Skeletal System

Definition

The skeletal system is a living, dynamic, bony framework of the body, with networks of infiltrating blood vessels.

Description

Inside every person is a skeleton, a sturdy framework of about 206 bones that protects the body's organs, supports the body, provides attachment points for muscles to enable body movement, functions as a storage site for minerals such as calcium and phosphorus, and produces blood cells. Living mature bone is about 60% calcium compounds and about 40% collagen. Hence, bone is strong, hard, and slightly elastic. Humans are born with over 300 bones but some bones, such as those in the skull and lower spine, fuse during growth, thereby reducing the number. Although mature bones consist largely of calcium—70% calcium salts and about 30% organic matrix, mostly collagen fibers—most bones in the skeleton of vertebrates, including humans, began as cartilage. Cartilage is a type of connective tissue, and contains collagen and elastin fibers. The hard outer part of bones are comprised mostly of a proteins such as collagen, in addition to a substance called hydroxyapatite. This substance is composed primarily of calcium and other minerals, and stores much of the body's calcium; it is primarily responsible for the strength of bones. At the center of each bone is the marrow, which is softer and less dense than the rest of the bone. The marrow contains specialized cells that produce blood cells that run through a bone, with nerves surrounding it.

Individual bones meet at areas called joints and are held in place by connective tissue. Most joints, such as the elbow, are called synovial joints, for the synovial membrane which envelopes the joint and secretes a lubricating fluid. Cartilage lines the surface of many joints and helps reduce friction between bones. The connective tissues linking the skeleton together at the joints are tendons and ligaments. Ligaments and tendons are both made up of collagen, but serve different functions. Ligaments link bones together and help prevent dislocated joints. Tendons link bone to muscle.

Because the bones making up the human skeleton are inside the body, the skeleton is called an endoskeleton. Some animals, such as the crab, have an external skeleton called an exoskeleton.

Types of bone

Bones may be classified according to their various traits, such as shape, origin, and texture. Four types are recognized based on shape. These are long bones, short bones, flat bones, and irregular bones. Long bones have a long central shaft, called the diaphysis, and two knobby ends, called the epiphysis. In growing long bones, the diaphysis and epiphysis are separated by a thin sheet of cartilage. Examples of long bones include bones of the arms and legs, the metacarpals of the hand, metatarsals of the foot, and the clavicle. Short bones are about as long as wide. The patella, carpels of the wrist, and tarsals of the ankle are short bones. Flat bones take several shapes, but are characterized by being relatively thin and flat. Examples include the sternum, ribs, hip bones, scapula, and cranial bones. Irregular bones are the odd-shaped bones of the skull, such as the sphenoid, the sacrum, and the vertebrae. The common characteristic of irregular bones is not that they are similar to each other in appearance, but that they cannot be placed in any of the other bone categories.

Bones may also be classified based on their origin. All bone (as well as muscles and connective tissue) originates from an embryonic connective tissue called mesenchyme, which makes mesoderm, also an embryonic tissue. Some mesoderm forms the cartilaginous skeleton of the fetus, the precursor for the bony skeleton. However, some bones, such as the clavicle and some of the facial and cranial bones of the skull, develop directly from mesenchyme, thereby bypassing the cartilaginous stage. These types of bone are called membrane bone (or dermal bone). Bone that originates from cartilage is called endochondral bone.

Finally, bones are classified based on texture. Smooth, hard bone called compact bone forms the outer layer of bones. Inside the outer compact bone is cancellous bone, sometimes called the bone marrow. Cancellous bone appears open and spongy, but is actually very strong, like compact bone. Together, the two types of bone produce a light, but strong, skeleton.

Structure

The human skeletal system is divided into two main groups: the axial skeleton and the appendicular skeleton. The axial skeleton includes bones associated with the body's main axis including:

• the skull
• the spine or vertebral column
• the ribs

The appendicular skeleton consists of the bones that anchor the body's appendages to the axial skeleton including:

• the pectoral girdle (shoulder area)
• the pelvic girdle (hip area)
• the upper extremities (arms)
• the lower extremities (legs)

AXIAL SKELETON. There are 28 bones in the skull. Of these, eight bones comprise the cranium and provide protection for the brain. In adults, these bones are flat and interlocking at their joints, making the cranium immobile. Fibrous joints, or sutures occur where the bony plates of the cranium meet and interlock. Cartilage-filled spaces between the cranial bones of infants, known as soft spots or fontanelles, allow their skull bones to move slightly during birth. This makes birth easier and helps prevent skull fractures, but may leave the infant with an odd-shaped head temporarily while the skull regains its shape. Eventually, the fontanelles in an infant's head are replaced by bone, and fibrous joints develop. In addition to protecting the brain, skull bones also support and protect the sensory organs responsible for sight, hearing, smell and taste.

The eight bones of the cranium are:

• frontal
• parietal (2)
• temporal (2)
• ethmoid
• sphenoid
• occipital

The frontal bone forms the forehead and eyebrows. Behind the frontal bone are the two parietal bones. Parietal bones form the roof of the cranium and curve down to form the sides of the cranium. Also forming the sides of the cranium are the two temporal bones, located behind the eyes. Each temporal bone encloses the cochlea and labyrinth of the inner ear, and the ossicles, three tiny bones of the middle ear which are not part of the cranium. The ossicles are the malleus (hammer), incus (anvil), and stapes (stirrups). The temporal bones also attach to the lower jaw, and this is the only moveable joint in the skull. Between the temporal bones is the irregular shaped sphenoid bone, which provides protection for the pituitary gland. The small ethmoid bone forms part of the eye socket next to the nose. Olfactory nerves, or sense of smell nerves, pass through the ethmoid bone on their way to the brain. Forming the base and rear of the cranium is the occipital bone. The occipital bone has a hole, called the foramen magnum, through which the spinal cord passes and connects to the brain.

Fourteen bones shape the cheeks, eyes, nose, and mouth. These include:

• the nasal (2)
• zygomatic (2)
• maxillae (2)
• mandible

The upper, bony bridge of the nose is formed by the nasal bones and provides an attachment site for the cartilage making up the softer part of the nose. The zygomatic bones form the cheeks and part of the eye sockets. Two bones fuse to form the maxillae, the upper jaw of the mouth. These bones also form the hard palate of the mouth. The mandible forms the lower jaw of the mouth and is moveable, enabling chewing of food and speech. The mandible is the bone which connects to the temporal bones.

Located behind these facial bones are other bones which shape the interior portions of the eyes, nose, and mouth. These include:

• lacrimal (2)
• palatine (2)
• conchae (2)
• vomer bones

In addition to these 28 skull bones is the hyoid bone, located at the base of the tongue. Technically, the hyoid bone is not part of the skull but it is often included with the skull bones. It provides an attachment site for the tongue and some neck muscles.

Several of the facial and cranial bones contain sinuses, or cavities, that connect to the nasal cavity and drain into it. These are the frontal, ethmoid, sphenoid, and maxillae bones, all located near the nose. Painful sinus headaches result from the build up of pressure in these cavities. Membranes that line these cavities may secrete mucus or become infected, causing additional aggravation for humans.

The skull rests atop of the spine, which encases and protects the spinal cord. The spine, also called the vertebral column or backbone, consists of 33 stacked vertebrae, the lower ones fused. Vertebra are flat with two main features. The main oval shaped, bony mass of the vertebra is called the centrum. From the centrum arises a bony ring called the neural arch which forms the neural canal (also called a vertebral foramen), a hole for the spinal cord to pass through. Short, bony projections (neural spines) arise from the neural arch and provide attachment points for muscles. Some of these projections (called transverse processes) also provide attachment points for the ribs. There are also small openings in the neural arch for the spinal nerves, which extend from the spinal cord throughout the body. Injury to the column of vertebrae may cause serious damage to the spinal cord and the spinal nerves, and could result in paralysis if the spinal cord or nerves are severed.

There are seven cervical, or neck, vertebrae. The first one, the atlas, supports the skull and allows the head to nod up and down. The atlas forms a condylar joint (a type of synovial joint) with the occipital bone of the skull. The second vertebra, the axis, allows the head to rotate from side to side. This rotating synovial joint is called a pivot joint. Together, these two vertebrae make possible a wide range of head motions.

Below the cervical vertebrae are the 12 thoracic, or upper back, vertebrae. The ribs are attached to these vertebrae. Thoracic vertebrae are followed by five lumbar, or lower back, vertebrae. Last is the sacrum, composed of five fused vertebrae, and the coccyx, or tail bone, composed of four fused bones.

The vertebral column helps to support the weight of the body and protects the spinal cord. Cartilaginous joints rather than synovial joints occur in the spine. Disks of cartilage lie between the bony vertebrae of the back and provide cushioning, like shock absorbers. The vertebrae of the spine are capable of only limited movement, such bending and some twisting.

A pair of ribs extends forward from each of the 12 thoracic vertebrae, for a total of 24 ribs. Occasionally, a person is born with an extra set of ribs. The joint between the ribs and vertebrae is a gliding (or plane) joint, a type of synovial joint, as ribs do move, expanding and contracting with breathing. Most of the ribs (the first seven pair) attach in the front of the body via cartilage to the long, flat breastbone, or sternum. These ribs are called true ribs. The next three pair of ribs are false ribs. False ribs attach to another rib in front instead of the sternum, and are connected by cartilage. The lower two pair of ribs which do not attach anteriorly are called floating ribs. Ribs give shape to the chest and support and protect the body's major organs, such as the heart and lungs. The rib cage also provides attachment points for connective tissue, to help hold organs in place. In adult humans, the sternum also produces red blood cells as well as providing an attachment site for ribs.

