the suprameatal triangle.
Tympanic part.
The floor and anterior wall of the meatus are formed by a curved lamina, the tympanic plate. In children, it is merely a tympanic ring.
Styloid part.
The styloid process, of variable length (fig. 42-9), extends downward and forward from under cover of the tympanic plate. The stylohyoid ligament on each side suspends the hyoid bone from the skull.
Mastoid part.
The mastoid is the back portion of the temporal. In the adult, the mastoid part generally contains air spaces, the mastoid air cells, which communicate with the middle ear by way of the mastoid antrum. The mastoid process, easily felt in vivo, projects downward and gives attachment to muscles (fig. 42-10).
Petrous part.
The petrous part is placed deeply and is described later.
Temporal Fossa.
The temporal line, to which the temporal fascia is attached, arches across the frontal and parietal bones (see fig. 42-8) and joins a ridge (supramastoid crest, see fig. 42-9) on the temporal bone. The temporal fossa, in which the temporal muscle is located, is bounded by the temporal line above and the zygomatic arch below. The floor of the fossa includes portions of the parietal and frontal bones, the greater wing of the sphenoid bone, and the squamous part of the temporal bone. The area where these four bones approach each other is known as the pterion (see fig. 42-8). The pterion overlies the anterior branch of the middle meningeal artery on the internal aspect of the skull, and it corresponds also to the stem of the lateral sulcus of the brain. The center of the pterion is about 4 cm above the midpoint of the zygomatic arch and nearly the same distance behind the zygomatic process of the frontal bone (see fig. 43-8).
Infratemporal Fossa (fig. 42-11).
The interval between the zygomatic arch and the rest of the skull is traversed by the temporal muscle, which thereby leaves the temporal fossa and enters the infratemporal fossa. The infratemporal fossa, an irregular space behind the maxilla, is limited medially by the lateral pterygoid plate of the sphenoid bone (see fig. 48-3) and laterally by the ramus of the mandible. Medial to its communication with the temporal fossa, the roof of the infratemporal fossa is formed by the infratemporal surface of the greater wing of the sphenoid bone (fig. 42-12). The infratemporal fossa contains the lower part of the temporalis and the lateral and medial pterygoid muscles; the maxillary artery and its branches and the pterygoid venous plexus; and the mandibular, maxillary, and chorda tympani nerves. The infratemporal fossa communicates with the orbit (table 42-1) through the inferior orbital fissure, which is continuous behind with the pterygomaxillary fissure. The infratemporal fossa communicates with the pterygopalatine fossa through the pterygomaxillary fissure, which is between the lateral pterygoid plate and the maxilla and which transmits the maxillary artery. The pterygopalatine fossa, between the pterygoid plates and the palatine bone, is below the apex of the orbit, and it contains the maxillary artery and nerve. It communicates with the nasal cavity through the sphenopalatine foramen (table 42-2).
Inferior aspect of skull (figs. 42-12 and 42-13)
Occipital Bone (fig. 42-14)
The lower surface of the base is formed behind by the occipital bone, which consists of four parts arranged around the foramen magnum: a squamous part behind, a lateral part on each side, and a basilar part in front. These four portions, separable at birth, become fused by the age of about six years. The foramen magnum is located midway between, and on a level with, the mastoid processes. Through it the posterior cranial fossa and contained brain become continuous with the vertebral canal and contained spinal cord. The foramen magnum contains the medulla oblongata, tonsils of the cerebellum, meninges, and the subarachnoid space; the spinal roots of the accessory nerve, meningeal branches (C.N.1 to 3), and sympathetic plexuses; the vertebral and anterior and posterior spinal arteries; and several ligaments (apical ligament of the dens, cruciform ligament of the atlas, and membrana tectoria).
Squamous part.
The squamous part of the occipital bone continues from the base onto the back of the skull. The external occipital crest, to which the ligamentum nuchae is attached, extends from the foramen magnum to the external occipital protuberance, which is readily palpable in vivo. Nuchal lines extend laterally and delimit areas of muscular insertion.
Lateral parts.
