146 FIRST PRINCIPLES OF AGRICULTURE. 



Respiration or Breathing. — The air taken into the 

 lungs consists chiefly of oxygen and nitrogen. When it 

 passes out it has lost about one-quarter of its oxygen, 

 and also contains a large amount of carbonic acid ; the 

 oxygen has been absorbed by the blood, and the carbonic 

 acid carried out is the product of the oxidation or burn- 

 ing of old tissue. By this oxidation the heat of the 

 body is maintained, and kept in a healthy condition. 



Excretion. — The products of oxidation of the animal 

 tissues are carbonic acid, water, urea, and mineral salts. 

 The carbonic acid, as already seen, is chiefly removed by 

 the lungs, and to some extent by the skin; the urea and 

 salts are removed by the kidneys, and the water by all 

 of the organs of excretion. 



What is Food ? — It is thus seen that, in the processes 

 of life the substances contained in the food are changed 

 into animal product, and that this change is always 

 accompanied by loss. Any material capable of replacing 

 this loss, in whole or in part, is called a "fodder" or a 

 "feed;" and any single compound, like albumen or fat, 

 is called a "nutrient." The first essential in a feed is, 

 then, nutritious compounds ; these must, however, be pal- 

 atable, that is, capable of being eaten, and must possess 

 a certain bulk, in order to properly distend the stomach 

 and supply the needs of the animal in this respect, and 

 must be capable of, at least, partial digestion. 



Fodders and Feeds. — Common usage has divided 

 animal foods into two classes, coarse fodders and con- 

 centrated fodders, or fodders and feeds. By fodders are 

 commonly understood those products whose chief charac- 

 teristic is bulk; hay, cornstalks, and straw, belong to 



