1. Your roommate says that the concept of homeostasis isbeing violated when the rate of respiration goes up duringexercise, because the rate of respiration clearly is not beingheld constant. Explain to him where his thinking is faulty.

2. What do you think would be some of the problems associ-ated with severe third-degree burns, in which both the epi-dermis and the dermis are severely damaged or destroyed?

3. Sherlock Holmes, the greatest fictional detective of all time,is talking to a woman in her late 40s, when he suddenlysays, "I see, my dear madam, you must have enjoyed yourcigarettes and your suntans." The woman is amazed, be-cause she mentioned nothing about these two formerfavorite activities. What physical characteristics mightMr. Holmes have seen in this woman to indicate she was anavid sun-worshiper and cigarette smoker? And how do thesecharacteristics develop?

4. Dieting is difficult. People who do manage to lose weight cangain it back if they're not careful. Are techniques such as li-posuction or lipodissolve a good way to keep the weight offpermanently? Why or why not?

5. Fibrous connective tissue consists of ground substance andfibers that provide strength, support, and fleXibility. Concreteis used to make tough, durable structures in constructionprojects. How is a concrete structure like or unlike fibrousconnective tissue?

6. By definition, an organ is a structure composed of two ormore tissue types that perform a specific function. Perfor-mance of that function often requires coordination amongmany cells. Why is it so important that cardiac muscle cellsof the heart be synchronized (coordinated) so that they beatnearly all at once?

he human body is capable of an awesome arrayof physical activities. With training, some individualscan run a mile in less than four minutes or lift more

than their own weight. Exquisitely sensitive motor skillsallow us to thread a needle, turn our head to focus on a sin-gle star, and throw a baseball into the strike zone. Consid-ered individually, anyone of these activities may not seemamazing, but for a single structure (the human body) to becapable of all of them is remarkable indeed. From an engi-neering standpoint it would be like designing a bulldozerthat is strong enough to flatten a building, yet delicateenough to pick up a dime.

This chapter describes the skeletal system, the organ sys-tem for support, protection, and movement. We examine thestructure and development of bones, and the way they re-model and repair themselves. We review how the bones fittogether to make the skeleton. We take a look at how jointsenable bones and muscles to work together. Finally, we con-sider what can go wrong with the skeletal system.

5.1 The skeletal system consistsof connective tissue

The skeletal system comprises three types of connectivetissue:

• Bones, the hard elements of the skeleton• Ligaments, dense fibrous connective tissue that binds the

bones to each other• Cartilage, a specialized connective tissue consisting pri-

marily of fibers of collagen and elastic in a gel-like fluidcalled ground substance

Bo es are the hard elements of the skeletonAlthough most of the mass of bones consists of nonlivingextracellular crystals of calcium minerals, bones also containseveral types of living bone cells, nerves, and blood vessels.Indeed, bones bleed when cut during orthopedic surgery orwhen they break.

Bones perform five important functions. The first threeare the same as the functions of the skeleton, which consistsprimarily of bone:

• Support. Bones form the structure (the skeleton) towhich skeletal muscles attach. The skeleton also sup-ports the soft organs.

• Movement. Bones support and interact with muscles,making it possible for our bodies to move.

• Protection. As the hard elements of the skeleton, bonessurround and protect many of our delicate internalorgans.

• Formation of blood cells. Some bones contain cells that areresponsible for producing different types of blood cells.

• Mineral storage. Bones store minerals, including calciumand phosphate, that are important to body metabolismand function.

Bones contribute to support, movement, and protection. They

also produce the blood cells and store minerals .•

Bone contains living cellsA typical long bone, so called because it is longer than it iswide, consists of a cylindrical shaft (called the diaphysis) withan enlarged knob called an epiphysis at each end (Figure 5.1).Dense compact bone forms the shaft and covers each end.A central cavity in the shaft is filled with yellow bone marrow.Yellow bone marrow is primarily fat that can be utilized forenergy. Inside each epiphysis is spongy bone. Spongy boneis less dense than compact bone, allowing the bones to belight but strong. Spongy bone is a latticework of hard, rela-tively strong trabeculae (from Latin, meaning "little beams")composed of calcium minerals and living cells. In certainlong bones, most notably the long bones of the upper armsand legs (humerus and femur, respectively), the spaces be-tween the trabeculae are filled with red bone marrow. Specialcells called stem cells in the red bone marrow are responsiblefor the production of red and white blood cells and platelets.

