CHAPTER 8: Skeletal System: Axial and Appendicular Skeleton

Identify the locations of cranial and facial bones in various views of the skull.

A cursory glance at the skull reveals numerous bone markings, such as canals, fissures, and foramina that serve as passageways for blood vessels and nerves. The major foramina of the cranial and facial bones are summarized in table 8.1. Refer to this table as we examine the skull from various directions. (This table also will be important when we study cranial nerves in section 13.9 and blood vessels in section 20.10.) An anterior view (figure 8.4) shows several major bones of the skull. The frontal bone forms the forehead. The left and right orbits (figure 8.3) are formed from a complex articulation of multiple skull bones. There are two large openings within each orbit called the superior orbital fissure and the inferior orbital fissure (figure 8.4). Superior to the orbits on the anterior surface of the frontal bone are the superciliary (suˉ -per-sil′ē- ār- ē; super = above, cilium = eyelid) arches, otherwise known as the brow ridges. Male skulls tend to have larger and more pronounced superciliary arches than do female skulls. The left and right nasal bones form the bony bridge of the nose. Superior to the nasal bones and between the orbits is a land- mark area called the glabella (gla ̆-bel′a ̆; glabellus = smooth). The left and right maxillae (mak-sil′ē; sing., maxilla, mak-sil-a ̆; jawbone), also called maxillary bones, fuse in the midline to form most of the upper jaw and the lateral boundaries of the nasal cavity. The maxillae also help form a portion of both the floor of each orbit and the roof of the oral cavity. Inferior to each orbit in the maxilla is an infraorbital foramen, which is a passageway for blood vessels and nerves to the face. The lower jaw is formed by the mandible. The prominent chin of the mandible is called the mental protuberance. The oral margins of the maxillae and mandible each have alveolar (al-vē′ō-lăr) processes that contain the teeth. The nasal cavity is also seen in an anterior view. Its inferior bor- der is marked by a prominent anterior nasal spine. The thin ridge of bone that divides the nasal cavity into left and right halves is called the nasal septum. Along the inferolateral walls of the nasal cavity are two scroll-shaped bones called the inferior nasal conchae (kon′kē; sing., concha, kon′ka ̆; shell). The superior view of the skull in figure 8.5a primarily shows four of the cranial bones: the frontal bone, both parietal (pa ̆-rī′eˇ-ta ̆l; paries = wall) bones, and the occipital (ok-sip′i-t a ̆ l; occiput = back of head) bone. The articulation between the frontal and pari- etal bones is the coronal suture, so named because it runs along a coronal plane. The sagittal suture connects the left and right parietal bones along the midline of the skull. Along the posterior one-third of the sagittal suture is either a single parietal foramen or paired parietal foramina, which serve as the passageway of small veins between the brain and the scalp. The lateral surface of each parietal bone exhibits a rounded, smooth area called the parietal eminence. The superior part of the lambdoid suture represents the articulation of the occipital bone with both parietal bones. The posterior view of the skull in figure 8.5b shows a portion of the occipital, parietal, and temporal bones, as well as the lambdoid suture between the occipital and parietal bones. Within the lambdoid suture, there may be one or more sutural bones. The external occipital protuberance (pr ō-t uˉ ′ber-ans) is a prominence on the posterior aspect of the skull. Palpate the back of your head; males tend to have a prominent, pointed external occipital protuberance, whereas females have a more subtle, rounded protuberance. Intersecting the external occipital protuberance are two horizontal ridges, the superior and inferior nuchal (nuˉ′ka ̆l) lines (see figure 8.7).

Learn key bone markings and features of each of the bones of the cranium.

A lateral view of the skull (figure 8.6) shows one parietal bone, temporal bone, and zygomatic (zī′gō-mat′ik; zygoma = a joining, a yoke) bone. This view also shows part of the maxilla, mandible, frontal bone, and occipital bone. The superior and inferior temporal lines arc across the surface of the parietal and frontal bones and mark the attachment site of the temporalis muscle (see section 11.3c). The small lacrimal (lak′ri-măl; lacrima = a tear) bone articulates with the maxilla anteriorly and with the ethmoid bone posteriorly. A portion of the sphenoid (sfē′noyd; wedge-shaped) bone articulates with the frontal, parietal, and temporal bones. This region is called the pterion (tĕ′rē-on; ptéron = wing) and is circled on figure 8.6. Pterion includes the H-shaped set of sutures of these four articulating bones. The temporal process of the zygomatic bone and the zygomatic process of the temporal bone fuse to form the zygomatic arch. Put your fingers along the bony prominences ("apples") of your cheeks and move your fingers posteriorly toward your ears; you are feeling the zygomatic arch. The zygomatic arch terminates superior to the point where the mandible articulates with the mandibular (man-dib′ū-lăr) fossa of the temporal bone. This articulation is called the temporomandibular joint (TMJ) and is described further in section 9.7a. By putting your finger anterior to your external ear opening and then opening and closing your jaw, you can feel that joint moving. The squamous part of the temporal bone lies directly inferior to the squamous suture. Immediately posterolateral to the mandibular fossa is the tympanic (tim-pan′ik; tympanon = drum) part of the temporal bone. This is a small, bony ring surrounding the external ear opening called theexternal acoustic meatus (mē-ā′tŭs; a passage), or external auditory canal (see section 16.5a). Inferior and posterior to this meatus is the mastoid (mas′toyd; masto = breast, eidos = resemblance) process, the bump you feel behind your external ear opening. The styloid (stī′loyd; stylos = pillar, post) process is a thin, pointed projection of bone located anteromedial to the mastoid process. It serves as an attachment site for several hyoid and tongue muscles (see section 11.3c). Cutting the skull along a sagittal sectional plane reveals bones that form the endocranium and the nasal cavity (figure 8.7a). The cranial cavity is formed from a complex articulation of the frontal, parietal, temporal, occipital, ethmoid (eth′moyd; ethmos = sieve), and sphenoid bones. Vessel impressions may be visible on the internal surface of the skull. The frontal sinus (a space within the frontal bone) and the sphenoidal sinus (open space within the sphenoid bone) are visible in a sagittal view. A sagittal sectional view also shows the bones that form the nasal septum more clearly. The perpendicular plate of the ethmoid forms the posterosuperior portion of the nasal septum, whereas the vomer (vō′mer; plowshare) forms the posteroinferior portion. (The anterior part of the nasal septum is cartilaginous.) The ethmoid bone serves as the division between the anterior floor of the cranial cavity and the roof of the nasal cavity. The palatine process of the maxillae and the palatine (pal′a-t ̄ın) bones form the hard palate (figure 8.7b), which acts as both the floor of the nasal cavity and a portion of the roof of the mouth. Move your tongue along the roof of your mouth; you are palpating the maxillae anteriorly and palatine bones posteriorly. In an inferior (basal) view, the most anterior structure is the hard palate (figure 8.7b). On the posterior aspect of either side of the palate are the medial and lateral pterygoid (ter′i-goyd; pteryx = winglike) plates of the sphenoid bone. Together, both plates form a pterygoid process. Medially adjacent to these structures are the internal openings of the nasal cavity, called the choanae (kō′an-ē; sing., choana, kō′an-a ̆; funnel). Between the mandibular fossa and the pterygoid processes are several paired foramina and canals. Typically, these openings provide passage for specific blood vessels and nerves. For example, the jugular (jŭg′uˉ-lar; jugulum = throat) foramen is an opening between the temporal and occipital bones that provides a passageway for the internal jugular vein and several nerves. The foramen lacerum (anteromedial to the carotid canal) extends between the occipital and temporal bones. This opening is covered by cartilage in a living individual. The entrance to the carotid (ka-rot′id; karoo = to put to sleep) canal is anteromedial to the jugular foramen; the internal carotid artery passes through this canal. The stylomastoid foramen lies between the mastoid process and the styloid process. The facial nerve (CN VII) extends through the stylomastoid foramen to innervate the facial mus- cles (see sections 11.3a and 13.9). The largest foramen of all is the foramen magnum, literally meaning big hole. Through this opening, the spinal cord enters the cranial cavity and is continuous superiorly with the brainstem. On either side of the foramen magnum are the rounded occipital condyles, which articulate with the first cervical vertebra of the vertebral column. At the anteromedial edge of each condyle is a hypoglossal canal through which the hypoglossal nerve (CN XII) extends to innervate tongue muscles (see sections 11.3c and 13.9). When the top of the skull is cut and removed, the internal view of the cranial base (figure 8.8) is revealed. Here we see the frontal bone surrounding the delicate cribriform (krib′ri-fōrm; cribrum = sieve, forma = form) plate of the ethmoid bone. The plate has numerous perfora- tions called the cribriform foramina, which provide passageways for the olfactory nerves (CN I; see sections 13.9 and 16.3a) into the superior portion of the nasal cavity. The anteromedial part of the cribriform plate exhibits a midsagittal elevation called the crista galli (kris′ta ̆ = crest; gal′l ē = of a rooster), to which the cranial dural septa of the brain attach (see section 13.2a). The relatively large sphenoid is located posterior to the frontal bone. It is often referred to as a "bridging bone" because it unites the cranial and facial bones. The lateral expansions of the sphenoid bone are called the greater wings and the lesser wings of the sphenoid. The pituitary gland (see section 17.7a) is suspended inferiorly from the brain into a prominent midline depres- sion between the greater and lesser wings. This depression is termed the hypophyseal fossa, and the bony enclosure around the hypophyseal fossa is called the sella turcica (sel′a ̆ = saddle; tur′si-ka = Turkish). Anterior to the sella turcica are the optic canals through which the optic nerves (CN II) extend from the eyes in the orbits to the brain (figure 8.8). The lateral regions of the cranial base are formed by the petrous (pet′rŭs; petra = a rock) part of each temporal bone, whereas the posterior region is formed by the occipital bone. The internal acoustic meatus (also called the internal auditory canal) opens in the more medial portion of the temporal bone and contains the facial nerve (CN VII) and the vestibulocochlear nerve (CN VIII; see section 13.9). An internal landmark of the occipital bone is the internal occipital protuberance. The internal occipital crest extends from the protuberance to the posterior border of the foramen magnum. Large grooves along the internal aspect of the cranium are formed from impressions from the dural venous sinuses of the brain that lie within them (see section 13.2a). Each bone of the cranium has specific surface features, and each specific bone is shown and summarized in table 8.2. The facial bones are summarized in table 8.3.

