INTRAMEMBRANOUS BONE. 



lOI 



sian spaces institute a secondary formative process, by which new bone is deposited 

 on the walls of the Haversian spaces. This process is continued until, layer after 

 layer, almost the entire Haversian space is filled with the resulting concentrically 

 disposed osseous lamellae ; the cavity remaining at the centre of the new bone per- 

 sists as the Haversian canal, while the concentrically arranged layers are the lamellae 

 of the Haversian system, the extent of the latter corresponding to the form and size 

 of the Haversian space in which the secondary deposit of bone occurs. It is evident 

 from the development of the compact substance that the interstitial or ground- 

 lamellae of the adult tissue correspond to the remains of the trabeculae of the primary 

 spongy bone ; these lamellae are, therefore, genetically older than those constituting 

 the Haversian systems. The details of the formation of the Haversian lamellae, in- 

 cluding the deposition of the matrix and the inclusion of the osteoblasts to form the 

 bone-cells, are identical with those of the production of the trabeculae of the earlier 

 bone. 



Intramembranous Bone. — The development of certain bones, as those con- 

 stituting the vault of the skull and the greater part of the skeleton of the face, 

 differs in its earliest details from that of the subperiosteal bone, although the essen- 

 tial features of the processes are identi- 



FiG. 132 





Parietal bone of human fcetus of three months, 

 trabecular net-work of intramembranous bone. 



showing 



X5- 



cal. The mode by which these mem 

 brane-bones are formed may claim, 

 therefore, a brief consideration. 



The early roof of the skull consists, 

 except where developing muscle occurs, 

 only of the integument, the dura mater, 

 and an intervening connective-tissue 

 layer in which the membranous bones 

 are formed. The earliest evidences of 

 ossification usually appear about the 

 middle of the area corresponding to the 

 later bone, delicate spicules of the new 

 bone radiating from the ossific centre 

 towards the periphery. As the tra- 

 beculae increase in size and number they 

 join to form a bony net-work (Fig. 

 132), close and robust at the centre and 

 wide-meshed and delicate towards the 

 margin where the reticulum fades into 

 the connective tissue. With the con- 

 tinued growth of the bony tissue the 



net-work becomes more and more compact until it forms an osseous plate, which 

 gradually expands towards the limits of the area devoted to the future bone. For a 

 time, however, until the completion of the earliest growth, the young bones are 

 separated from their neighbors by an intervening tract of unossitied connective tissue. 

 Subsequent to the earlier stages of the formation of the tabular bones, the continued 

 growth takes place beneath the periosteum in the manner already described for 

 other bones. 



On examining microscopically the connective tissue in which the formation of 

 membrane-bone has begun, this layer is seen to contain numerous osteogenetic fibres 

 around and upon which are grouped many irregularly oval or stellate cells ; the latter 

 correspond to the osteoblasts in other locations, since through the agency of these 

 elements the osseous matrix is deposited upon the fibres. As the stratum of bony 

 material increases some of the cells are enclosed to form the future bone-cor- 

 puscles. Although the osteogenetic fibres correspond to delicate bundles of fibrous 

 tissue, they are stiffer, straighter, and present less indication of fibrillar structure. 

 Since the fibres forming the ends of the bony spicules generally spread out, they fre- 

 quently unite and interlace with the fibres of adjacent spicules, thus early suggesting 

 the production of the bony net-work which later appears. 



Gro\A7th of Bone. — It is evident, since the new bone is deposited beneath the 

 periosteum, that the growth of the subperiosteal bone results in an increased diame- 