True. In addition to the hard elements of calcium minerals that compose most of a bone's bulk, bonecontains several types of living cells that are involved in bone growth, repair, and remodeling. Bone alsocontains nerves and blood vessels.

True. Regular exercise increases bone density and strengthens bone.

True. Experiments show that weak electrical current speeds the rate at which bone heals, possibly becauseit stimulates bone-forming cells. In this chapter, we'll look at how bones repair themselves after a fracture.

b. Apply cold at regUlar intervals during the first 24 hours after a sprain, then switch to heat. We'll see whyin this chapter.

C. The knee. The knee joint is adapted to absorb vertical forces of nearly seven times body weight.However, it is vulnerable to horizontal blows such as kicks and football tackles, especially when the footis firmly on the ground. In this chapter, we'll examine the structure of the knee and other joints.

Spongy bone(spaces containred bonemarrow)

Compactbone

Yellowbonemarrow

Bloodvessel

Periosteum

Central cavity(contains yellowbone marrow)

Figure 5.1 structure of bone. (a) A partial cut through a long bone. (b) A closer view of a sectionof bone shows that compact bone is a nearly solid structure with central canals for the blood vesselsand nerves. (c) A photograph of an osteon of compact bone showing osteocytes embedded withinthe solid structure. (d) A single osteocyte in a lacuna. Osteocytes remain in contact with each otherby cytoplasmic extensions into the canaliculi between cells.

The outer surface of the bone is covered by a tough layerof connective tissue, the periosteum, which contains special-ized bone-forming cells. If the end of a long bone forms amovable joint with another bone, the joint surface is coveredby a smooth layer of cartilage that reduces friction.

Taking a closer look, we see that compact bone is madeup largely of extracellular deposits of calcium phosphate en-closing and surrounding living cells called osteocytes (fromthe Greek words for "bone" and "cells"). Osteocytes arearranged in rings in cylindrical structures called osteons(sometimes called Haversian systems). As bone develops andbecomes hard, the osteocytes become trapped in hollowchambers called lacunae. However, the osteocytes remainin contact with each other via thin canals called canaliculi.Within the canaliculi, extensions of the cell cytoplasm and

the presence of gap junctions between adjacent cells keepthe osteocytes in direct contact with each other. Osteocytesnearest the center of an osteon receive nutrients by diffu-sion from blood vessels that pass through a central canal(Haversian canal). These cells then pass nutrients on toadjacent cells via the gap junctions. In this way, all theosteocytes can be supplied with nutrients even though mostosteocytes are not located near a blood vessel. Waste prod-ucts produced by the osteocytes diffuse in the opposite direc-tion and are removed from the bone by the blood vessels.

In spongy bone, osteocytes do not need to rely on cen-tral canals for nutrients and waste removal. The slender tra-becular structure of spongy bone gives each osteocyte accessto nearby blood vessels in red bone marrow.

Bone may be compact or spongy in appearance. Long boneshave a hollow shaft of compact bone filled with yellow marrow;spongy bone with red bone marrow is found in the epiphyses. _

Ligaments hold bones togetherLigaments attach bone to bone. Ligaments consist of densefibrous connective tissue, meaning that they are a regulararray of closely packed collagen fibers all oriented in thesame direction with a few fibroblasts in between. Ligamentsconfer strength to certain joints while still permitting move-ment of the bones in relation to each other.

Cartilage lends supportCartilage, as you already know, contains fibers of collagenand/ or elastin in a ground substance of water and other sub-stances. Cartilage is smoother and more flexible than bone.Cartilage is found where support under pressure is importantand where some movement is necessary.

