The Skeletal System

The word skeleton comes from the Greek word skeletos, meaning "dried up." The parts of the skeletal system—the bones and other structures that make up the joints of the skeleton—are anything but dried up. Strong yet light, the skeletal system is made of living material, with networks of blood vessels running throughout. The system protects body organs, supports the body, and provides attachment points for muscles to enable body movement. All bones act as storage sites for minerals such as calcium and phosphorus, and certain bones also produce blood cells.

DESIGN: PARTS OF THE SKELETAL SYSTEM

Because the bones making up the human skeleton are inside the body, the skeleton is called an endoskeleton (endo means "within"). In animals that have an external skeleton, such as the crab, the skeleton is called an exoskeleton (exo means "outside"). Exoskeletons restrict the movement of an organism and must be shed periodically in order for that organism to grow. Endoskeletons allow for freer movement and grow along with an organism.

All humans are born with over 300 bones. As an individual ages, certain bones (such as those in the skull and lower spine) fuse or join together, thereby reducing the number. By the time an individual reaches adulthood, the number of bones in the body totals about 206.

Structure of bones

Bone is living tissue that is constantly being renewed throughout life. Three types of bone cells take part in this process: osteoblasts, osteocytes, and osteoclasts (osteon is the Greek word meaning "bone").

Osteoblasts are the principal bone-building cells. They produce hard calcium compounds and flexible collagen (a fibrous protein), which combined form the nonliving part of bone called the bone matrix. The matrix makes bone strong, hard, and slightly elastic. In the process of forming the bone matrix, osteoblasts become trapped in it. Once they are trapped, they develop into osteocytes or mature bone cells. Osteocytes help maintain the hard bone tissue by removing and replacing the calcium compounds in the matrix. In mature adults (whose bones are no longer growing), osteocytes are the most numerous bone cells. Finally, osteoclasts are the bone-destroying cells. They break down bone matrix, releasing calcium and phosphate ions into the blood (this is important when blood calcium levels drop below normal).

The Skeletal System: Words to Know

Appendicular skeleton (ap-en-DIK-yoo-lar SKEL-i-ton):

Portion of the skeleton consisting of the pectoral girdle, the pelvic girdle, and the bones of the arms and legs.

Axial skeleton (ACK-see-uhl SKEL-i-ton):

Portion of the skeleton consisting of the skull, vertebral column, and rib cage.

Bursa (BURR-sah):

Sac filled with synovial fluid that decreases friction between a tendon and a bone.

Diaphysis (die-AFF-i-sis):

Shaft of a long bone containing a narrow canal filled with yellow bone marrow.

Epiphysis (e-PIFF-i-sis):

End of a long bone.

Fontanels (fon-tah-NELZ):

Also known as soft spots, fibrous connective tissue between flat bones in the developing cranium.

Joint:

Area where adjacent bones meet or articulate.

Ligament (LIG-a-ment):

Fibrous connective tissue that connects bone to bone.

Ossification (ah-si-fi-KAY-shun):

Process of bone formation.

Osteoblasts (OS-tee-oh-blasts):

Principal bone-building cells.

Osteoclasts (OS-tee-oh-klasts):

Large cells that break down bone matrix.

Osteocytes (OS-tee-oh-sites):

Mature bone cells.

Periosteum (per-ee-OS-tee-um):

Dense fibrous membrane covering the surface of bones except at the joints.

Synovial membrane (sin-OH-vee-uhl MEM-brain):

Connective tissue membrane that lines joint cavities and secretes synovial fluid.

Tendon (TEN-den):

Tough, white, cordlike tissue that attaches muscle to bone.

About 98 percent of the calcium and 90 percent of the phosphorus in the body are stored in bones and teeth. Although mature bones consist largely of calcium, most bones in the human skeleton begin as cartilage. Cartilage is a type of connective tissue that contains collagen and elastin fibers, which make it tough and elastic. In a developing fetus, cells in the cartilage skeleton begin to break down. They are replaced by osteoblasts, which begin producing bone matrix around the outer portion of the cartilage. For a while, the fetus has cartilage "bones" enclosed by "bony" bones. This bone formation process, known as ossification, continues until almost all the cartilage

has been replaced, usually by the time an individual reaches the end of puberty (cartilage remains at the ends of bones to prevent wear and tear).

Bones may be classified according to their various traits, such as shape and texture. Four types are recognized based on shape. These are long bones, short bones, flat bones, and irregular bones.

Long bones are found in the extremities: the arms, legs, hands, and feet (but not the wrists or ankles). As their name indicates, long bones have a long central shaft with knobby end portions. The shaft is called the diaphysis and each end is called the epiphysis. Short bones, which are cube-shaped, are found in confined spaces such as the wrist and ankle. Flat bones are thin and wide, providing surfaces for muscle attachment and protection for underlying organs. The ribs, shoulder blades, sternum (breastbone), pelvis (hips), and most of the bones of the skull are consider flat bones. Irregular bones are those that do not fit into the first three categories. Vertebrae (bones of the spinal column) and facial bones are types of irregular bones.

