THE FUNCTION OF RESPIRATION. 591 



of lime, phosphate of magnesia, common salt, and other saline com- 

 pounds. These same compounds exist, ready formed, in the vegetable 

 food, associated generally with the gluten, the albumen, or the casein, 

 it contains. The materials of the harder parts of the body, therefore — 

 (the phosphates) as well as the inorganic saline substances which are 

 found in the blood, and in the other fluids of the body — are all formed in 

 or by the plant, or are by it extracted fiOm the soil and incorporated with 

 the food on which the animal is to live. 



Not only, therefore, do the mere elements of which the parts of the 

 bodies of animals are formed, exist in the food — but they occur in it, put 

 together and combined, nearly in the state in which they are wanted, in 

 order to form the several solids and fluids of the body. The plant, in 

 short, is the compounder of the raw materials of living bodies. The ani- 

 mal uses up these raw materials— cutting them into shape when neces- 

 sary, and fitting them to the several places into which they are intended 

 to be built. 



This is a very simple, and yet a very beautiful view of one of the 

 many forms of chemical connection which exist between the processes 

 and purposes of animal and vegetable life. Nature seems to divide the 

 burden of building up living bodies between the vegetable and the animal 

 kingdoms — the lower appearing to exist and to labour only for the good 

 of the higher race of beings. 



§ 3. Of the respiration of animals, and of the purposes served hy the starchy 

 guirij and sugar, contained in vegetable food. 



But, besides the gluten of plants and seeds, which supplies the mate- 

 rials from which the muscular parts of animals are formed, the oil which 

 is converted into the fat of animals, and the saline and earthy matters 

 of plants which supply the salts of the blood and the earth of the bones — 

 vegetable food in general contains a large proportion of starch, sugar, 

 gum, and other substances which consist of carbon and the elements of 

 water only (p. 111). What purpose is served by this part of the food? 

 Is it merely taken into the stomach and again rejected, or is it decom- 

 posed and made to serve some vital purpose in the economy of the 

 living animal ? From the fact that so large a part of all vegetable food 

 consists of these substances, we might infer that they were destined to 

 .serve some important purpose in the animal economy. To the herbiv- 

 orous animal they are, in fact, almost necessary for the support of a 

 healthy life. 



In order to understand this fact, it will be necessary briefly to advert to 

 the respiration of animals — the chemical changes produced by it, and 

 the purposes it is su])posed to serve in the animal economy. 



1°. Of the function of respiration. — All animg.ls possessed of lungs al- 

 ternately inhale and exhale the atmospheric air. They breathe, that is, 

 or respire. The air they draw into their lungs, supposing it to be dry, 

 consisfjs by volume (pp. 32 aoi 148) very nearly of — 



Nitrogen 79-16 



Oxygen 20-80 



Carbonic acK-. 0-04 



100 