The lateral parts of the occipital bone present the occipital condyles at the sides of the foramen magnum. The condyles articulate with the lateral masses of the atlas and, through them, the weight of the head is transmitted to the vertebral column. The hypoglossal canal, which transmits the hypoglossal nerve, lies hidden above the front of each condyle. The jugular process extends laterally from each condyle to the temporal bone, and its concave anterior border is the posterior boundary of the jugular foramen. The transverse process of the atlas lies immediately below the jugular process and rarely may be fused with it.
Basilar part.
The basilar part of the occipital bone joins the sphenoid bone (fig. 42-15) at the spheno-occipital joint, which is cartilaginous until about puberty, when bony fusion occurs. [This is earlier than was previously believed (B. Ingervall and B. Thilander, Acta Odontol. Scand., 30:349-356, 1972; B. Melsen, Acta Anat., 83:112-118, 1972)]. In front of the foramen magnum, the ill-defined pharyngeal tubercle anchors the pharynx.
Goethe first pointed out clearly the resemblance of the occipital bone to a vertebra. The basilar part and foramen magnum represent the body and vertebral foramen, respectively; the squamous part, with its external occipital protuberance and crest, perhaps represents the laminae and spinous process; and the condyles and jugular processes are in series with the articular and transverse processes, respectively.
Temporal Bone (see figs. 42-9 and 42-19).
All five divisions of the temporal bone are seen on the lower surface of the base (see fig. 42-12). The temporal bone contains the middle and internal portions of the ear.
Squamous part.
The zygomatic process extends forward from the squamous part of the temporal bone to form the zygomatic arch. (see also) The mandibular fossa and articular tubercle are portions of the squamous part of the temporal bone.
Tympanic part.
The tympanic plate is separated from the head of the mandible by a portion of parotid gland. In the mandibular fossa, the tympanic plate is separated from the squamous part of the temporal bone by the squamotympanic fissure (see fig. 42-13), the medial portion of which is generally occupied by a small portion of the petrous part (tegmen tympani). The fissure is thereby divided into petrosquamous and petrotympanic fissures, and the latter allows the exit of the chorda tympani.
Styloid part.
The styloid process is separated from the mastoid process by the stylomastoid foramen, through which the facial nerve emerges.
Mastoid part.
The mastoid part is fused medially with the petrous part, from which it is developed. The mastoid process (see fig. 42-9) projects downward and is grooved medially by the mastoid notch for the digastric muscle (see fig. 42-10) and the groove for the occipital artery.
Petrous part.
The petrous part, shaped like a medially directed pyramid, contains the internal ear and contributes to the boundaries of the middle ear. Laterally, its base is fused with the rest of the temporal bone. The apex of the petrous part of the temporal bone is directed forward and medially between the sphenoid and occipital bones. The petrous part of the temporal bone presents anterior and posterior surfaces, which are described with the cranial cavity (see pp. 359 and 362), and an inferior surface, which is considered here. The jugular foramen is formed by the petrous part of the temporal bone jugular fossa) and by the occipital bone jugular notch) (see fig. 42-13). The jugular foramen is related to the carotid canal anteriorly, the styloid process laterally, the transverse process of the atlas posteriorly, and the hypoglossal canal medially. The foramen transmits the internal jugular vein (a continuation of the sigmoid sinus) and the glossopharyngeal, vagus, and accessory nerves, as well as a tributary (inferior petrosal sinus). The internal jugular is dilated (superior bulb) at its commencement. The tympanic nerve (from cranial nerve 9) and the auricular branch of cranial nerve 10 pierce the skull in or near the jugular fossa. The carotid canal, a tunnel in the petrous part of the temporal bone, transmits the internal carotid artery and its sympathetic plexus to the cranial cavity. The entrance to the canal lies immediately in front of the jugular foramen. The canal is close to the internal ear, and the beating of the artery after exertion may be heard as a thundering sound. Further medially, the quadrate area of the petrous part of the temporal bone gives origin to the levator veli palatini. The groove between the petrous part of the temporal bone and the greater wing of the sphenoid bone is occupied in vivo by the cartilaginous part of the auditory tube. The foramen lacerum is a jagged opening at the junction of the petrous part of the temporal bone and the sphenoid and occipital bones. It is closed by fibrous tissue in vivo and is a part of the prenatal cartilaginous skull rather than a foramen. The pterygoid canal passes from the anterior margin of the foramen lacerum to the pterygopalatine fossa.
