Ossification

Ossification is the formation of bone. Most human bones are first represented by cartilage, which, by a complicated series of changes, becomes transformed into bone. The bones of the vault of the cranium and the face, part of the clavicle, and the 'sesamoid' bones occurring in tendons, on the other hand, are developed from fibrous tissue, without passing through a cartilaginous stage, and are distinguished as membrane-bones. In the larger bones of the limbs at least three centres of ossification are found, one in the shaft, and one at each extremity. Growth of the bone takes place mainly at the lines between these elements, which long remain cartilaginous. Bony union becomes complete in each situation at a tolerably definite age (in some not till about twenty-five; see BONE). True Ossification sometimes occurs as a morbid process; but in many cases the term is incorrectly used (especially in the case of blood-vessels—see under ARTERIES) to designate a hard calcareous deposit, better called calcification, or calcareous degeneration, in which the characteristic microscopic appearances of true bone are absent. In one sense the osseous tissue that is formed in regeneration of destroyed or fractured bones (see FRACTURES) may be regarded as due to a morbid, although a restorative action. Hypertrophy of bone is by no means rare, being sometimes local, forming a protuberance on the external surface, in which case it is termed an exostosis; and sometimes extending over the whole bone or over several bones, giving rise to the condition known as hyperostosis. Again, true osseous tissue occasionally occurs in parts in which, in the normal condition, no bone existed, as in the dura mater, in the so-called permanent cartilages (as those of the larynx, ribs, &c.), in the tendons of certain muscles, and in some forms of tumours. The peculiar causes of the osseous formations which are unconnected with bone are not known.