SKULL

SUMMARY OF PROJECTIONS

Icons in the Essential column indicate projections frequently performed in the United States and Canada. Students should be competent in these projections.

Skull

The skull rests on the superior aspect of the vertebral column. It is composed of 22 separate bones divided into two distinct groups: 8 cranial bones and 14 facial bones. The cranial bones are divided further into the calvaria and floor (Box 20-1). The cranial bones form a protective housing for the brain. The facial bones provide structure, shape, and support for the face. They also form a protective housing for the upper ends of the respiratory and digestive tracts and, with several of the cranial bones, form the orbital sockets for protection of the organs of sight. The hyoid bone is commonly discussed with this group of bones.

BOX 20-1 Skull bones

The bones of the skull are identified in Figs. 20-1 to 20-3. The 22 primary bones of the skull should be located and recognized in the different views before they are studied in greater detail.

Fig. 20-1 Anterior aspect of cranium.

Fig. 20-2 A, Lateral aspect of cranium. B, Superior aspect of cranial base.

Fig. 20-3 A, Lateral aspect of interior of cranium. B, Sagittal MRI of cranium showing contents and position of brain. Note bony protective housing.

The bones of the cranial vault are composed of two plates of compact tissue separated by an inner layer of spongy tissue called diploë. The outer plate, or table, is thicker than the inner table over most of the vault, and the thickness of the layer of spongy tissue varies considerably.

Except for the mandible, the bones of the cranium and face are joined by fibrous joints called sutures. The sutures are named coronal, sagittal, squamosal, and lambdoidal (see Figs. 20-1 and 20-2). The coronal suture is found between the frontal and parietal bones. The sagittal suture is located on the top of the head between the two parietal bones and just behind the coronal suture line (not visible in Figs. 20-1 and 20-2). The junction of the coronal and sagittal sutures is the bregma. Between the temporal bones and the parietal bones are the squamosal sutures. Between the occipital bone and the parietal bones is the lambdoidal suture. The lambda is the junction of the lambdoidal and sagittal sutures. On the lateral aspect of the skull, the junction of the parietal bone, squamosal suture, and greater wing of the sphenoid is the pterion, which overlies the middle meningeal artery. At the junction of the occipital bone, parietal bone, and mastoid portion of the temporal bone is the asterion.

In a newborn infant, the bones of the cranium are thin and not fully developed. They contain a small amount of calcium, are indistinctly marked, and present six areas of incomplete ossification called fontanels (Fig. 20-4). Two of the fontanels are situated in the midsagittal plane at the superior and posterior angles of the parietal bones. The anterior fontanel is located at the junction of the two parietal bones and the one frontal bone at the bregma. Posteriorly and in the midsagittal plane is the posterior fontanel, located at the point labeled lambda in Fig. 20-2. Two fontanels are also on each side at the inferior angles of the parietal bones. Each sphenoidal fontanel is found at the site of the pterion; the mastoid fontanels are found at the asteria. The posterior and sphenoidal fontanels normally close in the 1st and 3rd months after birth, and the anterior and mastoid fontanels close during the 2nd year of life.

Fig. 20-4 Fontanels of a newborn.

The cranium develops rapidly in size and density during the first 5 or 6 years, after which a gradual increase occurs until adult size and density are achieved, usually by the age of 12 years. The thickness and degree of mineralization in normal adult crania show comparatively little difference in radiopacity from person to person, and the atrophy of old age is less marked than in other regions of the body.

Internally, the cranial floor is divided into three regions: the anterior, middle, and posterior cranial fossae (see Fig. 20-2, B). The anterior cranial fossa extends from the anterior frontal bone to the lesser wings of the sphenoid. It is associated mainly with the frontal lobes of the cerebrum. The middle cranial fossa accommodates the temporal lobes and associated neurovascular structures and extends from the lesser wings of the sphenoid bone to the apices of the petrous portions of the temporal bones. The deep depression posterior to the petrous ridges is the posterior cranial fossa, which protects the cerebellum, pons, and medulla oblongata (see Fig. 20-3, B).