Sphenoid Bone (fig. 42-15).
The sphenoid bone consists of a body and three pairs of processes or wings: greater wings, lesser wings, and pterygoid processes. The greater wing and pterygoid process are described here.
Greater wing.
The infratemporal surface of the greater wing (see figs. 42-12 and 42-13) forms the roof of the infratemporal fossa. It is continuous medially with the lateral pterygoid plate, and it presents two openings into the middle cranial fossa. The anterior and larger is the foramen ovale, which transmits the mandibular nerve. The posterior and smaller is the foramen spinosum (in front of a spur named the spine of the sphenoid), which transmits the middle meningeal vessels and the meningeal branch of the mandibular nerve. Medial to the foramen ovale and foramen spinosum, the sphenoid bone is separated from the petrous part of the temporal bone by a groove for the cartilaginous part of the auditory tube.
Pterygoid process.
The pterygoid process of each side projects downward from the greater wing, behind the maxilla. Each consists of a lateral and a medial plate separated by the pterygoid fossa. The medial pterygoid plate, to which the auditory tube is anchored, is prolonged as the pterygoid hamulus. Above, the medial plate presents the scaphoid fossa, which gives origin to the tensor veli palatini; the muscle hooks around the hamulus to gain the soft palate. The lateral pterygoid plate gives origin to the lateral and medial pterygoid muscles (see figs. 42-12 and 42-13).
Choanae.
The nasal cavities are continuous with the nasopharynx through the choanae. They are two large openings above the posterior margin of the bony palate and are separated from each other by the back of the nasal septum (vomer). Laterally, each is bounded by the medial pterygoid plate. Above is found an overlapping complex of vomer, palatine, and sphenoid bones (see fig. 42-12).
Bony Palate.
The bony palate (i.e., the skeleton of the hard palate) lies in the roof of the mouth and the floor of the nasal cavity. It is formed by the palatine processes of the maxillae in front and by the horizontal plates of the palatine bones behind (see fig. 42-12). These four processes are united by a cruciform suture (see fig. 42-13). The bony palate is covered below by mucoperiosteum. Behind the incisors, a depression (incisive fossa) allows the passage of nasopalatine nerves through incisive canals and foramina. The posterior border of the bony palate, which presents the posterior nasal spine, gives attachment to the soft palate (palatine aponeurosis). Posterolaterally, the bony palate allows the passage of palatine nerves and vessels through greater and lesser palatine foramina and canals. A cleft palate is one in which the right and left halves have failed to meet in the median plane during development.
Cranial cavity
The cranial cavity lodges the brain and its meninges, cranial nerves, and blood vessels. It is roofed by the skull cap, and its floor is the upper surface of the base of the skull. The floor of the cranial cavity is divisible into three "steps," known as the anterior, middle, and posterior cranial fossae, separated by two prominent bony ledges on each side: the "sphenoidal ridge" (the posterior border of the lesser wing of the sphenoid bone) in front and the "petrous ridge" (the superior border of the petrous part of the temporal bone) behind (fig. 42-17).
Calvaria (fig. 42-16).
In young persons, portions of the coronal, sagittal, and lambdoid sutures, as well as the emissary parietal foramina, can be seen. Digital impressions corresponding to gyri of the brain may be visible. A median sagittal groove for the superior sagittal sinus runs backward on the internal surface of the vault. Depressions termed granular pits are found on each side of the groove, and they lodge lateral lacunae and arachnoid granulations. Numerous vascular grooves for the meningeal vessels are present on the internal surface of the vault.
Anterior Cranial Fossa (fig. 42-17).
