|SECTION I||BONES OF THE HUMAN SKULL (WITH EMPHASIS ON THE SPHENOID, TEMPORAL, MAXILLA, AND MANDIBLE BONES)|
OBJECTIVES OF THIS SECTION
This section is designed to prepare the learner to perform the following:
- Describe and identify each bone seen on an intact human skull.
- Describe and identify each bony structure highlighted in bold in this chapter. (Emphasis is placed on structures of the mandible, maxillae, temporal, and sphenoid bones.)
- Describe and identify the location of the attachment of chewing muscles and ligaments that are attached to the bones of the skull.
- Describe and identify the foramen of the nerves and arteries that supply the teeth and oral cavity.
There are 206 distinct bones in our skeleton, 28 of which are in the skull if we count the malleus, stapes, and incus bones of each ear. To obtain a clear understanding of the bones of the skull and their relationship to one another and to the teeth, it is best to have a skull or skull model at hand to examine while reading this chapter. If you touch and trace each bone with your fingers as you read, you are not likely to forget its characteristics. Also, as you study this section, you should relate the location of each bony structure on the skull to its location on your own head, that is, where it is located under the skin of the face or under the mucosa of the mouth. This is important in order to fully appreciate where muscles attach and how they can move the lower jaw (mandible) in all directions and to figure out where to apply local anesthetic along the path of the nerves to the teeth and oral cavity, as described in more detail in Chapter 15.
When reading the description of each bone, there are many descriptive terms that are used to describe the bumps, depressions, holes, and relative location of important landmarks. Many terms have similar definitions, so they are defined here in groups to facilitate learning. Since anatomy terms are often similar to common familiar words, the new terms are compared to familiar words whenever possible.
BUMPS—TERMS USED TO DESCRIBE CONVEXITIES ON BONES AND/OR TEETH
Crest: a projecting ridge along a bone
Eminence: a prominence or elevation of bone
Process: a projection or outgrowth from a bone
Protuberance [pro TU ber ahns]: a prominence of bone
Ridge: linear, narrow, elevated portion of bone or tooth
Tubercle [TOO ber k’l]: a small rounded projection on a bone or tooth
DEPRESSIONS—TERMS USED TO DESCRIBE CONCAVITIES IN BONES AND/OR TEETH
Alveolus [al VEE o lus] (plural: alveoli [al VEE o lie]): small hollow space or socket where the tooth root fits within the jaw bones
Cavity: a hollow place within the body of bone (or within a tooth)
Fissure [FISH er]: a cleft or groove (crack) between parts
Fossa [FOS ah] (plural: fossae [FOS ee]): a small hollow or depressed area
Fovea [FO ve ah]: small pit or depression
Groove: linear depression or furrow
Sinus: hollow, air-filled cavity or space within skull bones, or a channel for venous blood
OPENINGS—TERMS USED TO DESCRIBE HOLES IN BONES AND/OR TEETH
Aperture: an opening; compare a camera lens aperture
Foramen [fo RAY men] (plural: foramina [fo RAM i nah]): a small hole through bone or tooth for passage of nerves and vessels
Foramen ovale [o VAL ee]: a specific oval or egg-shaped foramen
Foramen rotundum: a specific round foramen; recall the Capitol’s rotundum or dome is round when viewed from above
Meatus [me A tus]: a natural passage or opening in the body
TERMS USED TO DESCRIBE RELATIVE LOCATION—Figure 14-1 will be helpful in understanding terms with an asterisk (*).
Anterior*: toward the front of the body
Buccal [BUCK al]: related to or near the cheek; the buccal nerve innervates the cheek; the buccinator muscle is within the cheek; the buccal surface of a tooth is the side toward the cheek.
Cervix: of the neck or neck-like; compare cervical vertebrae in the neck
External: toward the outside of the body; seen from the outside
Facial*: toward the face; seen when viewing the face side
Inferior* or the prefix infra: located below or beneath; lower than
Lateral*: pertaining to, or situated at, the side
Medial*: the surface toward, or closest to, the midline (median) plane of the body; do not confuse medial with mesial.
Median plane*: a longitudinal plane that divides the body into relatively equal right and left halves
Midsagittal plane* [mid SAJ i t’l]: same as median plane
Posterior*: toward the rear of the mouth or body
Retro (prefix): back or behind
Sub (prefix): under or beneath; compare to infra
Superficial: closer to the surface
Superior* or the prefix supra: located above or over; higher or upper
GENERAL TERMS RELATED TO BONES
Acoustic [ah KOOS tik]: referring to sounds or hearing; near the ear
Cervical [SER vi kal]: related to the neck; like cervical vertebrae
Condyle [KON dile]: an articular prominence of a bone resembling a knuckle
Coronoid: where the king’s coronation crown fits or the shape of a crown (compare coronation); for example, the coronoid process of the mandible is shaped like the point of a coronation crown; or a coronoid suture is where the crown fits.
