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 Line Narrow ridge of bone; thigh epicondyle 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 fiss re 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 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) Explore Cadaver Figure 5.20 The bony thorax (thoracic cage). cartilages around the organs of the thoracic cavity formed at the level of the second ribs. It pro- (heart, lungs, and major blood vessels). vides a handy reference point for counting ribs to locate the second intercostal space for Sternum listening to certain heart valves. To clarify The sternum (breastbone) is a typical flat bone the location of these structures, you can pal- and the result of the fusion of three bones—the pate your own sternal angle and jugular manubrium (mah-nu′bre-um), body, and xiphoid notch. (zif′oid) process. It is attached directly to the first The xiphisternal (zi′fe-ster″nal) joint, the seven pairs of ribs via costal cartilages. point where the sternal body and xiphoid pro- The sternum has three important bony land- cess fuse, lies at the level of the ninth thoracic marks—the jugular notch, the sternal angle, and vertebra. the xiphisternal joint. Because the sternum is so close to the body The jugular notch (concave upper border surface, it is easy to obtain samples from it of of the manubrium) can be palpated easily; blood-forming (hematopoietic) tissue for the diag- generally it is at the level of the third thoracic nosis of suspected blood diseases. A needle is vertebra. inserted into the marrow of the sternum, and the The sternal angle results where the manu- sample is withdrawn; this procedure is called a brium and body meet at a slight angle to sternal puncture. Because the heart lies immedi- each other, so that a transverse ridge is ately posterior to the sternum, the physician must 158 Essentials of Human Anatomy and Physiology take extreme care not to penetrate beyond the the sternum medially (at its sternal end) and to the sternum during this procedure. scapula laterally, where it helps to form the shoulder joint. The clavicle acts as a brace to hold the arm Ribs away from the top of the thorax and helps prevent Twelve pairs of ribs form the walls of the bony shoulder dislocation. When the clavicle is broken, thorax. All the ribs articulate with the vertebral the whole shoulder region caves in medially, which column posteriorly and then curve downward and shows how important its bracing function is. toward the anterior body surface. The true ribs, The paired scapulae (skap′u-le), or shoulder the first seven pairs, attach directly to the sternum blades, are commonly called “wings” because they by costal cartilages. False ribs, the next five pairs, flare when we move our arms posteriorly. The either attach indirectly to the sternum or are not scapulae are not directly attached to the axial attached to the sternum at all. The last two pairs of skeleton (see Figure 5.21a); they are loosely held false ribs lack the sternal attachments, so they are in place by trunk muscles. Each triangular scapula also called floating ribs. has a flattened body with three borders—supe- The intercostal spaces (spaces between the rior, medial (vertebral), and lateral (axillary) ribs) are filled with the intercostal muscles, which (Figure 5.21c). It also has three angles—superior, aid in breathing. inferior, and lateral. The glenoid cavity, a shal- low socket that receives the head of the arm Did You Get It? bone, is in the lateral angle (Figure 5.21d). The 16. What are the five major regions of the vertebral scapula has two important processes—the acro- column? mion (ah-kro′me-on), which is the enlarged lat- 17. How can you distinguish a lumbar vertebra from a eral end of the spine of the scapula, and the cervical vertebra? beaklike coracoid (kor′ah-koid) process. The 18. What is a true rib? A false rib? 19. Besides the ribs and sternum, there is a third group of acromion connects with the clavicle laterally at bones forming the thoracic cage. What is it? the acromioclavicular joint. The coracoid pro- 20. What bone class do the ribs and skull bones fall into? cess points laterally over the top of the shoulder 21. Which spinal curvatures are present at birth? and anchors some of the muscles of the arm. Just 22. How does the shape of a newborn baby’s spine differ medial to the coracoid process is the large supra- from that