A thin white membrane, called the periosteum, covers the surface of bones except at the joints (areas where bones articulate or connect). Made of

connective tissue, the periosteum contains nerves and blood vessels. Underneath the periosteum lie two different types of bone tissue: compact bone and spongy (or cancellous) bone. Although dense and smooth, compact bone is filled with numerous microscopic passageways carrying nerves, blood vessels, and other structures that provide living bone cells with nutrients. Spongy bone has a lacy network of bone tissue with many cavities, making it look like a sponge. Although lighter in weight than compact bone, spongy bone is nearly as strong.

The diaphyses of long bones are made of compact bone with a hollow center that forms a canal. That canal is filled with yellow bone marrow, which is mostly adipose or fatty tissue. The epiphyses of long bones consist of spongy bone covered with a thin layer of compact bone. Yellow bone marrow also fills the cavities of spongy bone in long bones.

Short, flat, and irregular bones are all made of spongy bone covered by a thin layer of compact bone. The cavities of the spongy bone in these bones are filled with red bone marrow, which is the loose connective tissue that produces blood cells in certain bones. In adults, red blood cells, five types of white blood cells, and platelets are formed in the red bone marrow of portions of the ribs, vertebrae, sternum, and pelvis.

The bones forming the human skeletal system are divided into two divisions: the axial skeleton and the appendicular skeleton. The axial skeleton includes bones associated with the body's main axis—the spine. This includes the skull, the spine or vertebral column, and the rib cage. The appendicular skeleton is attached to the axial skeleton and consists of the bones associated with the body's appendages—the arms and legs. This includes the bones of the pectoral girdle (shoulder area), the arms, the pelvic girdle (hip area), and the legs.

Please note: in the naming of the major bones of the body on the following pages, pronunciations are provided in parenthesis when necessary.

THE BARE BONE FACTS

Over half of the bones in the body are located in the arms and legs—120 bones.

The hyoid bone in the throat is the only bone that does not touch another bone. It is usually broken when a person is hanged or strangled, and therefore will often figure in trials concerning such crimes.

The longest and strongest bone in the body is the femur.

The clavicle (collar bone) is one of the most frequently fractured bones in the body. Fractured clavicles are caused either by a direct blow or a transmitted force resulting from a fall on an outstretched arm.

The smallest bones in the body are the three bones found in each middle ear, collectively known as the ossicles (OS-si-kuls).

The skeleton of an average person accounts for about 20 percent of total body weight.

Axial skeleton

THE SKULL. The skull consists of two sets of bones: cranial bones and facial bones. In addition to protecting the brain, these bones protect and support the organs responsible for sight, hearing, smell, and taste.

The eight bones of the cranium (the part of the skull that encloses the brain) are thin and flat. Interlocking at their joints, they are immovable. The frontal bone forms the forehead and the upper part of the eye sockets. The two parietal (pah-RYE-ah-tul) bones form the sides and upper portion of the cranium. Lying underneath the parietal bones are the two temporal bones. The occipital (ok-SIP-i-tal) bone forms the back of the cranium.

In infants, fibrous connective tissue fills the spaces between the cranial bones. Known as fontanels or soft spots, these spaces allow the skull bones to move slightly during birth. This makes birth easier and helps prevent skull fractures. Eventually, the fontanels are replaced by bone by the age of eighteen to twenty-four months.

Fourteen bones compose the face. Of these bones, only the mandible (MAN-di-buhl) or lower jaw is movable. It houses the lower set of teeth. The upper jaw, the maxilla (MAK-sill-lah), is formed by the fusion of two bones. The maxilla also forms the inner lower portion of the eye sockets and houses the upper set of teeth. The two zygomatic (zie-go-MA-tik) bones are commonly called the cheekbones. They also form the outer lower portion of the eye sockets.

Certain facial bones contain hollow, air-filled spaces known as sinuses. The main functions of the sinuses are to lighten the skull and to provide resonance (sound quality) for the voice. The sinuses in the bones that surround the nasal cavity are called the paranasal sinuses. They are lined with mucous membrane. The mucus produced in the sinuses drains into the nasal cavity to help moisturize and warm air as it flows into the respiratory tract. Infections occurring in the nasal sinuses tend to move into the paranasal sinuses, causing a condition known as sinusitis.

ARE SOME PEOPLE ACTUALLY DOUBLE-JOINTED?

No. Only one joint can occur at an area where bones come together. People who can move their bones beyond normal range are able to do so because the ligaments attached to those particular bones can stretch farther than normal. Contortionists have "stretchy" ligaments, not "double joints."

