48 HAND-BOOK OF PHYSIOLOGY. 



starting from a "centre of ossification," and shooting out in all directions 

 toward the periphery; while the bone increases in thickness by the depo- 

 sition of successive layers beneath the periosteum. The finely fibrillar 

 network of the deeper or osteogenetic layer of the periosteum becomes, 

 transformed into bone-matrix (the minute structure of which has been 

 already (p. 46) described as reticular), and its cells into bone-corpuscles. 

 On the young bone trabeculaa thus formed, fresh layers of cells (osteo- 

 blasts) from the osteogenetic layer are developed side by side, lining the. 

 irregular spaces like an epithelium (Fig. 50, 1)). Lime-salts are deposited 

 in the circumferential part of each osteoblast, and thus a ring of osteo- 

 blasts gives rise to a ring of bone with the remaining uncalcified portions 

 of the osteoblasts imbedded in it as bone-corpuscles (Fig. 50). 



FIG. 50. Osteoblasts from the parietal bone of a human embryo, thirteen weeks old. a, bony 

 septa with the cells of the lacunae; b, layers of osteoblasts; c, the latter in transition to bone cor- 

 puscles. Highly magnified. , (Gegenbaur.) 



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 arranged 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. 46). The bulk of the primitive spongy bone is thus gradu- 

 ally converted into compact bony-tissue with Haversian canals. Those 

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

 are not converted into bone remain in the spaces of the cancellous tissue 

 as the red marrow. 



Ossification in Cartilage. Under this heading, taking the femur as 

 a typical example, we may consider the process by which the solid carti- 

 laginous rod which represents it in the foetus is converted into the hollow 

 cylinder of compact bone with expanded ends of cancellous tissue which 

 forms the adult femur; bearing in mind the fact that this foetal cartilag- 