of an adult? scapular notch, which is a nerve passageway. For answers, see Appendix A. The shoulder girdle is very light and allows the upper limb exceptionally free movement. This is Appendicular Skeleton due to the following factors: Each shoulder girdle attaches to the axial skel- ➔ Learning Objectives eton at only one point—the sternoclavicular □ Identify on a skeleton or diagram the bones of the joint. shoulder and pelvic girdles and their attached The loose attachment of the scapula allows it limbs. to slide back and forth against the thorax as □ Describe important differences between a male and muscles act. a female pelvis. The glenoid cavity is shallow, and the shoulder The appendicular skeleton (shaded gold in Figure joint is poorly reinforced by ligaments. 5.8) is composed of 126 bones of the limbs However, this exceptional flexibility also has a draw- (appendages) and the pectoral and pelvic girdles, back; the shoulder girdle is very easily dislocated. which attach the limbs to the axial skeleton. Bones of the Upper Limbs Bones of the Shoulder Girdle Thirty separate bones form the skeletal framework Each pectoral girdle, or shoulder girdle, con- of each upper limb, including the arm, forearm, sists of two bones—a clavicle and a scapula and hand. (Figure 5.21a). The paired clavicles (klav′ı̆-kulz), or collar- Arm bones, are slender, doubly curved bones (Figure The arm is formed by a single bone, the 5.21b). Each clavicle attaches to the manubrium of humerus (hu′mer-us), which is a typical long 160 Essentials of Human Anatomy and Physiology Head of Greater humerus tubercle Trochlear notch Lesser tubercle Olecranon Anatomical neck Surgical Head neck Coronoid Intertubercular process sulcus Neck Radial Proximal tuberosity radioulnar joint Radius Radial groove Deltoid Deltoid tuberosity tuberosity Ulna Inter- osseous membrane Radial Medial fossa epicondyle Olecranon fossa Coronoid Ulnar fossa Radial styloid styloid Distal process Lateral process radioulnar Capitulum Trochlea epicondyle joint (a) (b) (c) Figure 5.22 Bones of the right arm and forearm. (a) Humerus, anterior view. (b) Humerus, posterior view. (c) Anterior view of the bones of the forearm: the radius and the ulna. bone (Figure 5.22a and b). At its proximal end is a the tubercles is the surgical neck, so named rounded head that fits into the shallow glenoid because it is the most frequently fractured part of cavity of the scapula. Immediately inferior to the the humerus. In the midpoint of the shaft is a head is a slight constriction called the anatomical roughened area called the deltoid tuberosity, neck. Anterolateral to the head are two bony pro- where the large, fleshy deltoid muscle of the jections separated by the intertubercular sul- shoulder attaches. Nearby, the radial groove runs cus—the greater tubercle and lesser tubercle, obliquely down the posterior aspect of the shaft. which are sites of muscle attachment. Just distal to This groove marks the course of the radial nerve,  Chapter 5: The Skeletal System 161 an important nerve of the upper limb. At the distal end of the humerus is the medial trochlea (trok′le-ah), which looks somewhat like a spool, and the lateral ball-like capitulum (kah-pit′u- lum), which can be “outlined” with a “C” from the Distal anterior view. Both of these processes articulate Middle with bones of the forearm. Above the trochlea Phalanges anteriorly is a depression, the coronoid fossa; on (fingers) the posterior surface is the olecranon (o-lek′rah- Proximal non) fossa. These two depressions, which are 5 flanked by the medial epicondyle and lateral epicondyle, allow the corresponding processes of 4 3 2 Metacarpals 5 the ulna to move freely when the elbow is bent 1 (palm) and extended. Forearm Hamate Trapezium Two bones, the radius and the ulna, form the Carpals Pisiform Trapezoid skeleton of the forearm (Figure 5.22c). When the (wrist) Triquetrum Scaphoid body is in the anatomical position, the radius is Lunate Capitate the lateral bone; that is, it is on the thumb side of Ulna the forearm. When the hand is rotated so that the Radius palm faces backward, the distal end of the radius Figure 5.23 Bones of the right hand, anterior crosses over and ends up medial to the ulna. Both view. proximally and distally the radius and ulna articu- late at small radioulnar joints, and the two bones are connected along their entire length by the flexible interosseous