The Skeletal System PDF

5 WHAT The Skeletal System The skeletal system is the internal frame of the HOW body and includes bones, cartilages, and joints. In addition to...

5 WHAT The Skeletal System The skeletal system is the internal frame of the HOW body and includes bones, cartilages, and joints. In addition to providing structure, bones articulate, or come together, at joints to allow body movement. WHY The skeleton is essential for protecting organs, producing blood cells, storing essential INSTRUCTORS minerals, and anchoring skeletal New Building Vocabulary muscles so that their contractions Coaching Activities for this cause body movements. chapter are assignable in A lthough the word skeleton comes from the Greek word meaning “dried-up body,” our internal framework is beautifully formed and proportioned. Strong yet light, it is perfectly adapted for its functions of protecting the body appendicular skeleton, the bones of the limbs and girdles that attach them to the axial skeleton. Joints give these parts of the skeleton flexibility and allow movement to occur. and allowing motion. No other animal has such long legs (compared to the arms or forelimbs) or Bones: An Overview such a strange foot, and few have grasping hands ➔ Learning Objectives with opposable thumbs. □□ Identify the subdivisions of the skeleton as axial or The bones of the skeleton are part of the appendicular. skeletal system, which also includes joints, car- □□ List at least three functions of the skeletal system. tilages, and ligaments (fibrous cords that bind the □□ Name the four main classifications of bones. bones together at joints). The skeleton is divided into two parts: the axial skeleton, the bones that At one time or another, all of us have heard the form the longitudinal axis of the body, and the expressions “bone tired,” “dry as a bone,” or “bag 134  Chapter 5: The Skeletal System 135 of bones”—pretty unflattering and inaccurate images of some of our most phenomenal organs. Our brains, not our bones, convey feelings of fatigue, and living bones are far from dry. As for “bag of bones,” they are indeed more obvious in some of us, but without bones to form our internal skeleton, we would creep along the ground like slugs, lacking any definite shape or form. Let’s examine how our bones contribute to overall body homeostasis. 5 Functions of the Bones Besides contributing to body structure, our bones perform several important body functions: Support. Bones, the “steel girders” and “rein- Spongy forced concrete” of the body, form the internal bone framework that supports the body and cradles its soft organs. The bones of the legs act as pil- Compact bone lars to support the body trunk when we stand, and the rib cage supports the thoracic wall. Figure 5.1 Flat bones consist of a layer of Protection. Bones protect soft body organs. spongy bone sandwiched between two thin For example, the fused bones of the skull pro- layers of compact bone. vide a snug enclosure for the brain, allowing someone to head a soccer ball without worry- Explore Cadaver ing about injuring the brain. The vertebrae sur- round the spinal cord, and the rib cage helps protect the vital organs of the thorax. for blood to clot. Problems occur not only Allow movement. Skeletal muscles, attached when there is too little calcium in the blood, to bones by tendons, use the bones as levers but also when there is too much. Hormones to move the body and its parts. As a result, we control the movement of calcium to and from can breathe, walk, swim, and throw a ball. the bones and blood according to the needs of Before continuing, take a moment to imagine the body. Indeed, “deposits” and “withdrawals” that your bones have turned to putty. What if of calcium (and other minerals) to and from you were running when this change took bones go on almost all the time. place? Now imagine your bones forming a rigid metal framework inside your body (with- Blood cell formation. Blood cell formation, out joints). What problems could you envision or hematopoiesis (hem″ah-to-poi-e′sis), occurs with this arrangement? These images should within the marrow cavities of certain bones. help you understand how well our skeletal system provides support and protection while Classification of Bones allowing movement. The adult skeleton is composed of 206 bones. There are two basic types of osseous, or bone, tis- Storage. Fat is stored in the internal (marrow) sue: Compact bone is dense and looks smooth cavities of bones. Bone itself serves as a store- and homogeneous, whereas spongy bone has a house for minerals, the most important of spiky, open appearance like a sponge (Figure 5.1). which are calcium and phosphorus. Most of Additionally, bones come in many sizes and the body’s calcium is deposited in the bones as shapes. For example, the tiny pisiform bone of the calcium salts, but a small amount of calcium in wrist is the size and shape of a pea, whereas the its ion form (Ca2+) must be present in the femur, or thigh bone, is nearly 2 feet long and has blood at all times for the nervous system to a large, ball-shaped head. The unique shape of transmit messages, for muscles to contract, and 136 Essentials of Human Anatomy and Physiology (b) Flat bone (sternum) (a) Long bone (humerus) (d) Irregular bone (vertebra), right lateral view (c) Short bone (talus) Figure 5.2 Classification of bones on the basis of shape. each bone fulfills a particular need. Bones are them (see Figure 5.1). Most bones of the skull, the classified according to shape into four groups: ribs, and the sternum (breastbone) are flat bones. long, short, flat, and irregular (Figure 5.2). Short bones are generally cube-shaped and As their name suggests, long bones are typi- contain mostly spongy bone with an outer layer of cally longer than they are wide. As a rule, they compact bone. The bones of the wrist and ankle have a shaft with enlarged ends. Long bones are are short bones. Sesamoid (ses′ah-moyd) bones, mostly compact bone but also contain spongy which form within tendons, are a special type of bone at the ends. All the bones of the limbs, short bone. The best-known example is the patella. except the patella (kneecap) and the wrist and Bones that do not fit one of the preceding cat- ankle bones, are long bones. egories are called irregular bones. The vertebrae, Flat bones are thin, flattened, and usually which make up the spinal column, fall into this curved. They have two thin layers of compact group. Like short bones, they are mainly spongy bone sandwiching a layer of spongy bone between bone with an outer layer of compact bone.  