108 WALL'S MANUAL 



matters, and they undergo, during circulation, the 

 changes which have been described, that is, combus- 

 tion and decay. Their hydrogen and oxygen unite, 

 and form the moisture of the breath, while their 

 carbon is combined with the oxygen of the air, 

 breathed into the lungs, and is thrown out as carbonic 

 acid gas. The same consequence heat-^YGSultB in 

 this, as in the other cases, and this heat is produced 

 with sufficient rapidity for the necessities of the 

 animal. When he exercises violently, his blood 

 circulates with increased rapidity, thus carrying 

 carbon more rapidly to the lungs. The breath also 

 becomes quicker, thus supplying increased quantities 

 of oxygen. In this way, decomposition becomes more 

 rapid, and the animal is heated in proportion. 



Thus, \ve see food has another office, besides that 

 of forming animal matter, namely, to supply heat 

 When food does not contain a sufficient quantity of 

 starch, sugar, etc., to answer the demands of the 

 system, the animal's own fat is carried to the lungs, 

 und then used in the production of hcuL 



CHAPTER XXI, 



THE VALUE OP CROPS AS FOOD, 



The ingredients of plants which serve valuable 

 purposes as food, are starch, sugar, gum, protein mat- 

 ter, oil, woody fibre, water, and salts. Starch is the 

 most abundant element in grain crops, forming about 

 one-half the weight of the most common cereal grains ; 



