OSSIFICATION (from Lat. os, bone + fa cere, to make). The vital process by which calcareous matter is deposited in cartilage or membrane. giving rise to bone. At a very early period of embryonic life, as soon as any struc tural differences can be detected, the material front which the bones are to be formed becomes mapped out as a soft gelatinous substance. which may he distinguished from the other tissues by tieing rather less transparent, and soon becom ing decidedly opaque. From this beginning the hones arc formed in two ways: either the tissue just described becomes converted into cartilage. which is afterwards replaced by hone (intrarar tilaginons ossification). or a germinal membrane is formed, in which the ossifying process takes place (intramembranous ossification). The lat ter is the more simple and rapid mode of forming bone. When ossification commences. the mem brane becomes more opaque. and exhibits a de cided fibrous charaeter, the fibres being arranged more or less in a reticulated manner. These fibres become more distinct and granular from impregnation with lime salts, and are converted into incipient hone, while the cells which are scattered among them shoot out into the bone corpuscles, from which the canaliculi are ex tended, probably by resorption. The facial and cranial bones, with the exception of those at the base of the skull, are formed without the inter vention of any cartilage.
In intracartilaginons ossification, at the point where ossification begins the cartilage cells ar range themselves into rows, and become sepa rated by the growth of the matrix in which they lie. A deposit of calcareous material now takes place between the rows of cartilaginous cells, and the cartilage assumes a granular and opaque ap pearance. While this process is going on in the centre, a thin layer of bone is being formed between the surface of the cartilage and the vas cular membrane covering it—the periosteum—by the agency of cells called osteoblasts, in much the same way that intramembranous ossification takes place. From this outer shell prolongations
consisting of osteoblasts and blood-vessels pene trate toward the centre of the cartilage and form the permanent canals through which the hone is nourished. All the bones of the body except those of the face and the vault of the cranium are formed in cartilage. Certain bones at the base of the skull, as the occipital, are formed partly in cartilage and partly in membrane.
True ossification sometimes occurs as a patho logical process: but in many eases the term is incorrectly used (especially in the case of blood vessels) to designate a hard calcareous deposit, in which the characteristic microscopic appear ances of true hone are absent. The osseous tissue that is formed in regeneration of destroyed or fractured bones may be regarded as due to a morbid, although a restorative action. Hyper trophy of bone is by no means rare, being some times local, forming a protuberance on the ex ternal 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 (Jura mater, in the so-called permanent cartilages (as those of the larynx, ribs, etc.), in the tendons of certain muscles, and in certain tumors. The causes of these osseous formations a re not known.