Chapter 5: The Skeletal System 137 Did You Get It? remnant of the epiphyseal plate (a flat plate of 1. What is the relationship between muscle function and hyaline cartilage) seen in a young, growing bone. bones? Epiphyseal plates cause the lengthwise growth of 2. What are two possible functions of a bone’s marrow a long bone. By the end of puberty, when hor- cavity? mones inhibit long bone growth, epiphyseal plates 3. Where are most long bones found in the body? have been completely replaced by bone, leaving For answers, see Appendix A. only the epiphyseal lines to mark their previous location. Structure of Bone The inner bony surface of the shaft is covered ➔ Learning Objectives by a delicate connective tissue called endosteum. 5 □□ Identify the major anatomical areas of a long bone. In infants, the cavity of the shaft, called the med- ullary cavity, is a storage area for red marrow, □□ Describe the microscopic structure of compact bone. which produces blood cells. Children’s bones con- tain red marrow until the age of 6 or 7, when it is □□ Explain the role of bone salts and the organic matrix in making bone both hard and flexible. gradually replaced by yellow marrow, which stores adipose (fat) tissue. In adult bones, red mar- Gross Anatomy of a Long Bone row is confined to cavities in the spongy bone of ➔ the axial skeleton, the hip bones, and the epiphy- CONCEPTLINK ses of long bones such as the humerus and femur. As we learn about the structure and organization of Even when looking casually at bones, you can bones, remember the levels of structural organization see that their surfaces are not smooth but scarred (Figure 1.1, p. 3). Bones are organs, so they contain with bumps, holes, and ridges. These bone mark- not only osseous tissue but also other connective ings (described and illustrated in Table 5.1 on tissues: fibrous tissue, cartilage, adipose tissue, and p. 140) reveal where muscles, tendons, and liga- blood. ➔ ments attach and where blood vessels and nerves pass. There are two categories of bone markings: In a long bone, the diaphysis (di-af′ı̆-sis), or (a) projections, or processes, which grow out from shaft, makes up most of the bone’s length and is the bone surface, and (b) depressions, or cavities, composed of compact bone (Figure 5.3, p. 138). which are indentations in the bone. There is a lit- The diaphysis is covered and protected by a tle trick for remembering some of the bone mark- fibrous connective tissue membrane, the perios- ings listed in the table: All the terms beginning teum (per-e-os′te-um). Hundreds of connective with T are projections, and the terms beginning tissue fibers, called perforating fibers, or with F (except facet) are depressions. Sharpey’s fibers, secure the periosteum to the Microscopic Anatomy underlying bone. The epiphyses (ĕ-pif′ı̆-sˉez) are the ends of The appearance of spongy bone and compact long bones. Each epiphysis consists of a thin layer bone to the naked eye only hints at their underly- of compact bone enclosing an area filled with ing complexity. Under a microscope, you can see spongy bone. Instead of a periosteum, articular that spongy bone is composed of small needlelike cartilage covers its external surface. Because the pieces of bone called trabeculae and lots of “open” articular cartilage is glassy hyaline cartilage, it pro- space filled by marrow, blood vessels and nerves vides a smooth surface that decreases friction at (Figure 5.4a, p. 139). the joint when covered by lubricating fluid. In compact bone, the mature bone cells, Imagine how slick a marble floor (the articular osteocytes (os′te-o-sıˉtz″), are found within the cartilage) is when wet; this is analogous to the bone matrix in tiny cavities called lacunae (lah- lubrication of a joint. ku′ne). The lacunae are arranged in concentric In adult bones, there is a thin line of bony tis- circles called lamellae (lah-mel′e) around central sue spanning the epiphysis that looks a bit differ- canals (also called Haversian canals). Each com- ent from the rest of the bone in that area. This plex consisting of a central canal and matrix rings is the epiphyseal line. The epiphyseal line is a is called an osteon, or Haversian system, and is the structural and functional unit of compact bone. 138 Essentials of Human Anatomy and Physiology Articular cartilage Compact bone Proximal epiphysis Spongy bone Epiphyseal line Periosteum Compact bone Medullary cavity (lined by endosteum) (b) Diaphysis Endosteum Yellow bone marrow Compact bone Periosteum Distal Perforating epiphysis (Sharpey’s) fibers (a) Nutrient arteries (c) Figure 5.3 The structure of a three-dimensional view of spongy surface of the diaphysis is covered by long bone (humerus of arm). bone and compact bone of the a periosteum but that the articular (a) Anterior view with longitudinal epiphysis. (c) Cross section of the shaft surface of the epiphysis (see a and b) section cut away. (b) Pie-shaped, (diaphysis). Note that the external is covered with hyaline cartilage. Central canals run lengthwise through the bony supply and waste removal services through the matrix, carrying blood vessels and nerves to all hard bone matrix. Because of this elaborate net- areas of the bone. Tiny canals, canaliculi (kan″ah- work of canals, bone cells are well nourished in lik′u-li), radiate outward from the central canals to spite of the hardness of the matrix, and bone inju- all lacunae. The canaliculi form a transportation sys- ries heal quickly. The communication pathway tem that connects all the bone cells to the nutrient from the outside of the bone to its interior (and  Chapter 5: The Skeletal System 139 5 Trabeculae of spongy bone Osteon (Haversian Perforating system) (Volkmann’s) canal Blood vessel continues into medullary cavity containing marrow Blood vessel Lamellae Compact bone Central (Haversian) canal Perforating (Sharpey’s) fibers Periosteum Periosteal blood vessel (a) Lamella Osteon Osteocyte Interstitial lamellae Canaliculus Lacuna Central Bone matrix (Haversian) canal (b) (c) Figure 5.4 Microscopic structure structural units (osteons) and osteocytes in lacunae (cavities in the of bone. (a) Diagram of a pie- trabeculae, respectively. (b) Higher matrix). (c) Photo of a cross-sectional shaped segment of compact and magnification view of part of one view of an osteon. spongy bone illustrating their osteon. Notice the position of View Histology 140 Essentials of Human Anatomy and Physiology Table 5.1 Bone Markings Name of bone marking Description Illustration Projections that are sites of muscle and ligament attachment Tuberosity Large, rounded projection; may be Intertrochanteric Trochanter roughened line Iliac Crest Narrow ridge of bone; crest usually prominent Trochanter (tro-kan′ter) Very large, blunt, irregularly