Dura: hard, not soft (compare durable)
Glenoid [GLE noyd]: socket-like
Glosso: a prefix referring to the tongue
Labial [LAY bee al]: related to the lips; toward the lips
Lacrimal [LAK ri mal] (also spelled lachrymal): referring to the tears (compare lacrimosa)
Lamina: a thin layer (compare laminated wood)
Lingula [LING gyoo la]: tongue-shaped structure (compare to the word “lingual”)
Malar [MAY lar]: referring to the cheek or cheek bone (not to be confused with molar)
Meatus [mee A tus]: a pathway or opening
Palpebral [PAL pe bral]: referring to the eyelid
Piriform [PEER i form]: pear shaped
Septum: a partition (compare separate)
Suture [SOO chur] line: the line of union of adjoining bones of the skull
Symphysis [SIM fi sis]: fibrocartilaginous joint or connection where opposed bony surfaces are joined (a suture line may not be evident)
Trochlea [TROK lee ah]: pulley shaped
The skull bones can be divided into two broad categories: the bones of the neurocranium [NOOR o CRAY ne um] surrounding the brain and the facial bones that make up the face and mouth and are involved in respiration and eating. The neurocranium is the portion of the skull that supports, encloses, and protects the brain. The eight bones of the neurocranium are four single bones (sphenoid, occipital, ethmoid, and frontal) and two paired bones (one on each side): temporal and parietal.
Study Figure 14-2 while reading about the frontal and temporal bones. The frontal bone is a single, large midline bone that forms the “forehead” and eyebrow region. It contains two frontal sinuses just above the eyes (seen later in Fig. 14-13). Two parietal bones are large, paired bones that protect the brain superiorly, laterally, and posteriorly. A small portion of these bones make up a part of the large and shallow temporal fossa region (outlined in Fig. 14-2), which serves as part of the attachment for the superior end of one of the major muscles of chewing called the temporalis muscle.
Suture lines are lines of fibrous connective tissue that join two bones of the skull immovably together as seen in Figure 14-2. The coronal suture is located between the frontal and two parietal bones. The sagittal suture (best seen on the superior surface of the skull) joins the right and left parietal bones along the midsagittal plane of the skull.
The occipital bone, sphenoid bone, ethmoid bone, and two temporal bones support the base of the brain. They all have holes (foramen) for the passage of nerves to the face and mouth. However, the temporal bones play a most important part in the functioning of the jaw, so they will be discussed separately when discussing the TMJ.
The floor of the braincase can be divided into three large depressions or fossae, anterior, middle, and posterior (seen later in Fig. 14-4B), which house various portions of the brain. The shallowest anterior cranial fossa extends anteriorly from the lesser wing of the sphenoid bone (and houses the frontal lobes of the brain). The deeper middle cranial fossa extends from the lesser wing of the sphenoid bone posteriorly to the petrous portion of the temporal bone (and houses the temporal lobes of the brain and the pituitary gland). The largest and deepest posterior cranial fossa extends posteriorly from the petrous portion of the temporal bone (and houses the cerebellum).
1. Occipital Bone
The occipital bone provides the articulating surface between the skull and vertebral column at the occipital condyles [ahk SIP eh tal KON diles] (seen on the inferior surface in Fig. 14-3). The large foramen magnum serves as the passageway for the spinal cord that connects the peripheral nervous system with the brain. The hypoglossal canals are located on the right and left lateral walls of the foramen magnum. These canals are the passageways of the hypoglossal nerves (CN XII). Lateral to the foramen magnum (between the occipital and temporal bones) are the large jugular [JUG you lar] foramen (Fig. 14-3), the passageway of blood draining from the brain to the internal jugular vein, and the passageway of the glossopharyngeal nerve (CN IX).
The lambdoid [LAM doid] suture joins the occipital bone with the parietal bones (Fig. 14-2). Its shape from the posterior view resembles an upside-down “V” and can be compared to the shape of the Greek letter lambda (λ).
2. Ethmoid Bone
The ethmoid bone is a single, hollow bone that is located on the midline inferior to the anterior part of the brain and contains ethmoidal sinuses (also called air sacs) located between the eye orbits (seen later in Fig. 14-13). The superior aspect of this bone is visible within the braincase as the sieve-like cribriform [KRIB ri form] plate (Fig. 14-4A) surrounding the triangular projection called the crista galli [KRIS ta GAL li, meaning rooster comb]. The cribriform plate is full of holes providing the passage from the brain into the nasal cavity for the fibers of CN I, the olfactory nerve (the nerve for smell).
Inferior to the cribriform plate, the hollow ethmoid bone balloons out around its ethmoid sinuses to form part of the medial aspect of each eye orbit (the orbital lamina of the ethmoid bone is visible in Fig. 14-5 and later in Fig. 14-7). It also has scrolled processes extending into the nasal cavity similar in appearance to the inferior nasal concha described later in this section. Finally, a vertical midline plate of the ethmoid bone extends downward into the nasal cavity (along with the separate single vomer bone) to form the nasal septum (seen later in Fig. 14-7), which separates the right and left nasal cavities. The scrolled portions and midline plate of the ethmoid bone are visible through the piriform opening (anterior opening of the nasal passageways).
3. Sphenoid Bone
The sphenoid [SFE noid] bone is a single, irregularly shaped, midline bone that cradles the base of the brain (and forms the posterior part of the orbit or eye socket as seen later in Fig. 14-7). The complex shape of the sphenoid bone can only be appreciated by looking at it from several different views (seen from above in Fig. 14-4A, from below in Fig. 14-6, and seen later in the lateral surface of the orbit in Fig. 14-7). The sphenoid bone is important to dental professionals because it has processes that serve as part of the attachment for three of the four pairs of major chewing muscles. The sphenoid bone also has foramina (holes) that are the passageway for branches of the important fifth CN (trigeminal nerve) that supply all teeth and surrounding structures.