APPENDICULAR SKELETON. The appendicular skeleton joins with the axial skeleton at the shoulders and hips. Forming a loose attachment with the sternum is the pectoral girdle, or shoulder. Two bones, the clavicle (collar bone) and scapula (shoulder blade), form one shoulder. The scapula rests on top of the ribs in the back of the body. It connects to the clavicle, the bone that attaches the entire shoulder structure to the skeleton at the sternum. The clavicle is a slender bone that is easily broken. Because the scapula is so loosely attached, it is easily dislocated from the clavicle, hence the dislocated shoulder injuries commonly suffered by persons playing sports. 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.

Unlike the pectoral girdle, the pelvic girdle, or hips, is strong and dense. Each hip, left and right, consists of three fused bones, the ilium, ischium, and pubic. Collectively, these three bones are known as the innominate bone.

The innominates fuse with the sacrum to form the pelvic girdle. Specifically, the iliums shape the hips and the two ischial bones support the body when a person sits. The two pubic bones meet anteriorly at a cartilaginous joint. The pelvic girdle is bowl-shaped, with an opening at the bottom. In a pregnant woman, this bony opening is a passageway through which her baby must pass during birth. To facilitate the baby's passage, the body secretes a hormone called relaxin which loosens the joint between the pubic bones. In addition, the pelvic girdle of women is generally wider than that of men. This also helps to facilitate birth, but is a slight impediment for walking and running. Hence, men, with their narrower hips, are better adapted for such activities. The pelvic girdle protects the lower abdominal organs, such as the intestines, and helps supports the weight of the body above it.

The arms and legs, the upper and lower appendages of the body, are very similar in form. Each attaches to the girdle, pectoral or pelvic, via a ball and socket joint, a special type of synovial joint. In the shoulder, the socket, called the glenoid cavity, is shallow. The shallowness of the glenoid cavity allows for great freedom of movement. The hip socket, or acetabulum, is larger and deeper. This deep socket, combined with the rigid and massive structure of the hips, give the legs much less mobility and flexibility than the arms.

The humerus, or upper arm bone, is the long bone between the elbow and the shoulder. It connects the arm to the pectoral girdle. In the leg the femur, or thigh bone, is the long bone between the knee and hip which connects the leg to the pelvic girdle. The humerus and femur are sturdy bones, especially the femur, which is a weight bearing bone. Since the arms and legs are jointed, the humerus and femur are connected to other bones at the end opposite the ball and socket joint. In the elbow, this second joint is a type of synovial joint called a hinge joint. Two types of synovial joints occur in the knee region, a condylar joint (like the condylar joint in the first vertebra) which connects the leg bones, and a plane, or gliding joint, between the patella (knee cap) and femur.

At the elbow the humerus attaches to a set of parallel bones, the ulna and radius, bones of the forearm. The radius is the bone below the thumb that rotates when the hand is turned over and back. The ulna and radius then attach to the carpel bones of the wrist. Eight small carpel bones make up the wrist and connect to the hand. The hand is made up of five long, slender metacarpal bones (the palms) and 14 phalanges of the hand (fingers and thumb). Some phalanges form joints with each other, giving the human hand great dexterity.

Similarly, in the leg, the femur forms a joint with the patella and with the fibula and tibia bones of the lower leg. The tibia, or shin bone, is larger than the fibula and forms the joint behind the patella with the femur. Like the femur, the tibia is also a weight bearing bone. At the ankle joint, the fibula and tibia connect to the tarsals of the upper foot. There are seven tarsals of the upper foot, forming the ankle and the heel. The tarsals in turn connect to five long, slender metatarsals of the lower foot. The metatarsals form the foot's arch and sole and connect to the phalanges of the feet (toes). The 14 foot phalanges are shorter and less agile than the hand phalanges. Several types of synovial joints occur in the hands and feet, including plane, ellipsoid and saddle. Plane joints occur between toe bones, allowing limited movement. Ellipsoid joints between the finger and palm bones give the fingers circular mobility, unlike the toes. The saddle joint at the base of the thumb helps make the hands the most important part of the body in terms of dexterity and manipulation. A saddle joint also occurs at the ankles.

Bone development and growth

Since most bone begins as cartilage, it must be converted to bone through a process called ossification. The key players in bone development are cartilage cells (chondrocytes), bone precursor cells (osteoprogenitor cells), bone deposition cells (osteoblasts), bone resorption cells (osteoclasts), and mature bone cells (osteocytes).

During ossification, blood vessels invade the cartilage and transport osteoprogenitor cells to a region called the center of ossification. At this site, the cartilage cells die, leaving behind small cavities. Osteoblast cells form from the progenitor cells and begin depositing bone tissue, spreading out from the center. Through this process, both the spongy textured cancellous bone and the smooth outer compact bone forms. Two types of bone marrow, red and yellow, occupy the spaces in cancellous bone. Red marrow produces red blood cells, while yellow marrow stores fat in addition to producing blood cells. Eventually, in compact bone, osteoblast cells become trapped in their bony cavities, called lacunae, and become osteocytes. Neighboring osteocytes form connections with each other and thus are able to transfer materials between cells. The osteocytes are part of a larger system called the Haversian system. These systems are like long tubes, squeezed tightly together in compact bone. Blood vessel, lymph vessels, and nerves run through the center of the tube, called the Haversian canal, and are surrounded by layers of bone, called lamellae, which house the osteocytes. Blood vessels are connected to each other by lateral canals called Volkmann's canals. Blood vessels are also found in spongy bone, without the Haversian system. A protective membrane called the periosteum surrounds all bones.

Bone development is a complex process, but it is only half the story. Bones must grow, and they do so via a process called remodeling. Remodeling involves resorption of existing bone inside the bone (enlarging the marrow cavities) and deposition of new bone on the exterior. The resorptive cells are the osteoclasts and osteoblast cells lay down the new bone material. As remodeling progresses in long bones, a new center of ossification develops, this one at the swollen ends of the bone, called the epiphysis. A thin layer of cartilage called the epiphyseal plate separates the epiphysis from the shaft and is the site of bone deposition. When growth is complete, this cartilage plate disappears, so that the only cartilage remaining is that which lines the joints, called hyaline cartilage. Remodeling does not end when growth ends. Osteocytes, responding to the body's need for calcium, resorb bone in adults to maintain a calcium balance.

Function

The skeletal system has several important functions:

• It provides shape and form to the body, while allowing for body movement.
• It supports and protects vital organs and muscles.
• It produces red blood cells for the body in the bone marrow. Each second, an average of 2.6 million red blood cells are to replace worn out blood cells and those destroyed by the liver.
• It stores minerals including calcium and phosphorus. When excess are present in the blood, the bones will store minerals. When the supply in the blood runs low, minerals will be withdrawn from the bones to replenish the blood supply.

Common diseases and disorders

Even though bones are very strong, they may be broken. Most fractures do heal. The healing process may be stymied if bones are not reset properly or if the injured person is the victim of malnutrition. Osteoprogenitor cells migrate to the site of the fracture and begin the process of making new bone (osteoblasts) and reabsorbing the injured bone (osteoclasts). With proper care, the fracture will fully heal, and in children, often without a trace.

The joint between the mandible and the temporal bones, called the temporomandibular joint, is the source of the painful condition known as temporomandibular joint dysfunction, or TMJ dysfunction. Sufferers of TMJ dysfunction experience a variety of symptoms including headaches, a sore jaw, and a snapping sensation when moving the jaw. There are several causes of the dysfunction. The cartilage disk between the bones may shift, or the connective tissue between the bones may be situated in a manner that causes misalignment of the jaw. Sometimes braces on the teeth can aggravate TMJ dysfunction. The condition may be corrected with exercise, or in severe cases, surgery. Another condition, cleft palate, is due to the failure of the maxillary bones in the jaw to completely fuse in the fetus.

Bones are affected by poor diet and are also subject to a number of diseases and disorders. Some examples include scurvy, rickets, osteoporosis, arthritis, and bone tumors. Scurvy results from the lack of vitamin C. In infants, scurvy causes poor bone development. It also causes membranes surrounding the bone to bleed, forming clots that are eventually ossified, and thin bones that break easy. In addition, adults are affected by bleeding gums and loss of teeth. Before modern times, sailors were often the victims of scurvy, due to extended periods of time at sea with limited food. They consequently tried to keep a good supply of citrus fruits, such as oranges and limes, on board because these fruits supply vitamin C. By the twenty-first century, scurvy had become extremely rare in Western societies.

Rickets is a children's disease resulting from a deficiency of vitamin D. This vitamin enables the body to absorb calcium and phosphorus; without it, bones become soft and weak and actually bend, or bow out, under the body's weight. Vitamin D is found in milk, eggs and liver, and may also be produced by exposing the skin to sunlight. Pregnant women can also suffer from a vitamin D deficiency, osteomalacia, resulting in soft bones. The elderly, especially women who had several children in succession, sometimes suffer from osteoporosis, a condition in which a significant amount of calcium from bones is dissolved into the blood to maintain the body's calcium balance. Weak, brittle bones dotted with pits and pores are the result. Osteoporosis occurs most often in older people and in women after menopause. It affects nearly half of all those, men and women, over the age of 75. Women, however, are five times more likely than men to develop the disease. They have smaller, thinner bones than men to begin with, and they lose bone mass more rapidly after menopause (usually around age 50), when they stop producing a bone-protecting hormone called estrogen. In the five to seven years following menopause, women can lose about 20% of their bone mass. By age 65 or 70, though, men and women lose bone mass at the same rate. As an increasing number of men reach an older age, they are becoming more aware that osteoporosis is an important health issue for them as well.