The frontal lobes of the brain rest on the ethmoid, frontal, and sphenoid bones. The crista galli projects up from the ethmoid bone and gives attachment to the falx cerebri. Behind and at each side of the crista galli, the cribriform plate of the ethmoid bone transmits the filaments of the olfactory nerves from the nasal mucosa to the olfactory bulbs, which lie on the plate. The ethmoid bone articulates behind with the jugum sphenoidale, a part of the sphenoid bone that unites the right and left lesser wings. Laterally, the orbital plate of the frontal bone roofs the orbit and ethmoidal air sinuses and articulates behind with the lesser wing of the sphenoid bones. The sphenoidal ridge ofthe lesser wing projects into the lateral sulcus of the brain and ends medially in the anterior clinoid process.
Middle Cranial Fossa.
The floor resembles a butterfly in that a smaller median part is expanded on each side. The body of the sphenoid bone, which is united to the occipital bone posteriorly, supports the hypophysis above. The features of the cranial fossae seen in a median section are the jugum sphenoidale, limbus sphenoidalis, chiasmatic groove, tuberculum sellae, hypophysial fossa, dorsum sellae, and clivus (fig. 42-18). The shallow chiasmatic groove is close to but does not lodge the optic chiasma. The optic canal for the optic nerve and ophthalmic artery leads forward and laterally into the orbit. It is bounded by the body of the sphenoid bone and the two roots of the lesser wing. The sella turcica is the upper surface of the body of the sphenoid bone between (and including) the tuberculum sellae (the posterior limit of the chiasmatic groove) and the dorsum sellae, which juts upward and presents a posterior clinoid process on each side. The seat of the' 'Turkish saddle" is the hypophysial fossa, which lodges the hypophysis (pituitary gland) and roofs the sphenoidal airsinuses. Laterally, the carotid groove ascends from the foramen lacerum, then runs forward along the side of the body of the sphenoid bone, and finally ascends medial to the anterior clinoid process. The carotid groove contains the internal carotid artery, embedded in the cavernous sinus. Near each end of the tuberculum sellae a middle clinoid process may be detectable; it is united by a ligament (in some cases by bone) to the anterior clinoid process (caroticoclinoid foramen).
The lateral part of the middle cranial fossa is formed by the greater wing of the sphenoid bone and by the squamous and petrous parts of the temporal bone. It lodges the temporal lobe of the brain. It is limited by the sphenoidal ridge in front and by the petrous ridge behind. These ridges are closely related to venous sinuses (sphenoparietal and superior petrosal, respectively). The superior orbital fissure is a slit between the greater and lesser wings of the sphenoid bone. It transmits the oculomotor, trochlear, and abducent nerves, and the branches of the ophthalmic nerve (see fig. 45-3). The foramen rotundum, immediately below the medial end of the superior orbital fissure, transmits the maxillary nerve to the pterygopalatine fossa. Behind the foramen rotundum, the foramen ovale transmits the mandibular nerve to the infratemporal fossa. Nearby, the foramen spinosum transmits the middle meningeal vessels, a groove for which can be traced laterally and forward. The superior orbital fissure, foramen rotundum, foramen ovale, and foramen spinosum are arranged in a crescent on the greater wing of the sphenoid bone (see fig. 42-15). Only the last two of these four openings can be seen on the lower surface of the base.
The anterior surface of the petrous part of the temporal bone (fig. 42-19) lodges the trigeminal ganglion on the trigeminal impression medially. The abducent nerve bends sharply forward across the apex of the petrous part, medial to the trigeminal ganglion. Lateral to the ganglion, a rounded elevation (arcuate eminence) indicates the underlying anterior semicircular canal. Nearby, the hiatus for the greater petrosal nerve proceeds toward the foramen lacerum. The tympanic cavity and mastoid antrum are roofed by the petrous part of the temporal bone, this part being called the tegmen tympani. The foramen lacerum, occupied by fibrous tissue in vivo, is at the junction of the petrous part and the sphenoid bone. It marks the end of the carotid canal and the beginning of the carotid groove.
Posterior Cranial Fossa.