The hollow, midline body of the sphenoid bone contains sphenoidal sinuses located posterior to the ethmoidal sinuses (seen later in Fig. 14-13). It has a depression on the superior surface called the hypophyseal fossa or sella turcica [SELL a TER si ka] (meaning Turkish chair or saddle) that cradles the pituitary gland (Fig. 14-4A). This gland secretes hormones, which regulate many body functions. There are two pairs of processes or wings (greater and lesser wings) that project off of the body laterally and superiorly. The greater wings are visible internally in Figure 14-4A but are best viewed externally in Figure 14-2. If you put your thumb on the external surface of the greater wing, you can place your forefinger opposite to your thumb on the inner surface to confirm the location of the greater wing on the inner surface of the braincase. These wings extend superiorly from the body, posterior to the upper jawbones (maxillae), and medial to the lower jawbone and cheekbones. The external surface of the greater wing (along with part of the temporal, frontal, and parietal bones) forms part of each temporal fossa (outlined in Fig. 14-2), where a muscle of mastication, the temporalis muscle, attaches to the neurocranium. The lesser wings are located superior to a fissure in the posterior surface of the eye socket (seen in the braincase in Fig. 14-4A and on the posterior surface of the eye sockets seen in Fig. 14-7). The fissure between the greater wing and the lesser wing is called the superior orbital fissure, which is the passageway of the ophthalmic nerve (one part of the trigeminal nerve). Look at this fissure on the inside of the braincase, and then, look at the front of the skull to see this fissure on the posterior superior surface of the eye socket.
The sphenoid bone also has two important processes that project inferiorly from the base of the skull adjacent to the posterior surface of the upper jawbones (maxillae). These are called pterygoid [TER i goid] processes and are best seen in the lateral view of Figure 14-5. (Hint: To remember the name of this process, note that each has a scalloped border somewhat resembling the wings of a pterodactyl flying dinosaur.) When each pterygoid process is viewed from below (or posteriorly), you can see that it is made up of two thin plates of bone (a lateral pterygoid plate or lamina and a medial pterygoid plate or lamina) that surround a concavity about the size of your little finger called the pterygoid fossa (Fig. 14-6). This fossa is where one end of another major muscle of mastication, the medial pterygoid muscle, attaches. The lateral surface of the lateral pterygoid plate (visible in Fig. 14-5) is where one end of yet another muscle of mastication (the lateral pterygoid muscle) attaches. The medial plate has a hook-like projection just posterior and medial to the third molars and behind the palate, called the pterygoid hamulus (Figs. 14-5 and 14-6). The space just lateral to and posterior to the lateral plate and inferior to the temporal bone is called the infratemporal space (Fig. 14-5), and it is filled with muscles, ligaments, vessels, and nerves, which will be described later. Note that all of the terms presented here that contain “ptery” relate to the pterygoid process of the sphenoid bone, and the closest you can come to touching this process on your own head is by placing a clean finger in the mouth and sliding it posterior, superior, and medial to the maxillary third molars. Confirm this by looking at a skull.
Two pairs of foramina in the sphenoid bone are important to dental professionals: the foramen rotundum and the foramen ovale. The oval, more posterior foramen ovale is the passage of the important mandibular nerve (part of the trigeminal nerve) that passes from the brain to the mandibular teeth and jaw and the muscles of mastication. The foramen ovale is best seen internally in Figure 14-4 and externally in Figure 14-6. On a skull, if you carefully pass a pipe cleaner through this foramen, you will see that it drops inferiorly through the infratemporal space (beneath part of the temporal bone) toward the lower jaw (mandible). The foramen ovale can be easily identified by its proximity to the much smaller foramen spinosum [spy NO sum] that is located just posterior to it. Just posterior to the foramen spinosum on the inferior surface is a thorn-shaped bony prominence called the sphenoidal [SFE noid al] spine (or angular spine) (Fig. 14-6). This spine is the superior attachment of the sphenomandibular ligament, which extends inferiorly from the spine toward the medial surface of the lower jaw (mandible).
The foramen rotundum [ro TUN dum] is visible only internally in Figure 14-4 and is round, anterior, and just slightly medial to the foramen ovale. It is the opening for the passage of another important branch of the trigeminal nerve called the maxillary nerve that supplies all maxillary teeth. If you are able to carefully pass a pipe cleaner from the braincase through this foramen, it will be somewhat hidden in a space between the pterygoid process and upper jawbone (maxilla). This space between the pterygoid process and the posterior wall of the maxilla (which is covered in part by vertical projections of the palatine bones) is known as the pterygopalatine [TER i go PAL eh tine] space (also called the pterygomaxillary space) labeled in Figure 14-5. The maxillary nerve that exits the skull through the foramen rotundum proceeds through this pterygopalatine space as it gives off branches to the upper jaw (maxillae) and teeth.
At this time, look inside the braincase to review the location for the openings of the three branches of the trigeminal nerve on each half of the skull (Fig. 14-4). The trigeminal nerve begins within the braincase as one cranial nerve (CN) but splits to exit the neurocranium in three nerve branches through three openings. The most anterior opening is the superior orbital fissure for the ophthalmic branch. Posterior to it is the foramen rotundum for the maxillary branch, and the more posterior (and slightly lateral) foramen ovale is for the mandibular branch. Recall that the much smaller foramen spinosum is just posterior to the foramen ovale.
The form of 14 facial bones gives us our appearance. They function in both respiration and digestion. The facial bones are located inferior to the forehead and make up most of the anterior part of the skull. Five large bones of the face are the mandible, two maxillae, and two zygomatic (cheek) bones. The smaller bones of the face are the vomer, two palatines, two nasals, two lacrimal bones, and two inferior nasal conchae [KONG kee] (also called turbinates). The mandible and maxillae are most important when considering the foundation for teeth and tooth function, so they will be discussed in most detail. Although the temporal bones are not considered facial bones, they are being discussed here due to their importance in our understanding of the TMJ.