Arthritis is another condition commonly afflicting the elderly. This is an often painful inflammation of the joints. Arthritis is not restricted to the elderly, and even young people can suffer from this condition. There are several types of arthritis, such as rheumatoid, rheumatic, and degenerative. Arthritis basically involves the inflammation and deterioration of cartilage and bone at the joint surface. In some cases, bony protuberances around the rim of the joint may develop. Most people will probably develop arthritis if they live to a significant older age. Degenerative arthritis is the type that commonly occurs with age. The knee, hip, shoulder, and elbow are the major targets of degenerative arthritis. A number of different types of tumors, some harmless and others more serious, may also affect bones.

KEY TERMS

Bone— Composed primarily of a non-living matrix of calcium salts and a living matrix of collagen fibers, bone is the major component that makes up the human skeleton. Bone produces blood cells and functions as a storage site for elements such as calcium and phosphorus.

Calcium— A naturally occurring element that primarily combines with phosphate to form the nonliving matrix of bones.

Cartilage— A type of connective tissue that takes three forms: elastic cartilage, fibrocartilage, and hyaline cartilage. Hyaline cartilage forms the embryonic skeleton and lines the joints of bones.

Haversian system— Tubular systems in compact bone with a central Haversian canal that houses blood and lymph vessels surrounded by circular layers of calcium salts and collagen, called lamellae, in which reside osteocytes.

Marrow— A type of connective tissue that fills the spaces of most cancellous bone. It produces blood cells and stores fat.

Ossification— The process of replacing connective tissue such as cartilage and mesenchyme with bone.

Osteoblast— The bone cell that deposits calcium salts and collagen during bone growth, bone remodeling, and bone repair.

Osteoclast— The bone cell responsible for reabsorbing bone tissue in bone remodeling and repair.

Osteocyte— Mature bone cell whose main function is to regulate the levels of calcium and phosphate in the body.

Skeleton— Consists of bones and cartilage that are linked together by ligaments. The skeleton protects vital organs of the body and enables body movement.

Synovial joint— One of three types of joints in the skeleton and by far the most common. Synovial joints are lined with a membrane that secretes a lubricating fluid. Includes ball and socket, pivot, plane, hinge, saddle, condylar, and ellipsoid joints.

Vertebrates— Includes all animals with a vertebral column protecting the spinal cord such as humans, dogs, birds, lizards, and fish.

Resources

BOOKS

Adams, John S., and Barbara P. Lukertet. Osteoporosis: Genetics, Prevention and Treatment. Boston, Kluwer Academic, 1999.

Browner, Bruce D. et al., ed. Skeletal Trauma: Fractures, Dislocations, Ligamentous Injuries, 2nd ed. Vols 1 and 2. Philadelphia, PA: WB Saunders Co, 1998.

Ganong, William F. Review of Medical Physiology, 20th ed. New York: McGraw-Hill Professional Publishing, 2001.

Kessler, George J. et al. The Bone Density Diet: 6 Weeks to a Strong Body and Mind. New York: Ballantine Books, 2000.

Praemer, A., et al., ed. Musculoskeletal Conditions in the United States. Rosemont, IL: American Academy of Orthopaedic Surgeons; 1999.

PERIODICALS

Boskey, Adele L. "Musculoskeletal Disorders and Orthopedic Conditions" Journal of the American Medical Association 285 (2001): 619-623. 〈http://jama.ama-assn.org/issues/v285n5/ffull/jsc00335.html〉.

Feder, G. et al. "Guidelines for the prevention of falls in people over 65." British Medical Journal 321 (2000): 1007-1011.

McClung, Michael R. et al. "Effect of Risedronate on the Risk of Hip Fracture in Elderly Women." The New England Journal of Medicine 344, no. 5 (2001): 333-40.

ORGANIZATIONS

Arthritis Foundation. 1330 W. Peachtree St., PO Box 7669, Atlanta, GA 30357-0669. (800) 283-7800. 〈http://www.arthritis.org〉.

National Center for Complementary and Alternative Medicine (NCCAM), 31 Center Dr., Room #5B-58, Bethesda, MD 20892-2182. (800) NIH-NCAM, Fax (301) 495-4957. 〈http://nccam.nih.gov〉.

National Osteoporosis Foundation. 1150 17th Street, Suite 500 NW, Washington, DC 20036-4603. (800) 223-9994. 〈http://www.nof.org〉.

Osteoporosis and Related Bone Diseases-National Resource Center. 1150 17th St., NW, Ste. 500, Washington, DC 20036-4603. (800) 624-BONE. 〈http://www.osteo.org〉.

Skeletal System

Structure

Axial skeleton

Appendicular skeleton

Types of bone

Bone development and growth

Bones and medicine

Resources

A skeleton is a sturdy framework of about 206 bones that protects the body’s organs, supports the body, provides attachment points for muscles to enable body movement, functions as a storage site for minerals such as calcium and phosphorus, and produces blood cells.

The skeletal system is a living, dynamic system, with networks of infiltrating blood vessels. Living mature bone is about 60% calcium compounds and about 40% collagen. Hence, bone is strong, hard and slightly elastic. All humans were born with over 300 bones but some bones, such as those in the skull and lower spine, fuse during growth, thereby reducing the number. Although mature bones consist largely of calcium, most bones in the skeleton of vertebrates, including humans, began as cartilage. Some animals, such as sharks and sting rays, retain their cartilaginous skeleton in adulthood. Cartilage is a type of connective tissue, and contains collagen and elastin fibers.

Individual bones meet at areas called joints and are held in place by connective tissue. Most joints, such as the elbow, are called synovial joints, for the synovial membrane which envelopes the joint and secretes a lubricating fluid. Cartilage lines the surface of many joints and helps reduce friction between

bones. The connective tissues linking the skeleton together at the joints are tendons and ligaments. Ligaments and tendons are both made up of collagen, but serve different functions. Ligaments link bones together and help prevent dislocated joints. Tendons link bone to muscle.

Because the bones making up the human skeleton are inside the body, the skeleton is called an endoskeleton. Some animals, such as the crab, have an external skeleton called an exoskeleton.

Structure

The human skeletal system is divided into two main groups: the axial skeleton and the appendicular skeleton. The axial skeleton includes bones associated with the body’s main axis, the spine. This includes the spine and the skull and rib cage, which are connected to the spine. 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, or shoulder area, bones of the pelvic girdle, or hip area, and arm and leg bones.

Axial skeleton

There are 28 bones in the skull. Of these, 8 bones comprise the cranium and provide protection for the brain. In adults, these bones are flat and interlocking at their joints, making the cranium immobile. Fibrous joints, or sutures occur where the bony plates of the cranium meet and interlock. Cartilage-filled spaces between the cranial bones of infants, known as soft spots or fontanelles, allow their skull bones to move slightly during birth. This makes birth easier and helps prevent skull fractures, but may leave the infant with an odd-shaped head temporarily while the skull regains its shape. Eventually, the fontanelles in an infant’s head are replaced by bone and fibrous joints develop. In addition to protecting the brain, skull bones also support and protect the sensory organs responsible for sight, hearing, smell and taste.

The eight bones of the cranium are: frontal, parietal (2), temporal (2), ethmoid, sphenoid and occipital. The frontal bone forms the forehead and eyebrows. Behind the frontal bone are the two parietal bones. Parietal bones form the roof of the cranium and curve down to form the sides of the cranium. Also forming the sides of the cranium are the two temporal bones, located behind the eyes. Each temporal bone encloses the cochlea and labyrinth of the inner ear, and the ossicles, three tiny bones of the middle ear which are not part of the cranium. The ossicles are the malleus (hammer), incus (anvil), and stapes (stirrups). The temporal bones also attach to the lower jaw, and this is the only moveable joint in the skull. Between the temporal bones is the irregular shaped sphenoid bone, which provides protection for the pituitary gland. The small ethmoid bone forms part of the eye socket next to the nose. Olfactory nerves, or sense of smell nerves, pass through the ethmoid bone on their way to the brain. Forming the base and rear of the cranium is the occipital bone. The occipital bone has a hole, called the foramen magnum, through which the spinal cord passes and connects to the brain.

Fourteen bones shape the cheeks, eyes, nose and mouth. These include the nasal (2), zygomatic (2), maxillae (2), and the mandible. The upper, bony bridge of the nose is formed by the nasal bones and provides an attachment site for the cartilage making up the softer part of the nose. The zygomatic bones form the cheeks and part of the eye sockets. Two bones

fuse to form the maxillae, the upper jaw of the mouth. These bones also form hard palate of the mouth. Failure of the maxillary bones to completely fuse in the fetus results in the condition known as cleft palate. The mandible forms the lower jaw of the mouth and is moveable, enabling chewing of food and speech. The mandible is the bone which connects to the temporal bones. The joint between these bones, the temporo-mandibular joint, is the source of the painful condition known as temporomandibular joint dysfunction, or TMJ dysfunction. Sufferers of TMJ dysfunction experience a variety of symptoms including headaches, a sore jaw and a snapping sensation when moving the jaw. There a several causes of the dysfunction. The cartilage disk between the bones may shift, or the connective tissue between the bones may be situated in a manner that causes misalignment of the jaw. Sometimes braces on the teeth can aggravate TMJ dysfunction. The condition may be corrected with exercise, or in severe cases, surgery.

Located behind these facial bones are other bones which shape the interior portions of the eyes, nose and mouth. These are the lacrimal (2), palatine (2), con-chae (2), and vomer bones. In addition to these 28 skull bones is the hyoid bone, located at the base of the tongue. Technically, the hyoid bone is not part of the skull but it is often included with the skull bones. It provides an attachment site for the tongue and some neck muscles.

Several of the facial and cranial bones contain sinuses, or cavities, that connect to the nasal cavity and drain into it. These are the frontal, ethmoid, sphenoid and maxillae bones, all located near the nose. Painful sinus headaches result from the build up of pressure in these cavities. Membranes that line these cavities may secrete mucous or become infected, causing additional aggravation for humans.

