THE TISSUES 



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tissue outside, and of a spongy bony tissue, cancellous tisme, in- 

 side. This cancellous tissue is formed as a secondary process. 

 Into the block of calcareous matter, formed as above described, 

 processes of the fibrous tissue burrow, carrying in blood vessels, 

 lymphatics, and numerous cells. This burrowing process seems 

 to be carried on by the connective tissue cells, which eat up 

 the bony matter formed. In doing this they frequently 

 change their appearance, becoming large and multi-nucleated 

 (osleoclasts). Thus the centre of the bone is eaten out into 

 a series of channels, in which the marrow of the bone is 

 lodged, and between which narrow bridges of bone remain. 



FIG. 13. Intra-cartilaginous Bone Developmert. A phalanx of a foetal finger 

 showing the formation of periosteal bone round the shaft ; the opening 

 up of the cartilage at the centre of ossification ; the vascularisation of 

 the cartilage hy the invasion of periosteum ; and the calcification of the 

 cartilage round the spaces. 



It is by the extension of the calcifying process outwards, 

 and the burrowing out of the central part of the bone, that 

 the diploe and cancellous tissue are produced. 



Intra-cartilaginous Bone Development. In the bones pre- 

 formed in cartilage, the process is somewhat more complex, 

 although all the bone is formed in connection with fibrous 

 tissue, the cartilage merely playing the part of a scaffolding 

 and being all removed. Where the adult bone is to be pro- 

 duced, a minute model is formed in hyaline cartilage in the 

 embryo, and this is surrounded by a fibrous covering, the 

 perichondrium. In the deepest layers of this perichondrium 



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