7.2 Skull and 7.3 Hyoid Bone
Facial Bone: Mandible
or lower jawbone, is the largest, strongest facial bone (Figure 7.10). This large, arch-shaped bone is the only movable skull bone other than the auditory ossicles, the small bones of the ear and occipital bone.
Surface feature: Parietal bones
The external surface of each of these bones is slightly convex (curved outward, like the outside of a sphere), while the internal surface is concave. The internal surfaces of the parietal bones contain many protrusions and depressions that accommodate the blood vessels supplying the dura mater, the superficial membrane (meninx) covering the brain.
Facial Bone: Palatine
The two L-shaped palatine bones (PAL-a-tīn) form the posterior portion of the hard palate. In addition to the roof of the mouth they form the posterior portion of the floor and walls of the nasal cavity. The superior aspect of their perpendicular plate contributes to a small portion of the floor of the orbit bordering the optic foramen (canal)
Facial Bone: Inferior Nasal Conchae
The two inferior nasal conchae (turbinates) are inferior to the middle nasal conchae of the ethmoid bone (see Figures 7.2, 7.7a, 7.2, and 7.8c). These scroll-like bones form a part of the inferior lateral wall of the nasal cavity and project into the nasal cavity. The inferior nasal conchae are separate bones; they are not part of the ethmoid bone. All three pairs of nasal conchae (superior, middle, and inferior) increase the mucosal-covered surface of the nasal cavity and help swirl and filter air before it passes into the lungs. However, only the superior nasal conchae of the ethmoid bone are involved in the sense of smell.
Cranial bones: Parietal bones
The two parietal bones are large, quadrilateral (four-sided) bones that form the greater portion of the sides and roof of the cranial cavity (Figure 7.3). Each bone articulates with five other bones. The inferior border forms a beveled articular surface, while the anterior, posterior, and superior borders form deeply denticulate (toothlike) articular surfaces.
Cranial bones: Temporal bones
The two temporal bones (tempor-=temple) form the inferior lateral aspects of the cranium and part of the cranial floor. The terms temporal and temple are derived from the Latin word tempus, meaning "time," in reference to the graying of hair in the temple area, a sign of time's passing.
Facial Bone: Zygomatic Bones
The two zygomatic bones (zygo-=yokelike), commonly called cheekbones, form the prominences of the cheeks and part of the lateral wall and floor of each orbit (see Figures 7.9b and 7.12). They articulate with the frontal, maxilla, sphenoid, and temporal bones.
Facial bone: Vomer
The unpaired vomer (VŌ-mer=plowshare) is a roughly triangular bone on the floor of the nasal cavity that articulates superiorly with the perpendicular plate of the ethmoid bone and the inferior surface of the body of the sphenoid bone. Inferiorly, the vomer articulates with both the maxillae and palatine bones along the midline (see Figures 7.2, 7.4, 7.5, and 7.8e, f). It forms the inferior portion of the bony nasal septum, the partition that divides the nasal cavity into right and left sides.
Cranial bone: Frontal bone
forms the forehead (the anterior part of the cranium), the roofs of the orbits (eye sockets), and most of the anterior part of the cranial floor (Figure 7.2). In most individuals it is an unpaired bone. Soon after birth the left and right sides of the frontal bone are united by a suture called the metopic suture , which usually disappears between the ages of 6 and 8.
Cranial Bones: Occipital Bone
forms the posterior part and most of the base of the cranium. When viewed from behind, it appears as a platelike bone with a somewhat triangular shape. Its inferior portion is a thick, blocklike region that surrounds the junction of the brain and spinal cord
Nasal septum
he nasal cavity is a space inside the skull that is divided into right and left sides by a vertical partition called the nasal septum. The three components of the nasal septum are the vomer, the perpendicular plate of the ethmoid bone, and the septal cartilage (Figure 7.11). The inferior border of the perpendicular plate of the ethmoid bone joins the superoposterior border of the vomer to form the more posterior bony part of the septum, referred to as the bony nasal septum. The septal cartilage, which is hyaline cartilage, articulates with the anterior margins of the two bones to form the anterior portion of the septum. The term "broken nose" usually refers to damage to the septal cartilage rather than the nasal bones.