The average or so-called normal cranium is more or less oval in shape, wider in back than in front. The average cranium measures approximately 6 inches (15 cm) at its widest point from side to side, 7 inches (17.8 cm) at its longest point from front to back, and 9 inches (22 cm) at its deepest point from the vertex to the submental region. Crania vary in size and shape, with resultant variation in the position and relationship of internal parts.

Internal deviations from the norm are usually indicated by external deviations and can be estimated with a reasonable degree of accuracy. The length and width of the normally shaped head vary by 1 inch (2.5 cm). Any deviation from this relationship indicates a comparable change in the position and relationship of the internal structures. If the deviation involves more than a 5-degree change, it must be compensated for by a change in either part rotation or central ray angulation. This “rule” applies to all images except direct lateral projections. A ½-inch (1.3 cm) change in the 1-inch (2.5-cm) width-to-length measurement indicates an approximately 5-degree change in the direction of the internal parts with reference to the midsagittal plane.

It is important for the radiographer to understand cranial anatomy from the standpoint of the size, shape, position, and relationship of the component parts of the cranium so that estimations and compensations can be made for deviations from the norm.

Cranial Bones

FRONTAL BONE

The frontal bone has a vertical portion and horizontal portions. The vertical portion, called the frontal squama, forms the forehead and the anterior part of the vault. The horizontal portions form the orbital plates (roofs of the orbits), part of the roof of the nasal cavity, and the greater part of the anterior cranial fossa (Figs. 20-5 to 20-7).

Fig. 20-5 Anterior aspect of frontal bone.

Fig. 20-6 Lateral aspect of frontal bone.

Fig. 20-7 Inferior aspect of frontal bone.

On each side of the midsagittal plane of the superior portion of the squama is a rounded elevation called the frontal eminence. Below the frontal eminences, just above the supraorbital margins, are two arched ridges that correspond in position to the eyebrows. These ridges are called the superciliary arches. In the center of the supraorbital margin is an opening for nerves and blood vessels called the supraorbital foramen. The smooth elevation between the superciliary arches is termed the glabella.

The frontal sinuses (see Chapter 22) are situated between the two tables of the squama on each side of the midsagittal plane. These irregularly shaped sinuses are separated by a bony wall, which may be incomplete and usually deviates from the midline.

The squama articulates with the parietal bones at the coronal suture, the greater wing of the sphenoid bone at the frontosphenoidal suture, and the nasal bones at the frontonasal suture. The midpoint of the frontonasal suture is termed the nasion.

The frontal bone articulates with the right and left parietals, the sphenoid, and the ethmoid bones of the cranium.

The orbital plates of the horizontal portion of the frontal bone are separated by a notch called the ethmoidal notch. This notch receives the cribriform plate of the ethmoid bone. At the anterior edge of the ethmoidal notch is a small inferior projection of bone, the nasal spine, which is the superiormost component of the bony nasal septum. The posterior margins of the orbital plates articulate with the lesser wings of the sphenoid bone.

ETHMOID BONE

The ethmoid bone is a small, cube-shaped bone that consists of a horizontal plate; a vertical plate; and two light, spongy lateral masses called labyrinths (Figs. 20-8 to 20-11). Situated between the orbits, the ethmoid bone forms part of the anterior cranial fossa, the nasal cavity and orbital walls, and the bony nasal septum.

Fig. 20-8 Anterior aspect of ethmoid bone.

Fig. 20-9 Lateral aspect of ethmoid bone with labyrinth removed.

Fig. 20-10 Superior aspect of ethmoid bone.

Fig. 20-11 Axial CT scan of ethmoidal sinus and perpendicular plate.

