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CHAPTER 7 The Skeletal System: The Axial Skeleton 7.1 Divisions of the Skeletal System The adult human skeleton consists of 206 named bones, most of which are paired, with one member of each pair on the right and left sides of the body. The skeletons of infants and children have more than 206 bones because some of their bones fuse later in life. Examples are the hip bones and some bones (sacrum and coccyx) of the vertebral column (backbone). Bones of the adult skeleton are grouped into two principal divisions: the axial skeleton and the appendicular skeleton (appendic- � to hang onto). Table 7.1 presents the 80 bones of the axial skeleton and the 126 bones of the appendicular skeleton. Figure 7.1 shows how both divisions join to form the complete skeleton (the bones of the axial skeleton are shown in blue). You can remember the names of the divisions if you think of the axial skeleton as consisting of the bones that lie around the longitudinal axis of the human body, an imaginary vertical line that runs through the body’s center of gravity from the head to the space between the feet: skull bones, auditory ossicles (ear bones), hyoid bone (see Figure 7.5), ribs, sternum (breastbone), and bones of the vertebral column. The appendicular skeleton consists of the bones of the upper and lower limbs (extremities or appendages), plus the bones forming the girdles that connect the limbs to the axial skeleton. Functionally, the auditory ossicles in the middle ear, which vibrate in response to sound waves that strike the eardrum, are not part of either the axial or appendicular skeleton, but they are grouped with the axial skeleton for convenience (see Chapter 17). We will organize our study of the skeletal system around the two divisions of the skeleton, with emphasis on how the many bones of the body are interrelated. In this chapter we focus on the axial skeleton, looking first at the skull and then at the bones of the vertebral column and the chest. In Chapter 8 we explore the appendicular skeleton, examining in turn the bones of the pectoral (shoulder) girdle and upper limbs, and then the pelvic (hip) girdle and the lower limbs. Before we examine the axial skeleton, we direct your attention to some general characteristics of bones. TABLE 7.1 The Bones of the Adult Skeletal System AXIAL SKELETON       

Skull Cranium = 8 Face= 14 Hyoid bone = 1 Auditory ossicles (see Figure 17.18) = 6 Vertebral column = 26 Thorax  Sternum= 1  Ribs = 24

Number of bones = 80

TABLE 7.1 The Bones of the Adult Skeletal System APPENDICULAR SKELETON  Pectoral (shoulder) girdles  Clavicle = 2  Scapula = 2  Upper limbs  Humerus = 2  Ulna= 2  Radius= 2  Carpals= 16  Metacarpals=10  Phalanges=28  Pelvic (hip) girdle  Hip, pelvic, or coxal bone=2  Lower limbs  Femur=2  Patella=2  Fibula =2  Tibia =2  Tarsals =14  Metatarsals=10  Phalanges= 28 Number of bones Appendicular skeleton = 126 Total bones in an adult skeleton = 206

Figure 7.1 Divisions of the skeletal system. The axial skeleton is indicated in blue. (Note the position of the hyoid bone in Figure 7.5.) The adult human skeleton consists of 206 bones grouped into two divisions: the axial skeleton and the appendicular skeleton.