One maxilla is shaded red in Figure 14-7. Each maxilla [mak SILL a] (right or left) consists of one large, hollow, central mass called the body and four projecting processes or extensions of bone. The plural of maxilla is maxillae [mack SILL ee]. The two maxillae contain all of the maxillary teeth.
a. Body of the Maxilla (Structures Seen in Fig. 14-7)
The body of the maxilla is shaped like a four-sided, hollow pyramid with its base oriented vertically next to the nasal cavity and the apex or peak extending laterally to join the zygomatic bone, part of the cheekbone. The superior portion of the maxilla forms the floor of the orbit of the eye where an infraorbital fissure is located. This fissure disappears anteriorly to become the infraorbital canal (hidden within the bone in Fig. 14-7). Important branches of the fifth CN and vessels enter this fissure and canal and give off branches within the canal, which supply some of the maxillary teeth and surrounding tissue. The infraorbital nerves and vessels exit the infraorbital canal onto the face through the infraorbital foramen. This foramen is on the anterior surface of the body of the maxilla, inferior to the eye orbit and superior to the canine fossa, which is a shallow depression superior and lateral to the canine.
b. Maxillary Sinus or Antrum (and Other Paranasal Sinuses)
There are four pairs of paranasal sinuses (hollow spaces) located within bones surrounding the nasal cavity (nasal passages), and they all connect with the nasal cavity. Three pairs have been mentioned already: the sphenoidal, ethmoidal, and frontal sinuses (within the bones of the same names). The fourth pair are the maxillary sinuses, one located in each maxilla. The relative location of these paranasal sinuses are shown later in Figure 14-13. The maxillary sinuses are the largest of these sinuses and, together with the other paranasal sinuses, they function to (a) lighten the skull, (b) give resonance to the voice, (c) warm the air we breathe, and (d) moisten the nasal cavity. (The average capacity of each maxillary sinus in an adult is about 15 mL or about 1 tablespoon.1)
Refer to Figure 14-8 while reading about the maxillary sinus. This large, four-sided, pyramid-shaped cavity located within the body of each maxilla is important to dental health professionals because of the close relationship it has to the teeth. The sinus cavity floor extends inferiorly onto the superior portion of the maxillary alveolar process where projections of the apices of the molar roots, and sometimes premolar roots, may be found. This intimate relationship between the teeth and maxillary sinus space can be appreciated in Figure 14-9A and B. Only very thin bone lies between the floor of the sinus and the apices of the roots of the maxillary molars. In rare cases, no bone separates the root apices from the sinus, but there is always soft tissue between the root and the space of the cavity, made up of the periodontal ligament on the tooth root and the mucous membrane lining the sinus cavity. Sometimes, when a dentist extracts a molar and the root breaks off, he or she is unjustly accused of pushing the root into the sinus. It may have been located in the maxillary sinus prior to the extraction. The other three walls of the pyramid-shaped sinus are toward the orbit of the eye, toward the face, and, posteriorly and laterally, next to the infratemporal space.
The nerves to the maxillary molars (posterior superior alveolar [PSA] nerves) enter the maxilla and sinus lining through very small foramina called the alveolar [al VEE o lar] canals located posterior and superior to the maxillary third molars (Fig. 14-8). These nerves pass just beneath the membrane lining of the sinus or through bony canals within the walls of the sinus. An infection in either the sinus or these teeth can spread to the other. Pain caused by a maxillary sinus infection can be mistaken for pain originating in any one or all of the molars or premolars on that side. Unfortunately, healthy teeth are sometimes extracted in a futile attempt to alleviate pain that was caused by a chronic maxillary sinus infection.
All of the paranasal sinuses drain directly or indirectly into the nasal cavity, so an infection into the nose may spread into these sinuses. The opening from each maxillary sinus to the nasal cavity is located on its anterosuperior wall (Fig. 14-8). The maxillary sinus is lined with specialized cells (ciliated columnar epithelium) similar to those found in the respiratory tract. The lining secretes mucous that moves spirally and upward (against gravity) across the membrane toward the opening of the sinus, which is located on the anterosuperior wall (Fig. 14-8), where secretions can drains into the nasal cavity. If humans walked on all fours with the head forward like many animals, this opening for drainage would be on the floor of the sinus, not near the roof, and humans would have fewer sinus problems. Persons with a congested maxillary sinus may get pain relief by placing their head with the face downward for several minutes to permit more rapid drainage of the maxillary sinuses.
c. Bony Processes of Each Maxilla
There are four processes extending out from the body of the maxillae. The first three described below are best viewed in Figure 14-7.
FRONTAL (OR NASOFRONTAL) PROCESS
The frontal (or nasofrontal) process derives its name from the fact that its medial edge joins with the nasal bone, extending superiorly to also articulate with the frontal bone. The medial surface forms part of the lateral wall of the nasal cavity and half of the opening of the nasal cavity (called the piriform aperture because of its pear shape).
The bulky zygomatic process forms part of the anterior or facial surface of each maxilla. It extends laterally to join with the maxillary process of the zygomatic bone.