The skull rests atop of the spine, which encases and protects the spinal cord. The spine, also called the vertebral column or backbone, consists of 33 stacked vertebrae, the lower ones fused. Vertebra are flat with two main features. The main oval shaped, bony mass of the vertebra is called the centrum. From the centrum arises a bony ring called the neural arch that forms the neural canal (also called a vertebral foramen), a hole for the spinal cord to pass through. Short, bony projections (neural spines) arise from the neural arch and provide attachment points for muscles. Some of these projections (called transverse processes) also provide attachment points for the ribs. There are also small openings in the neural arch for the spinal nerves, which extend from the spinal cord throughout the body. Injury to the column of vertebrae may cause serious damage to the spinal cord and the spinal nerves, and could result in paralysis if the spinal cord or nerves are severed.

There are seven cervical, or neck, vertebrae. The first one, the atlas, supports the skull and allows the head to nod up and down. The atlas forms a condylar joint (a type of synovial joint) with the occipital bone of the skull. The second vertebra, the axis, allows the head to rotate from side to side. This rotating synovial joint is called a pivot joint. Together, these two vertebrae make possible a wide range of head motions.

Below the cervical vertebrae are the 12 thoracic, or upper back, vertebrae. The ribs are attached to these vertebrae. Thoracic vertebrae are followed by five lumbar, or lower back, vertebrae. Last is the sacrum, composed of five fused vertebrae, and the coccyx, or tail bone, composed of four fused bones.

The vertebral column helps to support the weight of the body and protects the spinal cord. Cartilaginous joints rather than synovial joints occur in the spine. Disks of cartilage lie between the bony vertebrae of the back and provide cushioning, like shock absorbers. The vertebrae of the spine are capable of only limited movement, such bending and some twisting.

A pair of ribs extends forward from each of the 12 thoracic vertebrae, for a total of 24 ribs. Occasionally, a person is born with an extra set of ribs. The joint between the ribs and vertebrae is a gliding (or plane) joint, a type of synovial joint, as ribs do move, expanding and contracting with breathing. Most of the ribs

(the first seven pair) attach in the front of the body via cartilage to the long, flat breastbone, or sternum. These ribs are called true ribs. The next three pair of ribs are false ribs. False ribs attach to another rib in front instead of the sternum, and are connected by cartilage. The lower two pair of ribs which do not attach anteriorly are called floating ribs. Ribs give shape to the chest and support and protect the body’s major organs, such as the heart and lungs. The rib cage also provides attachment points for connective tissue, to help hold organs in place. In adult humans, the sternum also produces red blood cells as well as providing an attachment site for ribs.

Appendicular skeleton

The appendicular skeleton joins with the axial skeleton at the shoulders and hips. Forming a loose attachment with the sternum is the pectoral girdle, or shoulder. Two bones, the clavicle (collar bone) and scapula (shoulder blade) form one shoulder. The scapula rest on top of the ribs in the back of the body. It connects to the clavicle, the bone which attaches the entire shoulder structure to the skeleton at the sternum. The clavicle is a slender bone that is easily broken. Because the scapula is so loosely attached, it is easily dislocated from the clavicle, hence the dislocated shoulder injuries commonly suffered by persons playing sports. 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.

Unlike the pectoral girdle, the pelvic girdle, or hips, is strong and dense. Each hip, left and right, consists of three fused bones, the ilium, ischium and pubic. Collectively, these three bones are known as the innominate bone. The innominates fuse with the sacrum to form the pelvic girdle. Specifically, the iliums shape the hips and the two ischial bones support the body when a person sits. The two pubic bones meet anteriorly at a cartilaginous joint. The pelvic girdle is bowl-shaped, with an opening at the bottom. In a pregnant woman, this bony opening is a passageway through which her baby must pass during birth. To facilitate the baby’s passage, the body secretes a hormone called relaxin that loosens the joint between the pubic bones. In addition, the pelvic girdle of women is generally wider than that of men. This also helps to facilitate birth, but is a slight impediment for walking and running. Hence, men, with their narrower hips, are better adapted for such activities. The pelvic girdle protects the lower abdominal organs, such as the intestines, and helps supports the weight of the body above it.

The arms and legs, appendages of the body, are very similar in form. Each attaches to the girdle, pectoral or pelvic, via a ball and socket joint, a special type of synovial joint. In the shoulder, the socket, called the glenoid cavity, is shallow. The shallowness of the glenoid cavity allows for great freedom of movement. The hip socket, or acetabulum, is larger and deeper. This deep socket, combined with the rigid and massive structure of the hips, give the legs much less mobility and flexibility than the arms.

The humerus, or upper arm bone, is the long bone between the elbow and the shoulder. It connects the arm to the pectoral girdle. In the leg the femur, or thigh bone, is the long bone between the knee and hip that connects the leg to the pelvic girdle. The humerus and femur are sturdy bones, especially the femur, which is a weight bearing bone. Since the arms and legs are jointed, the humerus and femur are connected to other bones at the end opposite the ball and socket joint. In the elbow, this second joint is a type of synovial joint called a hinge joint. Two types of synovial joints occur in the knee region, a condylar joint (like the condylar joint in the first vertebra) that connects the leg bones, and a plane, or gliding joint, between the patella (knee cap) and femur.

At the elbow the humerus attaches to a set of parallel bones, the ulna and radius, bones of the forearm. The radius is the bone below the thumb that rotates when the hand is turned over and back. The ulna and radius then attach to the carpel bones of the wrist. Eight small carpel bones make up the wrist and connect to the hand. The hand is made up of five long, slender metacarpal bones (the palms) and 14 phalanges of the hand (fingers and thumb). Some phalanges form joints with each other, giving the human hand great dexterity.

Similarly, in the leg, the femur forms a joint with the patella and with the fibula and tibia bones of the lower leg. The tibia, or shin bone, is larger than the fibula and forms the joint behind the patella with the femur. Like the femur, the tibia is also a weight bearing bone. At the ankle joint, the fibula and tibia connect to the tarsals of the upper foot. There are seven tarsals of the upper foot, forming the ankle and the heel. The tarsals in turn connect to five long, slender metatarsals of the lower foot. The metatarsals form the foot’s arch and sole and connect to the phalanges of the feet (toes). The 14 foot phalanges are shorter and less agile than the hand phalanges. Several types of synovial joints occur in the hands and feet, including plane, ellipsoid and saddle. Plane joints occur between toe bones, allowing limited movement. Ellipsoid joints between the finger and palm bones give the fingers circular mobility, unlike the toes. The saddle joint at the base of the thumb helps make the hands the most important part of the body in terms of dexterity and manipulation. A saddle joint also occurs at the ankles.

Types of bone

Bones may be classified according to their various traits, such as shape, origin, and texture. Four types are recognized based on shape. These are long bones, short bones, flat bones and irregular bones. Long bones have a long central shaft, called the diaphysis, and two knobby ends, called the epiphysis. In growing long bones, the diaphysis and epiphysis are separated by a thin sheet of cartilage. Examples of long bones include bones of the arms and legs, the metacarpals of the hand, metatarsals of the foot, and the clavicle. Short bones are about as long as wide. The patella, carpels of the wrist and tarsals of the ankle are short bones. Flat bones take several shapes, but are characterized by being relatively thin and flat. Examples include the sternum, ribs, hip bones, scapula and cranial bones. Irregular bones are the odd-shaped bones of the skull, such as the sphenoid, the sacrum and the vertebrae. The common characteristic of irregular bones is not that they are similar to each other in appearance, but that they can’t be placed in any of the other bone categories.

Bones may also be classified based on their origin. All bone (as well as muscles and connective tissue) originates from an embryonic connective tissue called mesenchyme, which makes mesoderm, also an embryonic tissue. Some mesoderm forms the cartilaginous skeleton of the fetus, the precursor for the bony skeleton. However, some bones, such as the clavicle and some of the facial and cranial bones of the skull, develop directly from mesenchyme, thereby bypassing the cartilaginous stage. These types of bone are called membrane bone (or dermal bone). Bone which originates from cartilage is called endochondral bone.

Finally, bones are classified based on texture. Smooth, hard bone called compact bone forms the outer layer of bones. Inside the outer compact bone is cancellous bone, sometimes called the bone marrow. Cancellous bone appears open and spongy, but is actually very strong, like compact bone. Together, the two types of bone produce a light, but strong, skeleton.

Bone development and growth

Since most bone begins as cartilage, it must be converted to bone through a process called ossification. The key players in bone development are cartilage cells (chondrocytes), bone precursor cells (osteoprogenitor cells), bone deposition cells (osteo-blasts), bone resorption cells (osteoclasts) and mature bone cells (osteocytes).

During ossification, blood vessels invade the cartilage and transport osteoprogenitor cells to a region called the center of ossification. At this site, the cartilage cells die, leaving behind small cavities. Osteoblast cells form from the progenitor cells and begin depositing bone tissue, spreading out from the center. Through this process, both the spongy textured cancellous bone and the smooth outer compact bone forms. Two types of bone marrow, red and yellow, occupy the spaces in cancellous bone. Red marrow produces red blood cells while yellow marrow stores fat in addition to producing blood cells. Eventually, in compact bone, osteoblast cells become trapped in their bony cavities, called lacunae, and become osteocytes. Neighboring osteocytes form connections with each other and thus are able to transfer materials between cells. The osteocytes are part of a larger system called the Haversian system. These systems are like long tubes, squeezed tightly together in compact bone. Blood vessel, lymph vessels and nerves run through the center of the tube, called the Haversian canal, and are surrounded by layers of bone, called lamellae, which house the osteocytes. Blood vessels are connected to each other by lateral canals called Volkmann’s canals. Blood vessels are also found in spongy bone, without the Haversian system. A protective membrane called the periosteum surrounds all bones.