There are three types of cartilage in the human skeleton.Fibrocartilage consists primarily of collagen fibers arranged inthick bundles. It withstands both pressure and tension well.The intervertebral disks between the vertebrae, and also

Fetus: First2 months

Fetus: At2-3 months

Cartilage -modelforms

Bloodvessel

Compact bone develops --starting at primaryossification site

certain disklike supportive structures in the knee joint calledmenisci, are made of fibrocartilage. Hyaline cartilage is asmooth, almost glassy cartilage of thin collagen fibers. Hyalinecartilage forms the embryonic structures that later becomethe bones. It also covers the ends of mature bones in joints,creating a smooth, low-friction surface. Elastic cartilage ismostly elastin fibers, so it is highly flexible. It lends structureto the outer ear and to the epiglottis, a flap of tissue that cov-ers the larynx during swallowing.

5.2 Bone development beginsin the embryo

In the earliest stages of fetal development, even before organsdevelop, the rudimentary models of future bones are createdout of hyaline cartilage by cartilage-forming cells calledchondroblasts. After about two to three months of fetal de-velopment, the cartilage models begin to dissolve and are re-placed by bone. This process is called ossification. Althoughossification is slightly different for flat bones and long bones,we will concentrate on the process for long bones.

Figure 5.2 illustrates how ossification occurs in a longbone. After the chondroblasts die, the matrix they producedgradually breaks down inside the future shaft of the bone,

Cartilage --growthplate

Compact--bone \containingosteocytes

Spongy bone developsat secondaryossification sites

The growth plates promotelongitudinal growth untilyoung adulthood

Figure 5.2 How bone develops. The first two phases occur in the fetus. Bones continue to growlonger throughout childhood and adolescence because of growth at the growth plates.

making room for blood vessels to develop. The blood vesselscarry bone-forming cells called osteoblasts (from the Greekwords for "bone" and "to build") into the area from the de-veloping periosteum. The osteoblasts secrete a mixture ofproteins (including collagen) called osteoid, which formsinternal structure and provides strength to bone. They alsosecrete enzymes that facilitate the crystallization of hardmineral salts of calcium phosphate, known as hydroxyapatite,around and between the osteoid matrix. As more and morehydroxyapatite is deposited, the osteoblasts become embed-ded in the hardening bone tissue. In mature compact bone,approximately one-third of the matrix is osteoid and two-thirds is crystals of hydroxyapatite.

Eventually the rate at which osteoblasts produce theosteoid matrix and stimulate the mineral deposits declines,and they become mature osteocytes embedded in their in-dividuallacunae. Mature osteocytes continue to maintainthe bone matrix, however. Without them the matrix wouldslowly disintegrate.

Bones continue to lengthen throughout childhood andadolescence. This is because a narrow strip of cartilage calledthe growth plate (or epiphyseal plate) remains in each epiph-ysis. Chondroblast activity (and hence the development ofnew cartilage as a model for the lengthening bone) is con-centrated on the outside of the plate, whereas the conversionof the cartilage model to bone by osteoblasts is concentratedon the inside of the plate (Figure 5.3). In effect, the bonelengthens as the two growth plates migrate farther and far-ther apart. Bones also grow in width as osteoblasts lay downmore bone on the outer surface just below the periosteum.

The bone development process is controlled by hor-mones, chemicals secreted by the endocrine glands. Themost important hormone in preadolescents is growth hor-mone, which stimulates the bone-lengthening activity of thegrowth plate. During puberty the sex hormones (testosteroneand estrogen) also stimulate the growth plate, at least ini-tially. But at about age 18 in females and 21 in males thesesame sex hormones signal the growth plates to stop growing,

Figure 5.3 How long bones increase in length. Chondrocytesproduce new cartilage at the outer surface, and cartilage is con-verted to bone at the inner surface.

and the cartilage is replaced by bone tissue. At this point thebones can no longer lengthen, though they can continue togrow in width.

Bone-forming cells called osteoblasts produce a protein mixture(including collagen) that becomes bone's structural frameworkThey also secrete an enzyme that facilitates mineral depositionwithin the protein matrix .•

5.3 Mature bone undergoesremodeling and repair

Even though bones stop growing longer, they do not remainthe same throughout life. Bone is a dynamic tissue that un-dergoes constant replacement, remodeling, and repair. Re-modeling may be so extensive that there is a noticeablechange in bone shape over time, even in adults.

