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 66% per cent. to 334 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 flat 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 lacune 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 issue, 
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 
