Structure of Bone.—The hard substance in bone is always arranged in lamellæ or bundles of bony fibers, which in cancellous tissue meet to form a kind of lattice-work, while in the dense tissue they are generally arranged in rings about the [Haversian canals], channels through which the blood-vessels pass through the bone longitudinally. Between the lamellæ are spaces called [lacunæ], in which lie branched cells, the spaces being connected with each other and with the Haversian canals by numerous tiny canals or [canaliculi], by which nutrient material finds its way from the Haversian canals to all parts of the bone.
Within the bone is the medulla or marrow, which is of two varieties: the yellow, which is largely fat and is found in the long bones of adults, and the red, which is nearly three-fourths water and is found in most of the other adult bones and in the bones of the fetus and of the infant.
Lining the medullary and cancellous cavities is a delicate connective tissue lining, the endosteum, which contains many bone-forming cells, while on the outside of the bone, except at the articular ends, is the periosteum with its outer protective layer and its inner vascular layer containing osteoblasts or bone-forming cells. The periosteum is essential for the growth of new bone where the old bone has died, and if the periosteum is removed from healthy bone the part beneath is liable to die, as it is by the constant growth of the osteoblasts that the bone grows and is renewed. In the repair of broken bones tissue is formed between and around the broken ends.
Bone Formation.—Most of the skull and face bones begin as membranes of connective tissue, that is, are formed in membrane. Bones are also formed in cartilage, the bone formation in this case beginning from centers of ossification, where the deposit of lime salts in the intercellular substance begins, the salts coming to the centers dissolved in the plasma. Such a center of growth in a bone is called the epiphysis and is separated from the main part of the bone or diaphysis by cartilage until full growth is attained, when ossification becomes complete. So in surgery, in working on the bones of children, part of the epiphysis should always be left for the sake of future growth. The outer shell of compact tissue is deposited by the periosteum.
Chemical Composition of Bone.—Chemically bone is composed of about one-third organic or animal matter, largely gelatine, and two-thirds inorganic matter, including various salts of calcium, magnesium, and sodium. In young children the animal matter predominates and the bones are soft and often bend instead of breaking, only the outside shell on one side giving way, as in “green-stick” fracture. In rickets there is a deficiency of lime salts, but the increased brittleness of the bones in old age is due, not to increase of mineral matter, but to the less spongy texture of old bones.
Classification and Function of Bones.—There are in the body some two hundred bones, which may be classified as long, short, flat, and irregular. Occasionally an irregular bone develops in a fontanelle, the membranous opening at the juncture of the sutures of the skull. This is known as a Wormian bone. It is not, however, included in the two hundred, as are not the sesamoid bones or bones developed in tendons, with the exception of the patella or knee-cap.
Long bones are developed in cartilage and consist of a shaft, two extremities, and various processes. They are more or less curved to give them strength and grace. They serve as supports and act as levers for purposes of motion and the exercise of power. Since a hollow cylinder is just as strong as a solid one of the same size, the weight coming only on the outer shell, the great bones which are accountable for weight and which need to be light themselves have hollow shafts, composed chiefly of compact tissue with a central medullary canal. The ends, however, are expanded in order to make better connection at the joints and to afford broad surfaces for muscular attachment, cancellous tissue being used in them for lightness and strength. The large spongy ends also give elasticity and lessen jar, and by bringing the tendons to the bone at a greater angle increase their effectiveness. Blood is brought to the long bones not only by the vessels of the periosteum but by the medullary artery, which penetrates the compact tissue by the nutrient foramen and divides into an ascending and a descending branch.
Short bones are spongy throughout. They are used for strength and where little motion is required.
Flat bones are composed of two thin layers of compact tissue with a varying amount of cancellous tissue between, and are for protection and muscular attachment. The cancellous material between the two layers or tablets of the skull is called the diploë.
Eminences and depressions occur on bones and when they are not articular are for the attachment of ligaments and muscles. If they are articular, they help to form joints.