66 DEVELOPMENT OF BONE: OSSIFICATION. 
extremities. Long after the ossification of the shaft and of 
the extremities has been completed, these parts remain sepa- 
rated from each other by the interposition of a thin layer of 
unconsolidated cartilage ; so that, although the bone appears 
firm and complete, its three portions fall apart, if it be 
macerated sufficiently long in water for the cartilage to 
decay. Now it is by the progressive consolidation of the 
cartilage at these two junctions, and by the continual forma- 
tion of new cartilage as the old is taken into the bone, that 
the length of the shaft continues to increase up to adult 
age ; and then, its full size having been attained, the whole 
thickness of the intervening layer of cartilage is replaced by 
bone, so that the shaft and extremities become firmly con- 
solidated.—The general history of the formation of the flat 
bones is nearly the same. In these, when they are large, or 
have projecting out-growths, there are several centres of ossi- 
fication ; and although the first ossification takes place in the 
substance of cartilage, yet the subsequent growth seems to 
be effected mainly by the consolidation of fibrous mem- 
brane. é 
53. The foregoing description applies chiefly to those 
higher and more complete forms of Bone, which are found in 
Birds and Mammals. In Reptiles and Fishes, the process of 
ossification is stopped short, as it were, at an early period; 
and thus the texture of their bones resembles that which w 
find the skeleton to present in the earlier life of the highe 
animals.—The long bones of Reptiles (with one remarkable 
exception in the Pterodact ylus, TS 669, which is adapted to 
the life of a Bird) have no one central cavity, but are pene- 
trated by numerous large Haversian canals, like those of very 
young bone ; and various pieces remain separate in them 
throughout life, which, originating in distinct centres of ossi- 
fication, subsequently coalesce in Birds and Mammals. This: 
permanent separation is still more remarkable in the bones, 
of Fishes ; and it is consequently in them that we can best 
study the real composition of the skeleton,—every piece 
which originates in a distinct centre of ossification, bei 
in the eye of the philosophical anatomist, a separate bone, 
Further, there is a large group of Fishes in which th 
skeleton retains the cartilaginous character through life ; ¢ 
certain quantity of mineral matter being deposited in 
