70 



THE ESSENTIALS OF HISTOLOGY. 



phalanges of the digits the ossification starts, not from the middle of 

 the cartilage, but from its distal extremity. 



For the regeneration of portions of bone which have been removed 

 by disease or operation it is important that the periosteum be left. 



Intramembranous ossification. In this variety of ossification, the 

 bone is not preceded by cartilage at all, and therefore no endochon- 

 dral bone is formed, but the calcification occurs in a sort of embryonic 

 fibrous tissue which contains numerous osteoblasts and blood-vessels 

 (fig. 79). The fibres of this tissue (osteogenic fibres), which, like those 

 of fibrous tissue, are collected into small bundles, become inclosed in 

 a calcareous matrix ; and as the fibres grow, the calcification extends 

 further and further, so that bony spicules are formed, which, as they 

 become thickened, run together to form reticular layers, leaving spaces 

 filled with osteoblasts around the blood-vessels. The osteogenic fibres 

 are covered with osteoblasts, and as the bone forms, some of these 

 become left as bone-corpuscles within lacunae. Thus in every particular 



FIG. 78. BONY TRABECUL^E FROM THE DEVELOPING LOWER JAW or A CALF, SHOWING 



OSTEOCLASTS AT THE EXTREMITIES WHERE ABSORPTION IS PROCEEDING, AND 

 OSTEOBLASTS COVERING THE SIDES WHERE DEPOSITION OF BONE IS GOING ON. 



the development of. these bones resembles that of the subperiosteal 

 layer of endochondral bone ; which is also to be considered as an instance 

 of intramembranous ossification, although taking place on the surface 

 of cartilage. Moreover, it is the same subperiosteal tissue which 

 deposits the true or secondary bone upon those parts of the calcified 

 cartilage-matrix which have escaped absorption; and this must also, 

 therefore, be reckoned as developed according to the same type. In 

 fact, even in intracartilaginous ossification, very little of the calcified 

 cartilage-matrix eventually remains ; this being almost wholly absorbed 



