OSSIFICATION. 341 



(lifter very materially from that by which a shell is 

 produced ; for a shell, as we have seen, is the result 

 of successive depositions of calcareous matter, form- 

 ing one layer after another, in union with a corre- 

 sponding deposit of animal membrane. But the 

 subsequent changes which occur show that the con- 

 stitution of bone is totally dissimilar to that of shell ; 

 for no portion of the shell that is once formed, and 

 has not been removed, is subject to any further 

 alteration. It is a dead, though perhaps not wholly 

 inorganic mass ; appended, indeed, to the living 

 system, but placed beyond the sphere of its influ- 

 ence. But a bone continues, during the whole of 

 life, to be an integrant part of the system, partaking 

 of its changes, moditied by its powers, and under- 

 going continual alterations of shape, and even re- 

 newals 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 growth ; and the general proportions 

 of the parts are still preserved ; the finished bone 

 exhibiting prominences and depressions 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, how- 

 ever, there is no stretching of elastic parts ; for all 

 the osseous fibres and laminae are rigid and un- 

 yielding, 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 re- 

 moval of such parts of the young bone as had 

 occupied the situations where vacuities are found to 