The horseshoe-shaped alveolar [al VEE o lar] processes of the right and left maxillae (also found on the mandible, described later) extend from the body of the maxillae to surround the roots of all maxillary teeth. The extension of these process from the body of each maxilla is best appreciated by viewing the maxillary from below since, when viewed laterally, the alveolar process appears to be continuous with the body of the maxilla (seen shaded on the right maxilla and the entire lower jaw or mandible in Fig. 14-7 and identified in cross-section in Fig. 14-10). Within each alveolar process, the roots of each tooth are embedded in an individual alveoli (or tooth sockets) that are visible in the mouth after a recent tooth extraction. The shape of each alveolus or thin bony socket naturally corresponds closely with the shape of the roots of the tooth it surrounds. Alveolar eminences are raised ridges of bone externally overlying prominent tooth root convexities. The alveolar eminence over the canine tooth on each side is called the canine eminence (Fig. 14-7). Medial to the canine eminence is a shallow fossa over the root of the maxillary lateral incisor called the incisive [in SI siv] fossa. Lateral and superior to the canine eminence is a fossa over the roots of maxillary premolars named the canine fossa.
The alveolar process is made up of several bony layers (seen in cross-section of the mandible in Fig. 14-10). The mandibular bone is made up of the thickened inner (lingual) and outer (facial) dense cortical plate with less dense trabecular [trah BEK u lar] bone sandwiched in between. Trabecular bone is composed of many plate-like bone partitions that separate the irregularly shaped marrow spaces located within this bone. Synonyms for trabecular bone include cancellous or spongy bone. Small nerve branches and vessels actually pass through this spongy bone to enter all teeth through their apical foramen. A thin, compact bony layer called alveolar bone proper or bundle bone lines the wall of each tooth socket (or alveolus) and shows up on radiographs as a white line called the lamina dura. Other terms used to describe this bony layer include bundle bone, alveolar bony socket, true alveolar bone, and cribriform plate of the alveolar process. The only space between the outer layer of tooth root (which is covered with cementum) and this alveolar bone is occupied by a periodontal ligament that suspends each tooth within its alveolus by attaching the circumference of each tooth root to the surrounding alveolar bony socket. The periodontal ligament is very thin (less than a third of a millimeter).
PALATINE PROCESS OF THE MAXILLA
The right and left palatine [PAL a tine] processes (Fig. 14-11) join to form the anterior three quarters of the bony roof of the mouth called the hard palate. The separate paired palatine bones, discussed later in this chapter, form the posterior one quarter of the hard palate. The palatine process of each maxilla is a thin, bony shelf that projects horizontally to join the process from the opposite side. The entire hard palate separates the nasal passageways from the oral cavity. That is, the hard palate forms the roof the mouth and the floor of the nasal passageways. The shape and relative location of these processes can be best appreciated by viewing them posteriorly between the two pterygoid processes of the sphenoid bone.
The anteroposterior line of fusion between the right and left palatine processes of the maxillae (and the horizontal plates of the palatine bones) is the intermaxillary (or midpalatine) suture. It is located on the midline running posteriorly from the incisive foramen (Fig. 14-11). The incisive foramen is a centrally located opening at the most anterior part of this suture, just posterior to the central incisors. It transmits branches of the nasopalatine nerve and artery that supply adjacent palatal mucosa. Just posterior to the maxillary alveolar process of the most posterior maxillary molar is a bulge of bone called the maxillary tuberosity. A notch that separates the maxillary tuberosity of each maxilla from the adjacent pterygoid process of the sphenoid bone is called the hamular notch. Recall that the pterygoid hamulus, the hook-like projection of the medial plate of the pterygoid process, is located just posterior to the hamular notch. In your mouth, the pterygoid hamulus might be felt with your tongue (or clean fingers) under the mucosa of the soft palate posterior to the hard palate and slightly medial to the maxillary tuberosity.
An embryonic premaxilla cannot normally be distinguished in the adult skull. It is the anterior part of the maxillary bone, which contains the incisors. When visible, a suture line separates the premaxilla from the palatine processes of the two maxillae.
2. Palatine Bones
Refer to Figure 14-11 while reading about the palatine bone. The horizontal processes of the paired palatine bones form the posterior one fourth of the hard palate. The entire hard palate is made up of these palatine bones, along with the right and left palatine processes of the maxillae. A palatomaxillary [PAL ah toe MACK si lar ee] (transverse palatine) suture, at right angles to the intermaxillary suture, is the junction between the palatine processes of the maxillae and the horizontal processes of the palatine bones. The shape of the palate and the shape of the maxillary arch vary in length, width, and height. The hard palate blends smoothly with the palatal portion of the maxillary alveolar process. Part or all of the palatine processes are absent in a person who was born with a cleft palate.
The greater palatine [PAL ah tine] foramina (Fig. 14-11) are located posteriorly on each side near the angle where the right and left palatine bones meet the alveolar processes of the hard palate. They transmit the descending palatine vessels and greater (anterior) palatine nerves to the palate. The lesser palatine foramina are located on the palatine bone just behind and lateral to the greater palatine foramen. They transmit the middle and posterior palatine nerves.
The palatine bones also have vertical processes that are practically hidden from view on the intact skull. These vertical processes form part of the posterior wall of the maxillary sinus in Figure 14-8. These vertical processes of the palatine bones are separated from the pterygoid process of the sphenoid bone by a space called the pterygopalatine [TER i go PAL ah tine] space, mentioned earlier when discussing the maxillae. Recall that this space is an important passageway of the maxillary nerve branches of CN V that exited from the cranium via the foramen rotundum on their way to the maxillary teeth and surrounding structures.
3. Zygomatic Bones
The zygomatic bones (also called malar bones) form the prominence of each cheek (one on each side of the face, shaded purple in Fig. 14-12). The temporal process of the zygomatic bone forms an arch along with the adjoining zygomatic process of the temporal bone. This zygomatic arch is where another muscle of mastication (the masseter muscle) attaches to the skull.
