268 THE MECHANICAL FUNCTIONS. 



The form which had at first been rudely sketched, slow- 

 ly advances towards perfection in the course of its 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 si- 

 milar internal cavities, but being frequently excavated in parts 

 which had before been solid. During all these gradual altera- 

 tions of shape, however, there is no stretching of elastic parts; 

 for all the osseous fibres and laminse are rigid and unyield- 

 ing, 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 occupied the situations 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 cavities, but the 

 whole is a uniform solid mass. At a certain stage of ossifi- 

 cation cells are excavated by the action of the absorbent ves- 

 sels, 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 oily matter, which is the mar- 

 row, as the growth proceeds, while new layers are deposited 

 on the outside of the bone, and at the ends of the long fibres, 

 the internal layers near the centre are removed by the ab- 

 sorbent vessels, so that the cavity is farther enlarged. In 

 this manner the outermost layer of the young bone gradual- 

 ly changes its relative situation, becoming more and more 

 deeply buried by the new layers which are successfully de- 

 posited, and which cover and surround it; until by the re- 

 moval of all the layers situated nearer to the centre, it be- 

 comes the innermost layer; and is itself destined in its turn 

 to disappear, leaving the new bone without a single particle 

 which had entered into the composition of the original struc- 

 ture. 



It has been found that by mixing certain colouring sub- 



• 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 

 development. 