Unique feature of skull:
sutures, paranasal sinuses, and fontanels.
Fontanels
At birth, bone formation is incomplete and the mesenchyme-filled spaces become dense connective tissue regions between the incompletely developed cranial bones called fontanels (fon-ta-NELZ=little fountains) (Figure 7.14). Commonly called "soft spots," fontanels are the areas where unossified mesenchyme develops into the dense connective tissues of the skull. As bone formation continues after birth, the fontanels are eventually replaced with bone by intramembranous ossification and the thin collagenous connective tissue junctions that remain between neighboring bones become the sutures. Functionally, the fontanels serve as spacers for the growth of neighboring skull bones and provide some flexibility to the fetal skull. They allow the skull to change shape as it passes through the birth canal and permit rapid growth of the brain during infancy. Although an infant may have many fontanels at birth, the form and location of six are fairly constant: 1.The unpaired anterior fontanel, located at the midline among the two parietal bones and the frontal bones, is roughly diamond-shaped and is the largest fontanel. It usually closes 18 to 24 months after birth. 2.The unpaired posterior fontanel is located at the midline among the two parietal bones and the occipital bone. Because it is much smaller than the anterior fontanel, it generally closes about 2 months after birth. 3.The paired anterolateral fontanels, located laterally among the frontal, parietal, temporal, and sphenoid bones, are small and irregular in shape. Normally, they close about 3 months after birth. 4.The paired posterolateral fontanels, located laterally among the parietal, occipital, and temporal bones, are irregularly shaped. They begin to close 1 to 2 months after birth, but closure is generally not complete until 12 months. The amount of closure in fontanels helps a physician gauge the degree of brain development. In addition, the anterior fontanel serves as a landmark for withdrawal of blood for analysis from the superior sagittal sinus (a large midline vein within the covering tissues that surround the brain
Surface features: Maxillae
Each maxilla has a central body, which is hollowed out internally to form the mucosal-lined maxillary sinus. Like the previously mentioned paranasal sinuses, the mucosa of the maxillary sinus is continuous with the mucosa of the nasal cavity (see Figure 7.13). The alveolar process (al-VĒ-ō-lar; alveol-=small cavity) is the ridge-like arch that contains the alveoli (sockets) for the maxillary (upper) teeth (see Figure 7.9a). The palatine process is a horizontal projection of the maxilla that forms the anterior three-quarters of the hard palate (see Figure 7.6c, d). The union and fusion of the maxillary bones is normally completed before birth. The infraorbital foramen (infra-=below; orbital=orbit), which can be seen in the anterior view of the skull in Figures 7.2 and 7.9a, is an opening in the maxilla inferior to the orbit. Through it passes the infraorbital blood vessels and nerve, a branch of the maxillary division of the trigeminal (V) nerve. Another prominent foramen in the maxilla is the incisive foramen (=incisor teeth) just posterior to the incisor teeth (see Figure 7.6c, d). It transmits branches of the greater palatine blood vessels and nasopalatine nerve. A final structure associated with the maxilla and sphenoid bone is the inferior orbital fissure, which is located between the greater wing of the sphenoid and the posterior aspect of the maxilla (see Figure 7.12).
Age related changes in skull
From fetus to newborn child to elderly adult, the skull undergoes significant changes as it ages. During fetal life, the skull, like the rest of the skeleton, consists of many more bones than are present later in life. Whether it is the femur in the thigh or the occipital bone at the back of the skull, when a bone first starts to develop and ossify it consists of multiple ossification centers that are each separate bony elements. For example, the occipital bone of the skull is a single bone in adults that typically arises from about a dozen or more ossification centers that fuse during the fetal period into five distinct bones. The five parts of the occipital bone are separated by connective tissue; these connective tissue spacers allow the occipital bone to increase in size during the early months and years of development to accommodate the growing brain. A few years after birth, as the individual bony parts enlarge, they begin to approach one another and soon fuse into a single bone. The final fusion of the five parts of the occipital bone is typically complete by the late teenage years. Similarly, the individual bones of the skull in the newborn and young skull are separated by connective tissue regions called fontanels, which were described in a previous section of this chapter. As neighboring bones develop and enlarge, the connective tissue is replaced and the fontanels decrease in size. Many of the resulting sutures will fuse and even obliterate the boundaries between neighboring bones later in adult life. Other age-related changes in the skull include the emergence of the paranasal sinuses and the eruption of teeth in the young skull, the wearing down and loss of teeth with advancing age, and the bone loss of the mandible and maxilla (the jaw bones) that accompanies tooth loss in older individuals.
