36 FEEDS AND FEEDING 



64. Disposal of body waste. In breaking up the food nutrients within 

 the body proper for the production of heat, and in the changes which 

 occur in building them into body tissues, carbon dioxid is evolved. 

 Most of this escapes into the capillaries and is carried in the blood by the 

 veins to the lungs, where it is eliminated in breathing, a portion, however, 

 escaping by way of the skin. Some of the marsh gas produced by fer- 

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

 thrown out by the lungs. 



Nearly all of the nitrogenous waste, representing the breaking down of 

 protein material in the body, is excreted in the urine thru the kidneys, 

 tho a trace is given off in the sweat and a more appreciable amount in 

 the feces. In mammals this waste takes the form principally of urea. 

 In calculating the total amount broken down in the body it is custom- 

 ary to determine the total nitrogen in the urine and multiply this by 

 6.25. This gives the amount of protein broken down, since it is assumed 

 that, on the average, nitrogen forms 16 per ct. of the total weight of 

 proteins. (17) 



A great variety of other end-products of metabolism are likewise 

 eliminated by the kidneys thru the urine. Most of the mineral matter, 

 such as common salt, is excreted in large part or principally in the urine. 

 However, calcium, magnesium, iron, and phosphorus are voided chiefly 

 in the feces. Small amounts of most of the substances eliminated in 

 the urine are also excreted by the skin thru the sweat glands. 



65. Summary. In Chapter 1 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 the substance of the plant. 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 undergoing more or less change, are conveyed 

 from the digestive tract to the body tissues and used for building the 

 body, for warming it, or in performing work. All the energy manifested 

 by living animals and the heat evolved in their bodies represent the 

 energy of the sun originally 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 be- 

 coming a part of the earth, air, and water as inert matter. After this 

 degradation 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 mysterious guiding prin- 

 ciple of life all play their parts. 