The horizontal portion of the ethmoid bone, called the cribriform plate, is received into the ethmoidal notch of the frontal bone. The cribriform plate is perforated by many foramina for the transmission of olfactory nerves. The plate also has a thick, conical process, the crista galli, which projects superiorly from its anterior midline and serves as the anterior attachment for the falx cerebri.

The vertical portion of the ethmoid bone is called the perpendicular plate. This plate is a thin, flat bone that projects inferiorly from the inferior surface of the cribriform plate and, with the nasal spine, forms the superior portion of the bony septum of the nose.

The labyrinths contain the ethmoidal sinuses, or air cells. The cells of each side are arbitrarily divided into three groups: the anterior, middle, and posterior ethmoidal air cells. The walls of the labyrinths form a part of the medial walls of the orbits and a part of the lateral walls of the nasal cavity. Projecting inferiorly from each medial wall of the labyrinths are two thin, scroll-shaped processes called the superior and middle nasal conchae.

The ethmoid bone articulates with the frontal and sphenoid bones of the cranium.

PARIETAL BONES

The two parietal bones are square and have a convex external surface and a concave internal surface (Figs. 20-12 and 20-13). The parietal bones form a large portion of the sides of the cranium. They also form the posterior portion of the cranial roof by their articulation with each other at the sagittal suture in the midsagittal plane.

Fig. 20-12 External surface of parietal bone.

Fig. 20-13 Internal surface of parietal bone.

Each parietal bone presents a prominent bulge, called the parietal eminence, near the central portion of its external surface. In radiography, the width of the head should be measured at this point because it is the widest point of the head.

Each parietal bone articulates with the frontal, temporal, occipital, sphenoid, and opposite parietal bone of the cranium.

SPHENOID BONE

The sphenoid bone is an irregularly wedge-shaped bone that resembles a bat with its wings extended. It is situated in the base of the cranium anterior to the temporal bones and basilar part of the occipital bone (Figs. 20-14 to 20-16). The sphenoid bone consists of a body; two lesser wings and two greater wings, which project laterally from the sides of the body; and two pterygoid processes, which project inferiorly from each side of the inferior surface of the body.

Fig. 20-14 A, Superior aspect of sphenoid bone. B, Axial CT scan of sphenoid bone. (B, From Kelley LL, Petersen CM: Sectional anatomy for imaging professionals, ed 2, St Louis, 2007, Mosby.)

Fig. 20-15 A, Oblique aspect of upper and lateroposterior aspects of sphenoid bone (right lateral pterygoid lamina removed). B, Sella turcica of sphenoid bone, lateral view.

Fig. 20-16 Sagittal CT scan of sella turcica and sphenoid sinus.

The body of the sphenoid bone contains the two sphenoidal sinuses, which are incompletely separated by a median septum. The anterior surface of the body forms the posterior bony wall of the nasal cavity. The superior surface presents a deep depression called the sella turcica and contains a gland called the pituitary gland. The sella turcica lies in the midsagittal plane of the cranium at a point ¾ inch (1.9 cm) anterior to and ¾ inch (1.9 cm) superior to the level of the external acoustic meatus (EAM). The sella turcica is bounded anteriorly by the tuberculum sellae and posteriorly by the dorsum sellae, which bears the posterior clinoid processes. The slanted area of bone posterior and inferior to the dorsum sellae is continuous with the basilar portion of the occipital bone and is called the clivus. The clivus supports the pons. On either side of the sella turcica is a groove, the carotid sulcus, in which the internal carotid artery and cavernous sinus lie.

The optic groove extends across the anterior portion of the tuberculum sellae. The groove ends on each side at the optic canal. The optic canal is the opening into the apex of the orbit for the transmission of the optic nerve and ophthalmic artery. The actual opening is called the optic foramen.