A few bones are not considered a part of the bones of the skull, but are associated with them. These include the bones of the middle ear and the hyoid (HI-oid) bone. Within each middle ear cavity are three auditory bones. They aid the hearing process by transmitting vibrations from the ear drum to receptors in the inner ear (for a further discussion of this process, see chapter 12). The hyoid bone is the only bone in the body that does not attach directly to any other bone. Horseshoe-shaped, it is suspended by ligaments (cords of fibrous tissue that connects bones) from the lower portions of the temporal bones. It lies in the neck about 1 inch (2.5 centimeters) above the larynx (voice box). The hyoid plays a major role in swallowing, supporting the tongue and larynx.

VERTEBRAL COLUMN. The skull rests atop the vertebral column, which encloses the spinal cord. Also called the spine or backbone, the vertebral column protects the spinal cord and helps to support the weight of the body, transmitting that weight to the lower limbs. It also provides attachment sites for the ribs as well as the muscles that move the trunk (main part of the body). The individual bones making up the column are collectively called vertebrae (VER-te-bray). A single bone is called a vertebra (VER-te-brah).

Before birth, thirty-three vertebrae compose the vertebral column. The nine lower vertebrae eventually fuse, forming the sacrum (SAY-krum) and the coccyx (KOK-siks). The sacrum is formed by the fusing of the first five of those nine vertebrae; the coccyx is formed by the fusing of the last four. The coccyx, or tailbone, is a remnant of the tail other vertebrate animals have.

The remaining twenty-four vertebrae are divided into regions based on their structure. The initial seven bones under the skull are referred to as cervical (SIR-vi-kul) vertebrae. They form the neck region of the spine. The first cervical vertebra is called the atlas (in Greek mythology, Atlas was a Titan who was forced by the gods to support the sky on his shoulders for eternity). Its special shape supports the skull and allows the head to nod "yes". The second cervical vertebra is called the axis. It acts as a pivot for the atlas (and skull) above. The twelve vertebrae below the cervical vertebrae are called the thoracic (thuh-RA-sik) vertebrae. The next five vertebrae are the lumbar vertebrae. The sacrum and coccyx then form the end of the vertebral column.

The vertebrae sit on top of each other to form the vertebral column. Although vertebrae in each region differ from each other, all vertebrae have the same basic structure. Each has a round body that bears the weight of the column. Discs of flexible cartilage lie between the bodies of vertebrae to provide cushioning, like shock absorbers. In a young person, the discs are about 90 percent water and are spongy and compressible. As a person ages, the water content decreases, and the discs become harder and less compressible. Processes or projections extending out from the bodies toward the back of the human body form a canal through which the spinal cord passes. The processes also allow the vertebrae to interlock with each other and serve as sites for muscle and ligament attachment.

The vertebral column is not rigid, but is capable of limited movement such as bending and some twisting. It is also not a straight structure. It has four major curves, forming a long S. This adds strength to the column, increasing the skeleton's balance and ability to hold the body upright. The out-ward curves (toward the back of the body) of the thoracic and sacral regions are known as primary curves because they are present at birth. The inward curves (toward the front of the body) of the cervical and lumbar regions develop when a baby begins to raise his or her head (cervical) and when the baby begins to walk (lumbar). By the time a child is ten years old, all four spinal curves are fully developed.

THE RIB CAGE. Twelve pairs of ribs (a total of twenty-four bones) extend forward from the thoracic vertebrae. Most of the ribs (the first seven pairs) attach in the front of the body by cartilage called costal cartilage to the long, flat sternum (STIR-num) or breastbone. These ribs are called true ribs. The next five pair of ribs are called false ribs. The first three pair of false ribs do not attach directly to the sternum, but to the costal cartilage of the seventh pair of ribs. The lower two pair of ribs of false ribs, also called floating ribs, do not attach to the sternum at all.

Ribs give shape to the chest and support and protect the body's major organs, such as the heart and lungs. The rib cage, formed by the ribs and sternum, also provides attachment points for connective tissue, to help hold organs in place.

Appendicular skeleton

THE PECTORAL GIRDLE. Forming a loose attachment with the sternum is the pectoral girdle, or shoulder. Each shoulder is formed by two bones: the scapula (SKAP-yoo-lah) or shoulder blade and the clavicle (KLAV-i-kul) or collar bone. The large triangular-shaped scapula anchors some of the muscles that move the upper arm. The S-shaped clavicle is small and light and relatively fragile. Each clavicle acts as a brace for its corresponding scapula, preventing the shoulder from coming too far forward.

The major advantage to the loose attachment of the pectoral girdle is that it allows for a wide range of shoulder motions and greater overall freedom of movement.

THE ARMS. Each arm or upper limb (composed of the upper arm, forearm, wrist, and hand) contains thirty bones. The upper arm contains only one bone, the humerus (HYOO-mer-us), which extends from the shoulder joint to the elbow joint. At the elbow joint, the humerus articulates or connects with the two bones of the forearm, the radius (RAY-dee-us) and the ulna (UL-na). When the arm is held out and palm faces upward, the radius and ulna are parallel to each other; the radius is on the thumb side and the ulna is on the little finger side. When the arm is turned over and the palm faces downward, the radius crosses on top of the ulna to form an X.