Cranial fossae
If the calvaria, the roof of the cranium, is removed, the floor of the cranium displays three distinct levels that correspond to the major contours of the inferior surface of the brain. The three distinct regions of the floor of the cranium are called cranial fossae (FAWS-ē) (Figure 7.15). Each cranial fossa is at a different level, creating the appearance of stairs that step down from anterior to posterior inside the skull. A specific structural region of the brain fits snugly into each cranial fossa. From anterior to posterior, they are named the anterior cranial fossa, middle cranial fossa, and posterior cranial fossa. The highest level, the anterior cranial fossa, is formed largely by the portion of the frontal bone that constitutes the roof of the orbits and nasal cavity, the crista galli and cribriform plate of the ethmoid bone, and the lesser wings and part of the body of the sphenoid bone. This fossa houses the frontal lobes of the cerebral hemispheres of the brain. The rough surface of the frontal bone can lead to tearing of the frontal lobes of the cerebral hemispheres during head trauma. The middle cranial fossa is inferior and posterior to the anterior cranial fossa. Like the sphenoid bone that surrounds it, it is shaped like a butterfly, with a small median portion and two expanded lateral portions. The median portion is formed by part of the body of the sphenoid bone, and the lateral portions are formed by the greater wings of the sphenoid bone, the temporal squama, and the parietal bone. The middle cranial fossa cradles the temporal lobes of the cerebral hemispheres and the pituitary gland. The last fossa, forming the most inferior level, is the posterior cranial fossa, the largest of the fossae. It is formed mostly by the occipital bone and the petrous and mastoid portions of the temporal bone. It is a very deep fossa that accommodates the cerebellum, pons, and medulla oblongata of the brain.
Cranial bones: Surface features of frontal bone
If you examine the anterior view of the skull in Figure 7.2, you will note the frontal squama, a thick, scalelike plate of bone that forms the forehead. It gradually slopes inferiorly from the coronal suture (its joint with the paired parietal bones) on top of the skull (see Figure 7.3b), then angles abruptly and becomes almost vertical above the orbits. At the superior border of the orbits the frontal bone thickens, forming the supraorbital margin (supra-=above; orbital=circular). From this margin the frontal bone extends posteriorly as a horizontal plate of bone to form the roof of the orbit and part of the floor of the cranial cavity. Within the supraorbital margin, slightly medial to its midpoint, is a hole called the supraorbital foramen through which the supraorbital nerve and artery pass. Sometimes this foramen is incomplete and is called the supraorbital notch. Near the midline, within the vertical portion of the frontal squama, the bone is hollow. These hollow spaces are the paranasal sinuses called the frontal sinuses. Paranasal sinuses, mucous membrane-lined cavities within certain skull bones,
Surface features: Temporal bones
In the lateral view of the skull (see Figure 7.3c, d), note the temporal squama, the thin, flat portion of the temporal bone that forms the anterior and superior part of the temple (the region of the cranium around the ear). Projecting anteriorly from the inferior portion of the temporal squama is the zygomatic process, which articulates (forms a joint) with the temporal process of the zygomatic (cheek) bone. Together, the zygomatic process of the temporal bone and the temporal process of the zygomatic bone form the zygomatic arch. You can easily palpate this horizontal arch of bone immediately anterior to the ear. On the inferoposterior surface of the zygomatic process of the temporal bone is a socket called the mandibular fossa. Anterior to the mandibular fossa is a rounded elevation, the articular tubercle (see Figure 7.3c, d). The mandibular fossa and articular tubercle articulate with the mandible (lower jawbone) to form the temporomandibular joint (TMJ). The mastoid portion (mastoid=breast-shaped) (see Figure 7.3c, d) is located posterior and inferior to the external auditory meatus (meatus=passageway), or ear canal, which directs sound waves into the ear. In the adult, this portion of the bone contains several mastoid air cells that communicate with the hollow space of the middle ear (tympanic cavity). These tiny air-filled compartments are separated from the brain by thin bony partitions. Middle ear infections that go untreated can spread into the mastoid air cells, causing a painful inflammation referred to as mastoiditis (mas′-toy-DĪ-tis). The mastoid