SEC. 3. ON NUTRITION IN GENERAL. 



434. It may now be profitable to take a brief survey of 

 the various conclusions at which we have arrived concerning 

 the problems of nutrition. 



We have seen that the several tissues, using lymph as a 

 medium, live upon the blood, taking up from the blood the mate- 

 rials for, and returning to the blood the products of, their meta- 

 bolism. The blood itself we have also seen to be replenished 

 with food from the alimentary canal and with oxygen from the 

 lungs, and to be freed from waste products by means of the 

 excretory organs. In this double action the raw material of 

 the food on the one hand undergoes, between its being placed 

 in the mouth and its taking part in the metabolism of the tissue 

 which ultimately uses it, many intermediate changes carried 

 on in various parts of the body, and the waste products simi- 

 larly undergo intermediate changes between leaving the tissue 

 and appearing in the urine, the sweat or the expired air. 



We have further seen reason to think that the metabolic 

 events of the body take place in the main in the tissues, not in 

 the blood stream on its way between the heart and the tissues. 

 Changes, proper to the blood itself, take place in the blood; 

 the corpuscles, red and white, with the plasma undergo like 

 the rest of the body, their proper metabolic cycles, and in this 

 sense blood may be called a tissue if there is any advantage in 

 using the phrase; but, apart from these intrinsic blood changes, 

 as far as we can see at present, the metabolism undergone dur- 

 ing their transit along the blood channels, by the substances 

 which are merely carried in the blood from place to place, is 

 an insignificant part of the total metabolism of the body. 



By metabolism of a tissue we understand the total chemical 

 changes taking place in the tissue ; and we divide these changes 

 into those which either directly or indirectly are concerned in 

 the building up (anabolic) and those which are in like manner 

 concerned in the breaking down (katabolic) of the living sub- 

 stance. We shall explain presently what we mean by the 



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