The lesser wings are triangular in shape and nearly horizontal in position. They arise, one on each side, from the anterosuperior portion of the body of the sphenoid bone and project laterally, ending in sharp points. The lesser wings form the posteromedial portion of the roofs of the orbits, the posterior portion of the anterior cranial fossa, the upper margin of the superior orbital fissures, and the optic canals. The medial ends of their posterior borders form the anterior clinoid processes. Each process arises from two roots. The anterior (superior) root is thin and flat, and the posterior (inferior) root, referred to as the sphenoid strut, is thick and rounded. The circular opening between the two roots is the optic canal.

The greater wings arise from the sides of the body of the sphenoid bone and curve laterally, posteriorly, anteriorly, and superiorly. The greater wings form a part of the middle cranial fossa, the posterolateral walls of the orbits, the lower margin of the superior orbital sulci, and the greater part of the posterior margin of the inferior orbital sulci. The foramina rotundum, ovale, and spinosum are paired and are situated in the greater wings. Because these foramina transmit nerves and blood vessels, they are subject to radiologic investigation for the detection of erosive lesions of neurogenic or vascular origin.

The pterygoid processes arise from the lateral portions of the inferior surface of the body of the sphenoid bone and the medial portions of the inferior surfaces of the greater wings. These processes project inferiorly and curve laterally. Each pterygoid process consists of two plates of bone, the medial and lateral pterygoid laminae, which are fused at their superoanterior parts. The inferior extremity of the medial lamina possesses an elongated, hook-shaped process, the pterygoid hamulus, which makes it longer and narrower than the lateral lamina. The pterygoid processes articulate with the palatine bones anteriorly and with the wings of the vomer, where they enter into the formation of the nasal cavity.

The sphenoid bone articulates with each of the other seven bones of the cranium.

OCCIPITAL BONE

The occipital bone is situated at the posteroinferior part of the cranium. It forms the posterior half of the base of the cranium and the greater part of the posterior cranial fossa (Figs. 20-17 to 20-19). The occipital bone has four parts: the squama, which is saucer-shaped, being convex externally; two occipital condyles, which extend anteriorly, one on each side of the foramen magnum; and the basilar portion. The occipital bone also has a large aperture, the foramen magnum, through which the inferior portion of the medulla oblongata passes as it exits the cranial cavity and joins the spinal cord.

Fig. 20-17 A, External surface of occipital bone. B, Coronal MRI showing atlantooccipital joint. (B, Courtesy Siemens Medical Systems, Iselin, NJ.)

Fig. 20-18 Internal surface of occipital bone.

Fig. 20-19 Lateroinferior aspect of occipital bone.

The squama curves posteriorly and superiorly from the foramen magnum and is curved from side to side. It articulates with the parietal bones at the lambdoidal suture and with the mastoid portions of the temporal bones at the occipitomastoid sutures. On the external surface of the squama, midway between its summit and the foramen magnum, is a prominent process termed the external occipital protuberance, or inion, that corresponds in position with the internal occipital protuberance.

The occipital condyles project anteriorly, one from each side of the squama, for articulation with the atlas of the cervical spine. Part of each lateral portion curves medially to fuse with the basilar portion and complete the foramen magnum, and part of it projects laterally to form the jugular process. On the inferior surface of the curved parts, extending from the level of the middle of the foramen magnum anteriorly to the level of its anterior margin, reciprocally shaped condyles articulate with the superior facets of the atlas. These articulations, known as the occipitoatlantal joints, are the only bony articulations between the skull and the neck. The hypoglossal canals are found at the anterior ends of the condyles and transmit the hypoglossal nerves. At the posterior end of the condyles are the condylar canals, through which the emissary veins pass. The anterior portion of the occipital bone contains a deep notch that forms a part of the jugular foramen (see Fig. 20-2, B). The jugular foramen is an important large opening in the skull for two reasons: It allows blood to drain from the brain via the internal jugular vein, and it lets three cranial nerves pass through it.

The basilar portion of the occipital bone curves anteriorly and superiorly to its junction with the body of the sphenoid. In an adult, the basilar part of the occipital bone fuses with the body of the sphenoid bone, resulting in the formation of a continuous bone. The sloping surface of this junction between the dorsum sellae of the sphenoid bone and the basilar portion of the occipital bone is called the clivus.

