506 NUTRITION. 



ON NUTRITION IN GENERAL. 



454. 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 appealing 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 it& 

 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 metab- 

 olism 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) r 

 and those which are, in like manner, concerned in the breaking down 

 (katabolic) of the living substance. We shall explain presently wha.t we 

 mean by the words " directly " and " indirectly," used in this connection. 

 And we may here repeat the caution ( 30) that though, for convenience 

 sake, we use the phrase " living substance," what is really meant by the 

 words is not a thing or body of a particular chemical composition, but 

 matter undergoing a series of changes. 



455. Since the several tissues originate through a differentiation of 

 the simpler primordial protoplasm, we may infer that we have a right to 

 speak of a general plan of metabolism common to all the tissues, modified 

 in various particulars in various tissues. It is more reasonable, for instance, 

 to suppose that there is such a general plan common to both muscle and 

 gland, than to suppose that the metabolism of the one differs wholly from 

 or only accidentally resembles that of the other. And we may profitably 

 take the nutrition of muscle as exemplifying, in the midst of the features 

 special to the muscle, the general plan of vital metabolism. The muscle, in 

 a normal state of things, lives ultimately on the proteids, fats, carbohydrates, 

 salts, and water of the food, and on the oxygen of the inspired air, but 

 lives directly on the blood which brings these things to it. Taking the pro- 

 teids first, we may ask the question, How does the blood supply the muscle 

 with proteids? 



The blood contains three classes of proteids : 1, serum-albumin ; 2, para- 



flobulin, and 3, fibrinogen. With regard to the function of these three 

 inds of proteids in the nutrition of muscle, our only conclusions at present 

 are indirect onces, based chiefly on the results of experiments as to the rel- 



