34 VETERINARY PHYSIOLOGY 



the process of calcification takes place, as described above, 

 and spreads outwards, thus encasing the cartilage in an ever- 

 thickening layer of bone (fig. 13). This was demonstrated 

 by inserting a silver plate under the periosteum, and showing 

 that bone was deposited outside of it. 



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

 is called the centre of ossification, the cells begin to divide 

 actively, and, instead of forming new cartilage, eat away 

 their capsules, and thus open out the cartilage spaces 

 (fig. 13). Into these spaces processes of the perichondrium 

 bore their way, carrying with them blood vessels, and thus 

 rendering the cartilage vascular (fig. 13). The vas- 

 cularisatidn of the centre of the cartilage having been 

 effected, the process of absorption extends towards the 

 two ends of the shaft of cartilage, which continues to 

 elongate. The cartilage cells divide and again divide, and, 

 by absorbing the material between them, form long irregular 

 canals running in the long axis of the bone, with trabeculae 

 of cartilage between them. Into these canals the processes 

 of the periosteum extend, and fill them with its fibrous 

 tissue. A deposition of lime salts takes place upon the 

 trabeculse, enclosing cells of the invading fibrous tissue, and 

 thus forming a crust of bone, while the cartilage also becomes 

 calcified. If this calcification of the cartilage and deposition 

 of bone were to go on unchecked, the block of cartilage 

 would soon be converted to a solid mass of calcified tissue. 

 But this does not occur. For, as rapidly as the trabeculse 

 become calcified, they are absorbed, while the active changes 

 extend farther and farther from the centre to the ends of 

 the shaft. The centre is thus reduced to a space filled by 

 fibrous tissue which afterwards becomes the bone marrow. 



The process of absorption does not stop at the original block 

 of cartilage; but after all of this has been absorbed, the bone 

 formed round the cartilage (the periosteal bone) is attacked 

 by burrowing processes from inside and outside, which hollow 

 out long channels running in the long axis of the bone. 

 These are the Haversian spaces (fig. 14). Round the inside 

 of these, calcification occurs, spreading inwards in layers, and 

 enclosing connective tissue cells, until, at length, only a 

 small canal is left, an Haversian canal, containing some 