membrane. Both the The palm of the hand consists of the meta- ulna and the radius have a styloid process at carpals. The metacarpals are numbered 1 to 5 their distal end. from the thumb side of the hand toward the little The disc-shaped head of the radius also forms finger. When the fist is clenched, the heads of the a joint with the capitulum of the humerus. Just metacarpals become obvious as the “knuckles.” below the head is the radial tuberosity, where The phalanges are the bones of the fingers. the tendon of the biceps muscle attaches. Each hand contains 14 phalanges. There are three When the upper limb is in the anatomical in each finger (proximal, middle, and distal), position, the ulna is the medial bone (on the little- except in the thumb, which has only two (proxi- finger side) of the forearm. On its proximal end mal and distal). are the anterior coronoid process and the poste- rior olecranon, which are separated by the troch- Did You Get It? lear notch. Together these two processes grip the 23. Contrast the general function of the axial skeleton to trochlea of the humerus in a curved “tongue-in- that of the appendicular skeleton. 24. What is the single point of attachment of the groove”–like joint. shoulder girdle to the axial skeleton? 25. What bone forms the skeleton of the arm? Hand 26. Where are the carpals found, and what type (long, The skeleton of the hand consists of the carpals, short, irregular, or flat) of bone are they? the metacarpals, and the phalanges (fah-lan′jˉez) 27. Which bones of the upper limb have a styloid (Figure 5.23). The eight carpal bones, arranged in process? two irregular rows of four bones each, form the For answers, see Appendix A. part of the hand called the carpus, or the wrist. The carpals are bound together by ligaments that restrict movements between them. 162 Essentials of Human Anatomy and Physiology Bones of the Pelvic Girdle The pubis (pu′bis) is the most anterior and The pelvic girdle is formed by two coxal inferior part of a coxal bone. Fusion of the rami of (kok′sal) bones, commonly called hip bones, the pubis anteriorly and the ischium posteriorly and the sacrum (described on p. 156). Together forms a bar of bone enclosing the obturator with the coccyx, the pelvic girdle forms the pelvis (ob′tu-ra″tor) foramen, an opening that allows (Figure 5.24a). Note that the terms pelvic girdle blood vessels and nerves to pass into the anterior (the coxal bones and the sacrum) and pelvis (the part of the thigh. The pubic bones of each hip coxal bones, sacrum, and coccyx) have slightly bone articulate anteriorly to form a cartilaginous different meanings. joint, the pubic symphysis (pu′bik sim′fı̆-sis). The bones of the pelvic girdle are large and The ilium, ischium, and pubis fuse at the deep heavy, and they are attached securely to the axial socket called the acetabulum (as″ĕ-tab′u-lum), skeleton via the sacral attachment to the L5 lumbar which means “vinegar cup.” The acetabulum vertebra. The sockets, which receive the thigh receives the head of the thigh bone. bones, are deep and heavily reinforced by liga- The bony pelvis is divided into two regions. ments that attach the limbs firmly to the girdle. The false pelvis is superior to the true pelvis; it Bearing weight is the most important function of is the area medial to the flaring portions of the ilia. this girdle because the total weight of the upper The true pelvis is surrounded by bone and lies body rests on the pelvis. The reproductive organs, inferior to the flaring parts of the ilia and the pel- urinary bladder, and part of the large intestine lie vic brim (Figure 5.24c). The dimensions of the true within and are protected by the pelvis. pelvis of a woman are very important because Each hip bone is formed by the fusion of three they must be large enough to allow the infant’s bones: the ilium, ischium, and pubis (Figure head (the largest part of the infant) to pass during 5.24b). The ilium (il′e-um), which connects poste- childbirth. The dimensions of the cavity, particu- riorly with the sacrum at the sacroiliac (sak″ro- larly the outlet (the inferior opening of the pelvis il′e-ac) joint, is a large, flaring bone that forms measured between the ischial spines) and the most of the hip bone (imagine a peacock’s fanned inlet (superior opening between the right and left tail feathers). When you put your hands on your sides of the pelvic brim), are critical, and