The occipital bone articulates with the two parietals, the two temporal bones and the sphenoid of the cranium, and the first cervical vertebra.

TEMPORAL BONES

The temporal bones are irregular in shape and are situated on each side of the base of the cranium between the greater wings of the sphenoid bone and the occipital bone (Figs. 20-20 to 20-24). The temporal bones form a large part of the middle fossa of the cranium and a small part of the posterior fossa. Each temporal bone consists of a squamous portion, a tympanic portion, a styloid process, a zygomatic process, and a petromastoid portion (the mastoid and petrous portions) that contains the organs of hearing and balance.

Fig. 20-20 Lateral aspect of temporal bone.

Fig. 20-21 Internal surface of temporal bone.

Fig. 20-22 A, Anterior aspect of temporal bone in relation to surrounding structures. B, Coronal section through mastoid and petrous portions of temporal bone.

Fig. 20-23 Coronal CT scan through temporal bones. (Courtesy Karl Mockler, RT[R].)

Fig. 20-24 Axial CT scan of petrous portion at level of external auditory meatus. (From Kelley LL, Petersen CM: Sectional anatomy for imaging professionals, ed 2, St Louis, 2007, Mosby.)

The squamous portion is the thin upper portion of the temporal bone. It forms a part of the side wall of the cranium and has a prominent arched process, the zygomatic process, which projects anteriorly to articulate with the zygomatic bone of the face and complete the zygomatic arch. On the inferior border of the zygomatic process is a rounded eminence, the articular tubercle, which forms the anterior boundary of the mandibular fossa. The mandibular fossa receives the condyle of the mandible to form the temporomandibular joint (TMJ).

The tympanic portion is situated below the squama and in front of the mastoid and petrous portions of the temporal bone. This portion forms the anterior wall, inferior wall, and part of the posterior walls of the EAM. The EAM is approximately ½ inch (1.3 cm) in length and projects medially, anteriorly, and slightly superiorly.

The styloid process, a slender, pointed bone of variable length, projects inferiorly, anteriorly, and slightly medially from the inferior portion of the tympanic part of the temporal bone.

Petromastoid portion

The petrous and mastoid portions together are called the petromastoid portion. The mastoid portion, which forms the inferior, posterior part of the temporal bone, is prolonged into the conical mastoid process (see Figs. 20-22 and 20-24).

The mastoid portion articulates with the parietal bone at its superior border through the parietomastoid suture and with the occipital bone at its posterior border through the occipitomastoid suture, which is contiguous with the lambdoidal suture. The mastoid process varies considerably in size, depending on its pneumatization, and is larger in males than in females.

The first of the mastoid air cells to develop is situated at the upper anterior part of the process and is termed the mastoid antrum. This air cell is quite large and communicates with the tympanic cavity. Shortly before or after birth, smaller air cells begin to develop around the mastoid antrum and continue to increase in number and size until around puberty. The air cells vary considerably in size and number. Occasionally, they are absent altogether, in which case the mastoid process is solid bone and is usually small.

The petrous portion, often called the petrous pyramid, is conical or pyramidal and is the thickest, densest bone in the cranium. This part of the temporal bone contains the organs of hearing and balance. From its base at the squamous and mastoid portions, the petrous portion projects medially and anteriorly between the greater wing of the sphenoid bone and the occipital bone to the body of the sphenoid bone, with which its apex articulates. The internal carotid artery in the carotid canal enters the inferior aspect of the petrous portion, passes superior to the cochlea, then passes medially to exit the petrous apex. Near the petrous apex is a ragged foramen called the foramen lacerum. The carotid canal opens into this foramen, which contains the internal carotid artery (see Fig. 20-2, B). At the center of the posterior aspect of the petrous portion is the internal acoustic meatus (IAM), which transmits the vestibulocochlear and facial nerves. The upper border of the petrous portion is commonly referred to as the petrous ridge. The top of the ridge lies approximately at the level of an external radiography landmark called the top of ear attachment (TEA).