List the fontanelles and the ages at which they close.

A neonatal (infant) cranium is shown in lateral and superior views in figure 8.15. The infant's cranial bones are not yet large enough to surround the brain completely, so some cranial bones are interconnected by flexible areas of dense regular connective tissue in regions called fontanelles (fon′ta ̆-nel = little spring; sometimes spelled fontanels). Fontanelles are sometimes referred to as the "soft spots" on a baby's head. The fontanelles enable some flexion in the bony plates within the skull during birth, thus allowing the child's head to pass through the birth canal to ease the baby's passage (see section 29.6d). Newborns frequently have a "cone-shaped" head due to this temporary deformation, but the cranial bones usually return to their normal position by a few days after birth. Some fontanelles, such as the small mastoid and sphenoidal fontanelles, close relatively quickly after birth. However others are present until many months after birth, when skull bone growth finally starts to keep pace with brain growth. The posterior fontanelle normally closes around 9 months of age; the larger anterior fontanelle doesn't close until about 15 months of age. The skull undergoes many more changes as we age. The maxil- lary sinus becomes more prominent after age 5, and by age 10 the frontal sinus is becoming well formed. Later, the cranial sutures start to fuse and ossify. As a person ages, the teeth start to wear down from use, a process called dental attrition. Finally, if an individual loses some or all of his or her teeth, the alveolar processes of the maxillae and mandible regress and eventually disappear.

Describe the phalanges and their relative locations.

A total of 14 bones are present in the digits; these are called phalanges (fă-lan′jēz; sing., phalanx, faˉ′langks; line of soldiers). Three phalanges are found in each of the second through fifth fingers, but only two phalanges are present within the thumb, also known as the pollex(pol′eks; thumb). The proximal phalanx articulates with the head of a metacarpal, whereas the distal phalanx is the bone in the very tip of the finger. The middle phalanx of each finger lies between the proximal and distal phalanges; however, a middle phalanx is not present in the pollex.

Compare and contrast the anatomy of male and female pelves.

Although it is possible to determine the sex of a skeleton by examining the skull, the most reliable indicator of sex is the pelvis, primarily the ossa coxae. The ossa coxae are the most sexually dimorphic bones of the body due to the demands of pregnancy and childbirth in females. For example, the female pelvis is shallower and wider than the pelvis of a male to accommodate the infant's head as it passes through the birth canal. Some of these differences are obvious, such as that males have nar- rower hips than females. But we can find many other differences by examining the shapes and orientations of the pelvic bones. For example, the female ilium flares more laterally, whereas the male ilium projects more superiorly, which is why males typically have narrower hips. Because the female pelvis is wider, the acetabulum projects more laterally, and the greater sciatic notch is much wider. In contrast, the male acetabu- lum projects more anteriorly, and the male greater sciatic notch is much narrower, deeper, and U-shaped. Females tend to have a preauricular sulcus, which is a depression or groove between the greater sciatic notch and the sacroiliac articulation. Males tend not to have this sulcus. The sacrum tends to be shorter and wider in females. The body of the pubis in females is much longer and almost rectangular in shape, compared to the shorter, triangular-shaped male pubic body. The subpubic angle (or pubic arch) is the angle formed when the left and right pubic bones are aligned at their symphysial surfaces. Because females have much longer pubic bones, the corre- sponding subpubic angle is much wider and more convex, usually much greater than 100 degrees. The male subpubic angle is much narrower and typically does not extend past 90 degrees. Several significant differences between the female and male pelves are shown and listed in table 8.6.

Explain how the radius, ulna, and humerus articulate.

Both the radius and the ulna exhibit interosseous borders, which face each other; the ulna's interosseous border faces laterally, whereas the interosseous border on the radius faces medially. These interosseous bor- ders are connected by an interosseous membrane (interosseous ligament) composed of dense regular connective tissue. This membrane helps keep the radius and ulna a fixed distance apart from one another and provides a pivot of rotation for the forearm. The bony joints that move during this rotation are the proximal and distal radioulnar joints.

Become familiar with terminology for common bone markings.

Distinctive bone markings are the surface features that characterize each bone in the body. Projections from the bone surface mark the points where muscles, tendons, and ligaments attach. Sites of articu- lation between adjacent bones tend to be smooth areas. Depressions, grooves, and openings through bones indicate sites where blood ves- sels and nerves travel. Anatomists use specific terms to describe these characteristics (figure 8.2). Knowing the names of bone markings will help you learn about specific bones described in this chapter. For example, when trying to locate the foramen magnum of the skull, you have an advantage if you know that foramen means hole or passageway.

Describe how the ossa coxae articulate with the femora and sacrum.

Each os coxae articulates posteriorly with the sacrum at the sacroiliac joint (figure 8.28). The femur articulates with a deep, curved depression on the lateral surface of the os coxae called the acetabulum (as-e ̆-tab′yuˉ-lŭm; shallow cup). The acetabulum contains a smooth, curved surface, called the lunate surface, which is C-shaped and articulates with the femoral head. The ilium, ischium, and pubis all contribute a portion to the acetabulum—thus, it represents a region where these bones have fused.

Identify skeletal features common to the upper and lower limbs.

Humans evolved from quadrupeds, which are animals that move on four feet. Quadruped limbs are very similar because all of the limbs are structured to support the body weight and move the animal. However, as our ancestors evolved into modern human beings, we became bipedal. Only our lower limbs normally support our body weight and are responsible for moving our bodies when we walk or run. In contrast, our upper limbs have been freed from these functions and are able to do other things, such as grasping objects and utilizing tools with our hands. Our upper and lower limb skeletons share some common features based on this evolutionary history, and they exhibit some differences based on the primary functions of each limb. Figure 8.22 summarizes the similarities. The proximal part of both upper and lower limbs are supported by a girdle of bones; the pectoral girdle (clavicles and scapulae) holds the upper limbs in place, whereas the pelvic girdle (i.e., both ossa coxae) articulates with the lower limb. The proximal part of each limb has one large bone: the humerus in the upper limb and the femur in the lower limb. The distal part of each limb contains two bones; these bones are able to pivot slightly about one another. Both the wrist and the proximal foot contain multiple bones (carpal and tarsal bones, respectively) that allow for a range of movement. Finally, the feet and hands are very similar in that both contain either 5 metacarpals (palm of hand) or 5 metatarsals (arch of foot), and each contains a total of 14 phalanges (bones of the fingers and toes, respectively).

Differentiate between supination and pronation of the forearm.

In anatomic position, the palm of the hand is facing anteriorly, and the bones of the forearm are said to be in supination (suˉ′pi-nā′shŭn) (figure 8.26c). Note that the radius and the ulna are parallel with one another. If you view your own supinated forearm, the radius is on the lateral (thumb) side of the forearm, and the ulna is on the medial (little finger) side. Pronation (prō-nā′shŭn) of the forearm requires that the radius cross over the ulna and that both bones pivot along the interosseous membrane (figure 8.26d). When the forearm is pronated, the palm of the hand is facing posteriorly and the head of the radius is still along the lateral side of the elbow, but the distal end of the radius has crossed over and become a more medial structure. When an individual is in anatomic position (i.e., has the upper limbs extended and forearms supinated), note that the bones of the forearm may angle laterally from the elbow joint. This positioning is referred to as the carrying angle of the elbow, and this angle posi- tions the bones of the forearms such that the forearms will clear the hips during walking as the forearms swing. Females have wider carrying angles than males, presumably because they have wider hips than males.

Compare and contrast upper and lower limb bud development.

