HISTOEY 



OF THE 



PRINCIPAL FUNCTIONS OF ANIMALS. 



I. OF THE FUNCTION OF NUTRITION. 



23. The nutrition of living beings consists in the intro- 

 duction of foreign matters into the interior of the tissues, and 

 the fixation, assimilation, and organization of the matters 

 so introduced. Every living animal is also the seat of a kind 

 of slow combustion, causing the unceasing destruction of a 

 certain quantity of organic matter. The matter thus de- 

 stroyed, being useless or even hurtful to the economy, is 

 expelled from it. 



It is evident, then, that the first condition necessary for 

 the due performance of this molecular composition and de- 

 composition, is the faculty of absorption, by which the 

 molecules are attracted and introduced into the centre of the 

 tissues. It is a function common to all living beings. 



24. In plants, this single faculty suffices for the intro- 

 duction from without of all matters requisite for their 

 nourishment. With animals, a portion only is directly intro- 

 duced into the tissues ; but a great portion requires being 

 elaborated by a process called digestion, by which the nutrient 

 molecules are fitted for absorption. This faculty of digestion 

 forms one of the characters which best distinguish animals 

 from plants. 



25. The liquids thus absorbed spread wherever they are 

 required, the distribution being in some effected slowly, in a 

 way analogous to the absorption. In others, by far the most 

 numerous, the distribution of the nutrient liquids is accom- 

 plished rapidly by the establishment of currents, which serve 

 also to remove the molecules eliminated from the organs. 