The temporal bone articulates with the parietal, occipital, and sphenoid bones of the cranium.

Ear

The ear is the organ of hearing and balance (Fig. 20-25). The essential parts of the ear are housed in the petrous portion of the temporal bone. The organs of hearing and equilibrium consist of three main divisions: the external ear, middle ear, and internal ear.

Fig. 20-25 A, Frontal view of face showing internal structures of ear (shaded area). B, External, middle, and internal ear.

EXTERNAL EAR

The external ear consists of two parts: (1) the auricle, the oval-shaped, fibrocartilaginous, sound-collecting organ situated on the side of the head, and (2) the external acoustic meatus (EAM), a sound-conducting canal. The superior attachment of the auricle is the top of ear attachment (TEA). The TEA is a reference point for positioning the lateral cervical spine. The auricle has a deep central depression, the concha, the lower part of which leads into the EAM. At its anterior margin, the auricle has a prominent cartilaginous lip, the tragus, which projects posteriorly over the entrance of the meatus. The outer rim of the ear is the helix. The EAM is about 1 inch (2.5 cm) long. The outer third of the canal wall is cartilaginous, and the inner two thirds are osseous. From the meatal orifice, the canal forms a slight curve as it passes medially and anteriorly in line with the axis of the IAM. The EAM ends at the tympanic membrane of the middle ear.

MIDDLE EAR

The middle ear is situated between the external ear and internal ear. The middle ear proper consists of (1) the tympanic membrane (or eardrum); (2) an irregularly shaped, air-containing compartment called the tympanic cavity; and (3) three small bones called the auditory ossicles (Fig. 20-26). The middle ear communicates with the mastoid antrum and auditory eustachian tube.

Fig. 20-26 Coronal CT scan through petrous portion of temporal bone showing middle and inner ear. (Courtesy Karl Mockler, RT[R].)

The tympanic membrane is a thin, concavoconvex, membranous disk with an elliptic shape. The disk, the convex surface of which is directed medially, is situated obliquely over the medial end of the EAM and serves as a partition between the external ear and middle ear. The function of the tympanic membrane is the transmission of sound vibrations.

The tympanic cavity is a narrow, irregularly shaped chamber that lies just posterior and medial to the mandibular fossa. The cavity is separated from the external ear by the tympanic membrane and from the internal ear by the bony labyrinth. The tympanic cavity communicates with the nasopharynx through the auditory (eustachian) tube, a passage by which air pressure in the middle ear is equalized with the pressure in the outside air passages. The auditory tube is about 1¼ inches (3 cm) long. From its entrance into the tympanic cavity, the auditory tube passes medially and inferiorly to its orifice on the lateral wall of the nasopharynx.

The mastoid antrum is the large air cavity situated in the temporal bone above the mastoid air cells and immediately behind the posterior wall of the middle ear.

The auditory ossicles, named for their shape, are the malleus (hammer), incus (anvil), and stapes (stirrup). These three delicate bones are articulated to permit vibratory motion. They bridge the middle ear cavity for the transmission of sound vibrations from the tympanic membrane to the internal ear. The handle of the malleus (the outermost ossicle) is attached to the tympanic membrane, and its head articulates with the incus (the central ossicle). The head of the stapes (the innermost ossicle) articulates with the incus, and its base is fitted into the oval window of the inner ear.

