CHAPTEK XLI 



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, Eespiration, 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 different parts of the body have very different compositions ; 

 still, speaking of the body as a whole, Volkmann and Bischoff state 

 that it contains 64 per cent, of water, 16 of proteins, 14 of fat, 5 of 

 salt, and 1 of carbohydrates. The carbohydrates are thus the smallest 

 constituent of the body; they are the glycogen of the liver and 

 muscles, and small quantities of dextrose in various parts. 



The most important, because the most abundant of the tissues of 

 the body, is the muscular tissue. Muscle forms about 42 per cent, 

 of the body-weight,* and contains, in round numbers, 75 per cent, of 

 water and 21 per cent, of proteins; thus about half the protein 

 material and of the water of the body exist in its muscles. 



The body, however, does not remain in a stable condition ; even 

 while nutrition is occurring, destructive changes are taking place 

 simultaneously; each cell may be considered to be in a state of 

 unstable equilibrium, undergoing anabolic, or constructive processes, 

 on the one hand, and destructive, or katabolic, processes on the other. 



* The following is in round numbers the percentage proportion of the different 

 structural elements of the body : skeleton, 16 ; muscles, 42 ; fat, 18 ; viscera, 9 ; 

 skin, 8; brain, 2; blood, 5. 

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