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.