7.2 Types of Bones Almost all bones of the body can be classified into five main types based on shape: long, short, flat, irregular, and sesamoid (Figure 7.2). As you learned in Chapter 6, long bones have greater length than width, consist of a shaft and a variable number of extremities or epiphyses (ends), and are slightly curved for strength. A curved bone absorbs the stress of the body’s weight at several different points, so that it is evenly distributed. If bones were straight, the weight of the body would be unevenly distributed, and the bone would fracture more easily. Long bones con- sist mostly of compact bone tissue in their diaphyses but have considerable amounts of spongy bone tissue in their epiphyses. Long bones vary tremendously in size and include those in the femur (thigh bone), tibia and fibula (leg bones), humerus (arm bone), ulna and radius (forearm bones), and phalanges (finger and toe bones). Short bones are somewhat cube-shaped and are nearly equal in length and width. They consist of spongy bone tissue except at the surface, which has a thin layer of compact bone tissue. Examples of short bones are most carpal (wrist) bones and most tarsal (ankle) bones. Flat bones are generally thin and composed of two nearly parallel plates of compact bone tissue enclosing a layer of spongy bone tissue. Flat bones afford considerable protection and provide extensive areas for muscle attachment. Flat bones include the cranial bones, which protect the brain; the sternum (breastbone) and ribs, which protect organs in the thorax; and the scapulae (shoulder blades). Irregular bones have complex shapes and cannot be grouped into any of the previous categories. They vary in the amount of spongy and compact bone present. Such bones include the vertebrae (backbones), hip bones, certain facial bones, and the calcaneus. Sesamoid bones (SES-a-moyd � shaped like a sesame seed) develop in certain tendons where there is considerable friction, tension, and physical stress, such as the palms and soles. They may vary in number from person to person, are not always completely ossified, and typically measure only a few millimeters in diameter. Notable exceptions are the two patellae (kneecaps), large sesamoid bones located in the quadriceps femoris tendon (see Figure 11.20a) that are normally present in everyone. Functionally, sesamoid bones protect tendons from excessive wear and tear, and they often change the direction of pull of a tendon, which improves the mechanical advantage at a joint. An additional type of bone is classified by location rather than shape. Sutural bones (SOO-chur-al; sutur� seam) are small bones located in sutures (joints) between certain cranial bones (see Figure 7.6). Their number varies greatly from person to person. Recall from Chapter 6 that in adults, red bone marrow is restricted to flat bones such as the ribs, sternum (breastbone), and skull; irregular bones such as vertebrae (backbones) and hip bones; long bones such as the proximal epiphyses of the femur (thigh bone) and humerus (arm bone); and some short bones.

Figure 7.2 Types of bones based on shape. The bones are not drawn to scale. The shapes of bones largely determine their functions.

7.3 Bone Surface Markings Bones have characteristic surface markings, structural features adapted for specific functions. Most are not present at birth but develop in response to certain forces and are most prominent in the adult skeleton. In response to tension on a bone surface from tendons, ligaments, aponeuroses, and fasciae, new bone is deposited, resulting in raised or roughened areas. Conversely, compression on a bone surface results in a depression. There are two major types of surface markings: (1) depressions and openings, which allow the passage of soft tissues (such as blood vessels, nerves, ligaments, and tendons) or form joints, and (2) processes, projections or outgrowths that either help form joints or serve as attachment points for connective tissue (such as ligaments and tendons). Table 7.2 describes the various surface markings and provides examples of each. TABLE 7.2 Bone Surface Markings MARKING DEPRESSIONS AND OPENINGS: SITES ALLOWING THE PASSAGE OF SOFT TISSUE (NERVES, BLOOD VESSELS, LIGAMENTS, TENDONS) OR FORMATION OF JOINTS Fissure (FISH-ur)

DESCRIPTION

EXAMPLE

Narrow slit between adjacent parts of bones through which blood vessels or nerves pass.

Superior orbital fissure of sphenoid bone (Figure 7.12).

Foramen (foˉ-RAˉ-men � hole; plural is foramina, foˉ -RAˉ M-ina) Fossa (FOS-a � trench; plural is fossae, FOS-eˉ) Sulcus (SUL-kus � groove; plural is sulci, SUL-sı O) Meatus (meˉ-Aˉ-tus � passageway; plural is meati, meˉ-Aˉ-tı O) MARKING PROCESSES: PROJECTIONS OR OUTGROWTHS ON BONE THAT FORM JOINTS OR ATTACHMENT POINTS FOR CONNECTIVE TISSUE, SUCH AS LIGAMENTS AND TENDONS Processes that form joints:  Condyle (KON-dı Ol; condylus � knuckle)

 Facet (FAS-et or fa-SET)  Head Processes that form attachment points for connective tissue  Crest  Epicondyle (epi- � above)  Line (linea)

 Spinous process  Trochanter (troˉ-KANter)  Tubercle (TOO-ber-kul; tuber- � knob)  Tuberosity

Opening through which blood vessels, nerves, or ligaments pass. Shallow depression. Furrow along bone surface that accommodates blood vessel, nerve, or tendon. Tubelike opening.

Optic foramen of sphenoid bone (Figure 7.12) Coronoid fossa of humerus (Figure 8.4a). Intertubercular sulcus of humerus (Figure 8.4a) External auditory meatus of temporal bone (Figure 7.4a).

