804 EXPERIMENT STATION RECORD. 



and permitted investigations based on other data than the study of the 

 gaseous excretory prod nets. 



Progress was hindered by the lack of a proper standard for compar- 

 ing the processes which take place in the animal body. The develop- 

 ment of organic chemistry and the isolation of a number of well char- 

 acterized chemical compounds which occur in the food and excretory 

 products furnished Liebig the foundation for his theories. He divided 

 nutrients into nitrogenous (plastic) and nitrogen-free (respiratory) 

 materials. 



To Voit and his fellows belongs the credit of discovering the impor- 

 tance of protein as a nutrient, *. e., that the animal requires daily in 

 the food a definite amount of nitrogenous material in order to sustain 

 life. The protein is broken down in the body at the same time the 

 processes of combustion are carried on, and eventually theanimal comes 

 into nitrogen equilibrium. Nitrogen equilibrium can be reached when 

 only protein is consumed, provided the amount is very large, but it can 

 be attained with a much smaller amount if fat and carbohydrates are 

 consumed at the same time. If the animal perforins work, accumu- 

 lates flesh and fat, or produces milk or wool, or nourishes a fetus, more 

 protein must be consumed than for maintenance or an equivalent 

 amount of nitrogen-free nutrients or both must be supplied. If work is 

 rjerforined nitrogen-free material is essential, if flesh is formed then 

 pi otein is required. 



Voit defined nutrients as materials which build and repair the ani- 

 mal body. This definition includes mineral matter and water. 



In investigations which have proved most useful for an understand- 

 ing of the processes of metabolism, the total income and outgo have 

 been taken into account. The elementary composition of the food, 

 urine, and feces has been determined, the respiratory products have 

 been measured and analyzed, and finally the oxygen consumed has 

 also been measured. The principal investigations along these lines 

 were made by Bidder and Schmidt, Petteukofer and Voit, Henneberg 

 and his followers, Regnault and lieiset, and Pfliiger and his associates. 



It is interesting to note some of the changes in theories of animal 

 feeding which have accompanied the growth of the knowledge of nutri- 

 tion. In this connection two points have always been of importance 

 and are important to day. The practical feeder (1) wishes to know the 

 cost of a method of feeding and the profit it will return, and (2) he 

 desires definite feeding standards or formulas which can be successfully 

 followed. In the early part of the present century feeding stuffs were 

 first examined on the basis of their content of protein, starch, gluten, 

 sugar, gums, mineral matters, etc. As these materials were regarded 

 as of equal value, it was customary to add them together and regard 

 the sum as the measure of the nutritive value of the feeding stuff. 

 The chemical methods followed were not exact, and for this and other 

 reasons the system was of little value. 