shaped Ischial process (the only spine Adductor examples are on the tubercle femur) Hip Ischial Femur of Medial bone tuberosity epicondyle Line Narrow ridge of bone; thigh less prominent than a crest Condyle Tubercle (too′ber-kl) Small, rounded projection or process Vertebra Epicondyle Raised area on or above a condyle Facet Spine Sharp, slender, often pointed projection Spinous Process Any bony prominence process Projections that help to form joints Head Bony expansion carried on a Head Condyle narrow neck Facet Smooth, nearly flat articular surface Facets Ramus Condyle (kon′dˉı l) Rounded articular projection Mandible Rib Ramus (ra′mus) Armlike bar of bone Depressions and openings For passage of blood vessels and nerves Meatus Groove Furrow Sinus Fissure Narrow, slitlike opening Fossa Foramen (fo-ra′men) Round or oval opening through a bone Notch Groove Notch Indentation at the edge of a structure Others Canal- or tunnel-like passageway Inferior Meatus (me-a′tus) orbital Sinus Cavity within a bone, filled with air and fissure lined with mucous membrane Foramen Fossa (fos′ah) Shallow, basinlike depression in a bone, often serving as an articular surface Skull  Chapter 5: The Skeletal System 141 Articular cartilage Hyaline Spongy cartilage bone New center of bone growth New bone Epiphyseal forming plate cartilage Growth Medullary in bone 5 cavity width Bone starting Invading to replace Growth blood cartilage in bone vessels length New bone Bone collar forming Hyaline Epiphyseal cartilage plate cartilage model In an embryo In a fetus In a child Figure 5.5 Stages of long-bone formation in an embryo, fetus, and young child. the central canals) is completed by perforating Bone Formation, Growth, canals (also called Volkmann’s canals), which and Remodeling run in the compact bone at right angles to the shaft (diaphysis) and central canals. ➔ Learning Objective Bone is one of the hardest materials in the □□ Describe briefly the process of bone formation in body, and although relatively light in weight, it has the fetus, and summarize the events of bone a remarkable ability to resist tension and other remodeling throughout life. forces acting on it. Nature has given us an Bone Formation and Growth extremely strong and exceptionally simple sup- porting system that also allows mobility. The cal- The skeleton is formed from two of the strongest cium salts deposited in the matrix give bone its and most supportive tissues in the body—cartilage hardness, which resists compression. The organic and bone. In embryos, the skeleton is primarily parts (especially the collagen fibers) provide for made of hyaline cartilage, but in young children, bone’s flexibility and great tensile strength (ability most of the cartilage has been replaced by bone. to be stretched without breaking). Cartilage remains only in isolated areas such as the bridge of the nose, parts of the ribs, and the joints. Did You Get It? Except for flat bones, which form on fibrous 4. What is the anatomical name for the shaft of a long membranes, most bones develop using hyaline car- bone? For its ends? tilage structures as their “models.” This process of 5. How does the structure of compact bone differ from bone formation, or ossification (os″ı̆-fı̆-ka′shun), the structure of spongy bone? involves two major phases (Figure 5.5). First, the 6. What is the importance of canaliculi? hyaline cartilage model is completely covered with For answers, see Appendix A. bone matrix (a bone “collar”) by bone-building cells called osteoblasts. So, as the embryo develops into a fetus, for a short period it has cartilage “bones” enclosed by actual bone matrix. Then, in 142 Essentials of Human Anatomy and Physiology Bone growth Bone remodeling Bone grows in Growing shaft is length because: remodeled as: 1 Cartilage Articular cartilage grows here. Epiphyseal plate 2 Cartilage 1 Bone is is replaced resorbed by by bone here. osteoclasts here. 3 Cartilage 2 Bone is added grows here. (appositional growth) by osteoblasts here. 4 Cartilage is replaced by 3 Bone is resorbed bone here. by osteoclasts here. Figure 5.6 Growth and articular cartilages and epiphyseal growth during long-bone remodeling of long bones. The plates as the bone grows in length. growth to maintain proper bone events on the left depict the process The events on the right show bone proportions. of ossification that occurs at the remodeling and appositional the fetus, the enclosed hyaline cartilage model is hormone and, during puberty, the sex hormones. It replaced by bone, and the center is digested away, ends during adolescence, when the epiphyseal opening up a medullary cavity within the newly plates are completely converted to bone. formed bone. By birth or shortly after, most hyaline cartilage Bone Remodeling models have been converted to bone except for Many people mistakenly think that bones are life- two regions—the articular cartilages (that cover less structures that never change once long-bone the bone ends) and the epiphyseal plates. In order growth has ended. Nothing could be further from for bones to increase in length as the infant grows the truth; bone is a dynamic and active tissue. into a child, new cartilage is formed continuously Bones are remodeled continually in response to on the external face (joint side) of the articular changes in two factors: (1) the calcium ion level in cartilage and on the epiphyseal plate surface that the blood and (2) the pull of gravity and muscles faces the bone end (is farther away from the med- on the skeleton. ullary cavity). At the same time, the old cartilage When the blood calcium ion level drops below abutting the internal face of the articular cartilage its homeostatic level, the parathyroid glands and the medullary cavity is broken down and (located in the throat) are stimulated to release replaced by bony matrix (Figure 5.6). parathyroid hormone (PTH) into the blood. PTH Growing bones also widen as they lengthen to activates osteoclasts, giant bone-destroying cells maintain proper proportion. How do they widen? in bones, to break down bone matrix and release Simply, osteoblasts in the periosteum add bone calcium ions into the blood. When the blood cal- matrix to the outside of the diaphysis as cells cium ion level is too high (hypercalcemia [hi″per- called osteoclasts in the endosteum remove bone kal-se′me-ah]), calcium is deposited in bone matrix from the inner face of the diaphysis wall, enlarg- as hard calcium salts by osteoblasts. ing the medullary cavity (see Figure 5.6). Because Bone remodeling is essential if bones are to these two processes occur at about the same rate, retain normal proportions and strength during the