60 



GENERAL ANATOMY. 



Intracartilaginous Ossification. Just before ossification begins the bone is 

 entirely cartilaginous, and in a long bone, which may be taken as an example, 

 the process commences in the centre and proceeds toward the extremities, which 

 for some time remain cartilaginous. Subsequently a similar process commences 

 in one or more places in those extremities and gradually extends through it. The 



extremity does not, however, 

 become joined to the shaft of 

 the bone until gi*owth has 

 ceased, but remains separated 

 by a layer of cartilaginous 



tissue termed epiphysial carti- 

 lage. 



The first step in the ossifica- 

 tion of the cartilage is that the 

 cartilage-cells, at the point 

 where ossification is commen- 

 cing and which is termed a cen- 

 tre of ossification, enlarge and 

 arrange themselves in rows (Fig. 

 31). The matrix in which they 

 are imbedded increases in quan- 

 tity, so that the cells become 

 further separated from each 

 other. A deposit of calcareous 

 material now takes place in this 

 matrix, between the rows of 

 cells, so that they become sepa- 

 rated from each other by longi- 

 tudinal columns of calcified 

 matrix, presenting a granular 

 and opaque appearance. Here 

 and there the matrix between 

 two cells of the same row also 

 becomes calcified, and thus we 

 have transverse bars of calcified 

 substance stretching across from 

 calcareous column to 



FIG. 3L Longitudinal section through the ossifying portion of 

 a long bone in the human embryo, o. Cartilaginous region. 6. 

 Region of calcified matrix. 



one 



another. Thus we have lon- 

 gitudinal groups of the cartilage- 

 cells enclosed in oblong cavities, the Avails of which are formed of calcified 

 matrix. These cavities are called the primary areolce (Sharpey). 



At the same time that this process is going on in the centre of the cartilage of 

 which the foetal bone consists, certain changes are taking place on its surface. 

 This is covered by a very vascular membrane, the periosteum, on the inner surface 

 of which that is to say, on the surface in contact with the cartilage are a number 

 of cells called osteoblasts. By the agency of these cells a thin layer of bony tissue 

 is being formed between the periosteum and the cartilage, by the intramem- 

 branous mode of ossification presently to be described. We have then, in this 

 first stage of ossification, two processes going on simultaneously : in the centre of 

 the cartilage the formation of a number of oblong spaces, enclosed by calcified 

 matrix and containing the cartilage-cells enlarged and arranged in groups, and on 

 the surface of the cartilage the formation of a layer of true membrane-bone. The 

 second stage consists in the prolongation into the cartilage of processes of the 

 deeper or osteogenetic layer of the periosteum (Fig. 32, ir). The processes consist 

 of blood-vessels and cells (osteoblasts). They excavate passages through the new- 

 formed bony layer by absorption, and pass through it into the calcified matrix (Fig. 

 32). Wherever these processes come in contact with the calcified walls of the 



