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.