process is a rounded projection of the mastoid portion of the temporal bone posterior and inferior to the external auditory meatus that serves as a point of attachment for several neck muscles (see Figure 7.3c, d). The internal auditory meatus (Figure 7.4) is the opening through which the facial (VII) and vestibulocochlear (VIII) cranial nerves pass. The styloid process (styl-=stake or pole) projects inferiorly from the inferior surface of the temporal bone and serves as a point of attachment for muscles and ligaments of the tongue and neck (see Figure 7.3c, d). Between the styloid process and the mastoid process is the stylomastoid foramen, through which the facial (VII) nerve and stylomastoid artery pass. At the floor of the cranial cavity (see Figure 7.6a, b) is the petrous portion (petrous=rock) of the temporal bone. This portion is pyramidal (having the shape of a pyramid) and located at the base of the skull between the sphenoid and occipital bones. The petrous portion houses the internal ear and the middle ear, structures involved in hearing and equilibrium. The middle ear contains three small auditory ossicles, the malleus, incus, and stapes. It also contains the carotid foramen, through which the carotid artery passes (see Figure 7.6a-d). Posterior to the carotid foramen and anterior to the occipital bone is the jugular foramen, a passageway for the jugular vein, formed by adjacent notches in the temporal and occipital bones
Surface Feature: Mandible
In the lateral views shown in Figure 7.10, you can see that the mandible consists of a curved, horizontal portion, the body, and two perpendicular portions, the rami (RĀ-mī=branches). The angle of the mandible is the area where each ramus (singular form) meets the body. Each ramus has a posterior condylar process (KON-di-lar) that articulates with the mandibular fossa and articular tubercle of the temporal bone (see Figures 7.3c, d and 7.6c, d) to form the temporomandibular joint (TMJ). It also has an anterior coronoid process (KOR-ō-noyd) to which the temporalis muscle attaches. The depression between the coronoid and condylar processes is called the mandibular notch. The alveolar process is the ridge-like arch containing the alveoli (sockets) for the mandibular (lower) teeth. The mental foramen (ment-=chin) is approximately inferior to the second premolar tooth. It is near this foramen that dentists reach the mental nerve when injecting anesthetics. Another foramen associated with the mandible is the mandibular foramen on the medial surface of each ramus, another site often used by dentists to inject anesthetics. The mandibular foramen is the beginning of the mandibular canal, which runs obliquely in the ramus and anteriorly to the body. The inferior alveolar nerves and blood vessels pass through the canal and are distributed to the mandibular teeth.
temporomandibular joint (TMJ) syndrome
It is characterized by dull pain around the ear, tenderness of the jaw muscles, a clicking or popping noise when opening or closing the mouth, limited or abnormal opening of the mouth, headache, tooth sensitivity, and abnormal wearing of the teeth. TMJ syndrome can be caused by improperly aligned teeth, grinding or clenching the teeth, trauma to the head and neck, or arthritis. Treatments include application of moist heat or ice, limiting the diet to soft foods, administration of pain relievers such as aspirin, muscle retraining, use of a splint or bite plate to reduce clenching and teeth grinding (especially when worn at night), adjustment or reshaping of the teeth (orthodontic treatment), and surgery. •
Orbits
Seven bones of the skull join to form each orbit (eye socket) or orbital cavity, which contains the eyeball and associated structures (Figure 7.12). The three cranial bones of the orbit are the frontal, sphenoid, and ethmoid; the four facial bones are the palatine, zygomatic, lacrimal, and maxilla. Each pyramid-shaped orbit has four regions that converge posteriorly: 1.Parts of the frontal and sphenoid bones comprise the roof of the orbit. 2.Parts of the zygomatic and sphenoid bones form the lateral wall of the orbit. 3.Parts of the maxilla, zygomatic, and palatine bones make up the floor of the orbit. 4.Parts of the maxilla, lacrimal, ethmoid, and sphenoid bones form the medial wall of the orbit. Associated with each orbit are five openings: 1.The optic foramen (canal) is at the junction of the roof and medial wall. 2.The superior orbital fissure is at the superior lateral angle of the apex. 3.The inferior orbital fissure is at the junction of the lateral wall and floor. 4.The supraorbital foramen is on the medial side of the supra-orbital margin of the frontal bone. 5.The lacrimal fossa is in the lacrimal bone.