they are hips, they are resting over the alae, or winglike carefully measured by the obstetrician. portions, of the ilia. The upper edge of an ala, the Of course, individual pelvic structures vary, iliac crest, is an important anatomical landmark but there are fairly consistent differences between that is always kept in mind by those who give a male and a female pelvis, including the follow- intramuscular injections. The iliac crest ends ante- ing characteristics (see Figure 5.24c): riorly in the anterior superior iliac spine and The female inlet is larger and more circular. posteriorly in the posterior superior iliac spine. The female pelvis as a whole is shallower, and Small inferior spines are located below these. the bones are lighter and thinner. The ischium (is′ke-um) is the “sit-down The female ilia flare more laterally, giving bone,” so called because it forms the most inferior women curvy hips. part of the coxal bone. The ischial tuberosity is a roughened area that receives body weight when The female sacrum is shorter and less curved. you are sitting. The ischial spine, superior to the The female ischial spines are shorter and far- tuberosity, is another important anatomical land- ther apart; thus, the outlet is larger. mark, particularly in the pregnant woman, because The female pubic arch is more rounded be- it narrows the outlet of the pelvis through which cause the angle of the pubic arch is greater. the baby must pass during birth. Another impor- tant structural feature of the ischium is the greater Did You Get It? sciatic notch, which allows blood vessels and the 28. What three bones form the hip bone? What bones large sciatic nerve to pass from the pelvis posteri- form each pelvic girdle? orly into the thigh. Injections in the buttock should 29. Describe three ways the bony pelvis of a woman always be given well away from this area to avoid differs from that of a man? possible nerve damage. For answers, see Appendix A. 164 Essentials of Human Anatomy and Physiology Surgical neck Intercondylar Greater Head trochanter eminence Inter- Inter- Lateral Medial trochanteric trochanteric Lesser trochanter condyle condyle line crest Head Tibial Gluteal tuberosity tuberosity Proximal tibiofibular joint Interosseous membrane Anterior border Fibula Tibia Intercondylar fossa Medial Lateral Lateral Distal condyle epicondyle condyle tibiofibular Medial joint epicondyle Patellar Medial Lateral surface malleolus malleolus (a) (b) (c) Figure 5.25 Bones of the right thigh and leg. Anterior view of femur (a). Posterior view of femur (b). Anterior view of leg (c). Bones of the Lower Limbs rated anteriorly by the intertrochanteric line and The lower limbs carry our total body weight posteriorly by the intertrochanteric crest). These when we are erect. Hence, it is not surprising that markings and the gluteal tuberosity, located on the bones forming the three segments of the the proximal end of the shaft, all serve as sites for lower limbs (thigh, leg, and foot) are much muscle attachment. The head of the femur articu- thicker and stronger than the comparable bones lates with the deep, secure socket of the acetabu- of the upper limb. lum of the hip bone. However, the surgical neck of the femur is still a common fracture site, especially Thigh in old age. The femur (fe′mur), or thigh bone, is the only The femur slants medially as it runs downward bone in the thigh (Figure 5.25a and b). It is the to join with the leg bones; this brings the knees in heaviest, strongest, and longest bone in the body. line with the body’s center of gravity. The medial Its proximal end has a ball-like head, a neck, and a course of the femur is more noticeable in women greater trochanter and lesser trochanter (sepa- because the female pelvis is typically wider than that  Chapter 5: The Skeletal System 165 Phalanges: Medial longitudinal arch Distal Transverse arch Middle Proximal Lateral longitudinal arch Tarsals: 5 Medial Metatarsals cuneiform Tarsals: Intermediate cuneiform Lateral cuneiform Figure 5.27 Arches of the foot. Navicular Cuboid forms the inner bulge of the ankle. The anterior surface of the tibia is a sharp ridge, the anterior Talus border, that is unprotected by muscles; thus, it is easily felt beneath the skin. The fibula, which lies alongside the tibia later- Calcaneus ally, forms joints with it both proximally and distally, and is thin and sticklike. The fibula has no part