SECTION VIII. 



NUTRITION AND HEAT PRODUCTION AND 

 REGULATION. 



CHAPTER XLVII. 

 GENERAL METHODS HISTORY OF THE PROTEIN FOOD. 



Under the head of nutrition or general metabolism we include 

 usually all those changes that occur in our foodstuffs from the time 

 that they are absorbed from the alimentary canal until they are 

 eliminated in the excretions. In many of these processes the oxygen 

 absorbed from the lungs takes a most important part, and the 

 changes directly due to this element, the physiological oxidations 

 of the body, can not be separated from the general metabolic phe- 

 nomena of the tissues. As was said in another place, the respiratory 

 history of oxygen ceases after this element has reached the tissues; 

 its subsequent participation in the chemical changes of the organ- 

 ism forms an integral part of the nutritional processes. These latter 

 processes are varied and complex and only partially understood. 

 For the sake of simplicity in presentation it is convenient to con- 

 sider separately each of the so-called foodstuffs, the proteins, 

 albuminoids, carbohydrates, fats, water, and inorganic salts, and 

 attempt to trace its nutritive history from the time it is absorbed 

 into the blood until it is eliminated from the body in the form of 

 excretory products. Before undertaking this description it is desir- 

 able to call attention to certain general methods and conceptions 

 that have been developed in connection with this part of physiology. 



Nitrogen Equilibrium. Among our main foodstuffs the pro- 

 teins (and albuminoids) are characterized by containing nitrogen. 

 After this material is metabolized in the body the nitrogen is elim- 

 inated in various forms, chiefly in the urine, but to a smaller ex- 

 tent in the feces and sweat. In the feces, moreover, there may be 

 present some undigested protein which, although taken with the 

 food, has never really entered the body. It is evident that the 

 urine, feces (and sweat) may be collected during a given period and 

 analyzed to determine their contents in nitrogen. The sweat i? 



808 



