Chap, vn.i BONE. 59 



but becomes gradually absorbed altogether, and this 

 process also starts from the points of ossification. Thus 

 a continuous cavity filled with marrow is formed, and 

 this first appears in the region of the centre of ossifi- 

 cation, and represents the rudiment of the future 

 continuous central marrow cavity of the shaft. Simul- 

 taneously with or somewhat previous to this absorp- 

 tion of the endochondral bone, new bone spongy 

 bone is deposited directly by the osteogenetic layer 

 of the periosteum on the outer surface of the en- 

 dochondral bone. This also commences at the centre of 

 ossification and proceeds from here gradually to further 

 points. This is the periosteal bone (Figs. 34, 35). It is 

 formed without the intervention of cartilage directly 

 by the osteoblasts of the osteogenetic layer. And as 

 fresh layers of osteoblasts multiply on the surface of 

 the periosteal bone, new layers of bone trabeculse 

 are formed, and the old trabeculse become increased 

 in thickness. In the meshes or Haversian spaces of 

 this spongy periosteal bone the same tissue is to be 

 found as constitutes the osteogenetic layer of the 

 periosteum, the one being derived from and con- 

 tinuous with the other. 



In these Haversian spaces concentric lamellae of 

 bone substance become formed by the osteoblasts, and 

 spongy is thus transformed into compact bone, while 

 at the same time the Haversian spaces, being nar- 

 rowed in by the deposit of the concentric lamellse, are 

 transformed into the Haversian canals. When this 

 compact bone is again absorbed, e.g., that next the 

 medullary cavity of the shaft of a long bone, the 

 concentric lamellae are first absorbed, the Haversian 

 canal being in this way widened out and again trans- 

 formed into a Haversian space. 



73. At birth all the primary endochondral bone 

 has already disappeared by absorption from the centre 

 of the shaft, and the bone present is all of periosteal 



