THE STRUCTURE AND STRENGTH OF BONE

 THE STRUCTURE AND STRENGTH OF BONE

 About one fifth of the weight of bone is water. Of the rest, about two thirds is mineral and one third organic matter. The minerals are chiefly compounds of calcium and phosphorus, with some of magnesium and other elements, while the organic matrix is chiefly a kind of protein fiber, collagen, which also is found in skin and connective tissue. The organic and inorganic components of bone along with cement- like substance are combined in a way that is sometimes compared to reinforced concrete. Bone is so strong that there are areas along the out- side of the femur, the long bone between hip and knee, which have been known to resist pressure of over 1,200 pounds per square inch.

 There are long bones like the femur (thigh bone), short ones like those in the wrist, flat ones like the shoulder blade, and irregular types like the vertebrae in the spinal column. All, however, have an outer layer of compact bone and an inner meshwork of less dense, actually porous material. Even the inner layer, sometimes called spongy because of its appearance, is remarkably strong. Far from being inert, the bones contain living cells and blood vessels. There are in fact three types of cells: the osteoblasts, which function in the construction of new bone material and repair of broken bones; the osteoclasts, which dissolve bone bits that are not required; and the osteocytes, which maintain the health of bone around them, using materials obtained from blood.

The long bones, ribs, and vertebrae are the sites, too, where the red blood cells of the body are produced. Each minute, millions of these vital elements are developed in the spongy area from red marrow, since the body requires billions and their lifetime is only a few months. Thus, the bony framework is not at all comparable to the framework of a building. It is a living framework. And while it has rigidity, it also has flexibility. Muscles are anchored to bone, and the anchoring must be strong. But there has to be movement, too. And it has been said aptly that man is a mixture of stiffness and relaxation, restriction and freedom, joints without movement and joints with great versatility. 

THE SKELETAL SYSTEM BONES

 THE SKELETAL SYSTEM BONES FORM the basic framework-the skeleton or chassis-of the body. The primary purpose of the framework is to provide support for the rest of the body through a combination of strength to resist great pressure and flexibility to absorb some shock without shattering. By means of rib cage and skull such vital organs as the heart and brain are guarded. Obviously, the skeletal system is important. Obviously, too, it is a source of much trouble-witness spinal disk problems and the proneness some people have toward easy bone fracture. Yet it is possible to use simple preventive measures to help maintain a healthy skeletal system. Much of the trouble people have is avoidable trouble.

THE BONES Generally, there are 206 bones in the body, although about 5 percent of people have an extra, or thirteenth, pair of ribs, and a much smaller percentage have only 11 pairs. At birth, there are actually about 350 bones, some of which later fuse. The fusion is usually complete by the end of the growth period. Each arm has 32 bones: a collarbone, shoulder blade, humerus, radius, ulna, 8 wristbones, 5 metacarpals in the palm, and 14 phalanges (3 to each finger, 2 to the thumb). Each leg has 31: hipbone, femur, kneecap, tibia, fibula, 7 tarsals in instep and heel, 5 metatarsals in the foot, and 14 phalanges (3 to each toe except for 2 to the biggest toe, the hallux).

The axial skeleton has 80 bones: there are 29 in the head, of which 8 are in the cranium, 14 in the face, 6 in the ears, and 1 in the throat between lower jaw and upper larynx; the spine has 33 bones in all-7 cervical, 12 dorsal or thoracic, 5 lumbar, 5 sacral vertebrae forming 1 bone, and 4 coccygeal vertebrae which, with the sacrum, are fused into one bone (thus, everyone is born with 33 vertebrae but in later life has only 26); there are also the 25 bones in the chest: the breastbone and 24 ribs.