44 



GENERAL ANA TOMY OF THE SKELETON 



which become the Haversian canals, so that the bone increases much in thickness. The process 

 spreads laterally to the region of the future suture, and here between the various bones a layer 

 of fibrous tissue, the cambium layer, is maintained until the full size of the bone is reached. 

 The cambium layer then ossifies and the bone ceases to grow at its edges. 



Intracartilaginous Ossification. Just before ossification begins the bone is entirely carti- 

 laginous, and in the 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 ossifies them. The extremities do not, however, become joined to the shaft bv honv 

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

 epiphyseal cartilage. 



The first step (proliferation) 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 ossification, multiply, 

 enlarge, and arrange themselves in rows (Fig. 11). The matrix in which they are embedded 



FIG. 13. Cross-section of a developing bone of a human fetus of four months, a. Periosteum, b. Boundary 

 between endochondral and periosteal bone. c. Perichondral bone. d. Remains of area of calcification. 

 e. Endochondral bone. /, f . Bloodvessels, g, g'. Developing Haversian spaces, h. Marrow, i. Bloodvessel. 

 (Radasch, after Stohr's Histology.) 



increases in quantity, so that the cells become further separated from each other. A deposit 

 of calcareous material (calcification) now takes place in this matrix, between the rows of cells, 

 so that they become separated from each other by longitudinal columns of calcified matrix. 

 These columns are connected to one another by transverse bars of calcareous substance, and 

 present a granular and opaque appearance. In the calcareous areas the cartilage cells repro- 

 duce so rapidly that a number of cells are seen in each large lacuna, or space, which is called a 

 primary areola. This process is succeeded by destruction of some of the columns between the 

 smaller spaces, forming thus a fewer number of larger spaces, the secondary areolae. Some of 

 the cells within the areolae disappear, others become osteoblasts, which apply themselves to the 

 columns and secrete a thin veneer of osseous tissue upon the calcareous matter; still others 

 of these cells become osteoclasts. 



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

 which the fetal bone consists, certain changes are taking place on its surface. This is covered 

 by a very vascular membrane, the perichondrium, entirely similar to the embryonic connective 

 tissue already described as constituting the basis of membrane bone, on the inner or cartilage 



