36 HUMAN PHYSIOLOGY 



is to be produced, a minute model is formed in hyaline car- 

 tilage in the embryo, and this is surrounded by a fibrous 

 covering, the perichondrium. In the deepest layers of this 

 perichondrium 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 peri- 

 osteum, and showing that bone was deposited outside of it. 







Flo. 13. Intra-cartilaginous Bone Development. 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 by the invasion of periosteum ; and the calcification of the carti- 

 lage round the .spaces. 



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- 

 cularisation 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 trabeculse 