Initially, the limb buds are cylindrical. The distal portion of the upper limb bud forms a rounded, paddle-shaped hand plate by the early fifth week. It later becomes both the palm and fingers. In the lower limb bud, a corresponding foot plate forms during the sixth week. These plates develop longitudinal thickenings called digital rays, which eventually form the digits. The digital rays in the hand plate appear in the late sixth week, and the foot digital rays appear during the early seventh week. The digital rays initially are connected by intermediately placed tissue, which later undergoes programmed cell death (apoptosis; see section 4.10). Thus, as this intermediate tissue dies, notching occurs between the digital rays, and separate digits are formed. This process occurs in the seventh week and is complete by the eighth week for both the fingers and the toes. As mentioned in section 7.4, bone forms by either intramembra- nous ossification or endochondral ossification. The flat bones of the skull; several facial bones, including the zygomatic, maxilla, and mandible; and the central part of the clavicle are formed from intra- membranous ossification, whereas almost all of the remaining bones of the skeleton form through endochondral ossification.

Identify the parts of a typical vertebra.

Most vertebrae share some common structural features (figure 8.17). The anterior region of each vertebra is a thick, cylindrical body, or centrum, which is the weightbearing structure of each vertebra. Posterior to the vertebral body is the vertebral arch, also called the neural arch. The body together with the vertebral arch enclose an opening called the vertebral foramen. All the stacked vertebral foramina collectively form a superior-to-inferior directed verte- bral canal that contains the spinal cord. Lateral openings between adjacent vertebrae are the intervertebral foramina. The interver- tebral foramina provide a horizontally directed passageway through which spinal nerves extend to various parts of the body (see section 14.5). The vertebral arch is composed of two pedicles and two lami- nae. The pedicles (ped′ĭ-kĕl; pes = foot) originate from the postero- lateral margins of the body, whereas the laminae (lam′i-nē; sing., lamina, lam′i-na ̆ = layer) extend posteromedially from the posterior edge of each pedicle. A spinous process projects posteriorly from the junction of the left and right laminae. Most of these spinous processes can be palpated along the skin of the back. Lateral projec- tions on both sides of the vertebral arch are called transverse processes. Each vertebra has superior and inferior articular processes that originate at the junction between the pedicles and laminae. Each articular process has a smooth surface called an articular facet (fas′et, fă-set′). The facets on the inferior articular processes of each vertebra articulate with the facets on the superior articular processes of the vertebra immediately inferior to it. The stack of vertebral bodies is stabilized and interconnected by ligaments. Adjacent vertebral bodies are separated by pads of fibrocartilage, called the intervertebral (in-ter-ver′te-brăl) discs. Intervertebral discs are composed of an outer ring of fibrocartilage, called the anulus fibrosus (an′ū-lŭs fī-brō′sus), and an inner gelatinous, circular region, called the nucleus pulposus (shown in Clinical View 8.4: "Herniated Discs"). Intervertebral discs make up approximately one-quarter of the entire vertebral column length. They act as shock absorbers between the vertebral bodies and per- mit the vertebral column to bend. For example, when you bend your torso anteriorly, the intervertebral discs are compressed at the bending (anterior) surface and pushed out at the opposite (posteri- or) surface. Over the course of a day, as body weight and gravity act on the vertebral column, the intervertebral discs become compressed and flattened. But while a person is lying horizontally during sleep, the intervertebral discs are able to expand and spring back to their origi- nal shape. In general, the vertebrae are smallest near the skull. They become gradually larger moving inferiorly through the body trunk as weight- bearing increases. Although vertebrae are divided into regions, there typically are no anatomically discrete "cutoffs" between the regions. For example, the most inferior cervical vertebra has some structural similarities to the most superior thoracic vertebra, as the two verte- brae are adjacent to one another. Likewise, the most inferior thoracic vertebra may look similar to the first lumbar vertebra. Table 8.5 compares the characteristics of the cervical, thoracic, and lumbar ver- tebrae and lists unique features of each regional group of vertebrae.

Describe the three arches of the foot and their functions.

Normally, the sole of the foot is arched, which helps it support the weight of the body and ensures that the blood vessels and nerves on the sole of the foot are not pinched when we are standing. The three arches of the foot are the medial longitudinal, lateral longitudinal, and transverse arches (figure 8.35). The medial longitudinal arch is the highest of the three arches and extends from the heel to the great toe. It is formed from the cal- caneus, talus, navicular, and cuneiform bones and metatarsals I-III. The medial longitudinal arch prevents the medial side of the foot from touching the ground and gives our footprint its characteristic shape (figure 8.35d). The lateral longitudinal arch is not as high as the medial arch, so the lateral part of the foot does contribute to a footprint. This arch extends between the little toe and the heel, and it is formed from the calcaneus and cuboid bones and metatarsals IV and V. The transverse arch runs perpendicular to the longitudinal arches. It is formed from the distal row of tarsals and the bases of all five metatarsals. The shape of the foot arches is maintained primarily by the foot bones themselves. These bones are shaped so that they can interlock and support their weight in an arch, much as the wedge-shaped blocks of an arched bridge can support the bridge without other mechanical supports. Secondarily, strong ligaments that attach to the bones and contracting muscles pull on the tendons, thereby helping to maintain the arches' shapes.

Identify key landmarks and features of the femur.

On the distal, inferior surface of the femur there are two smooth, oval articulating surfaces called the medial and lateral condyles. Superior to each condyle are projec- tions called the medial epicondyle and lateral epicondyle, respectively. When you flex your knee, you can palpate these epicondyles in the thigh on the sides of your knee joint. The medial and lateral supracondylar lines terminate at these epicondyles. On the distal pos- terior surface of the femur, a deep intercondylar fossa separates the two condyles. A smooth medial depression on the anterior surface, called the patellar surface, is the place where the patella articulates with the femur.

Differentiate between true ribs and false ribs.

Ribs 1-7 are called true ribs. True ribs articulate directly and individually to the sternum by separate cartilaginous extensions called costal (kos′tal; costa = rib) cartilages (figure 8.20). The smallest true rib is the first. Ribs 8-12 are called false ribs because their costal cartilages do not articulate directly to the sternum. The costal cartilages of ribs 8-10 fuse to the costal cartilage of rib 7 and thus indirectly articulate with the sternum. The last two pairs of false ribs (ribs 11 and 12) are called floating ribs because they have no articulation with the sternum. The vertebral bodies articulate with the head of a rib (figure 8.21). The articular surface of the head is divided into superior and inferior articular facets by an interarticular crest. The surfaces of these facets articulate with the costal facets or demifacets on the bodies of the thoracic vertebrae. The neck of the rib lies between the head and the tubercle. The tubercle of the rib has an articular facet for the costal facet on the transverse process of the thoracic vertebra. Figure 8.21b, c illustrates how most of the ribs articulate with the thoracic vertebrae. The angle (border) of the rib indicates the site where the tubular shaft begins to curve anteriorly toward the sternum. A prominent costal groove along its inferior internal border marks the path of spinal nerves (see section 14.5c) and blood vessels to the thoracic wall.

Describe the locations of the sutures between the cranial bones.

Sutures (suˉ′chuˉr; sutura = a seam) are immovable fibrous joints (see section 9.2b) that form the boundaries between the cranial bones (see figures 8.5-8.7). Dense regular connective tissue connects cranial bones firmly together at a suture. The sutures often have intricate, interlocking forms resembling puzzle pieces. Numerous sutures are present in the skull, each with a specific name. Many of the smaller sutures are named for the bones or features they interconnect. For example, the occipitomastoid suture connects the occipital bone with the portion of the temporal bone that houses the mastoid process. Here we discuss only the largest sutures—the coronal, lambdoid, sagittal, and squamous sutures: ∙ The coronal (kō-rō′nal; coron = crown) suture extends laterally across the superior surface of the skull along a coronal plane. It represents the articulation between the anterior frontal bone and the more posterior parietal bones. ∙ The lambdoid (lam′doyd) suture extends like an arc across the posterior surface of the skull. This suture is the site where the parietal bones and the occipital bone articulate. It is named for the Greek letter lambda, which its shape resembles. ∙ The sagittal (saj′i-tăl; sagitta = arrow) suture extends between the coronal and lambdoid sutures along the midsagittal plane. It is the site where the right and left parietal bones articulate. ∙ A squamous (skwā′mus) suture (or squamosal suture) on each side of the skull is the site where the temporal bone and the parietal bone of that side articulate. The squamous part of the temporal bone typically overlaps the parietal bone. One common variation in sutures is the presence of sutural bones (Wormian bones) (see figure 8.5b). Sutural bones may range in size from a tiny pebble to a quarter, but they can be much larger. Sutural bones represent independent bone ossification centers and are most common and numerous in the lambdoid suture. In adulthood, the sutures typically are obliterated (closed) as the adjoining bones fuse. This fusion starts internally and is followed by fusion on the skull's external surface. Although the timing of suture closure can be highly variable, the coronal suture typically is the first to fuse, usually in the late 20s to early 30s, followed by the sagittal suture (usually in the 30s or later) and then the lambdoid suture (usually in the 40s). The squamous suture usually does not fuse until late adulthood (60-plus years), or it may not fuse at all. Osteologists can estimate the approximate age at death of an individual by examining the extent of suture closure in the skull.

Name the bones that make up each os coxae.

