THE BONE. 



1103 



some time remain cartilaginous. Subsequently a similar process commences in one 

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

 extremities do not, however, become joined to the shaft by bony tissue until growth 

 has ceased, but are attached to it by a layer of cartilaginous tissue termed the 

 epiphyseal cartilage. 



The first step in the ossification of the cartilage is that the cartilage-cells, at 

 the point where ossification is commencing and which is termed a centre of ossifica- 

 tion, enlarge and arrange themselves in rows (Fig. 632). The matrix in which they 

 are imbedded increases in quantity, so that the cells become further separated from 

 each other. A deposit of calcareous material now takes place in this matrix, 





FIG. i',;:-.'. Section of foetal bone of cat. ir. Irruption of 

 the subperiosteal tissue, p. Fibrous layer of the perios- 

 teum, o. Layer of osteoblasts. im. Subperiosteal bony 

 . (Fro'ni Quain's Anatomy, E. A. Schafer.) 



FIG. 633. Part of a longitudinal section of 

 the developing femur of a rabbit, a. Flat- 

 tened cartilage-cells. 6. Enlarged cartilage- 

 cells, c. d. Newly-formed bone. e. Osteo- 

 blasts. /. Giant-cells or osteoclasts. g. h. 

 Shrunken cartilage-cells. (From Atlas oj His- 

 tology, Klein and Noble Smith.) 



between the rows of cells, so that they become separated 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 calci- 

 fied, and transverse bars of calcified substance stretch across from one calcareous 

 column to another. Thus there are longitudinal groups of the cartilage-cells 

 enclosed in oblong cavities, the walls of which are formed of calcified matrix, 

 which cuts off all nutrition from the cells, and they, in consequence, waste, 

 leaving spaces called the primary areolce (Sharpey). 



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

 cartilage of which the foetal bone consists, certain changes are taking place on 