Bone development is a complex process, but it is only half the story. Bones must grow, and they do so via a process called remodeling. Remodeling involves resorption of existing bone inside the bone (enlarging the marrow cavities) and deposition of new bone on the exterior. The resorptive cells are the osteoclasts and osteoblast cells lay down the new bone material. As remodeling progresses in long bones, a new center of ossification develops, this one at the swollen ends of the bone, called the epiphysis. A thin layer of cartilage called the epiphyseal plate separates the epiphysis from the shaft and is the site of bone deposition. When growth is complete, this cartilage plate disappears, so that the only cartilage remaining is that which lines the joints, called hyaline cartilage. Remodeling does not end when growth ends. Osteocytes, responding to the body’s need for calcium, resorb bone in adults to maintain a calcium balance. This process can sometimes have detrimental affects on the skeleton, especially in pregnant women and women who bear many children.

Bones and medicine

Even though bones are very strong, they may be broken, but fortunately, most fractures do heal. The healing process may be stymied if bones are not reset properly or if the injured person is the victim of malnutrition. Osteoprogenitor cells migrate to the site of the fracture and begin the process of making new bone

KEY TERMS

Bone —Composed primarily of a non-living matrix of calcium salts and a living matrix of collagen fibers, bone is the major component that makes up the human skeleton. Bone produces blood cells and functions as a storage site for elements such as calcium and phosphorus.

Calcium —An essential macro mineral necessary for bone formation and other metabolic functions.

Cartilage —A type of connective tissue that takes three forms: elastic cartilage, fibrocartilage and hyaline cartilage. Hyaline cartilage forms the embryonic skeleton and lines the joints of bones.

Haversian system —Tubular systems in compact bone with a central Haversian canal which houses blood and lymph vessels surrounded by circular layers of calcium salts and collagen, called lamellae, in which reside osteocytes.

Marrow —A type of connective tissue which fills the spaces of most cancellous bone and which functions to produce blood cells and store fat.

Ossification —The process of replacing connective tissue such as cartilage and mesenchyme with bone.

Osteoblast —The bone cell which deposits calcium salts and collagen during bone growth, bone remodeling and bone repair.

Osteoclast —The bone cell responsible for reabsorb-ing bone tissue in bone remodeling and repair.

Osteocyte —Mature bone cell which functions mainly to regulate the levels of calcium and phosphate in the body.

Skeleton —Consists of bones and cartilage which are linked together by ligaments. The skeleton protects vital organs of the body and enables body movement.

Synovial joint —One of three types of joints in the skeleton and by far the most common. Synovial joints are lined with a membrane which secretes a lubricating fluid. Includes ball and socket, pivot, plane, hinge, saddle, condylar and ellipsoid joints.

Vertebrate —Includes all animals with a vertebral column protecting the spinal cord such as humans, dogs, birds, lizards, and fish.

(osteoblasts) and reabsorbing the injured bone (osteo-clasts). With proper care, the fracture will fully heal, and in children, often without a trace.

Bones are affected by poor diet and are also subject to a number of diseases and disorders. Some examples include scurvy, rickets, osteoporosis, arthritis and bone tumors. Scurvy results from the lack of vitamin C. In infants, scurvy causes poor bone development. It also causes membranes surrounding the bone to bleed, forming clots which are eventually ossified, and thin bones which break easily. In addition, adults are affected by bleeding gums and loss of teeth. Before modern times, sailors were often the victims of scurvy, as they were at sea for long periods of time with limited food. Hence, they tried to keep a good supply of citrus fruits, such as oranges and limes, on board, as these fruits supply vitamin C.

Rickets is a children’s disease resulting from a deficiency of vitamin D. This vitamin enables the body to absorb calcium and phosphorus, and without it, bones become soft and weak and actually bend, or bow out, under the body’s weight. Vitamin D is found in milk, eggs and liver, and may also be produced by exposing the skin to sunlight. Pregnant women can also suffer from a vitamin D deficiency, osteomalacia, resulting in soft bones. The elderly, especially women who had several children in a row, sometimes suffer from osteoporosis, a condition in which a significant amount of calcium from bones is dissolved into the blood to maintain the body’s calcium balance. Weak, brittle bones dotted with pits and pores are the result.

Another condition commonly afflicting the elderly is arthritis, an often painful inflammation of the joints. Arthritis is not, however, restricted to the elderly, as even young people may suffer from this condition. There are several types of arthritis, such as rheumatoid, rheumatic, and degenerative. Arthritis basically involves the inflammation and deterioration of cartilage and bone at the joint surface. In some cases, bony protuberances around the rim of the joint may develop. Unfortunately, most people will probably develop arthritis if they live long enough. Degenerative arthritis is the type that commonly occurs with age. The knee, hip, shoulder and elbow are the major targets of degenerative arthritis. A number of different types of tumors, some harmless and others more serious, may also affect bones.

See also Orthopedics.

Resources

BOOKS

Abrahams, Peter H., Barry M. Logan, Ralph T. Hutchings, and Jonathan D. Spratt. McMinn’s The Human Skeleton. 2nd ed. Philadelphia: Mosby, 2006.

Elaine L. Martin

Skeletal system

A skeleton is a sturdy framework of about 206 bones that protects the body's organs, supports the body, provides attachment points for muscles to enable body movement, functions as a storage site for minerals such as calcium and phosphorus , and produces blood cells.

The skeletal system is a living, dynamic system, with networks of infiltrating blood vessels. Living mature bone is about 60% calcium compounds and about 40% collagen . Hence, bone is strong, hard and slightly elastic. All humans were born with over 300 bones but some bones, such as those in the skull and lower spine, fuse during growth, thereby reducing the number. Although mature bones consist largely of calcium, most bones in the skeleton of vertebrates , including humans, began as cartilage. Some animals, such as sharks and sting rays , retain their cartilaginous skeleton in adulthood. Cartilage is a type of connective tissue , and contains collagen and elastin fibers.

Individual bones meet at areas called joints and are held in place by connective tissue . Most joints, such as the elbow, are called synovial joints, for the synovial membrane which envelopes the joint and secretes a lubricating fluid. Cartilage lines the surface of many joints and helps reduce friction between bones. The connective tissues linking the skeleton together at the joints are tendons and ligaments. Ligaments and tendons are both made up of collagen, but serve different functions. Ligaments link bones together and help prevent dislocated joints. Tendons link bone to muscle.

Because the bones making up the human skeleton are inside the body, the skeleton is called an endoskeleton. Some animals, such as the crab, have an external skeleton called an exoskeleton.

Structure

The human skeletal system is divided into two main groups: the axial skeleton and the appendicular skeleton. The axial skeleton includes bones associated with the body's main axis, the spine. This includes the spine and the skull and rib cage, which are connected to the spine. 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, or shoulder area, bones of the pelvic girdle, or hip area, and arm and leg bones.

Axial skeleton

There are 28 bones in the skull. Of these, 8 bones comprise the cranium and provide protection for the brain . In adults, these bones are flat and interlocking at their joints, making the cranium immobile. Fibrous joints, or sutures occur where the bony plates of the cranium meet and interlock. Cartilage-filled spaces between the cranial bones of infants, known as soft spots or fontanelles, allow their skull bones to move slightly during birth . This makes birth easier and helps prevent skull fractures, but may leave the infant with an oddshaped head temporarily while the skull regains its shape. Eventually, the fontanelles in an infant's head are replaced by bone and fibrous joints develop. In addition to protecting the brain, skull bones also support and protect the sensory organs responsible for sight, hearing , smell and taste .

The eight bones of the cranium are: frontal, parietal (2), temporal (2), ethmoid, sphenoid and occipital. The frontal bone forms the forehead and eyebrows. Behind the frontal bone are the two parietal bones. Parietal bones form the roof of the cranium and curve down to form the sides of the cranium. Also forming the sides of the cranium are the two temporal bones, located behind the eyes. Each temporal bone encloses the cochlea and labyrinth of the inner ear , and the ossicles, three tiny bones of the middle ear which are not part of the cranium. The ossicles are the malleus (hammer), incus (anvil), and stapes (stirrups). The temporal bones also attach to the lower jaw, and this is the only moveable joint in the skull. Between the temporal bones is the irregular shaped sphenoid bone, which provides protection for the pituitary gland. The small ethmoid bone forms part of the eye socket next to the nose. Olfactory nerves, or sense of smell nerves, pass through the ethmoid bone on their way to the brain. Forming the base and rear of the cranium is the occipital bone. The occipital bone has a hole, called the foramen magnum, through which the spinal cord passes and connects to the brain.

Fourteen bones shape the cheeks, eyes, nose and mouth. These include the nasal (2), zygomatic (2), maxillae (2), and the mandible. The upper, bony bridge of the nose is formed by the nasal bones and provides an attachment site for the cartilage making up the softer part of the nose. The zygomatic bones form the cheeks and part of the eye sockets. Two bones fuse to form the maxillae, the upper jaw of the mouth. These bones also form hard palate of the mouth. Failure of the maxillary bones to completely fuse in the fetus results in the condition known as cleft palate. The mandible forms the lower jaw of the mouth and is moveable, enabling chewing of food and speech . The mandible is the bone which connects to the temporal bones. The joint between these bones, the temporomandibular joint, is the source of the painful condition known as temporomandibular joint dysfunction, or TMJ dysfunction. Sufferers of TMJ dysfunction experience a variety of symptoms including headaches, a sore jaw and a snapping sensation when moving the jaw. There a several causes of the dysfunction. The cartilage disk between the bones may shift, or the connective tissue between the bones may be situated in a manner that causes misalignment of the jaw. Sometimes braces on the teeth can aggravate TMJ dysfunction. The condition may be corrected with exercise , or in severe cases, surgery .

