32 FEEDS AND FEEDING, ABRIDGED 



from the food fats. The amino acids may be built up into body protein 

 or, if not needed for this purpose, a portion of their carbon, hydrogen, 

 and oxygen may be changed into fat, while the nitrogen is excreted 

 from the body. The highest use of the proteins, however, is the forma- 

 tion of nitrogenous tissues. 



Disposal of waste. — As we have seen, the undigested food, together 

 with some other w^aste material, is voided in the feces. Nearly all of 

 the nitrogenous waste resulting from the breaking down of protein in 

 the body is excreted in the urine thru the kidneys, tho a trace is given 

 off in the sweat and some in the feces. In mammals this waste takes 

 the form principally of urea. Some of the mineral matter, especially 

 calcium, magnesium, and phosphorus, is excreted in the feces. The 

 rest is voided in the urine. 



In breaking up the food nutrients within the body for the produc- 

 tion of heat and in the changes which occur in building them into body 

 tissues, carbon dioxid is evolved. Most of this is absorbed from the 

 stomach and intestines and is carried in the blood to the lungs, where 

 it is passed out in breathing. Some of the marsh gas produced by 

 fermentations in the stomach of herbivora is absorbed into the blood and 

 thrown out by the lungs. 



Summary. — In Chapter I we learned how the various inorganic 

 compounds taken by plants from earth, air, and water are built into 

 organic plant compounds, and how in such building the energy of the 

 sun becomes latent or hidden. In this chapter we have learned how 

 the animal, feeding on plants, separates the useful from the waste by 

 mastication and digestion, and how the digested nutrients, after under- 

 going more or less change, are carried from the alimentary canal to 

 the body tissues and used for building the body, for warming it, or in 

 perforaiing work. All the energy manifested by living animals and 

 the heat produced in their bodies represent the energy of the sun orig- 

 inally stored in food substances by plants. "With the breaking down of 

 the nutrients in the bodies of animals, and in the decay of the animal 

 substance itself, the organic matter loses the condition of life and falls 

 back to the inorganic condition, once more becoming a part of the earth, 

 air, and water. After this it is again gathered up by the plants and 

 once more starts on the upward path. Such is the eternal round of 

 Nature, in which plants, animals, the energy of the sun, and the mys- 

 terious guiding principle of life all play their parts. 



QUESTIONS 



1. State two fundamental differences in the composition of plants and animals. 



2. How does the composition of an animal's body change as it grows? As it 

 fattens ? 