The hindbrain (cerebellum, pons, and medulla oblongata) occupies the posterior cranial fossa, which is formed by the sphenoid, temporal, parietal, and occipital bones (see fig. 42-17). The fossa is limited above by a dural fold, the tentorium cerebelli, which is attached to the petrous ridge and which separates the cerebellum from the occipital lobes of the brain. Below, the foramen magnum is evident, and, above the margin of its front part, the hypoglossal canal on each side transmits cranial nerve 12. In front of the foramen magnum, the basilar part of the occipital bone ascends to meet the body of the sphenoid bone. This sloping surface, termed the clivus (see fig. 42-18), is related to the pons and medulla. Behind the foramen magnum, the internal occipital crest leads up to the internal occipital protuberance, near which the superior sagittal, straight, and transverse sinuses form a confluence (see fig. 43-21). A groove for the transverse sinus runs laterally on each side and then turns downward as the groove for the sigmoid sinus, which leads medially and forward into the jugular foramen (where the sigmoid sinus becomes the internal jugular vein), accompanied by cranial nerves 9, 10, and 11. The cerebellar fossa on each side lies between the transverse and sigmoid grooves and the foramen magnum. The occipital lobes of the brain lie on the occipital bone above the transverse sinuses.
The posterior surface of the petrous part of the temporal bone presents the internal acoustic meatus, which transmits the facial and vestibulocochlear nerves and the labyrinthine vessels. The internal meatus is almost directly medial to (in line with) the external acoustic meatus. Behind the internal meatus, a slit (the aqueduct of the vestibule) transmits the endolymphatic duct of the internal ear (see fig. 44-8). Below the internal meatus, a notch (the cochlear canaliculus) lodges the perilymphatic duct (aqueduct of the cochlea).
Mandible
The mandible (lower jaw) presents a body and a pair of rami (fig. 42-20). The obtuse angle of the mandible can readily be felt in vivo: its most prominent point is called the gonion. The mandible develops bilaterally, but bony union between the two halves occurs during the first postnatal year.
Body.
The U-shaped body (fig. 42-21) presents external and internal surfaces and superior and inferior borders. The line of fusion of the two halves of the mandible at the symphysis menti is generally visible on the external surface. An elevation below, termed the mental protuberance, leads laterally to a mental tubercle on each side, and, from this, the oblique line runs backward and upward to the anterior border of the ramus. The mental foramen, frequently below the second premolar, transmits the mental nerve and vessels. The upper border of the body of the mandible is the alveolar part and contains the lower teeth in sockets, or alveoli. The edge of the alveolar part is the alveolar arch. The lower border of the mandible is its base, on or behind which is a rough depression near the symphysis, the digastric fossa. The facial artery crosses the base (where its pulsations can be felt) a few centimeters in front of the angle. Near the symphysis, the internal surface shows an irregular elevation, the mental spine, which may consist of several genial tubercles. Further back, the mylohyoid line runs backward and upward to a point behind the third molar. Below the line, the submandibular fovea lodges the submandibular gland, whereas, further forward, the sublingual fovea (for the sublingual gland) lies above the mylohyoid line.
Ramus.
The ramus (fig. 42-21) is a quadrilateral plate with lateral and medial surfaces. The lateral surface gives insertion to the masseter. The medial surface shows the mandibular foramen, which leads downward and forward into the mandibular canal and transmits the inferior alveolar nerve and vessels. The foramen is limited medially by a projection, the lingula, to which the sphenomandibular ligament is attached. The mandibular canal gives off a side canal that opens at the mental foramen and then runs as far as the median plane. The mylohyoid groove (for the mylohyoid nerves and vessels) begins behind the lingula and descends to the submandibular fovea. The medial surface near the angle gives insertion to the medial pterygoid muscle. The sharp anterior border of the ramus can be felt inside the mouth. The rounded posterior border is related to the parotid gland. The concave upper border of the ramus, the mandibular notch, is bounded in front by the coronoid process, into which the temporalis is inserted. The notch is limited behind by the condylar process, that is, the head and neck of the mandible. The head, or condyle, covered with fibrocartilage, articulates indirectly with the squamous part of the temporal bone at the temporomandibular joint. The lateral end of the condyle can be felt in vivo. The neck gives insertion to the lateral pterygoid muscle.