4. Mandible: Forming the Inferior Portion of the Temporomandibular Joint
The single horseshoe-shaped mandible [MAN de b’l], seen anteriorly in Figure 14-13, is the largest and strongest bone of the face. Generally speaking, it is bilaterally symmetrical, and it contains all of the mandibular teeth. It is attached by ligaments and muscles to the relatively immovable bones of the temporal bone. The temporomandibular joint (TMJ) between the mandible and the temporal bones are movable articulations, the only visible movable articulations in the head. Therefore, the mandible is the only bone of the skull that can move. The other bones of the skull move only when the whole head is moved, and then, they move in unison.
The mandible has three parts: one horizontal, horseshoe-shaped body and two vertical rami [RAY mee] (singular, ramus [RAY mus]) (see Fig. 14-13). The landmarks of the mandible will be discussed according to their location: first those seen on the external surface of the body, then those on each ramus, and then those seen on the internal surface.
a. Body of the Mandible: External Surface
As with the maxillae, a horseshoe-shaped alveolar [al VEE o lar] process surrounds the tooth roots (shaded in Fig. 14-7), and alveolar eminences are visible as vertical elevations over tooth roots on the facial surface. The prominent elevations overlying the roots of the canines are called the canine eminences.
The symphysis [SIM fi sis] is the line of fusion of the right and left sides at the midline where the two halves of the mandible fused (joined together) during the first year after birth. It is therefore usually not visible. Near the symphysis, two mental tubercles and one mental protuberance make up the human chin (Fig. 14-13). No other mammal has a chin. Two mental tubercles lie on either side of the midline near the inferior border of the mandible. The mental protuberance is centered on the midline between the two mental tubercles but is about 10 mm superior. The protuberance and the tubercles are more prominent on men than on women.
An external oblique [ob LEEK] ridge (Fig. 14-12) extends from the anterior border of the ramus toward the canine region. The nearly horizontal ledge of bone in the molar region between the external oblique ridge and alveolar process is named the buccal shelf. The buccal (or buccinator) nerve is located in the cheek just superior to this shelf.
The mental foramen is located near the root end (apex) of the second premolar (Fig. 14-12). The nerve within the mandible (inferior alveolar nerve) gives off a branch (mental branch of the inferior alveolar nerve) that exits through this mental foramen to supply skin on that side of the chin. The mental nerve exits the mandible in an outward, upward, and posterior direction before it spreads anteriorly. Place a flexible probe carefully into this canal of the mandible to confirm the direction of this canal. The mental foramen is located at practically the same level on most humans: 13 to 15 mm superior to the inferior border of the mandible. (In a study of 40 skulls,9 the mental foramen was found most often to be directly under the second premolar (42.5% of the time) or between the apices of the first and second premolars (40%). Infrequently, it was located distal to the apex of the second premolar (17.5%) and was never found under the apex of the first premolar.) On dental radiographs (x-rays), this foramen appears as a small dark circle next to the premolar root and must be distinguished from a periapical abscess (infection destroying bone near the root apex), which may appear very similar to the normal mental foramen.
The bulky, curved, horizontal body and each flattened vertical ramus join at the angle of the mandible on either side. The angle of the mandible is where the inferior border of the body joins the posterior border of the ramus (Fig. 14-12). The roughened portion of the lateral surface near the angle of the mandible is where the inferior end of the powerful masseter muscle attaches. The posterior border of the ramus is the location of the attachment of one end of the stylomandibular ligament (whose other end attaches to the styloid process of the temporal bone).
b. Ramus of the Mandible: Lateral Surfaces
Refer to Figure 14-12 while reading about this surface. There are two processes on the superior end of each ramus. The coronoid [KOR o noyd] process is the more pointed, anterior process on the upper border. The second more rounded and posterior process of the ramus is the condyloid [KON di loyd] process (also called the mandibular condyle). This process is composed of a bulky condyle head and a narrow neck that attaches the head to the ramus. The sigmoid notch (also called the mandibular or semilunar notch) is located between the coronoid process and the condyloid process. An important muscle of mastication, the lateral pterygoid, attaches to the front of the neck of the condyloid process in a depression called the pterygoid fovea (Fig. 14-14). The head of the mandibular condyle fits into and functions beneath the articular (glenoid) fossa of the temporal bone (which is discussed in more detail later in this chapter).
c. Internal or Medial Surface of Mandible
Refer to Figure 14-14 while reading about this surface of the mandible. The mandibular foramen is a prominent opening located on the medial surface of the ramus inferior to the sigmoid notch near the middle of the ramus anteroposteriorly. It is the entrance into the mandibular canal where the inferior alveolar vessels and nerves pass from the infratemporal space into the mandible. The mandibular lingula [LING gu lah] is a tongue-shaped projection of bone just anterior and slightly superior to the mandibular foramen. This is where the inferior end of the sphenomandibular ligament attaches to the mandible. The superior end attaches to the angular (sphenoidal) spine on the sphenoid bone. The mylohyoid groove is a small groove running inferior and anterior from the mandibular foramen. The mylohyoid nerve rests in this groove.
The temporal crest is a ridge of bone extending from the tip of the coronoid process onto the medial surface of the ramus and terminating near the third molar. The tendon from the fibers of the wide, flat, fan-shaped temporalis muscle attaches here. The inferior one fourth of the temporal crest is called the internal oblique line. It is most important as a radiographic, rather than an anatomic, landmark. It appears on radiographs as a short, curved line somewhat inferior to the image of the external oblique line.
The retromolar fossa is a roughened shallow fossa distal to the last molar and bounded medially by the lowest portion of the temporal crest and laterally by the external oblique ridge. The retromolar triangle is in the lowest most anterior, and only horizontal, portion of the retromolar fossa. The most posterior fibers of the buccinator muscle (a pouch-shaped cheek muscle) attach within this retromolar triangle on a slight ridge of bone called the buccinator crest.
