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 fiat 

 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. 



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 we 

 find the skeleton to present in the earlier life of the higher 

 animals. The long bones of Eeptiles (with one remarkable 

 exception in the Pterodactylus, 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, being, 

 in the eye of the philosophical anatomist, a separate bone. 

 Further, there is a large group of Fishes in which the 

 skeleton retains the cartilaginous character through life; a 

 certain quantity of mineral matter being deposited in the 