circumference of the long bone expands, and long-bone growth as the body increases in size the bone widens. This process by which bones and weight. It also accounts for the fact that bones increase in diameter is called appositional growth, become thicker and form large projections to and like growth in length, is controlled by hor- increase their strength in areas where bulky mus- mones. The most important hormones are growth cles are attached. At such sites, osteoblasts lay  FOCUS Radiologic Technologist ON C AREERS Radiologic technologists supply might guess, radiologic technologists, critical information that allows especially in hospitals, must be pre- doctors to make accurate pared to spend a lot of time on their diagnoses. feet and to think quickly. Regalado described one case when a two-car “ Y ou never know what’s going to walk in the door, really,” says Maggie Regalado, a radiologic tech- accident sent five children to the trauma unit. The radiologic technolo- 5 gists had to work quickly to help the nologist at Dell Children’s Hospital in doctors see what injuries the children Austin, Texas. “In an emergency suffered—and equally important, to room, you see kids who swallowed make sure not to mix up anyone’s something, car accident victims, all X-ray exams. kinds of things.” Regalado and her “You don’t want to make errors, coworkers operate X-ray equipment because one thing you do wrong and must be ready to do everything could cost this patient his or her life,” from preparing patients for chest she says. “Even though radiology can X-ray exams to MRIs. get emotional, you have to stay tech- Fortunately for Regalado, anatomy nical with your job.” was her favorite class, because it’s an “We can’t see your bones with our important one for radiologic technolo- bare eyes, so we have to make sure gists. After getting her associate’s we position you correctly. Then also, if degree in diagnostic imaging, she you say, ‘It hurts here,’ I’ll call the doc- completed both state and national tor and see if he wants to do a differ- certification. To keep her certification ent type of X-ray exam.” current, she must complete 24 hours Regalado enjoys working with the of continuing education every 2 years. patients at Dell. Getting children to remain perfectly still and positioned schedule. Depending on the clinic, You don’t want to correctly is a challenge, but the imag- ing department has toys and televi- these technologists may also special- ize in areas such as ultrasound, mam- make errors, sions to distract them. For babies who cannot easily hold still or understand mography, magnetic resonance imaging (MRI), or computed tomogra- because one thing why they need to, there are various devices to position them appropriately. phy (CT). For more information, contact: you do wrong could “We have a lot of interaction with the patients and the patient’s family;  merican Society of Radiologic A cost this patient his we try to joke around and make them happy,” she says. “When we make Technologists 15000 Central Ave. SE or her life. the child happy, then the parents are happy.” Albuquerque, NM 87123-3909 (800) 444-2778 In a hospital setting, radiologic http://www.asrt.org “I didn’t realize how big a field it technologists are needed 24 hours a For additional information on this was,” she says. “With X-ray exams day and often are required to be on- career and others, click the Focus on you’re constantly moving from here to call in addition to their regular shifts. Careers link at. there, from surgery to the neonatal Technologists who work in clinics usu- intensive care unit and so on.” As you ally have a more traditional 9-to-5 143 144 Essentials of Human Anatomy and Physiology down new matrix and become trapped within it. Did You Get It? (Once they are trapped, they become osteocytes, 7. Bones don’t begin as bones. What do they begin as? or mature bone cells.) In contrast, the bones of 8. Which stimulus—PTH (a hormone) or mechanical bedridden or physically inactive people tend to forces acting on the skeleton—is more important in lose mass and to atrophy because they are no lon- maintaining the blood calcium ion level than in maintaining bone strength? ger subjected to stress. 9. If osteoclasts in a long bone are more active than These two controlling mechanisms—calcium osteoblasts, what change in bone mass is likely to occur? uptake and release as well as bone remodeling— For answers, see Appendix A. work together. PTH determines when bone is to be broken down or formed in response to the Bone Fractures need for more or fewer calcium ions in the blood. The stresses of muscle pull and gravity acting on ➔ Learning Objective the skeleton determine where bone matrix is to be □□ Name and describe the various types of fractures. broken down or formed so that the skeleton can remain as strong and vital as possible. Homeostatic Imbalance 5.2 Homeostatic Imbalance 5.1 For their relatively low mass, bones are amazingly strong. Consider, for example, the forces endured Rickets is a disease of children in which the in football and professional hockey. Despite their bones fail to calcify. As a result, the bones soften, remarkable strength, bones are susceptible to frac- and the weight-bearing bones of the legs become tures, or breaks, throughout life. During youth, bowed. Rickets is usually due to a lack of cal- most fractures result from exceptional trauma that cium in the diet or lack of vitamin D, which is twists or smashes the bones. Sports activities such needed to absorb calcium into the bloodstream. as football, skating, and skiing jeopardize the Rickets is not seen very often in the United bones, and automobile accidents certainly take States. Milk, bread, and other foods are fortified their toll. In old age, bones thin and weaken, and with vitamin D, and most children drink enough fractures occur more often. calcium-rich milk. However, it can happen in A fracture in which the bone breaks cleanly infants nursed by mothers who become vitamin but does not penetrate the skin is a closed (or D deficient, and it remains a problem in some simple) fracture. When the broken bone ends other parts of the world. penetrate through the skin, the fracture is open (or compound). Some of the many common types of fractures are illustrated and described in Table 5.2. ________________________________ ✚ A fracture is treated by reduction, which is the realignment of the broken bone ends, followed by immobilization. In a closed reduction, the bone ends are coaxed back into their normal position by the physician’s hands. In an open reduction, sur- gery is performed, and the bone ends are secured together with pins or wires. After the