INTERNAL EAR

The internal ear contains the essential sensory apparatus of hearing and equilibrium and lies on the densest portion of the petrous portion immediately below the arcuate eminence. Composed of an irregularly shaped bony chamber called the bony labyrinth, the internal ear is housed within the bony chamber and is an intercommunicating system of ducts and sacs known as the membranous labyrinth. The bony labyrinth consists of three distinctly shaped parts: (1) a spiral-coiled, tubular part called the cochlea, which communicates with the middle ear through the membranous covering of the round window (see Fig. 20-26); (2) a small, ovoid central compartment behind the cochlea, known as the vestibule, which communicates with the middle ear via the oval window; and (3) three unequally sized semicircular canals that form right angles to one another and are called, according to their positions, the anterior, posterior, and lateral semicircular canals (Fig. 20-27). From its cranial orifice, the internal acoustic meatus (IAM) passes inferiorly and laterally for a distance of about ½ inch (1.3 cm). Through this canal, the cochlear and vestibular nerves pass from their fibers in the respective parts of the membranous labyrinth to the brain. The cochlea is used for hearing, and the vestibule and semicircular canals are involved with equilibrium.

Fig. 20-27 Coronal CT scan of petromastoid portion of temporal bone showing semicircular canals and mastoid air cells. (Courtesy Karl Mockler, RT[R].)

Facial Bones

NASAL BONES

The two small, thin nasal bones vary in size and shape in different individuals (Figs. 20-28 and 20-29). They form the superior bony wall (called the bridge of the nose) of the nasal cavity. The nasal bones articulate in the midsagittal plane, where at their posterosuperior surface they also articulate with the perpendicular plate of the ethmoid bone. They articulate with the frontal bone above and with the maxillae at the sides.

Fig. 20-28 Anterior aspect of skull showing facial bones.

Fig. 20-29 Lateral aspect of skull showing facial bones.

LACRIMAL BONES

The two lacrimal bones, which are the smallest bones in the skull, are very thin and are situated at the anterior part of the medial wall of the orbits between the labyrinth of the ethmoid bone and the maxilla (see Figs. 20-28 and 20-29). Together with the maxillae, the lacrimal bones form the lacrimal fossae, which accommodate the lacrimal sacs. Each lacrimal bone contains a lacrimal foramen through which a tear duct passes. Each lacrimal bone articulates with the frontal and ethmoid cranial bones and the maxilla and inferior nasal concha facial bones. The lacrimal bones can be seen on PA and lateral projections of the skull.

MAXILLARY BONES

The two maxillary bones are the largest of the immovable bones of the face (see Figs. 20-28 and 20-29). Each articulates with all other facial bones except the mandible. Each also articulates with the frontal and ethmoid bones of the cranium. The maxillary bones form part of the lateral walls and most of the floor of the nasal cavity, part of the floor of the orbital cavities, and three fourths of the roof of the mouth. Their zygomatic processes articulate with the zygomatic bones and assist in the formation of the prominence of the cheeks. The body of each maxilla contains a large, pyramidal cavity called the maxillary sinus, which empties into the nasal cavity. An infraorbital foramen is located under each orbit and serves as a passage through which the infraorbital nerve and artery reach the nose.

At their inferior borders, the maxillae possess a thick, spongy ridge called the alveolar process, which supports the roots of the teeth. In the anterior midsagittal plane at their junction with each other, the maxillary bones form a pointed, forward-projecting process called the anterior nasal spine. The midpoint of this prominence is called the acanthion.

ZYGOMATIC BONES

The zygomatic bones form the prominence of the cheeks and a part of the side wall and floor of the orbital cavities (see Figs. 20-28 and 20-29). A posteriorly extending temporal process unites with the zygomatic process of the temporal bone to form the zygomatic arch. The zygomatic bones articulate with the frontal bone superiorly, with the zygomatic process of the temporal bone at the side, with the maxilla anteriorly, and with the sphenoid bone posteriorly.

PALATINE BONES

The two palatine bones are L-shaped bones composed of vertical and horizontal plates. The horizontal plates articulate with the maxillae to complete the posterior fourth of the bony palate, or roof of the mouth (see Fig. 20-3). The vertical portions of the palatine bones extend upward between the maxillae and the pterygoid processes of the sphenoid bone in the posterior nasal cavity. The superior tips of the vertical portions of the palatine bones assist in forming the posteromedial bony orbit.