in forming the knee joint. Its distal end, the lateral Figure 5.26 Bones of the right foot, superior malleolus, forms the outer part of the ankle. view. Foot The foot, composed of the tarsals, metatarsals, and of the male. Distally on the femur are the lateral phalanges, has two important functions. It sup- condyle and medial condyle, which articulate with ports our body weight and serves as a lever that the tibia below. Posteriorly these condyles are sepa- allows us to propel our bodies forward when we rated by the deep intercondylar fossa. Anteriorly walk and run. on the distal femur is the smooth patellar surface, The tarsus, forming the posterior half of the which forms a joint with the patella, or kneecap. foot, is composed of seven tarsal bones (Figure 5.26). Body weight is carried mostly by the two Leg largest tarsals, the calcaneus (kal-ka′ne-us), or Connected along their length by an interosseous heel bone, and the talus (ta′lus; “ankle”). The membrane, two bones, the tibia and fibula, form talus lies superior to the calcaneus, articulates with the skeleton of the leg (Figure 5.25c). The tibia, or the tibia, and allows pointing of the toes. Five shinbone, is larger and more medial. At the proxi- metatarsals form the sole, and 14 phalanges mal end, the medial condyle and lateral con- form the toes. Like the fingers of the hand, each dyle (separated by the intercondylar eminence) toe has three phalanges, except the great toe, articulate with the distal end of the femur to form which has two. the knee joint. The patellar (kneecap) ligament, The bones in the foot are arranged to form which encloses the patella, a sesamoid bone three strong arches: two longitudinal (medial and (look ahead to Figure 6.20c and d on p. 211), lateral) and one transverse (Figure 5.27). Ligaments, attaches to the tibial tuberosity, a roughened which bind the foot bones together, and tendons area on the anterior tibial surface. Distally, a pro- of the foot muscles help to hold the bones firmly cess called the medial malleolus (mal-le′o-lus) in the arched position but still allow a certain 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  A CLOSER Joint Ventures LOOK T he technology for creating the prostheses (artificial joints) used in medicine today developed, in relative terms, in a flash—less than 60 years. Over 1 million knee and hip joint replacement surgeries are performed each year, mostly because of the destructive effects of osteoarthritis or rheumatoid arthritis. The production of durable, mobile joints requires a substance that is strong, nontoxic, and resistant to the corrosive effects of organic acids in blood and that will not cause an immune response. In 1963, Sir John (a) A hip prosthesis. (b) X-ray image of right knee Charnley, an English orthopedic sur- showing total knee replacement geon, performed the first total hip methacrylate cement (bone cement). prosthesis. replacement using a modern pros- This cement proved to be exception- thetic, revolutionizing the therapy of ally strong and relatively problem free. Total hip and knee replacements arthritic hips. His device consisted of a Hip prostheses were followed by knee last over 20 years in elderly patients metal ball on a stem (the femoral prostheses (see photos a and b), and who do not excessively stress the head) and a cup-shaped polyethylene replacements are now available for joint but may not last as long in plastic socket (the acetabulum) many other joints, including shoul- younger (age 45 to 60), more active anchored to the pelvis by polymethyl ders, fingers, and elbows. people. Most such operations are amount of give, or springiness. Weak arches are one other bone. Joints, also called articula- referred to as “fallen arches,” or “flat feet.” tions, are the sites where two or more bones meet. They have two functions: They hold the Did You Get It? bones together securely but also give the rigid 30. What two bones form the skeleton of the leg? skeleton mobility. 