Sexual differences in skull
Sexual dimorphism, the structural differences that exist between males and females, is present in the skull, but skill and awareness of variations are essential when skull characteristics are used to determine the gender of an individual. Determining gender is only possible using the skull of a mature individual. In general, male skulls are always more massive and female skulls more gracile (delicate), but there is a zone where gracile males and robust females overlap that makes gender determination difficult. Many television shows and movies would have you believe this gender determination is easy, fast, and foolproof; however, even though the chances of correctly determining the gender of a skull increase with the number of characteristics considered, still only 80 to 90 percent accuracy is standard. Whenever possible, pelvic characteristics should be used in combination with skull characteristics to determine the gender of remains (see Section 8.4). Table 7.4 illustrates and describes some of the key visible characteristic differences in the male and female skulls.
Surface features: Occipital bone
The foramen magnum (=large hole) is in the inferior part of the bone. Within this foramen, the medulla oblongata (inferior part of the brain) connects with the spinal cord. The vertebral arteries, spinal arteries, and accessory (XI) nerve also pass through this foramen. The occipital condyles are two oval processes with convex surfaces, one on either side of the foramen magnum (see Figure 7.6a-d). They articulate with depressions on the first cervical vertebra (atlas) to form the atlanto-occipital joints. Superior to each occipital condyle on the inferior surface of the skull is the hypoglossal canal (hypo-=under; -glossal=tongue), through which the hypoglossal (XII) nerve and a branch of the ascending pharyngeal artery pass (see Figure 7.4). The external occipital protuberance is the most prominent midline projection on the posterior surface of the bone just superior to the foramen magnum. You may be able to feel this structure as a definite bump, the most prominent protrusion on the back of your head, just above your neck (Figure 7.5b). A large fibrous, elastic ligament, the ligamentum nuchae (nucha-=nape of neck), which helps support the head, extends from the external occipital protuberance to the seventh cervical vertebra. Extending laterally from the protuberance are two curved lines, the superior nuchal lines, and below these are two inferior nuchal lines, which are areas of muscle attachment (Figure 7.5b). It is possible to view the parts of the occipital bone, as well as surrounding structures, in the inferior view of the skull.
Surface feature: Lacrimal bones
The lacrimal bones each contain a lacrimal fossa, a vertical tunnel formed with the maxilla, that houses the lacrimal sac, a structure that gathers tears and passes them into the nasal cavity (see Figures 7.8b and 7.12). The medial surface of the bones is covered with mucous membrane and forms part of the upper wall of the nasal cavity.
Surface features: Ethmoid bone
The lateral masses of the ethmoid bone compose most of the wall between the nasal cavity and the orbits. They contain 3 to 18 air spaces called ethmoidal cells. The ethmoidal cells together form the ethmoidal sinuses (see Figure 7.13a, b). The midline perpendicular plate forms the superior portion of the nasal septum (see Figure 7.11). The horizontal cribriform plate (cribri-=sieve) lies in the anterior floor of the cranium and forms the roof of the nasal cavity. The cribriform plate contains the olfactory foramina (olfact-=smell), through which the olfactory (I) nerves pass. Projecting superiorly from the cribriform plate is a triangular process called the crista galli (crista=crest; galli=cock), which serves as a point of attachment for the falx cerebri, the membrane that separates the two hemispheres (sides) of the brain. The lateral masses of the ethmoid bone contain two thin, scroll-shaped projections lateral to the nasal septum. These are called the superior nasal concha (KONG-ka=shell) or superior nasal turbinate and the middle nasal concha (middle nasal turbinate). The plural form is conchae (KONG-kē). A third pair of conchae, the inferior nasal conchae, are separate bones (discussed shortly). The conchae increase the vascular and mucous membrane surface area in the nasal cavity, which warms, moistens, and humidifies inhaled air before it passes into the lungs. The conchae also cause inhaled air to swirl; the result is that many inhaled particles become trapped in the mucus that lines the nasal cavity. This action of the conchae helps cleanse inhaled air before it passes into the rest of the respiratory passageways. The superior nasal conchae are near the olfactory foramina of the cribriform plate where the sensory receptors for olfaction (smell) terminate in the mucous membrane of the superior nasal conchae. Thus, they increase the surface area for the sense of smell.
