DIGESTION AND USES OF FOOD. 169 



the organs that do this work, freeing the blood from these 

 refuse compounds and passing them off in the liquid excre- 

 ment or urine. This liquid excrement, then, is a solution of 

 material that comes from the blood, muscle, bone, etc., of the 

 body, and, therefore, we may conclude it will contain valuable 

 fertilizing material, more valuable as a rule than the solid ex- 

 crement. The liquid excrement consists of the dissolved 

 waste of the blood, muscle, bone, and other parts of the body ; 

 the solid excrement consists of the indigestible and undigested 

 portion of the food. None of the waste nitrogen or mineral 

 matter of the animal escapes from the body through the 

 lungs, but all passes off through the kidneys. Hence the 

 great importance of carefully saving, by litter or otherwise, all 

 the liquid excrement for use as a fertilizer. When we sell 

 grain, hay, straw, and roots, we take away from the soil of the 

 farm all the nitrogen and mineral matter which they contain, 

 we really sell part of the soil upon which these foods grew. 

 When we feed these to stock and sell the animals or their 

 products we sell but a small portion of these soil constituents ; 

 by far the larger portion is found in the solid and liquid ex- 

 crement. The economy of feeding stock upon the farm lies 

 then in the saving of all the excrement, especially the liquid, 

 and returning it to the soil upon which the plants originally 

 grew, and from which we wish to derive more food. 



CONCLUSIONS. The uses of food in the animal may now 

 be stated briefly as follows : 



1. To produce heat to keep the body warm. 



2. To produce force or energy to enable work to be done. 



3. To replace the waste from all parts of the body. 



4. To increase the body in bone, muscle, flesh and fat. 



5. To produce milk, wool, etc. 



Every animal must be kept warm. Every animal does some 

 work or uses up some energy even when standing still or lying 

 down ; all parts of the body are constantly wearing away and 

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