31. Which bone allows us to “point our toes”? The graceful movements of a ballet dancer 32. Which bone of the lower limb has an intertrochanteric and the rough-and-tumble grapplings of a football line and crest and an intercondylar fossa? player illustrate the great variety of motion that For answers, see Appendix A. joints allow. With fewer joints, we would move like robots. Nevertheless, the bone-binding func- tion of joints is just as important as their role Joints in mobility. The immovable joints of the skull, ➔ Learning Objective for instance, form a snug enclosure for the vital □ Name the three major structural categories of brain. joints, and compare the amount of movement Joints are classified in two ways—functionally allowed by each. and structurally. The functional classification focuses on the amount of movement the joint With one exception (the hyoid bone of the neck), every bone in the body forms a joint with at least 166  reduced the time and cost of creating individualized joints. The computer draws from a database of hundreds of healthy joints, generates possible designs, and produces a program to direct the machines that shape it. Equally exciting are techniques that call on the patient’s own tissues to regenerate, such as these three: Osteochondral grafting: Healthy bone and cartilage are removed from one part of the body and transplanted to the injured joint. (c) Physician with the ROBODOC machine used to perform hip Autologous chondrocyte implanta- joint replacement surgery. tion: Healthy chondrocytes are removed from the body, cultivated done to reduce pain and restore joint better-fitting hole and so achieve in the lab, and implanted at the function. better alignment for the femoral damaged joint. Replacement joints are not yet prosthesis in hip surgery. In unce- Stem cell regeneration: Undifferen- strong or durable enough for young, mented prostheses, the bone will tiated stem cells are removed from active people. The problem is that eventually grow so that it binds bone marrow and placed in a gel, the prostheses work loose over time. strongly to the implant, but this which is packed into an area of One solution is to strengthen the requires extended recovery time. eroded cartilage. cement that binds the implant to the Dramatic changes are also occur- These techniques offer hope for bone. Another solution is robot- ring in the way artificial joints are younger patients because they could assisted surgery using ROBODOC (see made. CAD/CAM (computer-aided stave off the need for a joint prosthe- photo c), which uses the information design and computer-aided manufac- sis for several years. from a CT scan to precisely drill a turing) techniques have significantly allows. On this basis, there are synarthroses Because the structural classification is more clear- (sin″ar-thro′sˉez), or immovable joints; amphiar- cut, we will focus on that classification scheme throses (am″fe-ar-thro′sˉez), or slightly movable here. The joint types are shown in Figure 5.28 on joints; and diarthroses (di″ar-thro′sˉez), or freely p. 168 and described next. ➔ movable joints. Immovable and slightly movable joints are restricted mainly to the axial skeleton, ConCeptLink where firm attachments and protection of internal To understand the structural classes of joints more organs are priorities. Freely movable joints pre- clearly, recall the properties of tissues that form the dominate in the limbs, where mobility is important. joints. Fibrous connective tissue contains many colla- Structurally, there are fibrous, cartilaginous, gen fibers for strength. The three types of cartilage and synovial joints. These classifications are based (hyaline, fibrocartilage, elastic) provide structure with on whether fibrous tissue, cartilage, or a joint cav- some degree of flexibility, and fibrocartilage also has ity separates the bony regions at the joint. As a the ability to absorb compressive shock (Chapter 3, general rule, fibrous joints are immovable, and pp. 94, 95). Synovial membranes contain areolar con- synovial joints are freely movable. Although carti- nective tissue and line the joint cavities of synovial laginous joints have both immovable and slightly joints (Chapter 4, pp. 110, 112). ➔ movable examples, most are amphiarthrotic. 