Facial bone: Lacrimal bones
The paired lacrimal bones (LAK-ri-mal; lacrim-=teardrops) are thin and roughly resemble a fingernail in size and shape (see Figures 7.2, 7.3, 7.8b, 7.2, 7.3, and 7.12). These bones, the smallest bones of the face, are posterior and lateral to the nasal bones and form a part of the medial wall of each orbit.
Facial Bone: Maxillae
The paired maxillae (mak-SIL-ē =jawbones; singular is maxilla) unite to form the upper jawbone. They articulate with every bone of the face except the mandible (lower jawbone) (see Figures 7.2, 7.3, and 7.9a). The maxillae form part of the floors of the orbits, part of the lateral walls and floor of the nasal cavity, and most of the hard palate. The hard palate is the bony roof of the mouth, and is formed by the palatine processes of the maxillae and horizontal plates of the palatine bones. The hard palate separates the nasal cavity from the oral cavity.
Surface Feature: Palatine Bones
The posterior portion of the hard palate, which separates the nasal cavity from the oral cavity, is formed by the horizontal plates of the palatine bones (see Figure 7.6c, d and 7.8d). The perpendicular plates of the palatine bones contribute to the walls of the nasal cavity and the orbital processes help form the orbits.
Surface features: Sphenoid Bone
The shape of the sphenoid resembles a butterfly with outstretched wings (Figure 7.6e, f). The body of the sphenoid is the hollowed, cubelike medial portion between the ethmoid and occipital bones. The hollow of the body is the sphenoidal sinus, which drains via a narrow opening into the superior aspect of the nasal cavity (see Figure 7.13c). The sella turcica (SEL-a TUR-si-ka; sella=saddle; turcica=Turkish) is a bony, saddle-shaped structure on the superior surface of the body of the sphenoid (Figure 7.6a, b). The anterior part of the sella turcica, which forms the horn of the saddle, is a ridge called the tuberculum sellae. The seat of the saddle is a depression, the hypophyseal fossa (hī-po-FIZ-ē-al), which contains the pituitary gland. The posterior part of the sella turcica, which forms the back of the saddle, is another ridge called the dorsum sellae. The greater wings of the sphenoid project laterally from the body, forming the anterolateral floor of the cranium (Figure 7.6a, b, e, f) The greater wings also form part of the lateral wall of the skull just anterior to the temporal bone and can be viewed externally. The lesser wings, which are smaller than the greater wings, form a ridge of bone anterior and superior to the greater wings. They form part of the floor of the cranium and the posterior part of the orbit of the eye. Between the body and lesser wing just anterior to the sella turcica is the optic foramen or canal (optic=eye), through which the optic (II) nerve and ophthalmic artery pass into the orbit (Figure 7.6e, f). Lateral to the body between the greater and lesser wings is an elongated, triangular slit called the superior orbital fissure. This fissure may also be seen in the anterior view of the orbit in Figure 7.12. Blood vessels and cranial nerves pass through this fissure. The pterygoid processes (TER-i-goyd=winglike) extend from the inferior part of the sphenoid bone (Figure 7.6c-f). These structures project inferiorly from the points where the body and greater wings unite and they form the lateral posterior region of the nasal cavity. Some of the muscles that move the mandible attach to the pterygoid processes. At the base of the lateral pterygoid process in the greater wing is the foramen ovale (=oval), an opening for the mandibular branch of the trigeminal (V) nerve (Figure 7.6a-d). Another foramen, the foramen spinosum (=resembling a spine, because of its proximity to the sharp spine of the sphenoid), lies at the posterior angle of the sphenoid and transmits the middle meningeal blood vessels. The foramen lacerum (=lacerated) is bounded anteriorly by the sphenoid bone and posteriorly by the temporal and occipital bones. This foramen is covered in part by a layer of fibrocartilage in living subjects and is a joint uniting the three bones. It transmits a branch of the ascending pharyngeal artery and numerous emissary veins. Another foramen associated with the sphenoid bone is the foramen rotundum (=round) located at the junction of the anterior and medial parts of the sphenoid bone. The maxillary branch of the trigeminal (V) nerve passes through the foramen rotundum
Surface features: zygomatic bones
The temporal process of the zygomatic bone projects posteriorly and articulates with the zygomatic process of the temporal bone to form the zygomatic arch (see Figures 7.3c, d and 7.9b). Posteriorly, the bone forms a concave temporal surface that contributes to the temporal region of the skull and houses the tendon of the strong jaw-closing muscle called the temporalis. A foramen called the zygomaticofacial foramen, located near the center of the zygomatic bone (Figure 7.9b), transmits the zygomaticofacial nerve and vessels.
