CHAPTER XXIX. 



NUTRITION. 



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 differ- 

 ent 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 go into 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 that then oxidized; or has it been 

 oxidized without 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 mus- 

 cular 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 elder views oxidation mainly took place in the blood while 

 flowing through the lungs. Those organs were considered a 

 sort of furnace in which heat was liberated by blood oxidation, 

 and then distributed by the circulation. But if this were so 

 the lungs ought to be the hottest parts of the Body, and the 

 blood leaving them by the pulmonary veins much hotter than 

 that brought to them by the pulmonary artery after it had 

 been cooled by warming all the tissues; and neither of these 



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