Laws of Nutrition 183 



constitutes the solid excrement or feces. The urea and 

 other nitrogenous compounds which are the unoxidized 

 portion of the protein, pass out wholly in the urine. All 

 digested nitrogen not stored is found here. The car- 

 bon dioxid is eliminated through the skin and lungs, 

 chiefly the latter, and water is disposed of through 

 the kidneys, skin and lungs. 



4. The digested food is used in two general direc- 

 tions, (a) for the protection of energy and (h) for 

 constructive purposes. 



(a) The food energy is made available through 

 combustion, i. e., the burning of the carbon compounds 

 of the food to simpler substances, carbon dioxid and 

 water, thus liberating the energy stored in the plant 

 during its growth. Protein is never fully oxidized, but 

 carbohydrates and fats may be. All the organic nu- 

 trients may be oxidized to produce energy, the total heat 

 values of protein, carboh^'drates and fats being approx- 

 imately as 1.5, 1, 2.4. This liberated energy finds ex- 

 pression in the animal organism in various ways, as heat, 

 mechanical energy or motion and chemical transforma- 

 tions. The total energy of food is never all available to 

 the animal because of a loss in the excreta and gases. 

 Moreover, the net energy gain seems not to be propor- 

 tional to the available energy, but is dependent upon the 

 work of digestion, which varies with different cattle foods. 



(&) The food compounds are used for constructive 

 purposes, either without changing their general char- 

 acter, as, for instance, the building of muscular tissue 

 from the plant albuminoids, or they may be reorgan- 

 ized into bodies of a very different character, as in the 



