CH. xxxix. J GENERAL METABOLISM. 58 1 



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 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. 



Supposing the body to remain in the condition produced by 

 these anabolic processes, what is its composition ? A glance 

 through the chapters on the cell, the blood, the tissues, and the 

 organs will convince the inquirer that different parts of the body 

 have very different compositions ; still, speaking of the body as a 

 whole, Volckmann and Bischoff state that it contains 64 per cent, 

 of water, 16 of proteids (including gelatin), 14 of fat, 5 of salt, 

 and i 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 2 1 per cent, of proteids ; thus about half 

 the proteid material and of the water of the body exist in its 

 muscles. 



The body, however, does not remain in this stable condition ; 

 even while nutrition is occurring, destructive changes are taking 

 place simultaneously ; each cell may be considered to be in a 



* 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. 



