THE SUPPORTING TISSUES 47 



certain points in the cartilage known as centers of ossification. 

 The cartilage itself becomes surrounded by a periosteum like that 

 which incloses membrane bones. At the center of ossification the 

 osteoblast layer of the periosteum begins to force its way into the 

 cartilage, absorbing much of the latter and leaving only a coarse 

 network, which is presently converted by the osteoblasts into true 

 bone. Meanwhile the periosteum has deposited on the surface of 

 the cartilage a layer of compact bone so that in time the cartilage 

 bone presents a structure not unlike that of membrane bones, a 

 spongy interior inclosed in a layer of compact bone. As the 

 cartilaginous network is being ossified, many osteoblasts are im- 

 prisoned within the bony substance. The spaces which they 

 occupy and the tiny canals which radiate therefrom into the bone 

 substances are among the most characteristic appearances of bone 

 viewed under the microscope (Fig. 12). 



The growth in thickness of bone is accomplished by the addition 

 of layer after layer of compact bone underneath the periosteum. 



FIG. 12. Cross-section of compact bone from the shaft of the humerus. (Sharpey, 

 from Bailey's Text Boole of Hiatology.) 



During this process blood-vessels of the periosteum often become 

 embedded within the bony mass. When this occurs the osteo- 

 blasts which accompany the blood-vessel surround it with con- 

 centric layers of bone. In this manner are formed the so-called 

 Hwersian Systems, each of which consists of the space through 