broken bone is reduced, it is immobilized by a cast or traction to allow the healing process to begin. The healing time for a simple fracture is 6 to 8 weeks but is much longer for large bones and for the bones of older people (because of their poorer circulation). This child suffering from rickets is a member of the el-Molo The repair of bone fractures involves four tribe in Kenya, whose diet consists primarily of fish. major events (Figure 5.7): ___________________________________________ ✚ 1 A hematoma forms. Blood vessels are rup- tured when the bone breaks. As a result, a  Chapter 5: The Skeletal System 145 Hematoma External Bony callus callus of spongy bone New Internal blood callus vessels Healed (fibrous fracture tissue and Spongy bone 5 cartilage) trabecula 1 Hematoma 2 Fibrocartilage 3 Bony callus 4 Bone remodeling forms. callus forms. forms. occurs. Figure 5.7 Stages in the healing of a bone fracture. blood-filled swelling, or bruise, called a dead tissue by phagocytes. As this goes on, ­hematoma (he-mah-to′mah) forms. Bone connective tissue cells of various types form 1 cells deprived of nutrition die. internal and external masses of repair tissue, 2 A fibrocartilage callus forms. Two early which collectively form the fibrocartilage events of tissue repair are the growth of new callus (kal′us). The internal and external 3 masses, called calluses, originate from cells of capillaries (granulation tissue) into the clotted blood at the site of the damage and disposal of the endosteum and periosteum, respectively, 4 5 6Table 5.2 Common Types of Fractures Fracture type Illustration Description Comment Comminuted Bone breaks into three or more Particularly common in older fragments people, whose bones are more brittle Compression Bone is crushed Common in porous bones (i.e., osteoporotic bones of older people) Depressed Broken bone portion is pressed Typical of skull fracture inward Impacted Broken bone ends are forced into Commonly occurs when each other someone attempts to break a fall with outstretched arms Spiral Ragged break occurs when Common sports fracture excessive twisting forces are applied to a bone Greenstick Bone breaks incompletely, Common in children, whose much in the way a green bones are more flexible than twig breaks those of adults 146 Essentials of Human Anatomy and Physiology 1 and contain several elements—cartilage matrix, joined together by sutures, which are interlocking, bone matrix, and collagen fibers—which act to immovable joints. Only the mandible (jawbone) is 2 “splint” the broken bone, closing the gap. attached to the rest of the skull by a freely mov- 1 3 The bony callus forms. As more osteoblasts able joint. 2 and osteoclasts migrate into the area and multi- 4 Cranium ply, the fibrocartilage callus is gradually replaced 3 The boxlike cranium is composed of eight large 5 by the bony callus made of spongy bone. flat bones. Except for two sets of paired bones (the 4 Bone remodeling occurs. Over the next few 6 parietal and temporal), they are all single bones. weeks to months, depending on the bone’s 5 size and site of the break, the bony callus is Frontal Bone The frontal bone forms the 6 remodeled in response to the mechanical forehead, the bony projections under the eye- stresses placed on it, so that it forms a strong, brows, and the superior part of each eye’s orbit permanent “patch” at the fracture site. (Figure 5.9). Did You Get It? Parietal Bones The paired parietal bones form 10. What is a fracture? What two fracture types are particularly common in older people? most of the superior and lateral walls of the cra- nium (see Figure 5.9). The sagittal suture is For the answer, see Appendix A. formed at the midline where the two parietal bones meet (shown in Figure 5.12, p. 150), and the coro- Axial Skeleton nal suture is formed where the paired parietal bones meet the frontal bone. As noted earlier, the skeleton is divided into two parts, the axial and appendicular skeletons. The Temporal Bones The temporal bones lie axial skeleton forms the longitudinal axis of the inferior to the parietal bones and join them at body (it is shown as the green portion of the squamous sutures. Several important bone Figure 5.8). It can be divided into three parts— markings appear on the temporal bones (see the skull, the vertebral column, and the thoracic Figure 5.9): cage. ➔ The external acoustic meatus is a canal that leads to the eardrum and the middle ear. It is CONCEPTLINK the route by which sound enters the ear. Recall the regional body terms you have already learned (look back at Figure 1.4, p. 12). Many of these The styloid process, a sharp, needlelike pro- terms can be associated with a bone name or group of jection, is just inferior to the external auditory bones. For example, the carpal region is the location of meatus. Many neck muscles use the styloid the carpals, or wrist bones. ➔ process as an attachment point. The zygomatic (zi″go-mat′ik) process is a Skull thin bridge of bone that joins with the cheek- ➔ Learning Objectives bone (zygomatic bone) anteriorly. □□ On a skull or diagram, identify and name the bones The mastoid (mas′toid) process, which is full of the skull and the four major skull sutures. of air cavities (the mastoid sinuses), is a rough □□ Describe how the skull of a newborn infant (or projection posterior and inferior to the external fetus) differs from that of an adult, and explain the acoustic meatus. It provides an attachment site function of fontanels. for some muscles of the neck. The mastoid sinuses are so close to the The skull is formed by two sets of bones. The middle ear—a high-risk spot for infections— cranium encloses and protects the fragile brain that they may become infected too, a condi- tissue. The facial bones form a cradle for the eyes tion called mastoiditis. Also, this area is so that is open to the anterior and allow the facial close to the brain that mastoiditis may spread muscles to show our feelings through smiles or to the brain. frowns. All but one of the bones of the skull are  Chapter 5: The Skeletal System 147 Cranium Skull Facial bones Bones of pectoral Clavicle girdle Thoracic cage Scapula (ribs and sternum) Sternum Upper 5 limb Rib Humerus Vertebra Vertebral column Radius Bones Ulna of pelvic Sacrum girdle Carpals Phalanges Metacarpals Femur Patella Lower limb Tibia Fibula Tarsals Metatarsals Phalanges (a) Anterior view (b) Posterior view Figure 5.8 The human skeleton. The bones of the axial skeleton are colored green. Bones of the appendicular skeleton are gold. 148 Essentials of Human Anatomy and Physiology Coronal suture Frontal bone Parietal bone Sphenoid bone Temporal bone Ethmoid bone Lambdoid