Located behind these facial bones are other bones which shape the interior portions of the eyes, nose and mouth. These are the lacrimal (2), palatine (2), conchae (2), and vomer bones. In addition to these 28 skull bones is the hyoid bone, located at the base of the tongue. Technically, the hyoid bone is not part of the skull but it is often included with the skull bones. It provides an attachment site for the tongue and some neck muscles.

Several of the facial and cranial bones contain sinuses, or cavities, that connect to the nasal cavity and drain into it. These are the frontal, ethmoid, sphenoid and maxillae bones, all located near the nose. Painful sinus headaches result from the build up of pressure in these cavities. Membranes that line these cavities may secrete mucous or become infected, causing additional aggravation for humans.

The skull rests atop of the spine, which encases and protects the spinal cord. The spine, also called the vertebral column or backbone, consists of 33 stacked vertebrae, the lower ones fused. Vertebra are flat with two main features. The main oval shaped, bony mass of the vertebra is called the centrum. From the centrum arises a bony ring called the neural arch which forms the neural canal (also called a vertebral foramen), a hole for the spinal cord to pass through. Short, bony projections (neural spines) arise from the neural arch and provide attachment points for muscles. Some of these projections (called transverse processes) also provide attachment points for the ribs. There are also small openings in the neural arch for the spinal nerves, which extend from the spinal cord throughout the body. Injury to the column of vertebrae may cause serious damage to the spinal cord and the spinal nerves, and could result in paralysis if the spinal cord or nerves are severed.

There are seven cervical, or neck, vertebrae. The first one, the atlas, supports the skull and allows the head to nod up and down. The atlas forms a condylar joint (a type of synovial joint) with the occipital bone of the skull. The second vertebra, the axis, allows the head to rotate from side to side. This rotating synovial joint is called a pivot joint. Together, these two vertebrae make possible a wide range of head motions.

Below the cervical vertebrae are the 12 thoracic, or upper back, vertebrae. The ribs are attached to these vertebrae. Thoracic vertebrae are followed by five lumbar, or lower back, vertebrae. Last is the sacrum, composed of five fused vertebrae, and the coccyx, or tail bone, composed of four fused bones.

The vertebral column helps to support the weight of the body and protects the spinal cord. Cartilaginous joints rather than synovial joints occur in the spine. Disks of cartilage lie between the bony vertebrae of the back and provide cushioning, like shock absorbers. The
vertebrae of the spine are capable of only limited movement, such bending and some twisting.

A pair of ribs extends forward from each of the 12 thoracic vertebrae, for a total of 24 ribs. Occasionally, a person is born with an extra set of ribs. The joint between the ribs and vertebrae is a gliding (or plane ) joint, a type of synovial joint, as ribs do move, expanding and contracting with breathing. Most of the ribs (the first seven pair) attach in the front of the body via cartilage to the long, flat breastbone, or sternum. These ribs are called true ribs. The next three pair of ribs are false ribs. False ribs attach to another rib in front instead of the sternum, and are connected by cartilage. The lower two pair of ribs which do not attach anteriorly are called floating ribs. Ribs give shape to the chest and support and protect the body's major organs, such as the heart and lungs. The rib cage also provides attachment points for connective tissue, to help hold organs in place. In adult humans, the sternum also produces
red blood cells as well as providing an attachment site for ribs.

Appendicular skeleton

The appendicular skeleton joins with the axial skeleton at the shoulders and hips. Forming a loose attachment with the sternum is the pectoral girdle, or shoulder. Two bones, the clavicle (collar bone) and scapula (shoulder blade) form one shoulder. The scapula rest on top of the ribs in the back of the body. It connects to the clavicle, the bone which attaches the entire shoulder structure to the skeleton at the sternum. The clavicle is a slender bone that is easily broken. Because the scapula is so loosely attached, it is easily dislocated from the clavicle, hence the dislocated shoulder injuries commonly suffered by persons playing sports. 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.

Unlike the pectoral girdle, the pelvic girdle, or hips, is strong and dense. Each hip, left and right, consists of three fused bones, the ilium, ischium and pubic. Collectively, these three bones are known as the innominate bone. The innominates fuse with the sacrum to form the pelvic girdle. Specifically, the iliums shape the hips and the two ischial bones support the body when a person sits. The two pubic bones meet anteriorly at a cartilaginous joint. The pelvic girdle is bowl-shaped, with an opening at the bottom. In a pregnant woman, this bony opening is a passageway through which her baby must pass during birth. To facilitate the baby's passage, the body secretes a hormone called relaxin which loosens the joint between the pubic bones. In addition, the pelvic girdle of women is generally wider than that of men. This also helps to facilitate birth, but is a slight impediment for walking and running. Hence, men, with their narrower hips, are better adapted for such activities. The pelvic girdle protects the lower abdominal organs, such as the intestines, and helps supports the weight of the body above it.

The arms and legs, appendages of the body, are very similar in form. Each attaches to the girdle, pectoral or pelvic, via a ball and socket joint, a special type of synovial joint. In the shoulder, the socket, called the glenoid cavity, is shallow. The shallowness of the glenoid cavity allows for great freedom of movement. The hip socket, or acetabulum, is larger and deeper. This deep socket, combined with the rigid and massive structure of the hips, give the legs much less mobility and flexibility than the arms.

The humerus, or upper arm bone, is the long bone between the elbow and the shoulder. It connects the arm to the pectoral girdle. In the leg the femur, or thigh bone, is the long bone between the knee and hip which connects the leg to the pelvic girdle. The humerus and femur are sturdy bones, especially the femur, which is a weight bearing bone. Since the arms and legs are jointed, the humerus and femur are connected to other bones at the end opposite the ball and socket joint. In the elbow, this second joint is a type of synovial joint called a hinge joint. Two types of synovial joints occur in the knee region, a condylar joint (like the condylar joint in the first vertebra) which connects the leg bones, and a plane, or gliding joint, between the patella (knee cap) and femur.

At the elbow the humerus attaches to a set of parallel bones, the ulna and radius, bones of the forearm. The radius is the bone below the thumb that rotates when the hand is turned over and back. The ulna and radius then attach to the carpel bones of the wrist. Eight small carpel bones make up the wrist and connect to the hand. The hand is made up of five long, slender metacarpal bones (the palms) and 14 phalanges of the hand (fingers and thumb). Some phalanges form joints with each other, giving the human hand great dexterity.

Similarly, in the leg, the femur forms a joint with the patella and with the fibula and tibia bones of the lower leg. The tibia, or shin bone, is larger than the fibula and forms the joint behind the patella with the femur. Like the femur, the tibia is also a weight bearing bone. At the ankle joint, the fibula and tibia connect to the tarsals of the upper foot. There are seven tarsals of the upper foot, forming the ankle and the heel. The tarsals in turn connect to five long, slender metatarsals of the lower foot. The metatarsals form the foot's arch and sole and connect to the phalanges of the feet (toes). The 14 foot phalanges are shorter and less agile than the hand phalanges. Several types of synovial joints occur in the hands and feet, including plane, ellipsoid and saddle. Plane joints occur between toe bones, allowing limited movement. Ellipsoid joints between the finger and palm bones give the fingers circular mobility, unlike the toes. The saddle joint at the base of the thumb helps make the hands the most important part of the body in terms of dexterity and manipulation. A saddle joint also occurs at the ankles.

Types of bone

Bones may be classified according to their various traits, such as shape, origin, and texture. Four types are recognized based on shape. These are long bones, short bones, flat bones and irregular bones. Long bones have a long central shaft, called the diaphysis, and two knobby ends, called the epiphysis. In growing long bones, the diaphysis and epiphysis are separated by a thin sheet of cartilage. Examples of long bones include bones of the arms and legs, the metacarpals of the hand, metatarsals of the foot, and the clavicle. Short bones are about as long as wide. The patella, carpels of the wrist and tarsals of the ankle are short bones. Flat bones take several shapes, but are characterized by being relatively thin and flat. Examples include the sternum, ribs, hip bones, scapula and cranial bones. Irregular bones are the oddshaped bones of the skull, such as the sphenoid, the sacrum and the vertebrae. The common characteristic of irregular bones is not that they are similar to each other in appearance, but that they can't be placed in any of the other bone categories.

Bones may also be classified based on their origin. All bone (as well as muscles and connective tissue) originates from an embryonic connective tissue called mesenchyme, which makes mesoderm, also an embryonic tissue. Some mesoderm forms the cartilaginous skeleton of the fetus, the precursor for the bony skeleton. However, some bones, such as the clavicle and some of the facial and cranial bones of the skull, develop directly from mesenchyme, thereby bypassing the cartilaginous stage. These types of bone are called membrane bone (or dermal bone). Bone which originates from cartilage is called endochondral bone.

Finally, bones are classified based on texture. Smooth, hard bone called compact bone forms the outer layer of bones. Inside the outer compact bone is cancellous bone, sometimes called the bone marrow. Cancellous bone appears open and spongy, but is actually very strong, like compact bone. Together, the two types of bone produce a light, but strong, skeleton.

Bone development and growth

Since most bone begins as cartilage, it must be converted to bone through a process called ossification . The key players in bone development are cartilage cells (chondrocytes), bone precursor cells (osteoprogenitor cells), bone deposition cells (osteoblasts), bone resorption cells (osteoclasts) and mature bone cells (osteocytes).