Development of skull (fig. 42-22)
The vault and portions of the base ossify intramembranously, whereas most of the base ossifies endochondrally. Postnatally, growth of the skull takes place at sutures and at the spheno-occipital junction, nasal septum, and condylar processes of the mandible.
Fonticuli, or fontanelles, are temporary membranous areas that bridge the gaps between the angles or margins of some of the ossifying bones ofthe skull. Usually six are present at birth, and these are situated at the angles of the parietal bones. The anterior fontanelle, the largest, is commonly seen to pulsate (because of the cerebral arteries) and is readily palpable in an infant. It is usually obliterated by the age of two years. It may be used to palpate the position of the fetal head, to estimate abnormal intracranial pressure in infancy, to assess cranial development, and to obtain blood from the superior sagittal sinus.
Hyoid bone (fig. 42-23)
The hyoid bone lies in the front of the neck at the upper limit of the larynx. It is suspended from the skull by the stylohyoid ligaments. It presents a body and paired greater and lesser horns. Each greater horn or cornu projects backward and upward from the body, to which it is united by cartilage or bone. When the neck is relaxed, the two greater horns of the hyoid can be gripped between index finger and thumb, and it can be moved from side to side. Each lesser horn or cornu projects upward from the junction of the body and the greater horn and gives attachment to the stylohyoid ligament.
Additional reading
Enlow, D. H., Handbook of Facial Growth, W.B. Saunders Company, Philadelphia, 1975. A nicely illustrated account of the developing skull.
Questions
42-1 What is the cranium?
42-2 What is the orbitomeatal plane?
42-3 Where may calcified areas be found normally in the interior of the head?
42-4 What forms the skeleton of the nasal septum?
42-5 What are the main parts of the temporal bone?
42-6 How far inferiorly do the cerebral hemispheres extend in terms of craniocerebral topography?
42-7 What are the limits of the temporal fossa?
42-8 Where is the pterion and what is its importance?
42-9 Where is the infratemporal fossa?
42-10 What are the main parts of the occipital bone?
42-11 In which ways does the occipital bone resemble a vertebra?
42-12 To which part of the temporal bone do the mandibular fossa and articular tubercle belong?
42-13 What is the petrous part of the temporal bone?
42-14 What are the main parts of the sphenoid bone?
42-15 What lies below and between the petrous part of the temporal bone and the greater wing of the sphenoid bone?
42-16 What are the choanae?
42-17 What forms the bony palate?
42-18 What is cleft palate?
42-19 What forms the "sphenoidal ridge" and the "petrous ridge"?
42-20 What is the calvaria?
42-21 What is the meaning of sella turcica and clinoid processes?
42-22 What are the main parts of the mandible, and how does the ramus terminate above?
42-23 List some important sites of growth of the skull.
42-24 Where are the main fontanelles situated?
42-25 What are the main parts of the hyoid bone?
42-26 Review the cranial exits of the cranial nerves.
Figure legends
Figure 42-1 Superior aspect of the skull. Note that some portions of a suture are more serrated than others.
Figure 42-2 Posterior aspect of the skull. Note the two sutural bones on the lambdoid suture.
Figure 42-3 Anterior aspect of the skull. Observe that the supra-orbital notch, infra-orbital foramen, and mental foramen are approximately in a vertical line- They transmit branches of divisions 1,2, and 3 of the trigeminal nerve, respectively.
Figure 42-4 Lateral radiograph of the head (from Meschan, I., Normal Radiographic Anatomy, 2nd ed., W.B. Saunders Company, Philadelphia, 1959; Courtesy of the author.)
Figure 42-5 Postero-anterior radiograph (from Meschan.)
Figure 42-6 Anteroposterior radiograph (TowneÕs projection). The subject is supine, the back of the head rests on the film and the cental ray is directed slightly caudally from the forehead to the external occipital protruberance (from Meschan.)
Figure 42-7 Anterior surface of the right maxilla. The maxilla consists of a body and four processes: zygomatic, frontal, palatine, and alveolar.
