NATURt OF LIVING BODIES. J 



\ In the second place, living bodies differ as to the mode of 

 their existence, insomuch as they are dependent upon other 

 things beside themselves for the continuance oi' that existence. 

 The matter of which they are composed is constantly chang- 

 ing. This matter is, in fact, only common matter endowed 

 for a certain period with the powers of life, in consequence of 

 being united to living systems. By the various internal ope- 

 rations constantly going on, part of this matter is expended, 

 is sent out of the system ; this loss must be repaired by the 

 addition of new matter. Hence the necessity of nourishment 

 to the support of life ; hence the necessity of a regular sup- 

 ply, to every thing living, of a certain quantity of food adapted 

 in kind to the nature of the individual. This food is ope- 

 rated upon by the organs of the animal or vegetable, is assimi- 

 lated to it, and its properties are modified until it becomes 

 fit to make a component part of it. ''This is nutrition, an es- 

 sential process of living bodies, by which they are enabled to 

 increase in size and strength, to modify the structure of their 

 different parts, and to maintain them in a fit state for perform- 

 ing the offices for which they are designed. Minerals, on the 

 contrary, have no such dependence ; the matter of which they 

 consist is always the same ; they contain within themselves 

 every thing which is essential to their existence, and have, 

 of course, no necessity for nutrition or growth. It is true 

 that these substances sometimes increase in size, as happens 

 with regard to stalactites, the deposition of crystals, and the 

 formation of alluvia. But there is this marked difference be- 

 tween all such instances of growth, and that of animals or 

 vegetables ; that, in the former case, it amounts to the mere 

 juxtaposition of similar particles, unchanged in their nature ; 

 whilst in the latter the particles are changed in their nature, 

 and subjected to the operation of entirely new laws. In the 

 former case, the growth depends upon a principle operating 

 from without ; in the latter, upon a principle operating from 

 within 



'But, in the third place, though dependent upon other sub- 

 stances in this way for the means of continuing their exist- 

 ence, living bodies possess, in another point of view, a kind of 

 independence upon all other matter. They are removed, by 

 the possession of the powers of life, in a certain degree, out of 

 tho influence of physical and chemical laws ; they contain 

 within themselves a principle by which they are enabled di- 

 rectly to resist the operation of those laws, which would other- 

 wise insure their speedy destruction. They depend upon 