DESCRIPTION

EXAMPLE

Large, round protuberance with a smooth articular surface at end of bone. Smooth, flat, slightly concave or convex articular surface. Usually rounded articular projection supported on neck (constricted portion) of bone.

Lateral condyle of femur (Figure 8.11a).

Prominent ridge or elongated projection. Typically roughened projection above condyle. Long, narrow ridge or border (less prominent than crest).

Iliac crest of hip bone (Figure 8.9b). Medial epicondyle of femur (Figure 8.11a). Linea aspera of femur (Figure 8.11b) Spinous process of vertebra (Figure 7.17). Greater trochanter of femur (Figure 8.11b) Greater tubercle of humerus (Figure 8.4a). Ischial tuberosity of hip bone (Figure 8.9b).

Sharp, slender projection. Very large projection. Variably sized rounded projection. Variably sized projection that has a rough, bumpy surface.

Superior articular facet of vertebra (Figure 7.18d). Head of femur (Figure 8.11a).

7.4 Skull The skull is the bony framework of the head. It contains 22 bones (not counting the bones of the middle ears) and rests on the superior end of the vertebral column (backbone). The bones of the skull are grouped into two categories: cranial bones and facial bones. The cranial bones (crani- � brain case) form the cranial cavity, which encloses and protects the brain. The eight cranial bones are the frontal bone, two parietal bones, two temporal bones, the occipital bone, the sphenoid bone, and the eth- moid bone. Fourteen (14) facial bones form the face: two nasal bones, two maxillae (or maxillas), two zygomatic bones, the mandible, two lacrimal bones, two palatine bones, two inferior nasal conchae, and the vomer. Exhibits 7.A through 7.G illustrate the bones of the skull from different views.

EXHIBIT 7.A Cranial Bones—Frontal Bone

(Figure 7.3)

The 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 7.3). Soon after birth, the left and right sides of the frontal bone are united by the metopic suture, which usually disappears between the ages of six and eight. Note the frontal squama, a scalelike plate of bone that forms the forehead of the skull (Figure 7.3). It gradually slopes inferiorly from the coronal suture, on the top of the skull, 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; -orbi� circle). From this margin, the frontal bone extends posteriorly to form the roof of the orbit, which is part of the floor of the cranial cavity. Within the supraorbital margin, slightly medial to its midpoint, is a hole called the supraorbital foramen. Sometimes the foramen is incomplete and is called the supraorbital notch. As you read about each foramen associated with a cranial bone, refer to Table 7.3 to note which structures pass through it. The frontal sinuses lie deep to the frontal squama. Sinuses, or more technically paranasal sinuses, are mucous membrane–lined cavities within certain skull bones that will be discussed later.

CLINICAL CONNECTION | Black Eye A black eye is 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.

Figure 7.3 Anterior view of the skull. The skull consists of cranial bones and facial bones.

EXHIBIT 7.B Bones (Figure 7.4)

Cranial Bones—Parietal

The two parietal bones (pa-RIˉ-e-tal; pariet- � wall) form the greater portion of the sides and roof of the cranial cavity (Figure 7.4). The internal surfaces of the parietal bones contain many protrusions and depressions that accommodate the blood vessels supplying the dura mater, the superficial connective tissue (me- ninx) covering of the brain. Figure 7.4 Superior and right lateral views of the skull. The zygomatic arch is formed by the zygomatic process of the temporal bone and the temporal process of the zygomatic bone.

The paired temporal bones (tempor- � temple) form the inferior lateral aspects of the cranium and part of the cranial floor. In Figure 7.4a, note the temporal squama (� scale), the thin, flat part of the temporal bone that forms the anterior and superior part of the temple (the region of the cranium around the ear). Projecting 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. A socket called the mandibular fossa is located on the inferior posterior surface of the zygomatic process of each temporal bone. Anterior to the mandibular fossa is a rounded elevation, the articular tubercle (Figure 7.4a). The mandibular fossa and articular tubercle articulate with the mandible (lower jawbone) to form the temporomandibular joint (TMJ).