167  Chapter 5: The Skeletal System 169 Q: How does this joint type differ structurally from cartilaginous and fibrous joints? Acromion of scapula Ligament Joint cavity containing Bursa syno ial uid Ligament 5 Articular (hyaline) Tendon cartilage sheath Synovial membrane Tendon of Fibrous layer of the biceps muscle articular capsule Humerus Figure 5.29 General structure of a synovial joint. Fibrous Joints hyaline cartilage. Examples include the epiphyseal In fibrous joints, the bones are united by fibrous plates of growing long bones and the joints between tissue. The best examples of this type of joint are ribs 1–7 and the sternum (Figure 5.28c). Symphyses the sutures of the skull (Figure 5.28a). In sutures, are amphiarthrotic (slightly moveable) joints linked the irregular edges of the bones interlock and are by discs of fibrocartilage. Examples include the bound tightly together by connective tissue fibers, intervertebral discs of the spinal column (Figure allowing no movement. Gomphoses (gom-fo′seˉ z), 5.28d) and the pubic symphysis of the pelvis are “peg-in-socket” fibrous joints that are found (Figure 5.28e). where the teeth meet the facial bones (not shown in Figure 5.28). In another type of fibrous joint, Synovial Joints syndesmoses (sin-dez-mo′seˉ z), the connecting Synovial joints are joints in which the articulat- fibers are longer than those of sutures; thus the ing bone ends are separated by a joint cavity con- joint has more “give.” The joint connecting the dis- taining synovial fluid (Figure 5.28f–h). All joints of tal ends of the tibia and fibula is a syndesmosis the limbs are synovial joints. (Figure 5.28b). All synovial joints have four distinguishing fea- tures (Figure 5.29): Cartilaginous Joints Articular cartilage. Articular (hyaline) carti- Cartilaginous joints come in two varieties, which lage covers the ends of the bones forming the differ in the type of cartilage involved. Synchondroses joint. are immoveable (synarthrotic) joints linked by Articular capsule. The joint surfaces are en- closed by a sleeve, or layer, of fibrous connec- articulating bones. tive tissue, which is lined with a smooth syno- vial membrane (the reason these joints are A: instead of cartilage or fibrous tissue separating the It has a joint cavity filled with lubricating fluid called synovial joints). 170 Essentials of Human Anatomy and Physiology Table 5.3 Summary of Joint Classes Structural class Structural characteristics Types Mobility Fibrous Bone ends/parts united by Suture (short fibers) Immobile (synarthrosis) collagenic fibers Syndesmosis (longer fibers) Slightly mobile (amphiarthrosis) and immobile Gomphosis (periodontal Immobile ligament) Cartilaginous Bone ends/parts united by Synchondrosis (hyaline Immobile cartilage cartilage) Symphysis (fibrocartilage) Slightly movable Synovial Bone ends/parts covered with Plane Condylar Freely movable (diarthrosis; articular cartilage and enclosed Hinge Saddle movements depend on design within an articular capsule lined Pivot Ball and socket of joint) with synovial membrane Joint cavity. The articular capsule encloses a Types of Synovial Joints Based cavity, called the joint cavity, which contains on Shape lubricating synovial fluid secreted by the sy- The shapes of the articulating bone surfaces deter- novial membrane (look back at Chapter 4, mine what movements are allowed at a joint. pp. 110, 112). Based on such shapes, our synovial joints can be Reinforcing ligaments. The fibrous layer of classified as plane, hinge, pivot, condylar (kon′dı̆- the capsule is usually reinforced with ligaments. ler), saddle, or ball-and-socket joints. Examples of Bursae and tendon sheaths are not strictly part these types of joints are shown in Figure 5.30; the of synovial joints, but they are often found closely arrows indicate movement around one or more associated with them (see Figure 5.29). Essentially axes in each type. bags of lubricant, they help cushion and act like In a plane joint (Figure 5.30a), the articular ball bearings to reduce friction between adjacent surfaces are essentially flat, and only short slip- structures during joint activity. Bursae (ber′se; ping or gliding movements are allowed. The “purses”) are flattened fibrous sacs lined with movements of plane joints are nonaxial; that synovial membrane and containing a thin film of is, gliding back and forth does not involve ro- synovial fluid. They are common where ligaments, tation around any axis. The intercarpal joints of muscles, skin, tendons, or bones rub together. A the wrist are the best examples of plane joints. tendon sheath is essentially an elongated bursa In a hinge joint (Figure 5.30b), the cylindrical that wraps completely around a tendon subjected end of one bone fits into a trough-shaped sur- to friction, like a bun around a hot dog. face on another bone. Angular movement is al- The structural joint types are summarized in Table 5.3. lowed in just one plane, like a door hinge. Examples are the elbow joint, ankle joint