Bone remodeling and repair is in part due to a third typeof bone cell called an osteoclast (from the Greek words for"bone" and "to break"). Osteoclasts cut through maturebone tissue, dissolving the hydroxyapatite and digesting theosteoid matrix in their path. The released calcium and phos-phate ions enter the blood. The areas from which bone hasbeen removed attract new osteoblasts, which lay down newosteoid matrixes and stimulate the deposition of new hy-droxyapatite crystals.

Table 5.1 summarizes the four types of cells that con-tribute to bone development and maintenance.

Bones can change in shape, size, and strengtOver time, constant remodeling can actually change theshape of a bone. The key is that compression stress on abone, such as the force of repeated jogging on the legs,causes tiny electrical currents within the bone. These elec-trical currents stimulate the bone-forming activity of os-teoblasts. The compressive forces and the electric currents aregreatest at the inside curvature of the long bone undergoingstress (Figure 5.4). Thus, in the normal course of bone

Table 5.1 Cells involved in bone development andmaintenance

Type of cell

Chondroblasts Cartilage-forming cells that build a modelof the future bone

Youngbone-forming cells that cause thehard extracellular matrix of bone to develop

Mature bone cells that maintain the structureof bone

Bone-dissolvingcells

I

IBone removed - :

h'" i··········.... .

New boneadded here

a) The application of forceto a slightly bent boneproduces a greatercompressive force on theinside curvature.Compressive forceproduces weak electricalcurrents which stimulateosteoblasts.

b) Over time, bone isdeposited on the insidecurvature and removedfrom the outsidecurvature.

turnover, new bone is laid down in regions under high com-pressive stress and bone is resorbed in areas of low compres-sive stress. The final shape of a bone, then, tends to matchthe compressive forces to which it is exposed.

Weight-bearing exercise increases overall bone mass andstrength. The effect is pronounced enough that the bonesof trained athletes may be visibly thicker and heavier thanthose of nonathletes. You don't have to be a professionalathlete to get this benefit, however. If you begin a regularprogram of any weight-bearing exercise, such as jogging orweight lifting, your bones will become denser and strongeras your osteoblasts produce more bone tissue.

The maintenance of homeostasis of bone structure de-pends on the precise balance of the activities of osteoclastsand osteoblasts. Osteoporosis is a common condition inwhich bones lose a great deal of mass (seemingly becoming

I"porous") because of an imbalanceover many years in the rates of activi-ties of these two types of bone cells.

Bone cells are regulatedby hormonesLike bone growth, the rates of activi-ties of osteoblasts and osteoclasts inadulthood are regulated by hormonesthat function to maintain calciumhomeostasis. When blood levels ofcalcium fall below a given point,parathyroid hormone (PTH) stimulatesthe osteoclasts to secrete more bone-dissolving enzymes. The increased ac-tivity of osteoclasts causes more boneto be dissolved, releasing calcium andphosphate into the bloodstream. Ifcalcium levels rise, then another hor-mone called calcitonin stimulates os-teoblast activity, causing calcium andphosphate to be removed from bloodand deposited in bone. Although thetotal bone mass of young adultsdoesn't change much, it's estimatedthat almost 10% of their bones maybe remodeled and replaced each year.We discuss this and other types ofhormonal regulation further inChapter 13.

1 1

c) The final result isa bone matched tothe compressiveforce to which it isexposed. Bones undergo repair

When you break (fracture) a bone,the blood vessels supplying the bonebleed into the area, producing a massof clotted blood called a hematoma.Inflammation, swelling, and pain

generally accompany the hematoma in the days immediatelyafter a fracture. The repair process begins within days as fi-broblasts migrate to the area. Some of the fibroblasts be-come chondroblasts, and together they produce a tough fi-brocartilage bond called a callus between the two brokenends of the bone. A callus can be felt as a hard raised ring atthe point of the break. Then osteoclasts arrive and begin toremove dead fragments of the original bone and the bloodcells of the hematoma. Finally, osteoblasts arrive to depositosteoid matrix and encourage the crystallization of calciumphosphate minerals, converting the callus into bone. Eventu-ally the temporary union becomes dense and hard again.Bones rarely break in the same place twice because the re-paired union remains slightly thicker than the original bone.

The repair process can take weeks to months, dependingon your age and the bone involved. In general, the repairprocess slows with age. Recently it has been discovered thatthe application of weak electrical currents to the area of abroken bone can increase the rate of healing. It is thought

that electrical current works by attracting osteoclasts and os-teoblasts to the area under repair.

