The skeleton

The human skeleton comprises more than 200 strong, bony rods, blocks, and plates buffered by cartilage and linked by joints. Between them, different bones provide internal scaffolding, muscle-operated levers, and shields protecting vital organs. Bones also contain marrow, which manufactures blood. Furthermore, despite a popular misconception, bone is a living substance that is constantly reconfigured and, like the brain, needs nourishment.

Bones link together to form the skeleton, which is symmetrical about a vertical, central plane. The illustration shows the skeleton seen from the front on one side, and from the back on the other. Bones in the center of the body have been shown cut in half.

Bone types and structures

Some anatomists classify bone into four types: long’, short, flat, and irregular. Long bones form the levers of the limbs. Short bones, as in the wrist and ankle, provide strong, compact structures. Flat bones, such as those of the skull and shoulder blades, protect other structures or provide broad surfaces for muscle anchorage. Irregular bones, such as the bones of the spine, are those too peculiarly shaped to be grouped with any of the rest. There are two main types of bony tissue.

The hard, solid, heavy walls are made of dense tissue called compact bone. Inside is a mesh of spongy, or cancellous, bone. The two combined give bones their strength and relative lightness. Bone’s hardness comes from layers of crystals of compounds of calcium, phosphorus, and other elements. Bone’s resilience depends upon pliable threads of the fibrous protein collagen, which has high tensile strength and forms a network within which the hard crystals are laid down. Strength, hardness, and flexibility help bone resist forces that would otherwise crush or bend it.

If you cut through a typical long bone you find first a thin covering of tough tissue, the periosteum. Then comes the hard compact bone built up of so-called Haversian systems: concentric rings with central canals carrying the blood vessels that supply oxygen and nutrients to the bone. Beyond the compact bone, a network of spongy bone occupies the bone’s interior and its bulging ends. The inside of the shaft and spaces between the “struts” of spongy bone, however, are filled with pulpy marrow, of connective tissue, blood vessels, and red blood cells. As long bones mature, fatty yellow marrow takes the place of red.

The ends of long bones are covered with cartilage, a dense, flexible connective tissue that is a precursor of bone in children and serves to protect bones from friction against other bones and from damage from jarring within the joints.

Axial skeleton

The axial skeleton of skull, vertebrae, and ribs provides the body’s structural core, supporting the appendicular skeleton of limb bones and providing the frame for the internal organs.

The skull’s 22 bones include 8 interlocking cranial bones, forming a dome that holds and shields the brain, and 14 facial bones, forming the basis of the face and jaws: a rigid maxilla (upper jaw) and hinged mandible (lower jaw). Air-filled cavities, called sinuses, lighten certain facial bones, and bony basins support and protect the eyes. The nasal cavity and mouth provide openings for the respiratory and digestive systems. The skull rests on the topmost of the 33 or so short, strong irregular bones called vertebrae. (Their numbers vary slightly in some individuals.) These and the fibrous disks between them form the spine. This flexible, weight-bearing column with a double bend supports the upper body’s weight; bony arches at the backs of vertebrae sheathe the fragile spinal cord. From top to bottom, the spine’s five sets of vertebrae comprise typically: 7 cervical (neck) vertebrae; 12 thoracic vertebrae at the back of the chest; 5 lumbar (lower back) vertebrae; 5 fused vertebrae forming the sacrum; and the 4 fused vertebrae of the coccyx, a vestigial tail.

The ribcage consists of 12 pairs of flat, curved bones projecting forward from the thoracic vertebrae. Pairs 1 -7, the true ribs, meet the sternum, or breastbone, at the front of the chest. Pairs 11-12, the floating ribs, stop short. Ribs and breastbone protect the heart and lungs inside the chest cavity and work with the diaphragm during breathing.

Compact bone, which forms the outer part of bones and gives them their strength, is made up of many columns—shown here in cross-section—of cells called osteoblasts, arranged concentrically around central Haversian canals.

Appendicular skeleton

Limb bones are anchored to the axial skeleton by frameworks known as girdles. The pectoral or shoulder girdle has two clavicles (collarbones), which help muscles hold the shoulders back, and two scapulae (shoulder blades), which lie at the back of the chest. A humerus (upper arm bone) fits into a socket in each scapula. Each forearm has two bones (radius and ulna) that are articulated so they can twist, allowing hand rotation. Each hand has 27 bones comprising carpals (wrist bones), meta-carpals (palm bones), and phalanges (fingers). The pelvic or hip girdle has two sets of three bones (ilium, ischium, and pubis) that flank and join the sacrum and, with it, form a bony, pelvic basin. This pelvis supports and protects internal organs, and each side of it has an acetabulum, a cup-shaped cavity, serving as a socket for a femur (thighbone). Below the femur are two other leg bones, the tibia (shinbone) and fibula. The tarsals, metatarsals, and phalanges form the bones of the back of the foot, the forefoot, and the toes, respectively.

Joints, ligaments, and tendons

Bones can transmit body weight and help muscles move limbs and other parts of the body only because they are linked at joints that is, where bone meets bone. Different types of joints serve different purposes. At fibrous joints, like those between the cranial bones, fibrous tissue knits bones almost rigidly together. At cartilaginous joints, like those between the vertebrae, however, springy cartilage buffers bone ends, and between these lies a fibrocartilage pad. Much greater movement occurs at synovial, or movable, joints, such as those of the ankle, elbow, and shoulder. Some are gliding joints, others are pivot, hinge, or ball-and-socket joints. Many are superbly designed to combat friction and the stress induced by sudden jolting. In each synovial joint, opposing bone ends are capped by cartilage and separated by a cavity walled by ligaments bands of flexible tissue that give the joint stability and strength. The inner lining of the joint capsule surrounding the cavity consists of smooth, slippery synovial membrane producing syrupy synovial fluid that lubricates joint surfaces. Active movement of many joints would be impossible without tendons the strong, tough, plaited fibers forming the cords that attach bone or cartilage to muscle.

The hip joint consists of a moving bone, the femur, attached to a group of fixed bones, known collectively as the pelvic girdle. Ligaments and muscles hold the femur in place and make it move. The head of the femur and the socket (acetabulum) into which it fits are lined with cartilage. Synovial fluid, produced by the enveloping synovial membrane, allows these two surfaces to move against each other without friction. The femur is a typical long bone, with a thick layer of compact bone around its shaft, and with spongy bone inside. The latter allows bones to be both strong and relatively light; it also contains a rich supply of blood vessels and the blood-forming tissue called marrow (not shown). The shafts of long bones contain yellow marrow, which is also a store of fat cells. Red marrow is found in the head of the femur, as well as in the flat bones of the body among them, the bones of the pelvic girdle.

Teeth and nails

Tooth enamel resembles bone in that it is made of mineral crystals in a mesh of protein, but enamel has a higher mineral content than bone and so is even harder. Tooth enamel, however, is not part of the skeleton: with the substance of toenails and fingernails, enamel forms the nearest human equivalent to an insect’s exoskeleton.