Each wrist is composed of eight bones known as carpal (CAR-pal) bones. They are arranged in two irregular rows of four bones each. Ligaments bind the carpals together, restricting their movement.

Nineteen bones form each hand. The bones forming the framework of the palm, which articulate with the carpals, are the five metacarpals (mehtah-CAR-pals). In turn, the metacarpals articulate with the fourteen finger bones or phalanges (fah-LAN-jees). The thumb has two phalanges, while the four fingers each have three.

WHAT HAPPENS WHEN YOU "CRACK" YOUR KNUCKLES?

When a person pulls quickly on his or her finger, a vacuum is created in the joint cavity between the phalanges, displacing the synovial fluid normally found in the cavity. The popping sound occurs when the fluid rushes back into the cavity.

THE PELVIC GIRDLE. Unlike the pectoral girdle, the pelvic girdle is strong and dense. It consists of two large coxal (KOK-sal) or hip bones. Each coxal bone, left and right, consists of three fused bones—the ilium (ILL-ee-yum), the ischium (ISH-ee-um), and the pubis (PEW-bis). The ilium is the flared, upper portion of a hip. Each ilium attaches at the rear to the sacrum, connecting the pelvic girdle to the vertebral column. The ischium is the ring-shaped lower part on which a person sits, and the pubis is the most forward portion at the bottom of a hip. These three bones generally have fused together by the time an individual reaches adolescence.

The pelvic girdle is bowl-shaped, with an opening at the bottom (pelvis is the Latin word meaning "basin"). In a pregnant woman, this bony opening is a passageway through which her baby must pass during birth. The pelvic girdle of women is generally wider than that of men, which helps to ease birth. The pelvic girdle protects the lower abdominal organs, such as the intestines, and helps support the weight of the body above it.

THE LEGS. Each leg or lower limb is similar in form to an arm or upper limb. Each leg (composed of the thigh, lower leg, and foot) also contains thirty bones. The thigh contains only one bone, the femur (FEE-mur), which extends from the hip joint to the knee joint. The bones of the lower limbs are thicker and stronger than the bones of the upper limbs. In fact, the femur is the longest, strongest, and heaviest bone in the body. As it runs down the upper part of the leg, the femur slants inward. This helps bring the knees in line with the body's center of gravity.

The patella (pah-TELL-ah) or kneecap is a thick, flat, triangular-shaped bone that lies above and protects the knee joint. At that joint, the femur articulates with the larger of the two bones of the lower leg, the tibia (TI-bee-ah) or shinbone. The fibula (FI-byoo-lah), which lies along the outer side of the tibia, is slender and sticklike. It has no part in forming the knee joint, but provides a surface for muscle attachment and helps keep the ankle bones from sliding laterally (side to side). The inner and outer bulges at the ankle are formed by the ends of the tibia (the inner bulge) and fibula (the outer bulge).

At the ankle joint, the tibia and fibula articulate with the seven tarsal (TAR-sal) bones forming the ankle and heel. The weight of the body is mostly carried by the two largest tarsals: the calcaneus (kal-KAY-nee-us) or heel bone and the talus (TAL-us), which lies between the tibia and fibula.

As in each hand, nineteen bones form each foot. The bones forming the framework of the sole, which articulate with the tarsals, are the five metatarsals (meh-tah-TAR-sals). In turn, the metatarsals articulate with the fourteen toe bones or phalanges (fah-LAN-jees). There are two phalanges in the big toe and three in each of the other toes.

Ligaments and tendons

Two types of dense connective or fibrous tissue are attached to bones—ligaments and tendons. Ligaments fasten bone to bone at joints, wrapping around the joints to hold the bones together. By doing so, they make joints more stable. Depending on their location in the body, they can be shaped like a thick strap, a rope, or a flat ribbon or bandage. Because they are bundles containing elastic fibers as well as collagen fibers, ligaments can stretch to a certain degree.

Tendons attach skeletal muscles to bone. A tendon is the tough, white, cordlike tissue that is formed when the layers of connective tissue that bundle the various parts of a muscle converge or come together at the end of the muscle. Tendons contain fibers of the tough protein collagen, but they cannot stretch as much as ligaments. Tendons are much stronger than muscle tissue. As muscles are used, the tendons are able to withstand the constant pulling and tugging of the muscles on the bones.

WORKINGS: HOW THE SKELETAL SYSTEM FUNCTIONS

The most obvious function of the skeletal system is to provide shape and form to the body. Like the steel girders of a building, the bones provide a framework around which the body is built. Delicate tissues and organs are attached to and protected by this bony framework. For example, the skull encloses the brain, the ribs protect the heart and lungs, and the vertebrae shield the spinal cord.

Bones are also storehouses for minerals, the most important of which are calcium and phosphorus. Both are used to build and maintain bones and teeth. Calcium serves other vital functions, too. Muscles need it in order to contract and blood needs it in order to clot. For these two bodily processes to occur, calcium must be present in the bloodstream at all times. When blood calcium levels drop below normal, hormones trigger osteoclasts to break down bone matrix, releasing calcium. When blood calcium levels rise above normal, hormones cause excess calcium to be deposited in the bones in the form of calcium salts.

