Ossification

Ossification is the term applied to the process by which, in the course of development, true bone is deposited and takes the place of the more rudimentary structures which represent this tissue in the embryo or growing child. Ossification is of two kinds: in the first place, there is what is called ossificaiion in membrane, such as occurs, for example, in the flat bones of the skull, which are developed from membranous structures; in the second place, there is ossification in cartilage, such as occurs in the case of the long bones, the representatives of which in the embryonic condition are rods of cartilage. The changes which the cartilaginous rod undergoes in the process of conversion into bone are somewhat complicated. They commence at definite points, which are termed centres of ossification. Here the cartilage becomes permeated by minute blood-vessels, and its substance is impregnated with lime-salts, i.e. calcified. The calcified cartilage is later reabsorbed, and a framework of spongy bone laid down in the ossifying tissue. This spongy bone becomes in its turn absorbed and replaced by compact bone. The centres of ossification are the points at which the most active changes are manifested. Such centres are usually found near the ends of long bones, and the increase of these bones in length is consequent upon the continued advance in the process of ossification at such centres. The bone increases in thickness by the formation of new bone beneath the fibrous investing layer of the bone called the periosteum.