METABOLISM, NUTRITION AND DIETETICS 



459 



learnt as to the relation between the proximate principles 

 of the tissues and the proximate principles of the food. 

 Inside the body we recognise representatives of the three groups 

 of organic food-substances in a typical diet proteids, carbo- 

 hydrates, and fats. But we should greatly err if we were to 

 imagine that the three streams of food-materials have flowed 

 from the intestines into the tissues each in its separate 

 channel, neither giving to nor taking from the others. The 

 fats of the body may, indeed, in part be composed of molecules 

 which were present as fat in the food ; but they may also be formed 

 from proteids, and from carbo-hydrates. The carbo-hydrates of 

 the body the glycogen of the liver and muscles, the sugar of the 

 blood may undoubtedly be derived from carbo-hydrates in the 

 food, but they may also be derived from proteids, although probably 

 not from fats. The proteids of the body arise solely from the pro- 

 teids of the food ; neither fats nor carbo-hydrates can form proteids, 

 although both can economize them and shield them from an over- 

 hasty metabolism. 



4. The Income and Expenditure of the Body. (i) Income and 

 Expenditure of Nitrogen. 



Preliminary Data. The purpose of food is to maintain the con- 

 stituents of the body upon the whole in their normal proportions. 

 A knowledge of the chemical composition of the body is. therefore, 

 an important datum in the consideration of the statistics of its 

 metabolism. The body of a man analyzed by Volkrnann had the 

 following compositipn : 



Inorganic substances 



Organic substances 



65-9 per cent. 

 4*4 >, 



297 



Water ... 

 Mineral matter - 

 Carbon 18-4 per cent. 

 Hydrogen 27 

 Nitrogen 2*6 ,, 

 Oxygen 6'o J 



The muscles, the adipose tissue, and the skeleton form nearly four-fifths 

 of the total body-weight in the adult. The following table shows the per- 

 centage amount of each of these tissues in a man, a woman, and a 

 child (Bischoff) : 