As stated earlier, some bones serve as production sites for blood cells. Red blood cells and other blood elements are formed in the red bone marrow of certain flat and irregular bones.

Perhaps the most dramatic function of the skeletal system is to allow the human body to move through space. Walking, running, jumping, dancing, and even breathing are all possible because of the combined actions of the muscles and the bones. Skeletal muscles are attached to bones, and when a muscle contracts, it pulls on its attached bone and movement occurs at a specific joint.

Joints

Joints, also called articulations, are the places where two or more bones meet or articulate. Every bone in the body (with the exception of the hyoid bone in the neck) forms a joint with at least one other bone.

Joints allow a great variety of motion, and the function of each joint is closely related to its structure. When movement is not required, such as between the bones of the cranium, the joint is immobile and strong. The bones are locked together as if they were a single bone. Other joints, such as those between the vertebrae, allow limited movement while still maintaining a certain amount of strength. Finally, in areas where movement is more important than strength, joints can be freely movable. The shoulder joint, for example, allows the arm to move in a variety of ways. Because it allows such a range of motion, the shoulder joint is relatively weak and prone to injury.

As stated, ligaments fasten bone to bone at joints. To prevent friction, the ends of bones at joints are covered in smooth cartilage. In freely movable joints, extra protection is afforded by a joint capsule under the ligaments that surrounds the joint. Made of fibrous connective tissue, the capsule encloses the joint like a sleeve. Lining the capsule is the synovial membrane, which secretes a thick and slippery fluid into the joint cavity. This fluid, called synovial fluid, helps prevent further friction as the bones move.

In areas where tendons cross bone, small sacs filled with synovial fluid lie between the tendon and the joint. These sacs, called bursae (singular: bursa), help cushion the tendon as it slides across the bone.

WHEN DID HUMANS BEGIN WALKING UPRIGHT?

The human species, or Homo sapiens, belongs to the hominid family tree. Hominid means "human types," and describes early creatures who split off from the apes and took to walking upright or on their hind legs.

The reasons that human ancestors started to walk upright are not known. Scientists believe it may have been a response to environmental changes: as tropical forests were beginning to shrink, walking might have been a better way to cross the grasslands to get to nearby patches of forest for food. Standing upright also may have been a means of defense that slowly evolved. By standing upright, animals appear bigger and more impressive in size than they normally are. In addition, the ability to stand up and get a wider view of the surroundings gives an animal an advantage in the tall grasses. Walking upright also frees up the hands to carry objects, such as tools.

The oldest known humanlike animals to have walked upright are believed to be Australopithecus afarensis, meaning the southern ape of the Afar region in Ethiopia, Africa, where the fossils were found. The most famous of these fossils, nicknamed Lucy, was found in 1974 near Hadar, Ethiopia, by a team of anthropologists led by American Donald Johanson (1943–). Lucy lived about 3.18 million years ago. Her skull, knees, and pelvis were more similar to humans than to apes. Her brain size was about one-third that of modern humans, yet larger than any apelike ancestor to have come before. She would have stood about 3.5 feet (1 meter) tall, with long arms, a V-shaped jaw, and a large projecting face.

Freely movable joints are the largest category of joints in the body, especially in the appendicular skeleton where mobility or movement is important. The five main types of freely movable joints are the ball-and-socket, saddle, hinge, pivot, and gliding.

BALL-AND-SOCKET JOINT. A ball-and-socket joint provides the most freedom of movement of any joint. In this type of joint, the round head of one bone (the ball) fits into a cup-shaped depression in another bone (the socket). The joint allows movement in all directions, including rotation. The shoulder and hip joints are examples of ball-and-socket joints.

SADDLE JOINT. A saddle joint allows the next greatest amount of movement. In a saddle joint, the bones are shaped like a horseback rider sitting in a saddle. All movement except rotation is possible with this type of joint. The joint at the base of the thumb is an example of a saddle joint.

HINGE JOINT. A hinge joint allows backward and forward movement in only one direction, much like a door opening and closing. In this type of joint, the convex surface of one bone fits in the concave surface of the other. The joints at the knees, elbows, and knuckles are hinge joints.

PIVOT JOINT. A pivot joint consists of a cylinder of one bone rotating within a ring formed by another bone. Movement occurs only around a single axis. The pivot joint between the atlas and the axis (first and second cervical vertebrae) allows the head to be turned from side to side.

GLIDING JOINT. A gliding or plane joint allows only a small amount of movement as the flattened or slightly curved surfaces of bones slide or glide over each other in various directions. The joints between the carpal bones in the wrist, the tarsal bones in the ankle, and the vertebrae in the spine are examples of gliding joints.