The adult pelvis (pel′vis; pl., pelves, pel′vēz; basin) is composed of four bones: the sacrum, the coccyx, and the right and left ossa coxae (os′ă kok′să; sing., os coxae; hip bone) (figure 8.28). The pelvis protects and supports the viscera in the inferior part of the ventral body cavity (see section 1.5e). The term pelvic girdle refers to both the left and right ossa coxae only. The pelvic girdle articulates with the trunk and provides an attachment point for each lower limb. When a person is standing upright, the pelvis is angled slightly anteriorly. The os coxae is commonly referred to as the hip bone (and sometimes as the coxal bone or the innominate bone). Each os coxae is formed from three separate bones: the ilium, ischium, and pubis (figure 8.29). These three bones fuse between the ages of 13 and 15 years to form the single os coxae.

Describe the functions of the vertebral column.

The adult vertebral column is composed of 26 bones, including 24 individual vertebrae (ver′te ̆-brē; sing., vertebra, ver′teˇ-bra ̆) and the fused vertebrae that form both the sacrum and the coccyx. Each vertebra (except the first and the last) articulates with one superior vertebra and one inferior vertebra. Here we consider the vertebral column's general function and regions, the curves of the spine, anatomy of a generalized vertebra, and anatomic details of the components of the five regions of the vertebral column. The vertebral column provides vertical support for the body and sup- ports the weight of the head. It helps maintain an upright body position. Most important, it houses and protects the delicate spinal cord.

Describe how the limb buds form.

The appendicular skeleton begins to develop during the fourth week of development, when limb buds appear as small ridges along the lateral sides of the embryo. The upper limb buds appear early in the fourth week (approximately day 26), and the lower limb buds appear a few days later (day 28) (figure 8.36). Lower limb development lags behind upper limb development by about 2 to 4 days. The upper and lower limbs form proximodistally, meaning that the more proxi- mal parts of the limbs form first (in weeks 4-5), whereas the more distal parts differentiate later. Early limb buds are composed of lateral plate mesoderm and covered by a layer of ectoderm (see section 5.6a). The musculature of the limbs forms from somitic mesoderm that migrates to the develop- ing limbs during the fifth week of development. At the apex of each limb bud, part of the ectoderm forms an elevated thickening called the apical ectodermal (ek-tō-der′ma ̆l) ridge. By mechanisms not completely understood, this ridge "signals" the underlying tissue to form the various components of the limb.

Locate and identify the auditory ossicles.

The auditory ossicles and the hyoid bone are bones of the axial skeleton associated with the skull. Auditory ossicles (os′i-kl) are three tiny ear bones housed within the petrous part of each temporal bone. These bones—the malleus (mal′ē-us), the incus (ing′kŭs), and the stapes (stā′pēz)—are discussed in detail in section 16.5a.

Describe the phalanges and their relative locations.

The bones of the toes (like the bones of the fingers and pollex) are called phalanges. The toes contain a total of 14 phalanges. The great toe is the hallux (hal′ŭks; hallex = great toe), and it has only 2 phalanges (proximal and distal); each of the other four toes has 3 phalanges (proximal, middle, and distal).

Locate and identify the tarsals and metatarsals.

The bones that form the ankle and foot are the tarsals, metatarsals, and phalanges (figure 8.34). The seven tarsals (tar′să ̆l; tarsus = flat surface) of the ankle and proximal foot are similar to the eight carpal bones of the wrist in some respects, although their shapes and arrangement are different from those of their carpal bone counterparts. The talus, calcaneus, and navicular bone are considered the proximal row of tarsal bones. The superiormost and second largest tarsal bone is the talus (tā′lŭs; ankle bone), which articulates with the tibia. The calcaneus (kal-kā′nē-ŭs) is the largest tarsal bone and forms the heel. Its posterior end is a rough, knob-shaped projection that is the point of attach- ment for the calcaneal (Achilles) tendon extending from the strong posterior leg muscles (see section 11.9c). The navicular (nă ̆-vik′yūˉ-lăr; navis = ship) bone is on the medial side of the ankle. The distal row of four tarsal bones includes the cuneiforms and the cuboid bone. The medial cuneiform (kū′nē-i-fōrm; cuneus = wedge), intermediate cuneiform, and lateral cuneiform bones are wedge-shaped bones that articulate with and are positioned anterior to the navicular bone. The laterally placed cuboid (kyuˉ′boyd; kybos = cube) bone articulates at its medial surface with the lateral cuneiform and at its posterior surface with the calcaneus. The metatarsals (met′a ̆-tar′sa ̆l) of the foot are five long bones similar in arrangement and name to the metacarpal bones of the hand. They form the arched sole of the foot and are identified with Roman numerals I-V, proceeding medially to laterally. The metatarsals articulate proximally with either the cuneiform bones or the cuboid bone. Distally, each metatarsal bone articulates with a proximal phalanx. At the head of the first metatarsal are two tiny sesamoid bones, which insert on the tendons of the flexor hallucis brevis muscle and help these tendons move more freely (see section 11.9d).

Locate and identify the carpals and metacarpals.

The bones that form the wrist and hand are the carpals, metacarpals, and phalanges (figure 8.27). The carpals (kar′pa ̆l; karpus = wrist) are small, short bones that form the wrist. They are arranged in two rows (a proximal row and a distal row) of four bones each and allow for the multiple movements possible at the wrist. The proximal row of carpal bones, listed from lateral to medial, are the scaphoid (skaf′oyd; skaphe = boat), lunate (lu-′nāt; luna = moon),triquetrum (trī-kwē′trŭm; triquetrus = three-cornered), and pisiform (pis′i-fōrm; pisum = pea, forma = appearance). The distal row of the carpal bones, listed from lateral to medial, are the trapezium (tra-pē′zē-ŭm; trapeza = table), trapezoid (trap′e ̆-zoyd), capitate (kap′i-tāt), and hamate (ha′māt; hamus = hook). Bones in the palm of the hand are called metacarpals (met′a ̆-kar′pa ̆l; meta = beyond). Five metacarpal bones articulate with the distal carpal bones and support the palm. Roman numerals I-V denote the metacarpal bones, with metacarpal I located at the base of the thumb, and metacarpal V at the base of the little finger.

List the bones that form the orbital and nasal complexes.

The bony cavities called orbits enclose and protect the eyes and the muscles that move them. The orbital complex consists of seven bones that form each orbit. The borders of the orbital complex are shown and listed in figure 8.11. The nasal complex is composed of bones and cartilage that enclose the nasal cavity and the paranasal sinuses (see section 23.2a). Most of these bones are best seen in sagittal section, as shown in figure 8.12.

Identify the three main components of the sternum and their features.

The bony framework of the chest is called the thoracic cage and consists of the thoracic vertebrae posteriorly, the ribs laterally, and the sternum anteriorly (figure 8.20). The thoracic cage acts as a protective enclosure around the thoracic organs and provides attachment points for many muscles. The sternum (ster′nŭm; sternon = the chest), also referred to as the breastbone, is a flat bone that forms in the anterior midline of the thoracic wall. Its shape has been likened to that of a sword. The sternum is composed of three parts: the manubrium, the body, and the xiphoid process. The manubrium (ma ̆-nuˉ′brē-ŭm) is the widest and most superior portion of the sternum (the handle of the bony sword). The two clavicular notches of the sternum articulate with the left and right clavicles. The shallow superior indentation between the clavicular notches is called the suprasternal (or jugular) notch. A single pair of costal notches represent articulations for the first ribs' costal cartilages. The body is the longest part of the sternum and forms its bulk (the blade of the bony sword). Individual costal cartilages from ribs 2-7 are attached to the body at indented articular costal notches. The body and the manubrium articulate at the sternal angle, a horizontal ridge that may be palpated under the skin. The sternal angle is an important landmark in that the costal cartilages of the second ribs attach there; thus, it may be used to count the ribs. The xiphoid (zi′foyd; xiphos = sword) process represents the very tip of the sword. This small, inferiorly pointed, cartilaginous projection often doesn't ossify until after age 40.

Compare and contrast the different types of vertebrae.