INFERIOR NASAL CONCHAE

The inferior nasal conchae extend diagonally and inferiorly from the lateral walls of the nasal cavity at approximately its lower third (see Fig. 20-28). They are long, narrow, and extremely thin; they curl laterally, which gives them a scroll-like appearance.

The upper two nasal conchae are processes of the ethmoid bone. The three nasal conchae project into and divide the lateral portion of the respective sides of the nasal cavity into superior, middle, and inferior meatus. They are covered with a mucous membrane to warm, moisten, and cleanse inhaled air.

VOMER

The vomer is a thin plate of bone situated in the midsagittal plane of the floor of the nasal cavity, where it forms the inferior part of the nasal septum (see Fig. 20-28). The anterior border of the vomer slants superiorly and posteriorly from the anterior nasal spine to the body of the sphenoid bone, with which its superior border articulates. The superior part of its anterior border articulates with the perpendicular plate of the ethmoid bone; its posterior border is free.

MANDIBLE

The mandible, the largest and densest bone of the face, consists of a curved horizontal portion, called the body, and two vertical portions, called the rami, which unite with the body at the angle of the mandible, or gonion (Fig. 20-30). At birth, the mandible consists of bilateral pieces held together by a fibrous symphysis that ossifies during the first year of life. At the site of ossification is a slight ridge that ends below in a triangular prominence, the mental protuberance. The symphysis is the most anterior and central part of the mandible. This is where the left and right halves of the mandible have fused.

Fig. 20-30 A, Anterior aspect of mandible. B, Lateral aspect of mandible.

The superior border of the body of the mandible consists of spongy bone, called the alveolar portion, which supports the roots of the teeth. Below the second premolar tooth, approximately halfway between the superior and inferior borders of the bone, is a small opening on each side for the transmission of nerves and blood vessels. These two openings are called the mental foramina.

The rami project superiorly at an obtuse angle to the body of the mandible, and their broad surface forms an angle of approximately 110 to 120 degrees. Each ramus presents two processes at its upper extremity, one coronoid and one condylar, which are separated by a concave area called the mandibular notch. The anterior process, the coronoid process, is thin and tapered and projects to a higher level than the posterior process. The condylar process consists of a constricted area, the neck, above which is a broad, thick, almost transversely placed condyle that articulates with the mandibular fossa of the temporal bone (Fig. 20-31). This articulation, the TMJ, slants posteriorly approximately 15 degrees and inferiorly and medially approximately 15 degrees. Radiographic projections, produced from the opposite side, must reverse these directions. In other words, the central ray angulation must be superior and anterior to coincide with the long axis of the joint. The TMJ is situated immediately in front of the EAM.

Fig. 20-31 CT scan of mandibular condyle situated in mandibular fossa. A, Coronal. B, Sagittal. (Courtesy Karl Mockler, RT[R].)

HYOID BONE

The hyoid bone is a small, U-shaped structure situated at the base of the tongue, where it is held in position in part by the stylohyoid ligaments extending from the styloid processes of the temporal bones (Fig. 20-32). Although the hyoid bone is an accessory bone of the axial skeleton, it is described in this chapter because of its connection with the temporal bones. The hyoid is the only bone in the body that does not articulate with any other bone.

Fig. 20-32 Anterior aspect of hyoid.

The hyoid bone is divided into a body, two greater cornua, and two lesser cornua. The bone serves as an attachment for certain muscles of the larynx and tongue and is easily palpated just above the larynx.

ORBITS

Each orbit is composed of seven different bones (Fig. 20-33). Three of these are cranial bones: frontal, sphenoid, and ethmoid. The other four bones are the facial bones: maxilla, zygoma, lacrimal, and palatine. The circumference of the orbit, or outer rim area, is composed of three of the seven bones—the frontal, zygoma, and maxilla. The remaining four bones compose most of the posterior aspect of the orbit.