During ossification, blood vessels invade the cartilage and transport osteoprogenitor cells to a region called the center of ossification. At this site, the cartilage cells die, leaving behind small cavities. Osteoblast cells form from the progenitor cells and begin depositing bone tissue, spreading out from the center. Through this process, both the spongy textured cancellous bone and the smooth outer compact bone forms. Two types of bone marrow, red and yellow, occupy the spaces in cancellous bone. Red marrow produces red blood cells while yellow marrow stores fat in addition to producing blood cells. Eventually, in compact bone, osteoblast cells become trapped in their bony cavities, called lacunae, and become osteocytes. Neighboring osteocytes form connections with each other and thus are able to transfer materials between cells. The osteocytes are part of a larger system called the Haversian system. These systems are like long tubes, squeezed tightly together in compact bone. Blood vessel, lymph vessels and nerves run through the center of the tube, called the Haversian canal, and are surrounded by layers of bone, called lamellae, which house the osteocytes. Blood vessels are connected to each other by lateral canals called Volkmann's canals. Blood vessels are also found in spongy bone, without the Haversian system. A protective membrane called the periosteum surrounds all bones.

Bone development is a complex process, but it is only half the story. Bones must grow, and they do so via a process called remodeling. Remodeling involves resorption of existing bone inside the bone (enlarging the marrow cavities) and deposition of new bone on the exterior. The resorptive cells are the osteoclasts and osteoblast cells lay down the new bone material. As remodeling progresses in long bones, a new center of ossification develops, this one at the swollen ends of the bone, called the epiphysis. A thin layer of cartilage called the epiphyseal plate separates the epiphysis from the shaft and is the site of bone deposition. When growth is complete, this cartilage plate disappears, so that the only cartilage remaining is that which lines the joints, called hyaline cartilage. Remodeling does not end when growth ends. Osteocytes, responding to the body's need for calcium, resorb bone in adults to maintain a calcium balance. This process can sometimes have detrimental affects on the skeleton, especially in pregnant women and women who bear many children.

Bones and medicine

Even though bones are very strong, they may be broken, but fortunately, most fractures do heal. The healing process may be stymied if bones are not reset properly or if the injured person is the victim of malnutrition . Osteoprogenitor cells migrate to the site of the fracture and begin the process of making new bone (osteoblasts) and reabsorbing the injured bone (osteoclasts). With proper care, the fracture will fully heal, and in children, often without a trace.

Bones are affected by poor diet and are also subject to a number of diseases and disorders. Some examples include scurvy, rickets, osteoporosis , arthritis and bone tumors. Scurvy results from the lack of vitamin C. In infants, scurvy causes poor bone development. It also causes membranes surrounding the bone to bleed, forming clots which are eventually ossified, and thin bones which break easy. In addition, adults are affected by bleeding gums and loss of teeth. Before modern times, sailors were often the victims of scurvy, as they were at sea for long periods of time with limited food. Hence, they tried to keep a good supply of citrus fruits , such as oranges and limes, on board, as these fruits supply vitamin C.

Rickets is a children's disease resulting from a deficiency of vitamin D. This vitamin enables the body to absorb calcium and phosphorus, and without it, bones become soft and weak and actually bend, or bow out, under the body's weight. Vitamin D is found in milk, eggs and liver, and may also be produced by exposing the skin to sunlight. Pregnant women can also suffer from a vitamin D deficiency, osteomalacia, resulting in soft bones. The elderly, especially women who had several children in a row, sometimes suffer from osteoporosis, a condition in which a significant amount of calcium from bones is dissolved into the blood to maintain the body's calcium balance. Weak, brittle bones dotted with pits and pores are the result.

Another condition commonly afflicting the elderly is arthritis, an often painful inflammation of the joints. Arthritis is not, however, restricted to the elderly, as even young people may suffer from this condition. There are several types of arthritis, such as rheumatoid, rheumatic and degenerative. Arthritis basically involves the inflammation and deterioration of cartilage and bone at the joint surface. In some cases, bony protuberances around the rim of the joint may develop. Unfortunately, most people will probably develop arthritis if they live long enough. Degenerative arthritis is the type that commonly occurs with age. The knee, hip, shoulder and elbow are the major targets of degenerative arthritis. A number of different types of tumors, some harmless and others more serious, may also affect bones.

See also Orthopedics.

Resources

books

Shipman, P., A. Walker, and D. Bichell. The Human Skeleton. Cambridge: Harvard University Press, 1985.

Steele, D.G., and C.A. Bramblett. The Anatomy and Biology of the Human Skeleton. College Station: A&M; University Press, 1988.

periodicals

Bower, B. "Fossils Put a New Face on Lucy's Species." Science News 145 (2 April 1994): 212.

Fischman, J. "Putting a New Spin on the Birth of Human Birth." Science 264:1082-1083, 1994.

Miller, A. "Collagen: The Organic Matrix of Bone." Phil. Trans. Roy Soc. Lond. ser. B 304:455-477, 1984.

Snow, C.C., B.P. Gatliff, and K.R. McWilliams. "Reconstruction of Facial Features from the Skull: An Evaluation of its Usefulness in Forensic Anthropology." American Journal of Physical Anthropology (1970).

Stevenson, J. "The Strong-boned Weavers of Spitalfields." Discover (August, 1993).

Elaine L. Martin

KEY TERMS

Bone

—Composed primarily of a non-living matrix of calcium salts and a living matrix of collagen fibers, bone is the major component that makes up the human skeleton. Bone produces blood cells and functions as a storage site for elements such as calcium and phosphorus.

Calcium

—An essential macro mineral necessary for bone formation and other metabolic functions.

Cartilage

—A type of connective tissue that takes three forms: elastic cartilage, fibrocartilage and hyaline cartilage. Hyaline cartilage forms the embryonic skeleton and lines the joints of bones.

Haversian system

—Tubular systems in compact bone with a central Haversian canal which houses blood and lymph vessels surrounded by circular layers of calcium salts and collagen, called lamellae, in which reside osteocytes.

Marrow

—A type of connective tissue which fills the spaces of most cancellous bone and which functions to produce blood cells and store fat.

Ossification

—The process of replacing connective tissue such as cartilage and mesenchyme with bone.

Osteoblast

—The bone cell which deposits calcium salts and collagen during bone growth, bone remodeling and bone repair.

Osteoclast

—The bone cell responsible for reabsorbing bone tissue in bone remodeling and repair.

Osteocyte

—Mature bone cell which functions mainly to regulate the levels of calcium and phosphate in the body.

Skeleton

—Consists of bones and cartilage which are linked together by ligaments. The skeleton protects vital organs of the body and enables body movement.

Synovial joint

—One of three types of joints in the skeleton and by far the most common. Synovial joints are lined with a membrane which secretes a lubricating fluid. Includes ball and socket, pivot, plane, hinge, saddle, condylar and ellipsoid joints.

Vertebrate

—Includes all animals with a vertebral column protecting the spinal cord such as humans, dogs, birds, lizards, and fish.

Skeletal system

Inside every person is a skeleton, a sturdy framework of 206 bones. The skeleton protects the body's organs, supports the body, and provides attachment points for muscles to enable body movement. Bones also produce blood cells and act as a storage site for minerals such as calcium and phosphorus.

All humans are born with over 300 bones. But some bones, such as those in the skull and lower spine, fuse (join together) during growth, thereby reducing the number. The skeletal system is made up of living material, with networks of blood vessels running throughout. Living mature bone is about 60 percent calcium compounds and about 40 percent collagen (a fibrous protein). Hence, bone is strong, hard, and slightly elastic. Although mature bones consist largely of calcium, most bones in the human skeleton began as cartilage. Cartilage is a type of connective tissue that contains collagen and elastin fibers.

Individual bones meet at areas called joints and are held in place by connective tissue. Cartilage lines the surface of many joints and helps reduce friction between bones. The connective tissues linking the skeleton together at the joints are ligaments and tendons. Both are made up of collagen, but serve different functions. Ligaments link bones together and help prevent dislocated joints. Tendons link bone to muscle.

Because the bones making up the human skeleton are inside the body, the skeleton is called an endoskeleton. Some animals, such as the crab, have an external skeleton called an exoskeleton.

Structure

The human skeletal system is divided into two main groups: the axial skeleton and the appendicular skeleton. The axial skeleton includes bones associated with the body's main axis, the spine. This includes the spine, the skull, 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 pelvic girdle (hip area), and the arms and legs.

Axial skeleton. There are 28 bones in the skull. In adults, the bones of the cranium (part of the skull that encloses the brain) are flat and interlocking at their joints. In infants, cartilage fills the spaces between the cranial bones. Known as soft spots or fontanelles, these spaces allow the skull bones to move slightly during birth. This makes birth easier and helps prevent skull fractures. Eventually, the fontanelles are replaced by bone. In addition to protecting the brain, skull bones also support and protect the organs responsible for sight, hearing, smell, and taste.

The skull rests atop the spine, which encases and protects the spinal cord. The spine, also called the spinal column or backbone, consists of 33 stacked vertebrae, the lower ones fused. The spinal column helps to support the weight of the body and protects the spinal cord. Disks of cartilage lie between the bony vertebrae of the back and provide cushioning, like shock absorbers. The vertebrae of the spine are capable of only limited movement, such as bending and some twisting.

Twelve pair of ribs (a total of 24) extend forward from the vertebrae of the upper back. Most of the ribs (the first seven pair) attach in the front of the body via cartilage to the long, flat breastbone, or sternum. These ribs are called true ribs. The next three pair of ribs, called false ribs, do not attach to the sternum. They are connected by cartilage to the ribs above them. The lower two pair of ribs that do not attach in the front are called floating ribs. Ribs give shape to the chest and support and protect the body's major organs, such as the heart and lungs. The rib cage also provides attachment points for connective tissue, to help hold organs in place.