The cervical vertebrae are the most superiorly located vertebrae. They typically have kidney-bean-shaped bodies and extend inferiorly from the occipital bone of the skull through the neck to the thorax. Because cervical vertebrae support only the weight of the head, their vertebral bodies are relatively small and light. Most cervical vertebrae are distinguished from other vertebrae by the presence of transverse foramina in their transverse processes that house the vertebral artery and vein (sometimes C7 does not have these foramina). Table 8.5 summarizes the key features of the typical cervical vertebra (C3-C6); the other cervical vertebrae are described here. Atlas (C1 ) The first cervical vertebra, called the atlas (at′las), supports the head through its articulation with the occipital condyles of the occipital bone (figure 8.18a). This vertebra is named for the Greek mythological figure Atlas, who carried the world on his shoulders. The articulation between the occipital condyles and the atlas, called the atlanto-occipital joint, permits us to nod our heads "yes." The atlas is readily distinguished from the other vertebrae because it lacks both a body and a spinous process. Instead, the atlas has lateral masses that are connected by semicircular anterior and posterior arches, each containing slight protuberances, the anterior and posterior tubercles (tuˉ′be ̆r-ke ̆l). The atlas has depressed, oval superior and inferior articular facets (table 8.5a) that articulate with the occipital condyles and the axis (C2), respectively. Finally, the atlas has an articular facet for dens on its anterior arch. Axis (C2) The body of the atlas separates from the atlas and fuses during development to the body of the second cervical vertebra, called the axis (ak′sis) (figure 8.18b). This fusion produces the most distinctive feature of the axis, the prominent dens, or odontoid (ō-don′toyd; odont = tooth). The dens acts as a pivot for the lateral rotation of both the atlas and the skull. This articulation between the atlas and axis, called the atlantoaxial joint, permits us to shake our heads "no" (figure 8.18c). This joint is stabilized by a transverse ligament. Vertebra Prominens (C ) The seventh cervical vertebra repre- 7 sents a transition from cervical to the thoracic vertebral region (see figure 8.16). The spinous processes of both C7 and all the thoracic vertebrae are nonbifid (not forked)—however, this process in C7 is much longer than it is within the other cervical vertebrae. It is easily palpated through the skin between the shoulder blades and inferior to the neck. Thus, C7 also is called the vertebra prominens (prom′ ̆ı- nens; prominent). This vertebra may or may not have transverse foramina. There are 12 thoracic vertebrae, and each articulates with the ribs (table 8.5). Thoracic vertebrae typically have heart-shaped bodies and are distinguished from all other types of vertebrae by the presence of costal facets or costal demifacets (semicircular facets) on the lateral side of the body and on the sides of the transverse processes. The head of the rib (see section 8.6b) articulates with the costal facet or demifacet on the body of the thoracic vertebra. The tubercle of the rib (see section 8.6b) articulates with the costal facets on the transverse processes of the vertebra. The thoracic vertebrae vary slightly with respect to their trans- verse costal facets. Vertebrae T1-T10 have transverse costal facets on their transverse processes; T11 and T12 lack these transverse costal facets because the eleventh and twelfth ribs do not have tubercles (and thus do not articulate with the transverse processes). The costal facets on the bodies of the thoracic vertebrae also display variations. Some vertebrae may have a single whole facet; others may have two demifacets. The largest vertebrae are the lumbar vertebrae, as they bear most of the weight of the body. A typical lumbar vertebra body is thicker than that of all the other vertebrae, and its body is oval or round (table 8.5). The lumbar vertebrae are distinguished by the features they lack— that is, lumbar vertebrae have neither transverse foramina (like the cervical vertebrae) nor costal facets (like the thoracic vertebrae). The thick spinous processes provide extensive surface area for the attachment of inferior back muscles that reinforce or adjust the lumbar curvature. The sacrum is an anteriorly curved, somewhat triangular bone that forms the posterior wall of the pelvic cavity (figure 8.19). The apex of the sacrum is a narrow, pointed portion of the bone that projects inferiorly, whereas the bone's broad superior surface forms the base. The sacrum is composed of five fused sacral vertebrae. These vertebrae start to fuse shortly after puberty and are usually completely fused between ages 20 and 30. The hori- zontal lines of fusion that remain are called transverse ridges. Superiorly, the sacrum articulates with L5 via a pair of superior articular processes. The vertebral canal becomes much narrower and continues through the sacrum on its posterior side as the sacral canal. The sacral canal terminates in an inferior opening called the sacral hiatus (hī-ā′tŭs; hio = to yawn). On either side of the sacral hiatus are bony projections called the sacral cornua. The anterosuperior edge of the first sacral vertebra bulges anteriorly into the pelvic cav- ity and is called the promontory. The paired anterior and posterior sacral foramina permit the passage of nerves to the pelvic organs and the gluteal region, respectively. A dorsal ridge, termed the median sacral crest, is formed by the fusion of the spinous pro- cesses of individual sacral vertebrae. On each lateral surface of the sacrum is the ala (meaning wing). On the lateral surface of the ala is the auricular surface, which marks the site of articulation with the os coxae of the pelvic girdle, forming the sacroiliac (sā-kr ō-il′ē-ak) joint (see table 9.5). Four small coccygeal vertebrae fuse to form the coccyx. These individual vertebrae begin to fuse by about age 25. The coccyx is an attachment site for several ligaments and some muscles. The first and second coccygeal vertebrae have unfused vertebral arches and transverse processes. The prominent laminae of the first coccygeal vertebrae are known as the coccygeal cornua, which curve to meet the sacral cornua. In males, the coccyx tends to project anteriorly, but in females it tends to project more inferiorly. In very elderly individuals, the coccyx may fuse with the sacrum.

Compare and contrast the locations and contents of three cranial fossae.

The contoured floor of the cranial cavity exhibits three curved depressions called the cranial fossae (figure 8.10). The anterior cranial fossa is the shallowest of the three depressions. It is formed by the frontal bone, the ethmoid bone, and the lesser wings of the sphenoid bone. The anterior cra- nial fossa houses the frontal lobes of the brain (see section 13.3b). The middle cranial fossa is inferior and posterior to the anterior cranial fossa. It ranges from the posterior edge of the lesser wings of the sphenoid bone (anteriorly) to the anterior region of the petrous part of the temporal bone (posteriorly). It houses the temporal lobes of the brain and the pituitary gland. The posterior cranial fossa is the most inferior and posterior cranial fossa and extends from the posterior region of the petrous part of the temporal bones to the occipital bone. This fossa supports part of the brainstem and the cerebellum (see sections 13.5 and 13.6).

Describe the structure and function of the hyoid bone.

The hyoid bone is a slender, curved bone located inferior to the skull between the mandible and the larynx (voice box) (figure 8.14). It does not articulate with any other bone in the skeleton. The hyoid has a medial body and two paired, hornlike processes, the greater cornua (kor′nū-ă = horn; sing. cornu, kōr′nū) and the lesser cornua. The cornua and body serve as attachment sites for tongue and anterior neck muscles and ligaments (see section 11.3d).

Describe landmarks and features of an os coxae.

The largest of the three coxal bones is the ilium (il′ē-ŭm; groin, flank), which forms the superior region of the os coxae and part of the acetabular surface. The wide, fan-shaped portion of the ilium is called the ala (ā′la ̆). The ala terminates inferiorly at a ridge called the arcuate (ar′kyuˉ-āt; arcuatus = bowed) line on the medial surface of the ilium. On the medial side of the ala is a depression termed the iliac fossa. On the lateral surface of the ilium, the anterior, posterior, and inferior gluteal (gluˉ′tē-a ̆l; gloutos = buttock) lines are attachment sites for the gluteal muscles of the buttock (see section 11.9a). The posteromedial side of the ilium exhibits a large, roughened area called the auricular (aw-rik′yuˉ-la ̆r; auris = ear) surface, where the ilium articulates with the sacrum. The superiormost ridge of the ilium is the iliac crest. Palpate the posterosuperior edges of your hips; the ridge of bone you feel on each side is the iliac crest. The iliac crest arises anteriorly from a projection called the anterior superior iliac spine and extends posteriorly to the posterior superior iliac spine. Located inferiorly to the ala of the ilium are the anterior inferior iliac spine and the posterior inferior iliac spine. The posterior inferior iliac spine is adjacent to a prominent greater sciatic notch (sī-at′ik; sciaticus = hip joint), through which the sciatic nerve extends to the lower limb (see section 14.5g). The ilium fuses with the ischium (is′kē-ŭm; ischion = hip) near the superior and posterior margins of the acetabulum. Posterior to the acetabulum, the prominent triangular ischial (is′kē-ăl) spine projects medially. The bulky bone superior to the ischial spine is called the body of the ischium. The lesser sciatic notch is a semicircular depression inferior to the ischial spine. The posterolateral border of the ischium is a roughened projection called the ischial tuberosity. The ischial tuberosities also are called the sitz bones by some health professionals and fitness instructors because they support the weight of the body when seated. If you palpate your buttocks while in a sitting position, you can feel the large ischial tuberosities. An elongated ramus (rā′mŭs; pl., rami, rā′mē) of the ischium extends from the ischial tuberosity toward its anterior fusion with the pubis. The pubis (pyuˉ′bis) fuses with the ilium and ischium at the acetabulum. The ramus of ischium fuses anteriorly with the inferior pubic ramus to form the ischiopubic ramus (figure 8.28). The superior pubic ramus originates at the anterior margin of the acetabulum. Between the superior and inferior pubic rami is an anteriorly placed mass of bone called the body of the pubis. The obturator (ob′tuˉ-rā-to ̆r; obturo = to occlude) foramen is a space in the os coxae that is encircled by both pubic and ischial rami. In a living individual, this foramen is covered with a connective tissue membrane. A roughened ridge, called the pubic crest, is located on the antero- superior surface of the superior pubic ramus, and it ends at the pubic tubercle. A roughened area on the body of the pubis, called the symphysial (sim-fiz′ē-a ̆l; growing together) surface or pubic symphysis, denotes the site of articulation between the pubic bones. On the medial surface of the pubis, the pectineal (pek-tin′ē-a ̆l) line originates and extends diagonally across the pubis to merge with the arcuate line.

Describe the articulations of the femur.

