CHAPTEE XXXIX 



GENERAL METABOLISM 



THE word metabolism has been often employed in the preceding 

 chapters, and, as there explained, it is used to express the sum total 

 of the chemical exchanges that occur in living tissues. The chemical 

 changes have been considered separately under the headings 

 Alimentation, Excretion, Kespiration, etc. We have now to put our 

 knowledge together, and consider these subjects in their relation to 

 one another. 



The living body is always giving off by the lungs, kidneys, and 

 skin the products of its combustion, and is thus always tending to 

 lose weight. This loss is compensated for by the intake of food and 

 of oxygen. For the material it loses, it receives in exchange fresh 

 substances. If, as in a normal adult, the income is exactly equal to 

 the expenditure, the body-weight remains constant. If, as in a 

 growing child, the income exceeds the expenditure, the body gains 

 weight; and if, as in febrile conditions, or during starvation, the 

 expenditure exceeds the income, the body wastes. 



The first act in the many steps which constitute nutrition is the 

 taking of food, the next digestion of that food, the third absorption, 

 and the fourth assimilation. In connection with these subjects, it is 

 important to note the necessity for a mixed diet, and the relative and 

 absolute quantities of the various proximate principles which are 

 most advantageous. Assimilation is a subject which is exceedingly 

 difficult to describe ; it is the act of the living tissues in selecting, 

 appropriating, and making part of themselves the substances brought 

 to them by the nutrient blood-stream from the lungs on the one 

 hand, and from the alimentary canal on the other. The chemical 

 processes involved in some of these transactions have been already 

 dwelt on in connection with the functions of the liver and other 

 secreting organs, but even there our information on the subject is 

 limited ; much more is this the case in connection with other tissues. 

 Assimilation, or the building up of the living tissues, may, to use 

 Gaskell's expression, be spoken of as anabolic. 



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