Healthy bone replacement and remodeling depend on the bal-ance of activities of bone-resorbing osteoclasts and bone-forming osteoblasts. When a bone breaks, a fibrocartilage callusforms between the broken ends and is later replaced withbone. _

5.4 The skeleton protects, supports,and permits movement

Now that we have reviewed the dynamic nature of bone tis-sue, we turn to how all of those bones are classified and or-ganized. Bones can be classified into four types based onshape: long, short, flat, and irregular. So far we have dis-cussed long bones, which include the bones of the limbs andfingers. Short bones (the bones of the wrists), are approxi-mately as wide as they are long. Flat bones (including the cra-nial bones, the sternum, and the ribs), are thin, flattened,and sometimes curved, with only a small amount of spongybone sandwiched between two layers of compact bone.Irregular bones such as the coxal (hip) bones and the verte-brae include a variety of shapes that don't fit into the othercategories. A few flat and irregular bones, including the ster-num and the hip bones, contain red bone marrow that pro-duces blood cells.

The 206 bones of the human body and the various con-nective tissues that hold them together make up the skeleton(Figure 5.5). The skeleton has three important functions.First, it serves as a structural framework for support of the softorgans. Second, it protects certain organs from physical in-jury. The brain, for example, is enclosed within the bonesof the skull, and the heart and lungs are protected by a bonycage consisting of ribs, the sternum, and vertebrae. Third, be-cause of the way that the bony elements of the skeleton arejoined together at joints, the presence of the skeleton permitsflexible movement of most parts of the body. This is particu-larly true of the hands, feet, legs, and arms.

The skeleton is organized into the axial skeleton and theappendicular skeleton.

The axial skeleton forms the midline of the bodyThe axial skeleton consists of the skull, vertebral column,ribs, and sternum.

The skull: Cranial and facial bones The human skull (cra-nium) comprises over two dozen bones that protect thebrain and form the structure of the face. Figure 5.6 illustratessome of the more important bones of the skull.

The cranial bones are flat bones in the skull that encloseand protect the brain. Starting at the front of the skull, thefrontal bone comprises the forehead and the upper ridges of

Maxilla

Mandible

ClavicleScapula

SternumRibs

UlnaRadius

Patella

Tibia

Fibula

TarsalsMetatarsalsPhalanges

the eye sockets. At the upper left and right sides of the skullare the two parietal bones, and forming the lower left andright sides are the two temporal bones. Each temporal bone ispierced by an opening into the ear canal that allows soundsto travel to the eardrum. Between the frontal bone and thetemporal bones is the sphenoid bone, which forms the backof both eye sockets. The ethmoid bone contributes to the eyesockets and also helps support the nose.

Curving underneath to form the back and base of theskull is the occipital bone. Near the base of the occipital boneis a large opening called the foramen magnum (Latin for"great opening"). This is where the vertebral column con-nects to the skull and the spinal cord enters the skull to com-municate with the brain.

The facial bones compose the front of the skull. On eitherside of the nose are the two maxilla (maxillary) bones, whichform part of the eye sockets and contain the sockets that an-chor the upper row of teeth. The hard palate (the "roof" ofthe mouth) is formed by the maxilla bones and the two

Zygomatic bone

Maxilla

External auditorymeatus

Zygomatic bone --'

Palatine bone

Sphenoid bone

Figure 5 6 The human skull. Except for the mandible, whichhas a hinged joint with the temporal bone, the bones of the skullare joined tightly together Their function is protection, notmovement.

palatine bones. Behind the palatine bones is the vomer bone,which is part of the nasal septum that divides the nose intoleft and right halves. The two zygomatic bones form the cheek-bones and the outer portion of the eye sockets. The twosmall, narrow nasal bones underlie only the upper bridge ofthe nose; the rest of the fleshy protuberance called the noseis made up of cartilage and other connective tissue. Part ofthe space formed by the maxillary and nasal bones is thenasal cavity. The small lacrimal bones, at the inner eye sockets,are pierced by a tiny opening through which the tear ductsdrain tears from the eye sockets into the nasal cavity.

