SEC. 3. ON NUTRITION IN GENERAL. 



541. 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 

 materials for, and returning to the blood the products of, their 

 metabolism. 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 similarly 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 during 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 