hyoid bone
U-shaped) is a unique component of the axial skeleton because it does not articulate with any other bone. Rather, it is suspended from the styloid processes of the temporal bones by ligaments and muscles (see Figure 11.7a, b). Located in the anterior neck between the mandible and larynx (Figure 7.16a), the hyoid bone supports the tongue, providing attachment sites for some tongue muscles and for muscles of the neck and pharynx. The hyoid bone consists of a horizontal, rectangular body and paired projections called the lesser horns and the greater horns (Figure 7.16b, c). Muscles and ligaments attach to the body and these paired projections. The hyoid bone, as well as cartilages of the larynx and trachea, are often fractured during strangulation. As a result, they are carefully examined at autopsy when manual strangulation is a suspected cause of death.
paranasal sinuses
are cavities within certain cranial and facial bones near the nasal cavity. They are most evident in a sagittal section of the skull (Figure 7.13). The paranasal sinuses are lined with mucous membranes that are continuous with the lining of the nasal cavity through small openings in the lateral wall of the nasal cavity. Secretions produced by the mucous membrane of the paranasal sinuses drain into the lateral wall of the nasal cavity. Paranasal sinuses are rudimentary or absent at birth and increase in size during two critical periods of facial enlargement—during the eruption of the teeth and at the onset of puberty. They arise as outgrowths of the mucosal lining of the nasal cavity that project into the surrounding bones. Skull bones containing the paranasal sinuses are the frontal, sphenoid, and ethmoid bones, and the maxillae. The paranasal sinuses allow the skull to increase in size without a corresponding change in the mass (weight) of the bone. In addition, the paranasal sinuses serve as resonating (echo) chambers within the skull that intensify and prolong sounds, thereby enhancing the quality of the voice. The influence of the paranasal sinuses on your voice becomes obvious when you have a cold; the passageways through which sound travels into and out of the paranasal sinuses become blocked by excess mucus production, changing the quality of your voice.
Facial bone: Nasal bones
are small, flattened, rectangular-shaped bones that form the bridge of the nose (see Figures 7.2 and 7.8a). These small bones protect the upper entry to the nasal cavity and provide attachment for a couple of thin muscles of facial expression. For those of you who wear glasses, they are the bones that form the resting place for the bridge of the glasses. The major structural portion of the nose consists of cartilage.
