OSSIFICATION. 383 



tions of shape, and even renewals of its substance, 

 by the actions of the living vessels. 



The form which had at first been rudely 

 sketched, slowly advances towards perfection in 

 the course of its growth ; and the general pro- 

 portions of the parts are still preserved ; the 

 finished bone exhibiting prominences and de- 

 pressions in the same relative situation as at first ; 

 and not only having similar internal cavities, but 

 being frequently excavated in parts which had 

 before been solid. During all these gradual 

 alterations of shape, however, there is no stretch- 

 ing of elastic parts ; for all the osseous fibres 

 and laminae are rigid and unyielding, and in this 

 respect retain an analogy with shell. The 

 changes thus observed can have been effected in 

 no other way than by the actual removal of such 

 parts of the young bone as had occupid the situ- 

 ations where vacuities are found to exist in the 

 old bone. We find, for instance, that in the 

 early state of a bone there are no internal cavi- 

 ties, but the whole is a uniform solid mass. At 

 a certain stage of ossification cells are excavated 

 by the action of the absorbent vessels, which 

 carry away portions of bony matter lying in the 

 axis of the cylindrical, or in the middle layer of 

 the flat bones.* Their place is supplied by an 



* The bones of the lower classes of vertebrated animals, as of 

 Fishes and Reptiles, seldom reach this stage of ossification, but 

 remain solid throughout ; corresponding to the bones of the 

 higher classes at the early periods of their developenient. 