The lower limb is made up of the thigh, leg, and foot. The structure of the foot enables it to support the body during bipedal walking and running. The arrangement and numbers of bones in the lower limb are similar to those of the upper limb. Each lower limb contains a total of 30 bones: ∙ 1 femur, located in the femoral region ∙ 1 patella (kneecap), located in the patellar region ∙ 1 tibia and 1 fibula, located in the crural region ∙ 7 tarsal bones, which form the bones of the ankle and proximal foot ∙ 5 metatarsal bones, which form the arched part of the foot ∙ 14 phalanges, which form the toes The femur (fē′mŭr; thigh) is the longest bone in the body as well as the strongest and heaviest (figure 8.31). The nearly spherical head of the femur articulates with the os coxae at the acetabulum. There is a small depression within the head of the femur, called the fovea (fō′vē-ă; a pit), or fovea capitis. Here a small ligament connects the head of the femur to the acetabulum. Distal to the head, an elongated, constricted neck joins the shaft of the femur at an angle. This results in a medial angling of the femur, which brings the knees closer to the midline. The greater trochanter (trō-kan′ter; a runner) projects laterally from the junction of the neck and shaft. A lesser trochanter is located on the femur's posteromedial surface. These are rough processes that serve as attachment sites for powerful gluteal and thigh muscles (see section 11.9a). The greater and lesser trochanters are connected on the poste- rior surface of the femur by a thick oblique ridge of bone called the intertrochanteric (in′ter-trō-kan-tār′ik) crest. Anteriorly, a raised intertrochanteric line extends between the two trochanters and marks the distal edge of the hip joint capsule. Inferior to the intertrochanteric crest, the pectineal line marks the attachment of the pectineus muscle; the gluteal (glŭ′tē-ăl; gloutos = buttock) tuberosity marks the attachment of the gluteus maximus muscle (see section 11.9a). The gluteal tuberosity and pectineal line merge inferiorly into an elevated, midline ridge called the linea aspera (lin′ē-ă as′pĕr-ă), where many thigh muscles attach. Distally, the linea aspera branches into medial and lateral supracondylar lines. A flattened, triangular area, called the popliteal (pop-lit′ē-a ̆l; poples = ham of knee) surface, is bordered by these lines. The medial supracondylar ridge terminates in the adductor tubercle. This is a rough, raised projection that is the site of attachment for the adductor magnus muscle (see section 11.9).

Describe changes to the ossa coxae as a person ages.

The ossa coxae are an excellent indicator of both sex and age, and they can provide a reliable estimate of age at death. These estimates are given in age ranges (as opposed to precise num- bers) because some variation may occur in how an os coxae exhibits the age-related changes. Osteologists have noted age-related changes to the auricular surface of the ilium. The auricular surface of a young adult typically has some billowing texture to it (e.g., appears to have "hills" and "valleys"), and the surface is fine-grained. As the auricular surface ages, the billowing flattens out and the surface becomes more coarse and granular. In much older individuals, the surface may develop some bony lipping (evidence of osteoarthritic changes) and the surface becomes even more rough and irregular. Osteologists also have documented that the symphysial surface of the pubis under- goes uniform, age-related changes as well. In fact, the symphysial surface has become one of the most reliable indicators for estimating age at death. In a young adult (age range 15-24), the symphysial surface is billowed, and no well-formed rim is found around the surface. As the person ages, this billowing becomes more flattened, and a bony rim begins to form around the circumference of the symphysial surface. This rim is completed about ages 35-50 for most individuals. Once the rim is complete, the sym- physial surface becomes depressed and concave and may become pitted in much older individuals. The rim or border may start to break down, and bony lipping (arthritis) develops along the edges of the symphysial surface. These last stages typically occur after age 50.

Describe the location and function of the patella.

The patella (pa-tel′a ̆; patina = shallow disk), or kneecap, is a large, roughly triangular sesamoid bone housed within the tendon of the quadriceps femoris muscle (figure 8.32). The patella allows the tendon to glide more smoothly, and it protects the knee joint. The superior base of the patella is broad, whereas its inferior apex is pointed. The posterior aspect of the patella has an articular surface that articulates with the patellar surface of the femur.

Identify and locate the clavicle and its landmarks.

The pectoral (pek′to ̆-ra ̆l; pectus = breastbone) girdle articulates with the trunk and supports the upper limbs. A pectoral girdle consists of the clavicles and the scapulae. The clavicle (klav′i-kĕl; clavis = key), commonly known as the collarbone, is an elongated, S-shaped bone that extends between the manubrium of the sternum and the acromion of the scapula (figure 8.23). Its sternal end (medial end) is roughly pyramidal in shape and articulates with the manubrium of the sternum, forming the sternoclavicular joint (see section 9.7b). The acromial end (lateral end) of the clavicle is broad and flattened. The acromial end articulates with the acromion of the scapula, forming the acromioclavicular joint. You can palpate your own clavicle by first locating the superior aspect of your sternum and then moving your hand laterally. The curved bone you feel under your skin, and close to the neck opening of your shirt, is your clavicle. The superior surface of the clavicle is relatively smooth and the inferior surface is roughened (figure 8.23). On the inferior surface, near the acromial end, is a rough tuberosity called the conoid (kō′noyd; konoeides = cone-shaped) tubercle for the conoid ligament (part of the coracoclavicular ligament of the shoulder joint; see section 9.7b). The inferiorly located prominence at the sternal end of the clavicle is the costal tuberosity, for the attachment of the shoul- der's costoclavicular ligament.

Differentiate between the true and false pelves.

The pelvic brim is a continuous, oval ridge that extends from the pubic crest, pectineal line, and arcuate line to the rounded inferior edges of the sacral ala and promontory. This pelvic brim helps subdivide the entire pelvis into a true pelvis and a false pelvis (figure 8.30). The true pelvis, also known as the lesser pelvis, lies inferior to the pelvic brim. It encloses the pelvic cavity and forms a deep bowl that contains the pelvic organs. The false pelvis, also known as the greater pelvis, lies superior to the pelvic brim. It is enclosed by the alae of the ilia. It forms the inferior region of the abdominal cavity and houses the inferior abdominal organs.

Compare and contrast the pelvic inlet and pelvic outlet.

The pelvis also has a superior and an inferior opening, and each has clinical significance. The pelvic inlet, also known as the superior pelvic aperture, is the superiorly positioned space enclosed by the pelvic brim. In other words, the pelvic brim is the bony, oval ridge of bone, whereas the pelvic inlet is the space surrounded by the pelvic brim. The pelvic inlet is the opening at the boundary between the true pelvis and the false pelvis. The pelvic outlet, also known as the inferior pelvic aperture, is the inferiorly placed opening bounded by the coccyx, the ischial tuberosities, and the inferior border of the symphysial surface. In males, the ischial spines commonly project into the pelvic outlet, thereby narrowing the diameter of this outlet. In contrast, female ischial spines less frequently project into the pelvic outlet (so the birth canal will not be obstructed by these bony prominences). The pelvic outlet is covered with muscles and skin, and it forms the body region called the perineum (per′i-nē′ŭm) (see figure 11.17). The width and size of the pelvic outlet are especially important in females, because the opening must be wide enough to accommodate the fetal head during childbirth (see section 29.6).

Compare and contrast the features of the radius and the ulna.

The radius and ulna form the forearm (figure 8.26). In anatomic position, these bones are parallel, and the radius (rā′dē-ŭs; spoke of a wheel, ray) is located more laterally. The proximal end of the radius has a distinctive disc-shaped head that articulates with the capitulum of the humerus. A narrow neck extends from the radial head to the radial tuberosity (or bicipital tuberosity). The radial tuberosity is an attachment site for the biceps brachii muscle. The shaft of the radius curves slightly and leads to a wider distal end, where there is a laterally placed styloid (stī′loyd) process. This bony projection can be palpated on the lateral side of the wrist, just proximal to the thumb. On the distal medial surface of the radius is an ulnar notch, which articulates with the medial surface of the distal end of the ulna at the distal radioulnar joint. The ulna (ŭl′nă) is the longer, medially placed bone of the forearm. At the proximal end of the ulna, a C-shaped trochlear notch interlocks with the trochlea of the humerus. The posterosuperior aspect of the trochlear notch has a prominent projection called the olecranon. The olecranon articulates with the olecranon fossa of the humerus and forms the posterior "bump" of the elbow. The inferior lip of the trochlear notch, called the coronoid process, articulates with the humerus at the coronoid fossa. Lateral to the coronoid pro- cess, a smooth, curved radial notch accommodates the head of the radius and helps form the proximal radioulnar joint. Also at the proximal end of this bone is the tuberosity of ulna. At the distal end of the ulna, the shaft narrows and terminates in a knoblike head that has a posteromedial styloid process. The styloid process of the ulna may be palpated on the medial (little finger) side of the wrist.

Describe the features found on all ribs.

The ribs are elongated, curved, flattened bones that originate on or between the thoracic vertebrae and end in the anterior wall of the thorax (figure 8.21a). Both males and females have 12 pairs of ribs.

Describe the landmarks and features of the scapula.