The mandible, or lower jaw, contains the sockets thathouse the lower row of teeth. All the bones of the skull arejoined tightly together except for the mandible, which at-taches to the temporal bone by a joint that, because it per-mits a substantial range of motion, allows us to speak andchew.

Several of the cranial and facial bones contain air spacescalled sinuses, which make the skull lighter and give thehuman voice its characteristic tone and resonance. Eachsinus is lined with tissue that secretes mucus, a thick, sticky

fluid that helps trap foreign particles in incoming air. Thesinuses connect to the nasal cavity via small passagewaysthrough which the mucus normally drains. However, if youdevelop a cold or respiratory infection, the tissue lining yoursinuses can become inflamed and block these passages.Sinus inflammation is called sinusitis. If fluid accumulates in-side the sinuses, the resulting sensation of pressure may giveyou a "sinus headache."

The vertebral column: The body's main axis The vertebralcolumn (the backbone or spine) is the main axis of the body(Figure 5.7). It supports the head, protects the spinal cord,and serves as the site of attachment for the four limbs andvarious muscles. It consists of a column of 33 irregular bonescalled vertebrae (singular: vertebra) that extends from theskull to the pelvis. When viewed from the side the vertebralcolumn is somewhat curved, reflecting slight differences instructure and size of vertebrae in the various regions.

We classify the vertebral column into five anatomicalregions:

• Cervical (neck)-7 vertebrae• Thoracic (the chest or thorax)-12 vertebrae

Cervicalvertebrae(7)

Thoracicvertebrae(12)

Intervertebraldisks

Lumbarvertebrae(5)

Sacrum(5 fused)

Coccyx--{(4 fused) ~-~-----

Figure 5.7 The vertebral column. Vertebrae are named andnumbered according to their location, The vertebral column ismoderately flexible because of the presence of joints and interver-tebral disks,

Articulationwith ribs

Main bodiesof vertebrae

Articulationswith anothervertebra

Intervertebraldisk

Herniated areapressing againsta nerve

a) Healthydisk b) Herniateddisk

Figure 5.8 vertebrae. (a) Two healthy vertebrae with their intervertebral disks. (b) A herniated disk.

• Lumbar (the lower portion or "small" of the back, whichforms the lumbar curve of the spine)-5 vertebrae

• Sacral (in the sacrum or upper pelvic region)-In thecourse of evolution, the 5 sacral vertebrae have becomefused.

• Coccygeal (the coccyx or tailbone)-4 fused vertebrae.The coccyx is all that remains of the tails of our ancientancestors. It is an example of a vestigial structure, mean-ing one that no longer has any function.

A closer look at vertebrae (Figure 5.8) shows how theyare stacked on each other and how they are joined. Vertebraeshare two points of contact, called articulations, located be-hind their main body. There are also articulations with theribs. The spinal cord passes through a hollow cavity betweenthe articulations and the main body. Neighboring vertebraeare separated from each other by a flat, elastic, compressibleintervertebral disk composed of a soft gelatinous centerand a tough outer layer of fibrocartilage. Intervertebral disksserve as shock absorbers, protecting the delicate vertebraefrom the impact of walking, jumping, and other movements.In conjunction with the vertebral joints, vertebral disks alsopermit a limited degree of movement. This lends the verte-bral column greater flexibility, allowing us to bend forward,lean backward, and rotate the upper body.

An especially strong impact or sudden movement cancompress an intervertebral disk, forcing the softer center toballoon outward, press against spinal nerves, and cause in-tense back pain. This condition is referred to as a "herniated"or "slipped disk" (Figure 5.8b), and it occurs most often inthe lumbar vertebrae. Occasionally the disk may rupture, re-leasing its soft pulpy contents. The pain that accompanies aherniated disk can be alleviated by surgery to remove thedamaged disk, relieving the pressure against the nerve. How-ever, surgical correction of a herniated disk reduces spinalflexibility somewhat because the two adjacent vertebrae mustbe fused together with bone grafts.

Generally the vertebral column does an effective job ofshielding the spinal cord. However, injury to the vertebralcolumn can damage the cord or even sever it, resulting inpartial or complete paralysis of the body below that point.Persons with suspected vertebral injuries should not bemoved until a physician can assess the situation, because anytwisting or bending could cause additional, perhaps perma-nent, damage to the spinal cord. You may have noticed thatwhen athletes are injured on the field, they are instructed tolie absolutely still until a trainer and physician have exam-ined them thoroughly.