Figure 42-8 Lateral aspect of the skull. The pterion is where the parietal and frontal bones, the greater wing of the sphenoid, and the squamous part of the temporal bone approach one another. The temporal fossa is bounded by the temporal lines above and the zygomatic arch below. The infratemporal fossa (see fig. 42-11) lies medial to the ramus of the mandible.
Figure 42-9 Lateral aspect of the right temporal bone. The styloid process is unusually long in this skull.
Figure 42-10 Lateral aspect of the occipitomastoid region of the skull, showing muscular attachments.
Figure 42-11 Medial wall of the right infratemporal fossa after removal of the mandible. The zygomatic arch is shown as if transparent; its borders are indicated by interrupted lines.
Figure 42-12 Photograph of the base of the skull. The features are identified in figure 42-13.
Figure 42-13 Keys to the previous figure (42-12). The upper drawing shows mainly formina and canals.
Figure 42-14 Inferior aspect of the occipital bone. The four chief parts-basilar, two lateral, and squamous-can be seen around the foramen magnum. The main muscular attachments are shown on the left side of the bone.
Figure 42-15 Superior aspect of the sphenoid and occipital bones. These two elements, separated by cartilage in the child, become united by bone at about puberty. The body, paired greater wings, and paired lesser wings of the sphenoid can be identified, but the pterygoid processes are not seen in this view. Note the crescentic row of openings in the greater wing: superior orbital fissure (arrow), foramen rotundum, foramen ovale, and foramen spinosum. Of these openings, only the last two would be visible from below (see fig. 42-13). The four chief parts of the occipital bone (basilar, two lateral, and squamous) can be seen around the foramen magnum. The area marked with an asterisk articulates with a corresponding area on the temporal bone. The wide gap between the greater wing of the sphenoid and the basilar part of the occipital bone would be occupied by the petrous part of the temporal bone (see fig. 42-19). The apical area at the junction of the three bones, however, remains cartilaginous and, in the dried skull, is known as the foramen lacerum.
Figure 42-16 Internal aspect of the calvaria.
Figure 42-17 Superior aspect of the base of the skull. The right temporal bone (see fig. 42-19) has been removed, thereby displaying the head of the mandible. The floor of the cranial cavity presents three "steps"- the anterior, middle, and posterior cranial fossae-separated by two ledges: (1) the lesser wings of the sphenoid and (2) the upper border of the petrous part of the temporal bone on each side.
Figure 42-18 Median section through the base of the skull, showing important features of the cranial fossae.
Figure 42-19 Superior aspect of the right temporal bone. (Cf. fig. 42-17.) The asterisk marks the portion of the petrous part (tegmen tympani) that turns downward into the squamotympanic fissure. The trigeminal impression, which lodges the trigeminal ganglion, is immediately behind the apex of the bone and the termination of the carotid canal.
Figure 42-20 Schemes of the mandible. A shows the muscular and alveolar parts added to a basic element. B shows the main structural portions. The area marked by the asterisk may be classified as a part of either the ramus or the body of the bone. (A is based on Symons.)
Figure 42-21 A and B, Right lateral aspect of the mandible. C and D, Medial aspect of the right half of the mandible. A and C show the main structural features. B and D show attachments and relations and are based on Frazer's Anatomy of the Human Skeleton.
Figure 42-22 The growth of the skull. A and B, Neonatal skull. C and D, Adult skull. The scale used for the adult is half that of the neonatal skull. The horizontal lines indicate the orbitomeatal plane. In the infant's skull, although the vault appears large, the facial region (chiefly below the horizontal line) is relatively small. The jaws, nasal cavities and paranasal sinuses are all small, and the orbits and teeth are close together. In the adult, however, the horizontal line approximately bisects the vertical height of the skull. Note the fontanelles in A and B. (Based chiefly on the work of J. C. Brash.)
Figure 42-23 The hyoid bone. A, Anterosuperior aspect, showing the parts of the bone. B and C, Anterosuperior and postero-inferior aspects of right half, showing attachments. (After Frazer.)