The formation of BONE. In early life, centres appear in the bones previously represented by cartilage or fibrous tissue; and these cells, called osteoblasts, initiate the formation of true bone, which includes the deposition of calcium salts. When a fracture occurs, the bone mends by ossification of the clot which forms between the fragments (see under BONE, DISORDERS OF). In old age, an unnatural process of ossification may take place in parts which normally remain cartilaginous – for example, in the cartilages of the larynx and of the ribs, making these parts unusually brittle.