Lacrimal bone suture Squamous suture Nasal bone Occipital bone Zygomatic process Zygomatic bone Maxilla External acoustic meatus Mastoid process Alveolar processes Styloid process Mandible (body) Mental foramen Mandibular ramus Figure 5.9 Human skull, lateral view. The jugular foramen, at the junction of the connect with the brain. Lateral to the foramen occipital and temporal bones (Figure 5.10), al- magnum on each side are the rockerlike occipital lows passage of the jugular vein, the largest condyles (see Figure 5.11), which rest on the first vein of the head, which drains blood from the vertebra of the spinal column. brain. Just anterior to it in the cranial cavity is the internal acoustic meatus (see Sphenoid Bone The butterfly-shaped sphe- Figure 5.10), which transmits cranial nerves VII noid (sfe′noid) bone spans the width of the skull and and VIII (the facial and vestibulocochlear forms part of the floor of the cranial cavity (see Figure nerves). Anterior to the jugular foramen on the 5.10). In the midline of the sphenoid is a small skull’s inferior aspect is the carotid canal (see depression, the sella turcica (sel′ah tur′sı̆-kah), or Figure 5.11), through which the internal ca- Turk’s saddle, which forms a snug enclosure for rotid artery runs, supplying blood to most of the pituitary gland. The foramen ovale, a large the brain. oval opening in line with the posterior end of the sella turcica (see Figure 5.10), allows fibers of cra- Occipital Bone The occipital (ok-sip′ı̆-tal) nial nerve V (the trigeminal nerve) to pass to the bone is the most posterior bone of the cranium chewing muscles of the lower jaw (mandible). (Figure 5.11). It forms the base and back wall of Parts of the sphenoid bone, seen exteriorly forming the skull. The occipital bone joins the parietal part of the eye orbits (Figure 5.12, p. 150), have bones anteriorly at the lambdoid (lam′doyd) two important openings, the optic canal, which suture. In the base of the occipital bone is a large allows the optic nerve to pass to the eye, and the opening, the foramen magnum (literally, “large slitlike superior orbital fissure, through which hole”). The foramen magnum surrounds the lower the cranial nerves controlling eye movements (III, part of the brain and allows the spinal cord to IV, and VI) pass (see Figure 5.12). The central part  Chapter 5: The Skeletal System 149 Frontal bone Cribriform plate Ethmoid Crista galli bone Sphenoid bone Optic canal Sella turcica Foramen ovale 5 Temporal bone Jugular foramen Internal acoustic meatus Parietal bone Occipital bone Foramen magnum Figure 5.10 Human skull, superior view (top of cranium removed). Maxilla Hard (palatine process) Incisive fossa palate Palatine bone Maxilla Zygomatic bone Sphenoid bone Temporal bone (greater wing) (zygomatic process) Foramen ovale Vomer Mandibular fossa Carotid canal Styloid process Mastoid process Jugular foramen Temporal bone Occipital condyle Parietal bone Foramen magnum Occipital bone Figure 5.11 Human skull, inferior view (mandible removed). 150 Essentials of Human Anatomy and Physiology Q: What bone articulates with every other facial bone? Coronal suture Frontal bone Parietal bone Nasal bone Superior orbital fissure Sphenoid bone Optic canal Ethmoid bone Temporal bone Lacrimal bone Zygomatic bone Middle nasal concha Infraorbital foramen of ethmoid bone Maxilla Inferior nasal concha Vomer Mandible Alveolar processes Mental foramen Sagittal suture Anterior view Parietal bone Lambdoid suture Occipital bone Mastoid process of temporal bone Posterior view Figure 5.12 Human skull, anterior and posterior views. A: The maxilla.  Chapter 5: The Skeletal System 151 of the sphenoid bone is riddled with air cavities, the sphenoidal sinuses (Figure 5.13). Frontal sinus Ethmoid Ethmoid Bone The ethmoid (eth′moid) bone sinus is very irregularly shaped and lies anterior to the Sphenoidal sphenoid (see Figure 5.10). It forms the roof of the sinus nasal cavity and part of the medial walls of Maxillary the orbits. Projecting from its superior surface is the sinus crista galli (kris′tah gah′le), literally “cock’s comb” (see Figure 5.10). The outermost covering of the 5 brain attaches to this projection. On each side of the crista galli are many small holes. These holey areas, the cribriform (krib′rı̆-form) plates, allow nerve fibers carrying impulses from the olfactory (smell) receptors of the nose to reach the brain. Extensions of the ethmoid bone, the superior nasal conchae (kong′ke) and middle nasal conchae, form part of (a) Anterior view the lateral walls of the nasal cavity (see Figure 5.12). Facial Bones Fourteen bones make up the face. Twelve are paired; only the mandible and vomer are single. Frontal (Figures 5.9 and 5.12 show most of the facial bones.) sinus Ethmoid Maxillae The two maxillae (mak-sı̆′le), or sinus maxillary bones, fuse to form the upper jaw. All Sphenoidal facial bones except the mandible join the maxillae; sinus thus they are the main, or “keystone,” bones of the Maxillary face. The maxillae carry the upper teeth in the sinus alveolar process. Extensions of the maxillae called the palatine (pal′ah-tıˉn) processes form the anterior part of the hard palate of the mouth (see Figure 5.11). Like many other facial bones, the maxillae contain sinuses, which drain into the nasal passages (see Figure 5.13). These paranasal sinuses, whose naming reveals their position surrounding the nasal cavity, lighten the skull bones and amplify (b) Medial view the sounds we make as we speak. Figure 5.13 Paranasal sinuses. Homeostatic Imbalance 5.3 They form the posterior part of the hard palate The paranasal sinuses cause many people a great (see Figure 5.11). Failure of these or the palatine deal of misery. Because the mucosa lining these processes to fuse medially results in cleft palate. sinuses is continuous with that in the nose and throat, infections in these areas tend to migrate Zygomatic Bones The zygomatic bones are into the sinuses, causing sinusitis. Depending on commonly referred to as the cheekbones. They which sinuses are infected, a headache or upper also form a good-sized portion of the lateral walls jaw pain is the usual result.