80 THE CHEMISTRY OF THE FAEM. 



The increase during the fattening stage of growth is 

 seen to be chiefly an increase in fat, eight to nine parts of 

 fat being laid on for one of nitrogenous matter. The 

 proportion of fat would be somewhat greater still in the 

 increase of highly fattened animals, as, for instance, of 

 pigs fed for bacon. 



The Processes of Nutrition. We have already seen 

 that the food of plants is of the simplest character. From 

 such simple substances as carbonic acid, nitric acid, water, 

 and a few salts, a plant is able to construct a great variety 

 of elaborate compounds. It accomplishes these surprising 

 transformations by a consumption of force (sunlight) 

 external to itself. An animal has no such constructive 

 power. The animal frame is built up of substances ex- 

 isting ready formed in the food, or produced by the 

 splitting up or partial combustion of some of the 

 food constituents in the body. The animal derives 

 no aid from external force. The temperature of the 

 animal (about 100 Fahr.) is maintained by heat gene- 

 rated within the body by the combustion of the materials 

 consumed as food. The force by which all the mechanical 

 work of the animal is performed is also derived from the 

 same source. The source of heat and force in the animal 

 is thus purely internal. 



It is evident from what has just been said that the 

 food of animals has duties to perform which are not de- 

 manded of the food of plants. In plants the food merely 

 provides the matter for building up the vegetable tissues. 

 In the animal, besides constructing tissue, the food has to 

 furnish the means of producing heat, and executing 

 mechanical work. 



