CHAPTER XXVIII. 

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The Problems of Animal Nutrition. We have in pre- 

 ceding chapters traced certain materials, consisting of foods 

 more or less changed by digestion, into the Body from the 

 alimentary canal, and oxygen into it from the lungs. We 

 have also detected the elements thus taken into the Body 

 in their passage out of it again by lungs, kidneys and skin; 

 and found that for the most part their chemical state was 

 different from that in which they entered; the difference 

 being expressible in general terms by saying that more 

 oxidized forms of matter leave the Body than enter it. 

 We have now to consider what happens to each food during 

 the journey through the Body: is it changed at all? is it 

 oxidized? if so where? what products does its oxidation 

 give rise to? Is the oxidation direct and complete at once 

 or does it occur in successive steps? Has the food been 

 used first to make part of a living tissue and is this then 

 oxidized; or has it been oxidized Avithout forming part of a 

 living tissue? if so, where? in the blood stream or outside 

 of it? Finally, if the chemical changes undergone in 

 the Body are such as to liberate energy, how has this energy 

 been utilized? to maintain the. temperature of the Body or 

 to give rise to muscular work, or for other purposes? This 

 is a long string of questions, the answers to many of which 

 Physiology has still to seek. 



The Seat of the Oxidations of the Body. According 

 to older views oxidation mainly took place in the blood 

 while flowing through the lungs. Those organs were con- 

 sidered a sort of furnace in which heat was liberated by 

 blood oxidation, and then distributed by the circulation. 



