DEVELOPMENT OF BONE. 



49 



conversion is the partial absorption of the trabeculae by the osteoclasts within 

 the primary marrow. As the result of this process, the robust and close 

 reticulum of subperiosteal bone is reduced to a delicate osseous framework 

 enclosing enlarged marrow-channels. The latter are now known as the 

 Haversian spaces and, in cross-section, are round or oval. After the 

 destructive work of the osteoclasts has progressed to the required extent, the 

 osteoblasts of the marrow-tissue within the Haversian spaces begin the for- 

 mation of new bone on the walls of the spaces. This process is continued 

 until, layer after layer, almost the entire space is filled with concentric 

 lamellae. The cavity remaining at the centre of the former space persists as 

 an Haversian canal, while the concentrically disposed layers of secondary 

 bone are the lamellae of the Haversian system, whose extent corresponds to 

 the form and size of the Haversian space. The interstitial or ground 

 lamellae of adult bone are the remains of the trabeculse of the primary sub- 

 periosteal spongy bone and are, evidently, genetically older than the Haver- 

 sian lamellae. The outer surface of the subperiosteal bone is beset with 

 depressions occupied by the primary marrow-tissue. As these pits deepen 

 in consequence of the increasing thickness of the growing bone, they are 

 converted into the nutrient channels which occupy the circumferential 

 and ground lamellae. They are, therefore, not surrounded by Haversian 

 layers and correspond to the 



:^ttb 



***> ' ' 



t'V'fV ' ;-,v;iij. 



' . ; . / . '- . . .'*3Lir*i1rm. 



Volkmann canals, through 

 which so many nutrient ves- 

 sels enter the bone. 



In tram embra nous 

 Bone -Development. 

 The bones not preceded by 

 masses of cartilage, as those 

 constituting the vault of the 

 cranium and the greater part 

 of the face, develop within 

 sheets of connective tissue 

 by a process which, although 

 differing in its earliest details, 

 essentially corresponds to 

 subperiosteal bone-forma- 

 tion. Except where develop- 

 ing muscle occurs, the early 

 roof of the skull consists of 

 the integument, the dura 

 mater and an intervening 

 stratum of young connective 

 tissue. The latter layer con- 

 tains numerous embryonal 

 cells and delicate bundles of fibres. 



Osteogenetic 

 tissue 



Blood-vessel 



Osteoblasts 



Young bone 



FIG. 62. Periphery of a developing membrane-bone (parietal), 

 showing trabeculae covered with osteoblasts. X 95- 



About the middle of the area corre- 

 sponding to the later bone, some of these fibrous strands undergo calcification 

 and thereby supply a radiating framework upon the surface of which the 

 osteoclasts, derived from the embryonal connective tissue cells, arrange 

 themselves and bring about the deposit of bone-matrix. Delicate spicule 

 of new bone radiate towards the periphery from the ossification centre 

 thus established. As the trabeculae increase in size and number, they join 

 to form a bony network, close and robust at the centre and wide meshed 

 and delicate towards the periphery where the osseous reticulum fades into 