AILMENTS: WHAT CAN GO WRONG WITH THE SKELETAL SYSTEM

The skeletal system is constructed to withstand the pressures and stresses of daily activities, protecting the body's delicate inner organs in the process. Sometimes, however, the system can be stressed beyond its capacity and injuries can result.

SKELETAL SYSTEM DISORDERS

Arthritis (ar-THRIGH-tis): Inflammation of the joints.

Dislocation (dis-low-KAY-shun): Condition in which a bone is forced out of its normal position in the joint cavity.

Fracture (FRAK-cher): Broken bone.

Osteoarthritis (os-tee-oh-ar-THRIGH-tis): Arthritis marked by the deterioration of the cartilage covering the bones in the joints.

Osteoporosis (os-tee-oh-po-ROW-sis): Condition in which bone matrix is lost and not replaced, resulting in an increased softening and weakening of the bones.

Rheumatoid arthritis (RUE-ma-toyd ar-THRIGH-tis): Arthritis characterized by the chronic inflammation of the lining of the joints.

Rickets (RI-kets): Disease in young children due to a deficiency in vitamin D that results in soft and deformed bones.

Scoliosis (sko-lee-OH-sis): Sideways curvature of the spine.

Spina bifida (SPY-na BI-fi-da): Birth defect in which a portion of the spinal cord protrudes through an opening in the vertebral column.

Sprain: Damage or tear in a ligament.

A common but serious skeletal injury is a fracture, a complete or incomplete break in a bone. A fracture usually occurs when excessive force is applied in some manner to the bone. Sports activities such as football, skiing, and skating often jeopardize the bones, putting them at risk for fractures. Car accidents and falls also take their toll on bones. As an individual ages, the bones become thin and weak, and fractures occur more easily.

When excessive force is applied to a joint, the ligaments that hold the bones together in the area may be torn or damaged. This results in a sprain. Its seriousness depends on how badly the ligaments are torn. Any joint can be sprained, but the most frequently injured joints are the ankle, knee, and finger.

A violent movement at a joint may also cause a dislocation, a condition in which a bone is forced out of its normal position in the joint cavity. When a bone is dislocated, its ligaments are often torn or overstretched in the process. Nerves in the area may also be pinched, causing pain. In a severe dislocation, small chips of bone may be torn away.

A herniated or "slipped" disc occurs when any direct and forceful pressure is applied to a vertebral disc (such as when lifting a heavy object), causing it to rupture. This most often occurs to a disc in the lumbar region of the vertebral column. When the disc is ruptured, pressure is placed on the spinal cord, causing considerable pain and damage to the nerve.

The following are just a few of the more serious disorders and diseases that can impair the functioning of the skeletal system or its parts.

COMMON FRACTURES

Comminuted: Bone has been broken into many fragments.

Compound: Bone has been broken and an end or ends of the bone protrude through the skin. Now often referred to as an open fracture.

Compression: Bone has been crushed.

Greenstick: Bone has not been broken completely, but only partly across its shaft, similar to the way a green stick or twig breaks. Now often referred to an incomplete fracture.

Impacted: Bone has been broken and a fragment of the bone has been firmly driven into another fragment.

Simple: Bone has been broken cleanly and does not penetrate or break the skin. Now often referred to as a closed fracture.

Spiral: Bone has been twisted apart.

Arthritis

Arthritis is a general term meaning an inflammation of a bone joint. More than 100 diseases have symptoms of joint inflammation or injury. This condition—the body's response to tissue damage—can cause pain, swelling, stiffness, and fatigue. Since the areas most commonly involved are the hands, arms, shoulders, hips, and legs, any action requiring movement of these parts becomes difficult. Arthritis is usually a chronic condition, meaning it persists throughout a person's life. In all its forms, arthritis is the most widespread, crippling disease in the United States.

Osteoarthritis and rheumatoid arthritis are the two most common forms of the disease. Osteoarthritis occurs as a result of aging or injury. Rheumatoid arthritis is an autoimmune disease, meaning that the body produces antibodies (proteins normally produced by the body to fight against foreign substances in the body) that act against its own tissues.

Osteoarthritis is the deterioration of the cartilage covering the bones in the joints of the body. It is most often seen in people who are forty years of age or older. Causes of osteoarthritis include wear and tear due to aging or overuse, injury, hereditary factors, and obesity. The wearing away of the cartilage results in the bones scraping against each other, causing the development of bony spurs and the deep joint pain characteristic of this disease.

The joints most commonly affected by osteoarthritis are those of the knees, hips, and fingers. Other areas can be affected by injury or overuse. The condition can cause minor stiffness and pain, or it can result in severe disability. Treatment of osteoarthritis includes the use of antiinflammatory drugs such as aspirin to reduce pain and swelling; supportive devices such as a brace, walker, or crutches; massage; moist heat; and rest.

