342 THE MECHANICAL FUNCTIONS. 



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 may be considered as a 

 uniform solid mass.* At a certain stage of ossifi- 

 cation 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, t Their place 

 is supplied by an oily matter, which constitutes the 

 marrow. 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 absorbent vessels, so 

 that the cavity is farther enlarged. In this manner 

 the outermost layer of the young bone gradually 

 changes its relative situation, becoming more and 

 more deeply buried by the new layers which are 

 successively deposited, and which cover and sur- 

 round it; until by the removal of all the layers 

 situated nearer to the centre, it becomes the inner- 

 most 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 structure. 



It has been found that by mixing certain colour- 

 ing substances with the food of animals the bones 

 will soon become deeply tinged by them. This 

 fact was discovered accidentally by Mr. Belchier, 



* In this general statement I do not take into account the canals 

 of Havers, formerly described, as they are so minute as to be disco- 

 verable only by the aid of the microscope : but they exist not only 

 in bones, but also in the cartilages which precede ossitication. 



t The bones of the lower classes of vetebrated 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 deveJopement. 



