The growth of a bone in length is due to a definite sequence of events which transpire at the junction of the shaft of the bone (di-aphysis) with its head (epiphysis). The shaft is composed of growing bone, the epiphysis of cartilage which is largely in a resting state. As the animal grows the cartilage of the epiphysis is converted into bone through (a) encroachment on it of the bony shaft and (b) through the expansion of one or more centers of ossification which form in it. The process of growth in length of the shaft is the same which results in the expansion of the centers of ossification in the epiphysis.

The cells of the resting cartilage have no particular arrangement until they come within a short distance of the shaft or center of ossification. The cartilage cells then arrange themselves into columns parallel to the long axis of the shaft or the radii of the ossification center. This columnar zone is known as the "proliferative cartilage." Lime salts are deposited in the matrix separating the cells of the thin layer of cartilage in contact with the shaft. This is known as the zone of provisional calcification. Cartilage cells in this zone lie in a matrix which is like a calcified honey-comb. As the shaft grows tiny blood vessels sprout evenly from it and each vessel opens a single compartment in the calcified honeycomb. The cartilage cell is freed to degenerate or to become an osteoblast or bone-forming cell. Large numbers of osteoblasts follow the blood-vessel sprouts into the cartilage. These settle on the spicules of calcified matrix which remain after the cartilage cell capsules are opened. These spicules furnish cores about which the trabecule of bone in the shaft are built. The osteoblasts on the spicules of calcified matrix bury themselves in a tissue known as osteoid which they manufacture. This receives a deposit of lime salts and becomes bone.

Growth in thickness of the shaft is accomplished by cells in a thick layer of connective tissue which surrounds the shaft and is called the periosteum. These connective-tissue cells are transformed directly into osteoblasts and lay down bone in layers beneath the periosteum. During growth, and to a lesser extent after growth has ceased, the structure of a bone is subject to constant change to meet the requirements of exercise and changing conditions of stress and strain. Old bone is constantly being removed (resorption) and new bone is always being formed. In a normal, healthy bone these processes are always in a dynamic equilibrium.