Appendicular skeleton. The appendicular skeleton joins with the axial skeleton at the shoulders and hips. Forming a loose attachment with the sternum is the pectoral girdle, or shoulder. Two bones, the clavicle (collar bone) and scapula (shoulder blade) form one shoulder. 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.

Unlike the pectoral girdle, the pelvic girdle, or hips, is strong and dense. Each hip, left and right, consists of three fused bones—the ilium, ischium, and pubic. The pelvic girdle is bowl-shaped, with an opening at the bottom. 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 supports the weight of the body above it.

The arms and legs, appendages of the body, are very similar in form. The upper arm bone, the humerus, is the long bone between the elbow and the shoulder. It connects the arm to the pectoral girdle. In the leg, the thigh bone, or femur, is the long bone between the knee and hip that connects the leg to the pelvic girdle. The humerus and femur are sturdy bones, especially the femur, which is the longest bone in the body.

At the elbow the humerus attaches to a set of parallel bones—the ulna and radius—the bones of the forearm. These bones attach to the eight small carpal bones of the wrist. The hand is made up of 19 bones.

Similarly, in the leg, the femur attaches to a set of bones of the lower leg, the fibula and tibia. The tibia, or shin bone, is larger than the fibula and forms the joint behind the patella (kneecap) with the femur. At the ankle joint, the fibula and tibia connect to the seven tarsal bones forming the ankle and heel. These, in turn, are connected to the 19 bones that make up the foot.

Bone structure

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. The smooth, hard outer layer of bones is called compact bone. Inside the compact bone is cancellous bone, sometimes called the bone marrow. Cancellous bone appears open and spongy, but is actually very strong, like compact bone. Together, these two types of bone produce a light, but strong, skeleton.

Bones and medicine

Even though bones are very strong, they may be broken. Fortunately, most fractures will fully heal with proper care. In children, bones often heal without a trace.

Bones are affected by poor diet and are also subject to a number of diseases and disorders. Some examples include scurvy, rickets, osteoporosis, and arthritis. Scurvy results from the lack of vitamin C. In infants, scurvy causes poor bone development. It also causes membranes surrounding the bone to bleed. Rickets is a children's disease resulting from a deficiency of vitamin D. This vitamin enables the body to absorb calcium and phosphorus. Without it, bones become soft and weak and actually bend, or bow out, under the body's weight.

The elderly, especially women who had several children in a row, sometimes suffer from osteoporosis. This condition develops when a

body's calcium level is low and calcium from bones is dissolved into the blood to maintain a proper balance. Weak, brittle bones dotted with pits and pores are the result.

Another condition commonly afflicting the elderly is arthritis, an often painful inflammation of the joints. Arthritis is not, however, restricted to the elderly, as even young people may suffer from this condition. Arthritis basically involves the inflammation and deterioration of cartilage and bone at the joint surface.

[See also Orthopedics ]

Skeletal System

The skeletal system is the structural framework that supports an animal's body. It also provides protection for an animal's soft tissues and internal organs and serves as an attachment for the body's muscles that push against it and apply force, resulting in movement. The skeletal system of invertebrate animals (animals without a backbone) is on the outside and is called an exoskeleton. Most vertebrates (animals with a backbone) have a skeleton on the inside of the body, called an endoskeleton.

Without some type of strong, rigid frame for support, an animal's body would be a soft mass of tissue without any real shape. Only animals have skeletons, and because of them, their muscles have an anchor or something to push against. Muscles and bones are really inseparable, since they were designed to always work together to allow an animal to move. The skeletal system of vertebrates has been described mechanically as bones that act as levers that, in turn, apply a force that muscles have generated.

HYDROSKELETON

The simplest and most primitive of all skeletal systems, the hydrostatic skeleton, does not resemble any type of rigid framework. Rather, it is what a soft-bodied animal, like an earthworm or a jellyfish, uses to keep its shape. Since these and other animals, like the hydra, have no rigid parts and are composed of fluid and soft tissue, they maintain their shape and get their support from the internal pressure of fluid pushing against the outer walls of their body cavity. Like air inside a balloon, the fluid that fills the animal's body cavity pushes against its sides and keeps it "inflated." These animals often live in water and move about by contracting their bodies and squeezing water out. Thus, when a jellyfish propels water from one opening, it darts quickly in the opposite direction. Earthworms make their internal fluid move inside their many segments in a particular, coordinated way and are able to lengthen and shorten their bodies and, as a result, crawl.

EXOSKELETON

Evolution transformed the primitive hydrostatic skeleton into an exoskeleton by simply hardening its outer wall. An exoskeleton covers all or part of an animal's body like a suit of armor, and gives it support from the outside. All arthropods (such as crabs, lobsters, spiders, and insects) have exoskeletons, and their muscles are attached to its inside and pull against it to create movement. The typical arthropod body is divided into several jointed regions all composed of a hard shell of dead tissue called chitin. This shell covers every part of the arthropod, and is especially thick and hard around its vital organs. However, it is thin and flexible at the joints so that the animal can move. Although a hard outer shell provides excellent protection, it also limits the size that an animal can reach since the larger it gets, the heavier its shell becomes. It also makes growing difficult and complicated, since the exoskeleton is made up of dead tissue and itself cannot grow. Arthropods achieve growth by periodic molts or splitting and shedding of an outer case. Molting is always a time of danger for arthropods, since they are vulnerable to attack and cannot move well. Mollusks (like clams and snails) have solved this problem since the space inside their extremely hard, outer shell gets bigger as the animal grows.

ENDOSKELETON

Unlike an exoskeleton, an endoskeleton cannot be seen since it is found inside the soft flesh of an animal. It is also very different from most exoskeletons because it grows in step with the rest of the body. Since endoskeletons are lighter than those carried on the outside, they can grow much larger in size. It is no surprise therefore that the largest animals on Earth have endoskeletons. An interior skeleton may be light, but it also provides little protection. While some vital organs may be surrounded by bones (like the ribs around the heart and lungs), other organs (like those below the ribs) are totally vulnerable to injury by force.

Surprisingly, there are some animals that, despite being invertebrates, nonetheless have an endoskeleton. Sponges do not have backbones, but they do have a form of endoskeleton. Other invertebrates, such as echinoderms like sea urchins and starfish, seem to have an exoskeleton, but their spines are really only extensions of their endoskeleton that lies just below their skin. Both squid and octopi are also invertebrates with an endoskeleton. There are also animals that are considered to be vertebrates but who do not have a single, real bone in their body. This is because their supporting framework is usually made up of cartilage. Cartilage is a tough, slippery substance that is both strong and flexible. The shark, ray, and lamprey are examples of vertebrates with an endoskeleton of cartilage, and they swim in a noticeably different way than true fish do.

PARTS OF AN ENDOSKELETON

All other vertebrates have an endoskeleton made of bone. As a living substance, bone is made up of cells surrounded by layers of hard mineral salts, such as calcium phosphate. Most bones have three structural parts: the periosteum or compact bone that is its dense outer layer and gives it strength; spongy bone, which is the light, softer inside layer; and marrow that fills its inside core and makes red and white blood cells. Before vertebrates are born, their skeleton exists in a cartilage form, which "ossifies" or hardens and turns into real bone at birth. After birth, some of the bones fuse together. For example, a human baby has 270 bones at birth. However, by the time the baby reaches adulthood many of the bones will have fused together, leaving only 206 separate bones in the human skeletal system.

For a skeleton to really work, however, it cannot be all bone. Bones must be connected to one another to make it all work together, and ligaments are the connective material that links bone to bone at the joints. A ligament is made up of many bands of tissue that, if torn, can heal; but if severed, must be surgically sewn back together. Muscles are connected to bones by connective tissue called tendons. Although tendons act like a very tough cord, they cannot be stretched. If severed, they must be surgically repaired or the muscle will not work the bone (since the connection is broken). Cartilage is also a form of connective tissue and serves to fill the spaces between bones and prevents them from scraping painfully against one another. Cartilage is slippery (called "gristle" in animals) and it allows bones to slide over each other at the joints.

Because vertebrate skeletons are jointed, they allow movement. There are six types of moving joints in vertebrate skeletons: ball-and-socket joints (like the hip and shoulder joint); gliding joints (wrist and ankle); pivotal joints (allow two kinds of movement—side-to-side and up-and-down); saddle joints (thumb joint); hinge joint (elbows, fingers, and knees); ellipsoid joints or condyloid joints (movement in two axes such as the tiny bones in people's fingers, toes, and jaw). Joints move easily because of their slippery cartilage but also because they are lubricated by a special fluid called synovial fluid. There are also some body joints that do not move, such as the bones of the skull and hip. In these joints, bones come together in joints that are really more like seams.

The basic form of all vertebrate skeletons is essentially the same for many different types of animals and is made up of an axial skeleton and

an appendicular skeleton. The standard axial model (for a typical mammal) has a vertebral column made up of individual small bones (vertebrae) that provide up-and-down or lengthwise support while also being flexible. The vertebrae (singular, vertebra) also connect the skull to the rest of the body. Ribs, also part of the axial skeleton, provide support and protection for the chest area.

The appendicular skeleton is made up of the forelimbs (arms and hands), the shoulders, and the pelvic bones to which the hindlimbs are attached. Finally, ligaments keep the bones securely attached at the joints. There are naturally many variations to this model. For instance, a snake is a vertebrate but it has no limbs at all. A manatee and a whale have only forelimbs (flippers). Humans have arms and birds have wings. The long, flexible human thumb allows us to perform many precise movements, whereas the chimpanzee, our close relative, has a shorter, much less useful thumb. While the number, size, and shape of skeletal elements may vary greatly within species during their development, and sometimes even between sexes, the basic vertebrate model is always the same.