384 THE MECHANICAL FUNCTIONS. 



oily matter, which is 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 

 surround it; until by the removal of all the 

 layers situated nearer to the centre, it becomes 

 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 structure. 



It has been found that by mixing certain 

 colouring substances with the food of animals 

 the bones will soon become deeply tinged by 

 them. This fact was discovered accidentally by 

 Mr. Belchier, who gives the following account 

 of the circumstances that led him to notice it.* 

 Happening to be dining with a calico printer on 

 a leg of fresh pork, he was surprised to ob- 

 serve that the bones, instead of being white as 

 usual, were of a deep red colour ; and on inquir- 

 ing into the circumstances, he learned that the 



* Philosophical Transactions for 1736, vol. xxxix. 287 and 



289. 