____________________✚ of the orbits. Palatine Bones The paired palatine bones lie Lacrimal Bones The lacrimal (lak′rı̆-mal) bones posterior to the palatine processes of the maxillae. are fingernail-sized bones forming part of the medial 152 Essentials of Human Anatomy and Physiology The Hyoid Bone Though not really part of the skull, the hyoid (hi′oid) bone (Figure 5.14) is closely related to the mandible and temporal bones. The hyoid bone is unique in that it is the only bone of the body that does not articulate (form a joint) with any other bone. Instead, it is suspended in the midneck region about 2 cm (1 inch) above the larynx (voicebox), Greater horn where it is anchored by ligaments to the styloid pro- cesses of the temporal bones. Horseshoe-shaped, Lesser horn with a body and two pairs of horns, the hyoid bone serves as a movable base for the tongue and as an Body attachment point for neck muscles that raise and lower the larynx when we swallow and speak. Figure 5.14 Anatomical location and structure of the hyoid bone. Anterior view. Did You Get It? 11. What are the three main parts of the axial skeleton? 12. Johnny was vigorously exercising the only joints in the wall of each orbit. Each lacrimal bone has a groove skull that are freely movable. What would you guess that serves as a passageway for tears (lacrima = tear). he was doing? 13. Which skull bone(s) form the “keystone of the face”? Nasal Bones The small rectangular bones 14. Which bone has the cribriform plate and crista galli? forming the bridge of the nose are the nasal 15. Which bones are connected by the coronal suture? By the sagittal suture? bones. (The lower part of the skeleton of the nose is made up of hyaline cartilage.) For answers, see Appendix A. Vomer Bone The single bone in the median Vertebral Column (Spine) line of the nasal cavity is the vomer. (Vomer means ➔ Learning Objectives “plow,” which refers to the bone’s shape.) The □□ Name the parts of a typical vertebra, and explain in vomer forms the inferior part of the bony nasal general how the cervical, thoracic, and lumbar septum, which separates the two nostrils. vertebrae differ from one another. □□ Discuss the importance of the intervertebral discs Inferior Nasal Conchae The inferior nasal con- and spinal curvatures. chae (kon′ke) are thin, curved bones projecting □□ Explain how the abnormal spinal curvatures (scoliosis, medially from the lateral walls of the nasal cavity. lordosis, and kyphosis) differ from one another. (As mentioned earlier, the superior and middle con- Serving as the axial support of the body, the verte- chae are similar but are parts of the ethmoid bone.) bral column, or spine, extends from the skull, which it supports, to the pelvis, where it transmits Mandible The mandible, or lower jaw, is the the weight of the body to the lower limbs. Some largest and strongest bone of the face. It joins the people think of the vertebral column as a rigid temporal bones on each side of the face, forming supporting rod, but that picture is inaccurate. the only freely movable joints in the skull. You can Instead, the spine is formed from 26 irregular find these joints on yourself by placing your fin- bones connected and reinforced by ligaments in gers just anterior to your ears and opening and such a way that a flexible, curved structure results closing your mouth. The horizontal part of the (Figure 5.15). Running through the central cavity of mandible (the body) forms the chin. Two upright the vertebral column is the delicate spinal cord, bars of bone (the rami) extend from the body to which the vertebral column surrounds and ­protects. connect the mandible with the temporal bone. Before birth, the spine consists of 33 separate The lower teeth lie in alveoli (sockets) in the alve- bones called vertebrae, but 9 of these eventually olar process at the superior edge of the mandib- fuse to form the two composite bones, the sacrum ular body. and the coccyx, that construct the inferior portion of  Anterior Posterior 1st cervical vertebra (atlas) Cervical curvature 2nd cervical (concave) vertebra (axis) 7 vertebrae, C1 – C7 1st thoracic vertebra 5 Transverse process Figure 5.16 The C-shaped spine typical Spinous Thoracic of a newborn. process curvature (convex) Remembering common meal times, 7 a.m., 12 noon, 12 vertebrae, and 5 p.m., may help you to recall the number of T1 – T12 bones in these three regions of the vertebral column. Intervertebral disc The individual vertebrae are separated by pads of flexible fibrocartilage—intervertebral discs— that cushion the vertebrae and absorb shock while Intervertebral allowing the spine flexibility. In a young person, the foramen discs have a high water content (about 90 percent) and are spongy and compressible. But as a person 1st lumbar ages, the water content of the discs decreases (as it vertebra does in other tissues throughout the body), and the Lumbar discs become harder and less compressible. curvature (concave) 5 vertebrae, Homeostatic Imbalance 5.4 L1 – L 5 Drying of the discs, along with a weakening of the ligaments of the vertebral column, predisposes older people to herniated (“slipped”) discs. Sacral However, herniation also may result when the ver- curvature (convex) tebral column is subjected to exceptional twisting 5 fused forces. If the protruding disc presses on the spinal vertebrae cord or the spinal nerves exiting from the cord, numbness and excruciating pain can result.______✚ Coccyx 4 fused The discs and the S-shaped structure of the ver- vertebrae tebral column work together to prevent shock to the head when we walk or run. They also make the Figure 5.15 The vertebral column. Thin discs between the thoracic vertebrae allow great flexibility in body trunk flexible. The spinal curvatures in the the thoracic region; thick discs between the lumbar thoracic and sacral regions are referred to as pri- vertebrae reduce flexibility. Notice that the terms convex mary curvatures because they are present when and concave refer to the curvature of the posterior we are born. Together the two primary curvatures aspect of the vertebral column. produce the C-shaped spine of the newborn baby (Figure 5.16). The curvatures in the cervical and lumbar regions are referred to as secondary curva- the vertebral column. Of the 24 single bones, the 7 tures because they develop some time after birth. vertebrae of the neck are cervical vertebrae, the next In adults, the secondary curvatures allow us to 12 are the thoracic vertebrae, and the remaining ­center our body weight on our lower limbs with 5 supporting the lower back are lumbar vertebrae. 