The Animal Body — Digestion — Metabolism. 37 



up and a portion of their carbon, hydrogen, and oxygen turned into 

 fat, while the nitrogen is wasted as urea. The highest and most 

 general use of the proteins, however, is the formation of nitroge- 

 nous tissues— the muscles, nerves, skin, hair, and various organs of 

 the body. 



56. 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 elimi- 

 nated in breathing, a portion, however, escaping by way of the 

 skin. Some of the marsh gas produced by fermentations 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 appre- 

 ciable amount in the feces. In mammals this waste takes the form 

 principally of urea. In calculating the total amount of protein 

 metabolism it is customary 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 M^eight of the protein molecule. 



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

 eliminated by the kidneys thru the urine. The inorganic salts, such 

 as common salt, also escape from the body principally in the urine. 

 Small amounts of most of the substances eliminated in the urine 

 are also excreted by the skin thru the sweat glands. A consider- 

 able portion of certain inorganic salts containing calcium, magne- 

 sium, and phosphorus are eliminated by way of the intestines. 



57. Summary. — In Chapter I we learned how the various inor- 

 ganic 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 alimentary tract to the body 

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

 forming work. All the energy manifested by living animals and 

 the heat evolved in their bodies represent the energy of the sun 



