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HANDBOOK OF PHYSIOLOGY. 



bony femur may however be divided for the sake of clearness into the 

 following six stages : 



Stage 1. Proliferation and Calcification. As ossification is 

 commencing the cartilage cells in and near the centre of ossification be- 

 come enlarged and proliferate, arranging themselves in rows correspond- 

 ing to the long axis of the bone (fig. 67). Lime salts are next deposited 

 in the form of fine granules in the hyaline matrix of the cartilage, and 

 this gradually becomes transformed into a number of calcified trabeculgp, 



FIG. 68. Transverse section of a portion of a metacarpal bone of a foetus, showing 1, fibrous 

 layer of periosteum ; 2, psteogenetic layer of ditto ; 3, periosteal bone ; 4, cartilage, with matrix 

 gradually becoming calcified, as at 5, with cells in primary areolae; beyond 5the calcified matrix is 

 being entirely replaced by spongy bone. X 200. (V. D. Harris.) 



(fig. 68, 5), inclosing alveolar spaces, which are the primary areolm, and 

 which contain cartilage cells. The cartilage cells, gradually enlarging, 

 become more transparent, and finally undergo disintegration. During 

 this stage the perichondrium has become the periosteum, and is be- 

 ginning to deposit bone on the outside of the cartilage. 



Stage 2. Vascularization of the Cartilage. Processes from 

 the osteogenetic or cellular layer of the periosteum containing blood- 

 vessels break into the substance of the cartilage and grow much as ivy 

 insinuates itself into the cracks and crevices of a wall. This begins ai, 

 the "centres of ossification," from which the blood-vessels spread chiefly 



