116 TENDENCY TO DECAY IN OROANISI'P STRUCTURES. 



stances, of which vegetable and animal structures are made up, 

 that they have a tendency to separate themselves into their ele- 

 ments, under the ordinary circumstances of warmth, moisture, 

 c., to which mineral bodies may be exposed for centuries with- 

 out change. This tendency to separation it is, which causes the 

 decay of animal and vegetable substances after death ; for the 

 elements that were previously combined, in ways which no 

 chemical processes can imitate, then pass off in simpler forms, of 

 which carbonic acid is one of the chief. 



161. Different parts of the Animal and Vegetable frame- 

 work display this tendency in varying degrees. Thus the bones 

 of an animal, and the heart-wood of a tree, may remain almost 

 unchanged for centuries, and thus exhibit nearly the same per- 

 manence as the limestone rock ; whilst the soft flesh of the 

 animal, and the pulpy portions of the plant, pass into decom- 

 position almost immediately upon the death of the being. This 

 decomposition, however, is chiefly remarkable after death ; only 

 because it is not then counteracted by the processes, which form 

 an essential part of the functions of life. The object of those 

 functions, is not only to provide for the growth of the structure, 

 and for the production of new individuals which shall continue 

 and extend the race ; but to maintain in constant perfection and 

 vigour the parts already formed. This is accomplished by the 

 removal of the portions, which have exhibited the slightest 

 tendency to decay ; and by the deposition of freshly-formed 

 substances, of a similar character, in their place. The particles 

 which are removed, are carried off in the blood of the animal or 

 the sap of the plant ; and are separated from this in part by the 

 process of respiration, which gets rid of the carbonic acid, and in 

 part by other means of a corresponding nature. 



1 62. The rapidity of these processes of deposition and removal, 

 in the several parts of the living body, bears a very close propor- 

 tion with the natural tendency to decay, which they respectively 

 manifest. Thus, the bones of an Animal are in general sparingly 

 supplied with blood, and seem to undergo little change, except as 

 the result of disease or injury ; but the supply of blood is greatly 

 increased, when any circumstances demand a new formation of 



