112 ANIMALS AND THEIR PRODUCTS. 



same substances that would pass up the chimney, if 

 the corn were burnt on a hearth. And as heat would 

 be produced by the latter operation, and diffused 

 through the room, so animal heat is produced and 

 diffused through the system by the former. Starch 

 undergoes the same changes in the lungs of animals 

 as when thrown upon burning coals — in both cases 

 it is converted into carbonic acid (CO''), and watery 

 vapor (HO) ; in both cases these are sent afloat in the 

 surrounding air ; and in both cases the same heating 

 effects are exhibited. 



199. The lungs may not inaptly be compared to a 

 stove constantly burning ; for as the stove converts 

 fuel into carbonic acid and water, and thereby gener- 

 ates heat, so the lungs convert starch, sugar, or gum, 

 which may be regarded as the fuel of the animal sys- 

 tem, into the same substances, and thereby generate 

 heat and diffuse it through the system. 



200. This explains why animals should be kept 

 warm in order to grow ; for if they are exposed to 

 severe cold, it takes a large portion of their food to 

 k^ep the lungs in sufficiently active operation to pre- 

 vent their freezing. Little is left to supply the natural 

 waste of the body, and perhaps none at all to furnish 

 material for new growth. It explains also why work- 

 ing animals require more food than others. In conse- 

 quence of exercise they breathe more, and a larger 

 proportion of their food is exhaled from the lungs. If 

 you were to give a young, thrifty ox, weighing 1,000 

 lbs., live weight, 25 lbs. of good hay daily, and to keep 