Rheumatoid arthritis is one of the most crippling forms of arthritis. It is characterized by chronic inflammation of the lining of joints. It also affects the muscles, tendons, ligaments, and blood vessels surrounding these joints. Deformities can result from the deterioration of bone, muscle, and tissue, impairing function and affecting mobility. Rheumatoid arthritis can occur at any age but usually appears between the ages of thirty and sixty. Three times more women than men are stricken with this disease.

The cause of the chronic inflammation of rheumatoid arthritis is not known. Scientists believe that a bacterial or viral infection may trigger an autoimmune response in genetically predisposed people. People with rheumatoid arthritis produce antibodies that attack their own body tissues. This sets off an immune response that results in the body's release of chemicals that produce inflammation.

Treatment of rheumatoid arthritis includes aspirin therapy to reduce inflammation and relieve pain, application of heat to joints and muscles, rest, and physical therapy. In some cases, surgery may be required to reconstruct joints that have been destroyed.

Osteoporosis

Osteoporosis (which literally means "porous bones") occurs when a body's blood calcium level is low and calcium from bones is dissolved into the blood to maintain a proper balance. Over time, bone mass and bone strength decrease. As a result, bones become dotted with pits and pores. Weak and fragile, they break easily. Even a sneeze or a sudden movement can cause a fracture in someone with severe osteoporosis.

About 28 million people in the United States are affected by this disease, which causes about 1.5 million fractures each year. Any bone can be affected, but common locations include the hip, spine, and wrist. Osteoporosis occurs in nearly half of all people over the age of seventy-five. However, women are five times more likely than men to develop the disease. After a woman goes through menopause (period in a woman's life when menstrual activity ceases), her body stops producing estrogen, a hormone that helps maintain the health and density of a woman's skeleton.

Other factors besides age can lead to osteoporosis. These include a diet low in calcium and protein, a lack of vitamin D, smoking, excessive alcohol drinking, and insufficient weight-bearing exercises to stress the bones.

There is no cure for osteoporosis, but drugs are available that stop further bone loss and even help build new bone. For some people, though, these drugs may not help build enough bone to replace that already lost in the body. The best way to prevent osteoporosis is to maintain a healthy lifestyle

throughout one's life: adhering to a diet with the proper amounts of calcium and vitamin D, avoiding smoking and heavy alcohol drinking, and exercising regularly.

Rickets

Rickets is a disease in young children that is brought about by a deficiency of vitamin D (the disease is also called vitamin D deficiency). When the deficiency occurs for a period of many months in children whose bones are still developing, softened bones and other bone defects occur.

While some vitamin D is supplied by the diet, most of it is made in the body from the Sun's rays. In the lower layers of the epidermis (thin, outer layer of the skin), cells contain a form of cholesterol or a fatlike substance produced by the liver that is an essential part of cell membranes and body chemicals. When exposed to UV radiation from the Sun, that cholesterol changes into vitamin D. The body then alters vitamin D to produce a hormone that keeps the concentration of calcium at a constant level in the bloodstream by stimulating the absorption of calcium from digested food in the intestines.

When there is a vitamin D deficiency, though, the calcium level in the blood is too low to help create hardened bone. The result is soft bone.

The symptoms of rickets include bowed legs and arms. The bowed appearance is due to the softening of the bones and their bending if the bones are weight-bearing (such as the legs). Other symptoms include a distorted sternum (which produces a protruding pigeon breast), bony bumps on the ribs, and knock-knees. Seizures may also occur because of reduced levels of dissolved calcium in the bloodstream.

Rickets is almost always treated with oral supplements of vitamin D. Children suffering from the disease are also encouraged to play outside with their faces exposed to sunlight for at least twenty minutes a day. This type of treatment heals rickets promptly. Bone abnormalities generally disappear gradually over a period of three to nine months.

Foods that are good sources of vitamin D include cod liver oil, egg yolks, butter, and oily fish. Some food, including milk and breakfast cereals, are also fortified with synthetic vitamin D. Food fortification (the adding of vitamin to food by the manufacturer) has almost completely eliminated rickets in the United States.

Scoliosis

Scoliosis is a sideways curvature of the spine or vertebral column. Normally, the spine has a set of front-to-back curves. When viewed from rear, a normal spine usually appears straight.

A small degree of lateral (sideways) curvature in the spine does not cause any medical problems, but larger lateral curves can cause imbalance and lead to muscle fatigue and pain. More severe scoliosis can interfere with breathing and lead to arthritis of the spine.

Approximately 10 percent of all adolescents have some degree of scoliosis. Fewer than 1 percent, however, have curves that require medical attention. Scoliosis is found in both boys and girls, but a girl's spinal curve is much more likely to progress. The cause behind 80 percent of scoliosis cases is unknown. Some cases can be linked to birth defects, while others are caused by a loss of control of the nerves or muscles that support the spine. Scientists do know that scoliosis is not caused by poor posture, diet, or carrying a heavy bag exclusively on one shoulder. There is no known way to prevent scoliosis.