The ribs and sternum: protecting the chest cavity Humanshave 12 pairs of ribs (Figure 5.9). One end of each ribbranches from the thoracic region of the vertebral column.The other ends of the upper seven pairs attach via cartilage tothe sternum, or breastbone, a flat blade-shaped bone com-posed of three separate bones that fuse during development.Rib pairs 8-10 are joined to the seventh rib by cartilage, andthus attach indirectly to the sternum. The bottom two pairsof ribs are called floating ribs because they do not attach tothe sternum at all.

The ribs, sternum, and vertebral column form a protec-tive rib cage that surrounds and shields the heart, lungs, andother organs of the chest (thoracic) cavity. They also help usbreathe, because muscles between the ribs lift them slightlyduring breathing, expanding the chest cavity and inflatingthe lungs. The base of the sternum is connected to the di-aphragm, a muscle that is important to breathing (seeChapter 10).

The skull protects the brain, the vertebral column protects thespinal cord and supports the appendicular skeleton, and the ribcage protects the organs of the chest cavity. _

Sternum(breastbone)

Figure 5.9 Ribs.The 12 pairs of ribs are numbered according totheir attachment to the thoracic vertebrae. Only the first sevenpairs attach directly to the sternum.

The appendicular skeleton: Pectoral girdle,pelvic girdle, and limbsThose parts of the body that attach to the axial skeleton arecalled appendages, from the Latin word meaning "to hangupon." The second division of the human skeleton, theappendicular skeleton, includes the arms, legs, and theirattachments to the trunk, which are the pectoral and pelvicgirdles.

The pectoral girdle lends flexibility to the upper limbs Thepectoral girdle, a supportive frame for the upper limbs,consists of the right and left clavicles (collarbones) and rightand left scapulas (shoulder blades). The clavicles extendacross the top of the chest and attach to the scapulas, trian-gular bones in the upper back.

The arm and hand consist of 30 different bones (Figure5.10). The upper end of the humerus, the long bone of theupper arm, fits into a socket in the scapula. The other end ofthe humerus meets with the ulna and radius, the two bonesof the forearm, at the elbow. If you've ever hit your elbowand experienced a painful tingling, you know why this areais nicknamed the "funny bone"; you've just struck the ulnarnerve that travels along the elbow.

The lower ends of the ulna and radius meet the carpalbones, a group of eight small bones that makes up the wrist.The five metacarpal bones form the palm of the hand, andthey join with the 14 phalanges, which form the fingers andthumb.

The pectoral girdle and arms are particularly welladapted to permit a wide range of motion. They connect to

Clavicle(collar bone)

Humerus(upper arm)

_Uln}Forearm

Radius

=r-- 8 Carpals (wrist)--.J-- 5 Metacarpals (hand)

~ 14 Phalaeg., (liog.' boo.,)

Figure 5.10 Bones of the right side of the pectoral girdleand the right arm and hand.

the rest of the body via muscles and tendons-a relativelyloose method of attachment. This structure gives the upperbody of humans a degree of dexterity unsurpassed amonglarge animals. We can rotate our upper arms almost 360degrees-a greater range of movement than with any otherjoint in the body. The upper arm can rotate in roughly a cir-cle, the arm can bend in one dimension and rotate, and thewrist and fingers can all bend and rotate to varying degrees.We also have" opposable thumbs," meaning we can placethem opposite our other fingers. The opposable thumb hasplayed an important role in our evolutionary history, as itmakes it easier to grasp and manipulate tools and otherobjects.

We pay a price for this flexibility, because freedom ofmovement also means relative instability. If you fall on yourarm, for example, you might dislocate your shoulder joint orcrack a clavicle. In fact, the clavicle is one of the most fre-quently broken bones in the body.

Although our upper limbs are well adapted to a widerange of movements, too much of one kind of motion canbe harmful. Repetitive motions-performing the same taskover and over-can lead to health problems called overuseor repetitive stress syndromes. Depending on the part of thebody that is overused, these injuries can take many forms.