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, includ-