Treatment for scoliosis depends on the degree of curvature. If the curvature is moderate, a brace may be worn. Bracing cannot correct curvature, but may be effective in halting or slowing the progression of the curve. Surgery is often required if the curvature is severe, if the curve has progressed despite bracing, or if there is significant pain. During surgery, the spine is straightened as much as possible, then vertebrae are fused together to prevent further curvature. Spinal fusion leaves the involved area of the vertebral column permanently stiff.

Spina bifida

Spina bifida is the common name for a range of birth defects caused by problems with the early development of the vertebral column or spine. The main defect of spina bifida is an abnormal opening somewhere along the vertebral column due to a failure of the vertebrae to wrap completely around the spinal cord. This leaves the spinal cord unprotected and vulnerable to either injury or infection.

In North America, spina bifida is much more common among whites than African Americans. It occurs in 1 of every 700 births to whites, but only in 1 in every 3,000 births to African Americans. Scientists are unsure of the reasons for this difference.

Different levels of the spinal cord control different functions in the body. Therefore, the location and size of the defect in spina bifida will determine what kind of disabilities an individual will experience. Most will have some degree of weakness in the legs. Depending on the condition of the spinal cord, that weakness may lead to paralysis. The higher up in the spine the defect occurs, the more severe the disabilities. These may include problems with bladder and bowel function, abnormal curves in the spine (scoliosis), clubfeet, hip dislocations, and water on the brain (a condition called hydrocephalus).

Treatment for spinal bifida is aimed first at surgically closing the spinal defect to prevent infection. Further operations are often necessary to repair the hip dislocations, clubfeet, scoliosis, or other conditions that accompany spina bifida. The success of treatments is still dependent on the severity of the original spinal defect. Current care for children with spina bifida usually enables them to live into adulthood.

TAKING CARE: KEEPING THE SKELETAL SYSTEM HEALTHY

Like every other system in the body, the skeletal system is affected by age. As people grow older, bone tissue tends to lose more calcium than is replaced. This is especially true for women who have gone through menopause (period in a woman's life when menstrual activity ceases).

An individual can lessen the effects of aging on the skeletal system (like every other system in the body) by following a healthy lifestyle. This includes getting adequate rest, reducing stress, drinking healthy amounts of good-quality drinking water, not smoking, drinking moderate amounts of alcohol (or not drinking at all), following a proper diet, and exercising regularly.

A proper diet and exercise are key factors in maintaining the health of the skeletal system. To help slow the rate of bone loss later in life, it is important to build as much bone mass as possible early in life. Without calcium, phosphorus, protein, certain vitamins (A, C, and D), and other nutrients, bones cannot grow properly.

Bones are specialized to bear or carry weight. Without this stress, they lose calcium. It cannot be emphasized too strongly that bones have to be physically stressed to remain healthy. The more they are used, the stronger they become. Exercising regularly builds and strengthens bones. Weight-bearing exercises—where bones and muscles are used against gravity—are best. These include aerobics, dancing, jogging, stair climbing, walking, tennis, and lifting weights. The exercise need not be too strenuous, but it should be engaged in regularly.

FOR MORE INFORMATION

Books

Arnau, Eduard. The Skeletal System. New York: Chelsea House, 1995.

Ballard, Carol. The Skeleton and Muscular System. Austin, TX: Raintree/Steck-Vaughn, 1997.

Parker, Steve. Skeleton. New York: Random Library, 1990.

Silverstein, Alvin, Virginia Silverstein, and Robert Silverstein. The Skeletal System. New York: Twenty-First Century Books, 1994.

Walker, Richard. The Visual Dictionary of the Skeleton. London, England: DK Publishing, 1995.

WWW Sites

The Bones of the Body
http://www.msms.doe.k12.ms.us/biology/anatomy/skeletal/skeletal.html
Site provides links to a list of all the bones in the body, a discussion of the physiology of bones, and a short look at the gross anatomy of the skeletal system.

Cyber Anatomy: Skeletal System
http://tqd.advanced.org/11965/html/cyber-anatomy_skeletal.html
Geared for students in grades 6 through 12, site provides an broad discussion of the skeletal system. Also includes a diagram of the skeleton with the major bones identified.

Muscular and Skeletal Systems
http://gened.emc.maricopa.edu/bio/BIO181/BIOBK/BioBookMUSSKEL.html
Site presents a detailed chapter (with extensive images) on the muscular and skeletal systems from the On-Line Biology textbook.

Skeletal System
http://hyperion.advanced.org/2935/Natures_Best/Nat_Best_Low_Level/skeletal_page.L.html
Site offers an extensive discussion of the skeletal system and its various parts, including a discussion of the composition of bone.

Skeletal System
http://www.lasallehs.org/courses/science/anatomy/html/skeletal_system.html
Site provides an image of the human skeleton; all major bones in that image may be clicked on, leading to a larger, more detailed image of the bone or bones in that area with all parts identified.

Your Gross and Cool Body—Skeletal System
http://www.yucky.com/body/index.ssf?/systems/skeletal
Site presents facts and answers questions about the skeletal system and its various parts.