Genial [JEE ne al] and mental spines or tubercles are located on either side of the midline on the internal surface of the mandible. Two large muscles (the genioglossus and the geniohyoid) attach to these spines and the elevated, roughened bone near them.
The mylohyoid ridge extends downward and forward from the molar region to the genial tubercles. It is not synonymous with the internal oblique line as incorrectly stated in some radiographic textbooks. The mylohyoid muscle, which forms part of the floor of the mouth, attaches from the mylohyoid ridge on the right medial side of the mandible to the ridge on the left medial side (somewhat like a hammock). The mylohyoid ridge separates two fossae, one above and one below. A very broad, shallow sublingual [sub LING gwal] fossa is found just superior to the mylohyoid ridge and lateral to the genial tubercles on each side. The sublingual salivary gland rests in this fossa. A shallow submandibular fossa is found just inferior to the mylohyoid ridge in the premolar and molar regions. This is where the large submandibular salivary gland rests. On the inferior border of the mandible, a shallow notch (called the antegonial [an te GO nee al] notch seen in Fig. 14-14) is located anterior to the angle of the mandible and is where the facial arteries and veins pass from the neck to the face. You should be able to feel a pulse at this location of your own lower jaw.
5. Temporal Bones: Forming the Superior Part of the Temporomandibular Joint
The temporal bones are a pair of complex bones that form part of the sides and base of the neurocranium (braincase) (best seen laterally in Fig. 14-15 where one is shaded blue). Laterally, the temporal fossa (outlined in Fig. 14-15) is a large, very shallow depression in the temple region of the face formed primarily by the lateral part of the temporal bone (also called the squamous part because it is shaped like a large fish scale) and also by the greater wing of the sphenoid bone and parts of the parietal and frontal bones that were discussed earlier. The temporal fossa is where the superior end of the major muscle of mastication (the temporalis muscle) attaches. Squamosal [skwa MO sal] sutures join the temporal bones to the parietal bones.
The paired temporal bones are especially important to dental professionals since each has a mandibular fossa (one is labeled on the right side of Fig. 14-16) located on the inferior aspect of the temporal bones. It is within these fossae that the mandibular condyles articulate with the temporal bones on the base of the neurocranium. This jaw joint (actually a joint on each side) is called the temporomandibular joint (commonly abbreviated TMJ) where the temporal bone and mandible articulate. Each mandibular fossa can be divided into two parts by the petrotympanic fissure (Fig. 14-16). The anterior two thirds of each mandibular fossa (that portion anterior to the petrotympanic fissure) is the important articular fossa (or glenoid fossa). Each articular fossa has a ridge of bone forming its anterior border, which is called the articular eminence.
Each temporal bone has several processes. The zygomatic [zy go MAT ik] process (Fig. 14-15) is the finger of bone extending anterior and lateral to the mandibular fossa of the TMJ. It joins with the temporal process of the zygomatic bone (and the zygomatic process of the maxillae) to form an arch called the zygomatic arch. This arch shape of bones, seen from beneath in Figure 14-16, is the attachment of one end of the large muscle of mastication (masseter muscle). The prominent mastoid process or portion (Fig. 14-15), seen inferiorly and posteriorly to the mandibular fossa, is the attachment for one end of a major neck muscle, the sternocleidomastoid muscle. You can feel the bump of the mastoid process behind your earlobe. Also, on the inferior surface of the temporal bones but more medial is the styloid process (Fig. 14-15), shaped like a small skinny pencil (or stylus). It is the attachment for one end of a ligament (stylomandibular ligament) that extends to the mandible.
Several paired foramina are of importance on this bone. Laterally, the large external acoustic meatus [a KOO stik me A tus] or auditory meatus is the entrance into the ear canal (Fig. 14-15). Note the proximity of the TMJ to the ear canal opening. This petrous portion of the temporal bone contains the auditory canal with the minute bones of hearing known as the malleus, incus, and stapes. The facial nerve (CN VII) exits the braincase by entering this petrous portion of the temporal bone through the internal acoustic meatus (Fig. 14-17) and exits the temporal bone into the infratemporal space through the stylomastoid foramen (Fig. 14-16), which is located between the styloid and mastoid processes. The carotid canal is the passageway of the internal carotid artery into the braincase, and the jugular foramen (between the temporal and occipital bones) is where the glossopharyngeal nerve (CN IX) passes out of the braincase (seen externally in Fig. 14-16 and internally in Fig. 14-17).
The vomer bone is a midline bone that, along with the vertical projection of the ethmoid bone, forms the nasal septum. The vomer is visible posteriorly between the two halves of the nasal passageways in Figure 14-16, and the nasal septum is visible anteriorly in Figure 14-18. The nasal septum separates the right and left halves of the nasal cavity. A deviated septum may limit breathing and require surgery.
The two nasal bones form the bony bridge of the nose (Fig. 14-18). These nasal bones along with the two maxilla bones, surround the opening (piriform aperture) to the nasal cavity, also known as the nasal passages.
The lacrimal [LAK ri mal] bones (also spelled lachrymal) are small rectangular bones at the medial corner of each orbit that contain a depression for tear glands (Fig. 14-18).
The inferior nasal conchae [CONG kee] or singular concha [KONG kah] (or turbinates) are scrolled bones (like the cross-section scroll shape of a conch shell) in the nasal cavity forming part of the maxillary sinus wall. These are best seen through the piriform aperture (Fig. 14-18). Along with other scrolled processes of the ethmoid bone described earlier, they increase the area of mucous membrane inside the nasal cavity to warm and moisten air that we breathe.
