64 COMPOSITION AND DEVELOPMENT OF BONE. 



it corresponds closely with the "white fibrous tissue ( 23), both 

 in structure and composition ; and so far from this view of its 

 nature being inconsistent with the history of the formation of 

 bone, it will be found to be in entire harmony with it. The 

 proportion which the mineral bears to the animal substance of 

 bone is very constant, when the proper "osseous tissue alone is 

 taken into account ; being almost exactly two of the former to 

 one of the latter, or 66f per cent, to 33|- per cent. But when 

 the composition of entire bones, including the contents of the 

 Haversian canals and cancelli, is compared, the proportion of 

 mineral to animal matter is found to vary greatly in different 

 classes of animals, in the same animal at different ages, and 

 even in different bones of the same individual ; the mineral 

 matter predominating in bones of a compact texture, and the 

 animal in those whose substance is more spongy. 



52. In the first development of the embryo, a sort of mould 

 of cartilage is laid down for the greater part of the bones; 

 though, in the case of the fiat bones, this mould is generally 

 limited to the central portion, the place of their marginal part 

 being occupied by a fibrous membrane only. The process of 

 ossification, or bone-formation, commences with the deposit 

 of calcareous matter in the intercellular substance of the 

 cartilage, so as to form a sort of network, in the interspaces 

 of which are seen the remains of the cartilage-cells. The 

 tissue thus formed can scarcely be considered as true bone, 

 for it contains neither lacunce nor canaliculi. Before 

 long, however, it undergoes very important changes; for 

 many of the partitions are removed, so that the minute 

 chambers which they separated coalesce into larger ones ; and 

 thus are formed the cancelli of the spongy substance, and the 

 Haversian canals of the more compact. These are at first 

 much larger than they are subsequently to become ; for they 

 are gradually narrowed by deposits of true bony tissue, 

 which successively take place upon their interior walls, at the 

 expense of the materials supplied by the blood brought 

 thither by their contained vessels ; and it is by this forma- 

 tion of concentric layers around the cavities of the Haversian 

 canals, that the appearance of concentric rings is produced, 

 which we have just seen to be presented by transverse sec- 

 tions of long bones. In old bones the Haversian canals are 

 so nearly filled by these deposits, that there is barely room 



