50 



HANDBOOK OF PHYSIOLOGY. 



new-formed bone. On the bony trabeculae first formed, fresh layers of 

 cells (osteoblasts) from the osteogenetic layer are developed side by side, 

 lining the irregular spaces like an epithelium (Fig. 57, #). Lime-salts 

 are deposited in the circumferential part of each osteoblast, and thus a 

 ring of osteoblasts give rise to a ring of bone with the remaining uncal- 

 cified portions of the osteoblasts imbedded iu it as bone corpuscles, as in 

 the first formation. 



Thus, the primitive spongy bone is formed, whose irregular branch- 

 ing spaces are occupied by processes from the osteogenetic layer of the 

 periosteum with numerous blood-vessels and osteoblasts. Portions of 

 this primitive spongy bone are re-absorbed ; the osteoblasts being ar- 





I - ' ^ - -^r ' ^B'U ' ; F*. ^ ^ 



~~ ' 



FIG. 57. 





FIG. 58. 



FIG. 57. Osteoblasts from the parietal bone of a human embryo, thirteen weeks old 

 septa with the cells of the lacunae ; 6, layers of osteoblasts ; c, the latter 



a, bony 



in transition to bone cor- 

 puscles. Highly magnified. (Gegenbaur.) 



FIG. 58. From a transverse section through part of the f ratal jaw near the extreme periosteum 

 in the state of spongy bone, p, fibrous layer of periosteum , 6, osteogenetic layer of periosteum ; 

 o, osteoblasts; c, osseous substance, containing many bone corpuscles. X 300. (Schofield.) 



ranged in concentric successive layers and thus giving rise to concentric 

 Haversian lamellae of bone, until the irregular space in the centre is 

 reduced to a well-formed Haversian canal, the portions of the primitive 

 spongy bone between the Haversian systems remaining as interstitial, or 

 ground lamellae (p. 48). The bulk of the primitive spongy bone is thus 

 gradually converted into compact bony tissue with Haversian canals. 

 Those portions of the in-growths from the deeper layer of the periosteum 