The scapula (skap′yū-lă) is a broad, flat, triangular bone that forms the shoulder blade (figure 8.24). You can palpate your scapula by putting your hand on your superolateral back region and moving your upper limb; the bone you feel moving is the scapula. The spine of the scapula is a ridge of bone on the posterior aspect of the scapula. It is easily palpated under the skin. The spine is continuous with a larger, posterior process called the acromion (a-krō′mē-on; akron = tip, omos = shoulder), which forms the bony tip of the shoulder. Palpate your upper shoulder; the prominent bump you feel is the acromion. The coracoid (kōr′ă-koyd) process is the smaller, more anterior, hook-shaped projection that is a site for muscle attachment. The triangular shape of the scapula forms three sides, or borders. The superior border is the horizontal edge of the scapula superior to the spine of the scapula; the medial border (also called the vertebral border) is the edge of the scapula closest to the vertebrae; and the lateral border (also called the axillary border) is closest to the axilla. A suprascapular notch (which in some individuals is a suprascapular foramen) in the superior border provides passage for the suprascapular nerve and blood vessels. Between these borders are the superior, inferior, and lateral angles. The superior angle is located between the superior and medial, while the inferior angle is positioned between the medial and lateral borders. The lateral angle is primarily made up of the cupshaped, shallow glenoid (glē′noyd; glen′oyd; resembling a socket) cavity, or glenoid fossa, which articulates with the humerus, the bone of the arm. The broad, relatively smooth anterior surface of the scapula is called the subscapular (sŭb-skap′yuˉ-la ̆r; sub = under) fossa. A large muscle called the subscapularis overlies this fossa. The spine subdi- vides the posterior surface of the scapula into two shallow fossae. The depression superior to the spine is the supraspinous (suˉ- pra ̆-spī′nŭs; supra = above) fossa; inferior to the spine is a broad, extensive surface called the infraspinous fossa. The supraspinatus and infraspinatus muscles, respectively, occupy these fossae (see section 11.8b).

Compare the structure of fetal, child, and adult skulls.

The shape and structure of cranial elements differ between infants and adults, causing variations in their proportions and size. The most significant growth in the skull occurs before age 5, when the brain is still growing and exerting pressure against the developing skull bones' internal surface. Brain growth is 90-95% complete by age 5, at which time cranial bone growth is close to completion, and the cranial sutures are almost fully developed. Note that early in life the skull grows at a much faster rate than does the rest of the body. Thus, a young child's cranium is relatively larger compared to the rest of its body than that of an adult.

Compare and contrast the composition and functions of the axial and appendicular skeletons.

The skeletal system is organized into two divisions: the axial skeleton and the appendicular skeleton (figure 8.1). The axial skeleton is so named because it is composed of the bones along the central axis of the body, which include the bones of the skull, vertebral column, sternum, and ribs. The main func- tion of the axial skeleton is to form a framework that supports and protects the organs. Additionally, the spongy bone of most of the axial skeleton contains hemopoietic tissue that is responsible for blood cell formation (see section 18.3a). The appendicular skeleton includes the bones of the upper and lower limbs, and the girdles of bones that attach the upper and lower limbs to the axial skeleton. The pectoral girdle consists of bones that hold the upper limbs in place, whereas the pelvic girdle consists of bones that hold the lower limbs in place.

Describe the features of the tibia and fibula.

The skeleton of the leg (crural region) has two parallel bones: the thick, strong tibia and the slender fibula (figure 8.33). Like the radius and ulna, these two bones are connected by an interosseous membrane that extends between their interosseous borders. The interosseous membrane stabilizes the relative positions of the tibia and fibula, and provides a pivot of minimal rotation for the two bones. The tibia (tib′ē-ă; large shinbone) is the medially placed bone and the only weight- bearing bone of the leg (crural region). Its broad, superior head has two relatively flat sur- faces, the medial and lateral condyles, which articulate with the medial and lateral condyles of the femur, respectively. Separating the condyles of the tibia is a prominent ridge called the intercondylar eminence (em′i-nens). On the proximal posterolateral side of the tibia is a fibular articular facet, where the head of the fibula articulates to form the superior (or proximal) tibiofibular joint. The rough anterior surface of the tibia near the proximal condyles is the tibial tuberosity, which can be palpated just inferior to the patella and marks the attachment site for the patellar ligament. The anterior border (or margin), often referred to as the shin, is a prominent ridge that extends distally along the anterior tibial surface from the tibial tuberosity. The tibia narrows distally, but at its medial border it forms a large, prominent process called the medial malleolus (ma-lē′ō-lŭs; malleus = hammer). Palpate the medial side of your ankle; the bump you feel is your medial malleolus. There is a fibular notch on the distal posterolateral side of the tibia where the fibula articulates and forms the inferior (or distal) tibiofibular joint. On the inferior distal surface of the tibia is the smooth inferior articular surface for the talus, one of the tarsal bones. The fibula (fib′yū-lă; buckle, clasp) is the long, thin, lateral bone of the leg. The fibula does not bear any weight, but several muscles attach to it. The rounded, knoblike head of the fibula is slightly inferior and posterior to the lateral condyle of the tibia. Distal to the fibular head is the neck of the fibula, followed by its shaft. The fibula's distal tip, called the lateral malleolus, extends laterally to the ankle joint, where it pro- vides lateral stability. Palpate the lateral side of your ankle; the bump you feel is your lateral malleolus.

Identify the similarities and differences between the male and female skulls.

The skull can provide insight into the sex and age of an individual. We first describe some diagnostic features of the skull used to determine the sex of an individual. Then we com- pare how the skull changes through the fetal period, childhood, early adulthood, and old age. Human female and male skulls display some obvious differences in general shape and size, a phenomenon known as sexual dimorphism. Typical "female" features tend to be gracile (delicate, small), whereas "male" features tend to be more robust (larger, sturdier, bulkier). Table 8.4 summarizes the general sex differences seen in the skull. However, caution is required when a skull and other skeletal remains are used to deter- mine an individual's sex. Both skeletons and skeletal features vary in their general size and robusticity among populations. For example, some male Asian skeletal remains may be less robust than those of a female Native American. Further, it often is difficult or impossible to determine the sex of infant and juvenile remains, because skull characteristics appear female-like until well after puberty. The most accurate method of determining sex is to look at multiple skeletal features and make a judgment based on the majority of features present. For example, if a skull displays two femalelike characteristics and four malelike characteristics, the skull will likely be clas- sified as male. If your anatomy lab uses real skulls, use table 8.4 to try to determine the sex of the skull you are studying.

Explain the sequence of curvature development.

The spinal curvatures appear sequentially during fetal, newborn, and child developmental stages. The primary curves are the thoracic and sacral curvatures, and they are present at birth. These curvatures arch posteriorly and result in the vertebral column being C-shaped. The secondary curves are called the cervical and lumbar curvatures, and they appear after birth. These curvatures arch anteriorly and are also known as compensation curves because they help shift the trunk weight over the legs. The cervical curvature appears when the child is first able to hold up its head without support (usually around 3-4 months of age). The lumbar curvature appears when the child is learning to stand and walk (typically by the first year of life). These curvatures become accentuated as the child becomes more adept at walking. The sacral curvature is less pronounced in females than in males, to allow for a greater pelvic outlet to accommodate the passage of an infant through the birth canal.

Describe the functional reasons for differences between the upper and lower limb skeletons.

The structural differences between the upper and lower limb skeletons arise from the functional differences. Understanding these gen- eral differences between upper and lower limbs will make the study of their individual bones easier. Because the lower limb is weight bearing and is used for locomotion, some mobility at specific joints has been lost for greater stability. The upper limb is not weight bearing, so both arm and forearm bones are relatively smaller and lighter than the similar respective lower limb bones. Additionally, the upper limb joints are relatively more mobile than the respective lower limb joints, so we may utilize the upper limbs for a wide range of activities. Unfortunately, more mobile joints are less stable, and that is why some of the upper limb joints (such as the shoulder joints) are the most frequently injured.

Explain how the function of the tibia differs from that of the fibula.

The tibia (tib′ē-ă; large shinbone) is the medially placed bone and the only weight- bearing bone of the leg (crural region). Its broad, superior head has two relatively flat sur- faces, the medial and lateral condyles, which articulate with the medial and lateral condyles of the femur, respectively. Separating the condyles of the tibia is a prominent ridge called the intercondylar eminence (em′i-nens). On the proximal posterolateral side of the tibia is a fibular articular facet, where the head of the fibula articulates to form the superior (or proximal) tibiofibular joint. The rough anterior surface of the tibia near the proximal condyles is the tibial tuberosity, which can be palpated just inferior to the patella and marks the attachment site for the patellar ligament. The anterior border (or margin), often referred to as the shin, is a prominent ridge that extends distally along the anterior tibial surface from the tibial tuberosity. The tibia narrows distally, but at its medial border it forms a large, prominent process called the medial malleolus (ma-lē′ō-lŭs; malleus = hammer). Palpate the medial side of your ankle; the bump you feel is your medial malleolus. There is a fibular notch on the distal posterolateral side of the tibia where the fibula articulates and forms the inferior (or distal) tibiofibular joint. On the inferior distal surface of the tibia is the smooth inferior articular surface for the talus, one of the tarsal bones. The fibula (fib′yū-lă; buckle, clasp) is the long, thin, lateral bone of the leg. The fibula does not bear any weight, but several muscles attach to it. The rounded, knoblike head of the fibula is slightly inferior and posterior to the lateral condyle of the tibia. Distal to the fibular head is the neck of the fibula, followed by its shaft. The fibula's distal tip, called the lateral malleolus, extends laterally to the ankle joint, where it pro- vides lateral stability. Palpate the lateral side of your ankle; the bump you feel is your lateral malleolus.