Sutures
is an immovable joint in most cases in an adult skull that holds most skull bones together. Sutures in the skulls of infants and children, however, often are movable and function as important growth centers in the developing skull. In older individuals, many of the sutures in the skull fuse and become completely immovable. The names of many sutures reflect the bones they unite. For example, the frontozygomatic suture is between the frontal bone and the zygomatic bone. Similarly, the sphenoparietal suture is located between the sphenoid bone and the parietal bone. In other cases, however, the names of sutures are not so obvious. Of the many sutures found in the skull, the following are the most prominent: •The coronal suture (ko-RŌ-nal; coron-=relating to the frontal or coronal plane) unites the frontal bone and both parietal bones (see Figure 7.3). •The sagittal suture (SAJ-i-tal; sagitt-=arrow) unites the two parietal bones on the superior midline of the skull (see Figure 7.3a, b). The sagittal suture is so named because in the infant, before the bones of the skull are firmly united, the suture and the fontanels (soft spots) associated with it resemble an arrow. •The lambdoid suture (LAM-doyd) unites the two parietal bones to the occipital bone. This suture is so named because of its resemblance to the Greek letter lambda (Λ), as can be seen in Figure 7.5 (with the help of a little imagination). Sutural or wormian bones may occur within the sagittal and lambdoid sutures (see Figure 7.5 and Section 6.2). •The two squamous sutures (SKWĀ-mus; squam-=flat, like the flat overlapping scales of a snake) unite the parietal and temporal bones on the lateral aspects of the skull (see Figure 7.3c, d).
Sinusitis
is an inflammation of the mucous membrane of one or more paranasal sinuses. It may be caused by a microbial infection (virus, bacterium, or fungus), allergic reactions, nasal polyps, or a severely deviated nasal septum. If the inflammation or an obstruction blocks the drainage of mucus into the nasal cavity, fluid pressure builds up in the paranasal sinuses, and a sinus headache may develop. Other symptoms may include nasal congestion, inability to smell, fever, and cough. Treatment options include decongestant sprays or drops, oral decongestants, nasal corticosteroids, antibiotics, analgesics to relieve pain, warm compresses, and surgery.
Cranial bone: Sphenoid Bone
lies at the middle part of the base of the skull (Figure 7.6a-d). This bone is called the keystone of the cranial floor because it articulates with all the other cranial bones, holding them together. When you view the floor of the cranium superiorly (Figure 7.6a, b), note the sphenoid articulations: anteriorly with the frontal and ethmoid bones, laterally with the temporal and parietal bones, anterolaterally with the parietal bones, and posteriorly with the occipital bone. The sphenoid bone lies posterior and slightly superior to the nasal cavity and forms part of the floor, sidewalls, and rear wall of the orbit
Formina
most of the foramina (openings for blood vessels, nerves, or ligaments) of the skull in the descriptions of the cranial and facial bones that they penetrate. As preparation for studying other systems of the body, especially the nervous and cardiovascular systems, these foramina and the structures passing through them are listed in Table 7.3. For your convenience and for future reference, the foramina are listed alphabetically.
black eye
s a bruising around the eye, commonly due to an injury to the face, rather than an eye injury. In response to trauma, blood and other fluids accumulate in the space around the eye, causing the swelling and dark discoloration. One cause might be a blow to the sharp ridge just superior to the supraorbital margin that fractures the frontal bone, resulting in bleeding. Another is a blow to the nose. Certain surgical procedures (face lift, eyelid surgery, jaw surgery, or nasal surgery) can also result in black eyes.
Cranial Bones: Ethmoid Bone
s a delicate bone located in the anterior part of the cranial floor between the two orbits and is spongelike in appearance (Figure 7.7). It is anterior to the sphenoid bone and posterior to the nasal bones. The ethmoid bone forms (1) part of the anterior portion of the cranial floor; (2) the thin, medial wall of the orbits; (3) the superior portion of the nasal septum, a partition that divides the nasal cavity into right and left sides; and (4) most of the superior sidewalls of the nasal cavity. The ethmoid bone is a major superior supporting structure of the nasal cavity and forms extensive surface area in the nasal cavity.
deviated nasal septum
s one that does not run along the midline of the nasal cavity. It deviates (bends) to one side. A blow to the nose can easily damage, or break, this delicate septum of bone and displace and damage the cartilage. Often, when a broken nasal septum heals, the bones and cartilage deviate to one side or the other. This deviated septum can block airflow into the constricted side of the nose, making it difficult to breathe through that half of the nasal cavity. The deviation usually occurs at the junction of the vomer bone with the septal cartilage. Septal deviations may also occur due to developmental abnormality. If the deviation is severe, it may block the nasal passageway entirely. Even a partial blockage may lead to infection. If inflammation occurs, it may cause nasal congestion, blockage of the paranasal sinus openings, chronic sinusitis, headache, and nosebleeds. The condition usually can be corrected or improved surgically. •