The mastoid portion (mastoid � breast-shaped; Figure 7.4a) of the temporal bone is located posterior and inferior to the external auditory meatus (meatus � passageway), or ear canal, which directs sound waves into the ear. In an adult, this portion of the bone contains several mastoid air cells that communicate with the hollow space of the middle ear. 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 called mastoiditis (mas�-toy-DIˉ -tis). The mastoid process is a rounded projection of the mastoid portion of the temporal bone posterior and inferior to the external auditory meatus. It is the point of attachment for several neck muscles. The internal auditory meatus (Figure 7.5) is the opening through which the facial (VII) nerve and vestibulocochlear (VIII) nerve 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 7.4a). Between the styloid process and the mastoid pro- cess is the stylomastoid foramen, through which the facial (VII) nerve and stylomastoid artery pass (see Figure 7.7). At the floor of the cranial cavity (see Figure 7.8a) is the petrous portion (petrous � rock) of the temporal bone. This triangular part, located at the base of the skull between the sphenoid and occipital bones, houses the internal ear and the middle ear, structures involved in hearing and equilibrium (balance). It also contains the carotid foramen, through which the carotid artery passes (see Figure 7.7). Posterior to the carotid foramen and anterior to the occipital bone is the jugular foramen, a passageway for the jugular vein. Figure 7.5 Medial view of sagittal section of the skull. Although the hyoid bone is not part of the skull, it is included here for reference. The cranial bones are the frontal, parietal, temporal, occipital, sphenoid, and ethmoid bones. The facial bones are the nasal bones, maxillae, zygomatic bones, lacrimal bones, palatine bones, inferior nasal conchae, mandible, and vomer.

EXHIBIT 7.D

The occipital bone (ok-SIP-i-tal; occipit- � back of head) forms the posterior part and most of the base of the cranium (Figure 7.6; also see Figure 7.4). Also view the occipital bone and surrounding structures in the inferior view of the skull in Figure 7.7. The foramen magnum (� large hole) is in the inferior part of the bone. The medulla oblongata (inferior part of the brain) connects with the spinal cord within this foramen, and the vertebral and spinal arteries also pass through it along with the accessory (XI) nerve. The occipital condyles, oval processes with convex sur- faces on either side of the foramen magnum (Figure 7.7), articulate with depressions on the first cervical vertebra (atlas) to form the atlanto-occipital joint, which allows you to nod your head “yes.” Superior to each occipital condyle on the inferior surface of the skull is the hypoglossal canal (hypo- � under; -glossal � tongue). (See Figure 7.5.) The external occipital protuberance is the most prominent midline projection on the posterior surface of the bone just above the foramen magnum. You may be able to feel this structure as a bump on the back of your head, just above your neck. (See Figure 7.4a.) A large fibrous, elastic ligament, the ligamentum nuchae (nucha- � nape of neck), extends from the external occipital protuberance to the seventh cervical vertebra to help support the head. Extending laterally from the protuberance are two curved ridges, the superior nuchal lines, and below these are two inferior nuchal lines, which are areas of muscle attachment (Figure 7.7).

Figure 7.6 Posterior view of the skull. The sutures are exaggerated for emphasis. The occipital bone forms most of the posterior and inferior portions of the cranium.

Figure 7.7 Inferior view of the skull. The mandible (lower jawbone) has been removed. The occipital condyles of the occipital bone articulate with the first cervical vertebra to form the atlantooccipital joint.

EXHIBIT 7.E Cranial Bones—Sphenoid Bone

(Figure 7.8)

The sphenoid bone (SFEˉ -noyd � wedge-shaped) lies at the middle part of the base of the skull (Figures 7.7 and 7.8). This bone is called the keystone of the cranial floor because it articulates with all the other cranial bones, holding them together. View the floor of the cranium superiorly (Figure 7.8a) and note the sphenoid articulations. The sphenoid bone joins anteriorly with the frontal and ethmoid bones, laterally with the temporal bones, and posteriorly with the occipital bone. The sphenoid lies posterior and slightly superior to the nasal cavity and forms part of the floor, side walls, and rear wall of the orbit (see Figure 7.12).

The shape of the sphenoid resembles a butterfly with out- stretched wings (Figure 7.8b). The body of the sphenoid is the hollowed cubelike medial portion between the ethmoid and occipital bones. The space inside the body is the sphenoidal sinus, which drains into the nasal cavity (see Figure 7.13). 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 7.8a). 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 (h Oı-poˉ-FIZ-eˉ-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.

Figure 7.8 Sphenoid ...