Describe how the tibia and fibula articulate.

The tibia (tib′ē-ă; large shinbone) is the medially placed bone and the only weight- bearing bone of the leg (crural region). Its broad, superior head has two relatively flat sur- faces, the medial and lateral condyles, which articulate with the medial and lateral condyles of the femur, respectively. Separating the condyles of the tibia is a prominent ridge called the intercondylar eminence (em′i-nens). On the proximal posterolateral side of the tibia is a fibular articular facet, where the head of the fibula articulates to form the superior (or proximal) tibiofibular joint. The rough anterior surface of the tibia near the proximal condyles is the tibial tuberosity, which can be palpated just inferior to the patella and marks the attachment site for the patellar ligament. The anterior border (or margin), often referred to as the shin, is a prominent ridge that extends distally along the anterior tibial surface from the tibial tuberosity. The tibia narrows distally, but at its medial border it forms a large, prominent process called the medial malleolus (ma-lē′ō-lŭs; malleus = hammer). Palpate the medial side of your ankle; the bump you feel is your medial malleolus. There is a fibular notch on the distal posterolateral side of the tibia where the fibula articulates and forms the inferior (or distal) tibiofibular joint. On the inferior distal surface of the tibia is the smooth inferior articular surface for the talus, one of the tarsal bones. The fibula (fib′yū-lă; buckle, clasp) is the long, thin, lateral bone of the leg. The fibula does not bear any weight, but several muscles attach to it. The rounded, knoblike head of the fibula is slightly inferior and posterior to the lateral condyle of the tibia. Distal to the fibular head is the neck of the fibula, followed by its shaft. The fibula's distal tip, called the lateral malleolus, extends laterally to the ankle joint, where it provides lateral stability. Palpate the lateral side of your ankle; the bump you feel is your lateral malleolus.

Describe the articulations of the humerus.

The upper limb consists of the brachium (arm), antebrachium (fore- arm), and hand. The complex structure of the hand in particular gives humans capabilities beyond those of most other vertebrates. Each upper limb contains a total of 30 bones: ∙ 1 humerus, located in the brachium region ∙ 1 radius and 1 ulna, located in the antebrachium region ∙ 8 carpal bones, which form the wrist ∙ 5 metacarpal bones, which form the palm of the hand ∙ 14 phalanges, which form the fingers The humerus (hyuˉ′me ̆r-ŭs) is the longest and largest upper limb bone (figure 8.25). Its proximal end has a hemispherical head that articulates with the glenoid cavity of the scapula. The prominent greater tubercle is positioned lateral to the head and helps form the rounded contour of the shoulder. The lesser tubercle is smaller and located more medial to the head. Between the two tubercles is the intertubercular sulcus (also called bicipital sulcus or bicipital groove), a depression that contains the tendon of the long head of the biceps brachii muscle (see section 11.8c). Between the tubercles and the head of the humerus is the anatomical neck, an almost indistinct groove that marks the location of the former epiphyseal plate. The surgical neck is a narrowing of the bone immediately distal to the tubercles, at the transition from the head to the shaft. This feature is called the "surgical" neck because it is a common fracture site. The shaft of the humerus has a roughened area, termed the deltoid (del′toyd; deltoides = like the Greek letter Δ) tuberosity (tuˉ′be ̆r-os′i- tē), which extends along its lateral surface for about half the length of the humerus. The deltoid muscle of the shoulder attaches to this roughened surface (see section 11.8b). The radial groove (or spiral groove) is located adjacent to the deltoid tuberosity and is the location of the radial nerve (see section 14.5e) and some blood vessels.

Name the four spinal curvatures of an adult vertebral column.

The vertebral column has some flexibility because it is not straight and rigid. When viewed from a lateral perspective, the adult vertebral column has four spinal curvatures: the cervical, thoracic, lumbar, and sacral curvatures. This arrangement better supports the weight of the body when standing than could a straight spine.

List the five types of vertebrae.

The vertebral column is partitioned into five divisions, or regions (figure 8.16). Vertebrae are identified by using a capital letter to denote their region, followed by a numerical subscript that indicates their sequence, going from a superior to an inferior location: ∙ Seven cervical (ser′v ̆ı-kal; cervix = neck) vertebrae (designated C1-C7) form the bones of the neck (cervical region). The first cervical vertebra (C1) articulates superiorly with the occipital condyles of the skull. The seventh cervical vertebra articulates inferiorly with the first thoracic vertebra. ∙ Twelve thoracic vertebrae (designated T1-T12) form the superior region of the back (thoracic region). Each thoracic vertebra articulates laterally with one or two pairs of ribs. The twelfth thoracic vertebra articulates inferiorly with the first lumbar vertebra. ∙ Five lumbar vertebrae (designated L1-L5) form the inferior concave region ("small") of the back (lumbar region). The fifth lumbar vertebra articulates inferiorly with the first sacral vertebra. ∙ The sacrum (sā′krŭm) is formed from five sacral vertebrae (designated S1-S5) that fuse into a single bony structure by the mid to late 20s. The sacrum articulates with L5 superiorly,the first coccygeal vertebra inferiorly, and laterally with the two ossa coxae (hip bones). ∙ The coccyx (kok′siks) is commonly called the tailbone and is formed from four coccygeal vertebrae (designated Co1-Co4) that start to unite during puberty and is complete by the mid 20s. The first coccygeal vertebra (Co1) articulates with the inferior end of the sacrum. In much later years, the coccyx also may fuse to the sacrum.

List landmarks and features of the humerus.

Together, the bones of the humerus, radius, and ulna form the elbow joint (figure 8.25c). The medial and lateral epicon- dyles (ep-i-kon′dīl; epi = upon, kondylos = a knuckle) are bony side projections on the distal humerus that provide surfaces for muscle attachment. Palpate the sides of your elbow; the bumps you feel are the medial and lateral epicondyles. Placed posterior to the medial epicondyle is the ulnar nerve (see table 14.4 and section 14.5e). (You actually are hitting this nerve when you hit your funny bone.) The distal end of the humerus has two smooth, curved surfaces for articulation with the bones of the forearm. The rounded capitulum (ka ̆-pit′yū-lŭm;caput=head)islocatedlaterallyandarticulates with the head of the radius. The pulley-shaped trochlea (trok′l ē- a ̆ ; trochileia = a pulley) is located medially and articulates with the trochlear notch of the ulna. Additionally, the distal end of the humer- us exhibits three depressions, two on its anterior surface and one on its posterior surface. The anterolaterally placed radial fossa accom- modates the head of the radius; the anteromedially placed coronoid (kōr′o ̆-noyd; korone = a crow, eidos = resembling) fossa accommo- dates the coronoid process of the ulna. The posterior depression, called the olecranon (ō-lek′ra ̆-non; olene = ulna) fossa, accom- modates the olecranon of the ulna when the elbow is extended (i.e., straightened).

Distinguish between the cranial and the facial bones.

We begin our examination of the skeleton by discussing its most complex structure, the skull. The skull is made up of 22 bones. Here we describe the anatomy and landmarks of the skull, the sutures (a type of fibrous joint; see section 9.2) that connect the bones of the cranium, and the specialized features of the orbital and nasal complexes and paranasal sinuses. The skull is composed of both cranial and facial bones. Cranial bones form the rounded cranium (krā′nē-um; kranion = skull), which completely surrounds and encloses the brain.1 The cranium consists of eight bones that form a roof and a base. The roof of the cranium, called the calvaria (kal-vā′rē-ă), is composed of part of the frontal bone, the parietal bones, and part of the occipital bone. The base of the cranium is composed of portions of the eth- moid, sphenoid, occipital, and temporal bones. Some skulls in the anatomy lab have had their calvariae cut away, making the distinction between the calvaria and base easier to distinguish. Facial bones form the face. They also protect the entrances to the digestive and respira- tory systems. Touch your cheeks, your jaws, and the bridge of your nose; these bones are facial bones. The facial bones give shape and individuality to the face, form part of the orbit and nasal cavities, support the teeth, and provide for the attachment of muscles involved in facial expression and mastication (chewing). There are 14 facial bones, including the paired zygomatic bones, lacrimal bones, nasal bones, inferior nasal conchae, palatine bones, maxil- lae, and unpaired vomer and mandible. The skull contains several prominent cavities (figure 8.3). The largest is the cranial cavity (or endocranium) that encloses, protects, and supports the brain. (The volume of an adult cranial cavity ranges from approximately 1300 to 1500 cubic centimeters, which is about 50 fluid ounces.) The skull also forms and has several smaller cavities, including the orbits (eye sockets), the oral cavity, the nasal cavity, and the paranasal sinuses.

Describe the location and function of the paranasal sinuses.

We have already described the ethmoidal, frontal, maxillary, and sphenoidal sinuses in connection with the bones where they are located. As a group, these air-filled chambers that open into the nasal cavities are called the paranasal sinuses (sī′nŭs; cavity, hollow) (figure 8.13). The term paranasal refers to their being located adja- cent to the nasal cavity. The sinuses have a mucous membrane lining that helps to humidify and warm inhaled air. Additionally, the sinus spaces reduce the weight of the skull bones in which they are located, and they provide resonance to the voice.