The hyoid [HI oid] bone (see later in Fig. 14-35) is not really a bone of the skull but is located in the neck above the laryngeal prominence of the thyroid cartilage (known to many as the Adam’s apple or voice box). The hyoid bone is not connected to the bones of the skull except via soft tissue. A group of muscles that extend from the hyoid bone superiorly to attach to the mandible are called suprahyoid muscles (such as the geniohyoid muscles that also attach to the genial tubercles), and another group that extend inferiorly from the hyoid bone to attach to the sternum (breastbone) or clavicle (collar bone) are called infrahyoid muscles.
Select the one best answer.
- Which of the following bones does not form part of the temporal fossa?
- The mental foramen is located where?
- On the external surface of the mandible
- On the internal surface of the mandible
- On the palatal surface of the maxilla
- On the external surface of the maxillae
- On the sphenoid bone
- What space does the maxillary nerve pass through immediately after exiting the foramen rotundum?
- Nasopalatine canal
- Mandibular canal
- Maxillary sinus
- Infraorbital canal
- Pterygopalatine space
- What bony process of the maxilla surrounds tooth roots?
- Nasofrontal process
- Frontal process
- Alveolar process
- Zygomatic process
- Palatine process
- Which structure is not located on the sphenoid bone?
- Foramen ovale
- Foramen rotundum
- Greater wing
- Pterygoid process
- Articular fossa
- Which teeth are most likely to have the roots in proximity with the maxillary sinus?
- Maxillary molars and premolars
- Maxillary canines
- Maxillary incisors
- Mandibular posterior teeth
- The suture line joining the two parietal bones is called the
- Squamosal suture.
- Coronoid suture.
- Sagittal suture.
- Intermaxillary suture.
- Lambdoid suture.
- Which of the following is not a bone that contains paranasal sinuses?
ANSWERS: 1–e; 2–a; 3–e; 4–c; 5–e; 6–a; 7–c; 8–a
Each of the following bony landmarks can be seen or felt underneath the soft tissue on the face or in the mouth and could be used to describe the location of abnormalities during a clinical examination. First, describe the location; then, identify each of the following landmarks on your own face and on an actual skull (or figures within this text). Use the referenced figures to confirm that you have correctly located each landmark. Use clean fingers when palpating structures within the mouth.
- Canine eminence of the mandible and maxillae—Figure 14-7
- Mental protuberance—Figure 14-13
- Maxillary tuberosity—Figure 14-11
- External auditory meatus—Figure 14-12
Each of the following landmarks is the attachment of a major muscle or ligament of importance to the dental professional. First, describe the location; then, identify each of the following landmarks on an actual skull (or figures within this text). Use the referenced figures to confirm that you have correctly described the location of the attachment on the skull. When possible, also feel or point to the landmark’s location on your own head or within your mouth (using clean fingers).
- Angle of the mandible, lateral surface (lower end of masseter muscle)—Figure 14-12
- Zygomatic arch (upper end of masseter muscle)—Figure 14-12
- Angle of the mandible, medial surface (lower end of medial pterygoid muscle)—Figure 14-14
- Medial surface of the lateral pterygoid plate and adjacent pterygoid fossa of the sphenoid bone (upper end of medial pterygoid muscle)—Figure 14-6
- Temporal fossa (upper end of temporalis muscle)—Figure 14-15
- Coronoid process and temporal crest of mandible (lower end of temporalis muscle)—Figure 14-14
- Lateral surface of the lateral pterygoid plate of the sphenoid bone (anterior end of lateral pterygoid muscle)—Figure 14-5
- Pterygoid fovea: anterior neck of mandibular condyle (posterior end of lateral pterygoid muscle)—Figure 14-14
- Angular spine of the sphenoid (upper end of sphenomandibular ligament)—Figure 14-6
- Lingula of the mandible (lower end of sphenomandibular ligament)—Figure 14-14
- Styloid process of the temporal bone (upper end of stylomandibular ligament)—Figure 14-15
- Mastoid process of the temporal bone (upper end of sternocleidomastoid muscle)—Figure 14-15
- Mylohyoid ridge of the mandible (mylohyoid muscle)—Figure 14-14
- Genial spines of the mandible (upper end of some suprahyoid muscles)—Figure 14-14
Each of the following foramen or spaces is the passageway for nerves and blood vessels of importance to the dental professional. First, describe the location; then, identify each of the following foramina or spaces on an actual skull (or figures within this text). Use the referenced figures to confirm that you have correctly located the foramen, canal, or space on the skull. Then, try to place your finger as close as possible to that opening, realizing that sometimes you cannot get very close with your finger but might get closer with the needle of a hypodermic syringe.
- Foramina rotundum in the sphenoid bone (for the maxillary division of trigeminal nerve)—Figure 14-4
- Pterygopalatine space (for the maxillary division of trigeminal nerve)—Figure 14-5
- Foramina ovale in the sphenoid bone (for the mandibular division of trigeminal nerve)—Figures 14-4 and 14-6
- Mandibular foramina in the mandible (for the inferior alveolar nerve)—Figure 14-14
- Mental foramina in the mandible (for the mental nerve)—Figure 14-12
- Greater palatine foramina in the palatine bones (for the greater palatine nerve)—Figure 14-11
- Incisive foramen between the maxillary bones (for the nasopalatine nerve)—Figure 14-11
- Infraorbital foramina in the maxillae (for the infraorbital nerve)—Figure 14-7