46 VETERINARY PHYSIOLOGY 



not known, and how far it is dependent on the action of 

 cells has not been clearly determined ; but in front of the 

 process, as it shoots outwards from the centre in all direc- 

 tions, accumulations of cells are to be seen, and these cells 

 have been called osteoblasts. The cells get enclosed in 

 definite spaces, lacunae, and become hone cells. Narrow 

 branching channels of communication are left between these 

 lacunas, the canaliculi. 



The fully formed adult bone, however, is not a solid 

 block, but is composed of a compact tissue outside, and of a 

 spongy bony tissue, cancellous tissue, inside. This cancellous 

 tissue is formed as a secondary process. Into the block of 

 calcareous matter, formed as above described, processes of 

 the surrounding fibrous tissue burrow, carrying in blood- 

 vessels, lymphatics, and numerous cells (fig. 13, lower surface). 

 This burrowing process seems to be carried on by the cells, 

 which eat up the bony matter formed. In doing this they 

 frequently change their appearance, becoming large and 

 multi-nucleated (osteoclasts). Thus the centre of the bone is 

 eaten out into a series of channels, in which the marrow of 

 the bone is loJged, and between which narrow bridges of 

 bone remain. 



It is by the extension of the calcifying process outwards, 

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

 the dense diploe and spongy cancellous tissue are produced. 



B. Intra-cartilaginous Bone Development. — In the bones 

 preformed in cartilage, the process is somewhat more complex. 

 But all the bone is developed in connection with fibrous 

 tissue, and the cartilage merely plays the part of a scatiblding 

 and is all removed. 



Where the adult bone is to be produced, a minute model 

 is formed in hyaline cartilage in the embryo, and this is 

 surrounded by a fibrous covering, the 'peTichondrium. In 

 the deepest layers of this perichondrium the process of calcifi- 

 cation takes place as described above, and spreads outwards, 

 thus encasing the cartilage in an ever-thickening layer of 

 bone (fig. 14, a). 



At the same time, in the centre of the cartilage, at what is 

 called the centre of ossiiication, the cells begin to divide 



