206 THE FEEDING OF ANIMALS 



physical and chemical properties, the compounds into 

 which these bodies break under certain conditions, the 

 chemical changes to which they are subject through cer- 

 tain agencies, and their relation to one another. Investi- 

 gations along these lines have for years occupied the 

 time of some of our ablest scientists, and, while such 

 researches when they were conducted may have seemed 

 to the extreme utilitarian to be of little value, we now see 

 how directly they are contributing to human progress 

 and welfare. 



To the above information has been added through 

 physiological investigations a knowledge of the ways in 

 which the several food compounds are transformed in 

 digestion and in other metabolic changes, the avenues 

 along which these compounds travel, and the ways in 

 which their products of decomposition are discharged 

 from the animal organism. We have learned how to dis- 

 tinguish between the digested and undigested food, have 

 demonstrated that all the nitrogen of the decomposed 

 proteins passes off in the urine, have measiu-ed the com- 

 bustion of the nutrients and have learned how to strike a 

 balance between the income and outgo of the animal 

 body. It is now possible to determine with reasonable 

 accuracy just how much substance is retained or lost 

 from the body of the experimental animal while eating a 

 given ration, and what is the nature of the gain or loss. 

 Very recently means have also been devised for measur- 

 ing the heat given off by a man or an animal in order to 

 ascertain the actual physiological values of different 

 feeding-stuffs. 



293. More accurate methods of investigation than 

 practical feeding tests. — In applying the principles and 

 facts of chemistry and physiology, the first advance from 