154 Essentials of Human Anatomy and Physiology minimum effort. The cervical curvature appears Posterior when a baby begins to raise its head, and the lum- Lamina Vertebral arch bar curvature develops when the baby begins to Transverse Spinous walk. process process Homeostatic Imbalance 5.5 Were you ever given a “spine check” in middle school? There are several types of abnormal spinal Superior curvatures that can be identified by simple observa- articular tion. Three of these are scoliosis (sko″le-o′sis), process and kyphosis (ki-fo′sis), and lordosis (lor-do′sis). facet These spinal abnormalities may be congenital (pres- ent at birth) or may result from disease, poor pos- Pedicle Vertebral ture, unequal muscle pull on the spine, or injury. foramen Generally, unless there is a congenital problem, Body young healthy people have no skeletal problems, assuming that their diet is nutritious and they stay Anterior reasonably active. As you look at these photos, try to pinpoint how each of these conditions differs Figure 5.17 A typical vertebra, superior view. from a normal healthy spine. The usual treatments (Inferior articulating surfaces are not shown.) for these abnormal curvatures are braces, casts, or surgery. Transverse processes: two lateral projections from the vertebral arch. Spinous process: single projection arising from the posterior aspect of the vertebral arch (actually the fused laminae). Superior articular process and inferior ­articular process: paired projections lateral to the vertebral foramen, allowing a vertebra to form joints with adjacent vertebrae. In addition to these common features, verte- brae in the different regions of the spine have very specific structural characteristics, which we (a) Scoliosis (b) Kyphosis (c) Lordosis describe next. Abnormal spinal curvatures. Cervical Vertebrae ______________________________________________✚ The seven cervical vertebrae (identified as C1 to All vertebrae have a similar structural pattern C7) form the neck region of the spine. The first (Figure 5.17). The common features of vertebrae two vertebrae (atlas and axis) are different include the following: because they perform functions not shared by the Body, or centrum: disclike, weight-bearing other cervical vertebrae. As you can see in Figure part of the vertebra facing anteriorly in the 5.18a, the atlas (C1) has no body. The superior ­vertebral column. surfaces of its transverse processes contain large depressions that receive the occipital condyles of Vertebral arch: arch formed from the joining the skull. This joint allows you to nod “yes.” The of all posterior extensions, the laminae and axis (C2) acts as a pivot for the rotation of the pedicles, from the vertebral body. atlas (and skull) above. It has a large upright Vertebral foramen: canal through which the spinal cord passes.  Chapter 5: The Skeletal System 155 (a) ATLAS AND AXIS (b) TYPICAL CERVICAL VERTEBRAE Facet on superior Spinous Transverse Posterior articular process process process arch Vertebral foramen Anterior Transverse arch process Transverse 5 Superior view of atlas (C1) foramen Superior view Superior Spinous articular Body Transverse process process process Facet on Spinous superior process articular Transverse process process Dens Facet on inferior Body articular process Superior view of axis (C2) Right lateral view (c) THORACIC VERTEBRAE (d) LUMBAR VERTEBRAE Spinous process Spinous process Transverse Vertebral process foramen Vertebral foramen Transverse process Facet Facet on for rib superior articular Facet on process superior Body articular Body process Superior view Superior view Facet on Body superior Superior Body articular articular process process Facet on transverse process Costal facet Spinous for rib process Spinous Facet on inferior process articular process Right lateral view Right lateral view Figure 5.18 Regional characteristics of vertebrae. (The types of vertebrae are not shown to scale with respect to one another.) 156 Essentials of Human Anatomy and Physiology Figure 5.19 Sacrum and are distinguished by the fact that they are the only coccyx, posterior view. vertebrae to articulate with the ribs. The body is somewhat heart-shaped and has two costal facets (articulating surfaces) on each side, which receive the heads of the ribs (dotted line in Figure 5.18c). The transverse processes of each thoracic vertebra articulate with the knoblike tubercles of the ribs. The spinous process is long and hooks sharply downward, causing the vertebra to look like a giraffe’s head viewed from the side. Lumbar Vertebrae Superior Auricular articular surface The five lumbar vertebrae (L1 to L5) have mas- Ala Sacral sive, blocklike bodies that are somewhat kidney process canal bean–shaped. Their short, hatchet-shaped spi- nous processes (dotted line in Figure 5.18d) make them look like a moose head from the lateral aspect. Because most of the stress on the verte- Body bral column occurs in the lumbar region, these are the sturdiest of the vertebrae. Median Sacrum Sacrum sacral The sacrum (sa′krum) is formed by the fusion of crest five vertebrae (Figure 5.19). Superiorly it articulates with L5, and inferiorly it connects with the coccyx. Each winglike ala articulates laterally with the hip Posterior bone, forming a sacroiliac joint. The sacrum forms sacral the posterior wall of the pelvis. Its posterior mid- foramina line surface is roughened by the median sacral Sacral crest, the fused spinous processes of the sacral Coccyx hiatus vertebrae. This is flanked laterally by the posterior sacral foramina. The vertebral canal continues inside the sacrum as the sacral canal and termi- nates in a large inferior opening called the sacral hiatus. process, the dens, which acts as the pivot point. The joint between C1 and C2 allows you to rotate Coccyx your head from side to side to indicate “no.” The coccyx is formed from the fusion of three to The “typical” cervical vertebrae are C3 through five tiny, irregularly shaped vertebrae (see Figure C7 (Figure 5.18b). They are the smallest, lightest ver- 5.19). It is the human “tailbone,” a remnant of the tebrae, and most often their spinous processes are tail that other vertebrate animals have. short and divided into two branches. The transverse processes of the cervical vertebrae contain foramina Thoracic Cage (openings) through which the vertebral arteries ➔ Learning Objectives pass on their way to the brain above. Any time you □□ Name the components of the thoracic cage. see these foramina in a vertebra, you should know □□ Describe how a true rib differs from a false rib. immediately that it is a cervical vertebra. The sternum, ribs, and thoracic vertebrae make up Thoracic Vertebrae the bony thorax (Figure 5.20). The bony thorax is The 12 thoracic vertebrae (T1 to T12) are all typi- routinely called the thoracic cage because it cal. They are larger than the cervical vertebrae and forms a protective cage of slender bones and  Chapter 5: The Skeletal System 157 T1 vertebra Jugular notch Clavicular notch Manubrium Sternal angle Body Xiphisternal Sternum True joint ribs Xiphoid 5 (1–7) process T2 Jugular T3 notch T4 Sternal angle False ribs (8–12) Heart Intercostal spaces T9 Xiphisternal L1 joint Floating vertebra Costal cartilage ribs (11, 12) (a) (b) Explo