62 HANDBOOK OF PHYSIOLOGY. 



uncalcified portions of the osteoblasts imbedded in it as bone corpuscles, 

 as in the first formation; then the central portion of the bony plate 

 becomes harder and less cancellous. At the same time, the plate in- 

 creases at the periphery not only by the extension of the bony spiculae, 

 but also by deposits taking place from the osteogenetic layer of the 

 periosteum. 



The primitive spongy bone is formed, and its irregular branching 

 spaces are occupied by processes from the osteogenetic layer of the peri- 

 osteum consisting of numerous blood-vessels and osteoblasts. Portions 

 of this primitive spongy bone are re-absorbed. The osteoblasts are 

 arranged in concentric successive layers and give rise to concentric 

 Haversian lamella? of bone, while the irregular space in the centre is 

 reduced to a well-formed Haversian canal, containing the usual blood- 

 vessels, the portions of the primitive spongy bone between the Haversian 



Fig. 65. Osteoblasts from the parietal bone of a human embryo, thirteen weeks old. a, Bony 

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

 puscles. Highly magnified. (Gegenbaur.) 



systems remaining as interstitial or ground-lamellae (p. 59). The bulk 

 of the primitive spongy bone is thus gradually converted into compact 

 bony-tissue of Haversian systems. Those portions of the ingrowths 

 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 

 cartilaginous rod which represents the bone in the foetus is converted 

 into the hollow cylinder of compact bone with expanded ends formed 

 of cancellous tissue of which the adult femur is made up. We must 

 bear in mind the fact that this fcetal cartilaginous femur is many times 

 smaller than the medullary cavity even of the shaft of the mature bone, 

 and, therefore, that not a trace of the original cartilage can be present 

 in the femur of the adult. Its purpose is indeed purely temporary; and, 

 after its calcification, it is gradually and entirely absorbed as will be 

 presently explained. 



