Chap, vi i.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 by multiplication appear on 

 the surface of the periosteal bone, new layers of bone 

 trabeculae are formed, and also the old trabeculse become 

 increased in thickness. In the meshes or Haversian 

 spaces of this spongy periosteal bone the same tissue 

 is of course to be found as constitutes the osteo- 

 genetic layer of the periosteum, being derived from 

 and continuous with it. 



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 in them of the concentric 

 lamellae, are transformed into the Haversian canals. 

 When this compact bone is again absorbed e.g., that 

 next the central marrow cavity of the shaft of a 

 tubular bone the concentric lamellae are first ab- 

 sorbed, the Haversian canal being in this way again 

 transformed into a Haversian space. 



73. At birth all the primary endochondral bone has 

 already disappeared by absorption from the centre of 

 the shaft, while the bone present is all of periosteal 



