ORGANIC CHEMISTRY. 9 



perties, are in their turn discarded and replaced 

 by others. Hence the continued interchange 

 and renewal of particles which take place in 

 the more active organs of the system, especially 

 in the higher classes of animals. In the fabric 

 of those animals which possess an extensive 

 system of circulating and absorbing vessels, the 

 changes that are effected are so considerable and 

 so rapid, that even in the densest textures, such 

 as the bones, scarcely any portion of the sub- 

 stance which originally composed them is per- 

 manently retained in their structure. To so 

 great an extent is this renovation of materials 

 carried on in the human system, that doubts 

 may very reasonably be entertained as to the 

 identity of any portion of the body after the 

 lapse of a certain time. The period assigned by 

 the ancients for this entire change of the sub- 

 stance of the body was seven or eight years : but 

 modern inquiries, which show us the rapid re- 

 paration that takes place in injured parts, and 

 the quick renewal of the bones themselves, tend 

 to prove that even a shorter time than this is 

 adequate to the complete renovation of every 

 portion of the living fabric* 



Imperfect as is our knowledge of organic 

 chemistrj^ we see enough to convince us that a 



* See the article *'Age" in the Cyclopaedia of Practical 

 Medicine, where I have enlarged upon this subject. 



