194 NUTRITION OF FARM ANIMALS 



down (katabolize) at once for the sake of their energy or store 

 up as a reserve of energy (fat, glycogen) for future use. Finally, 

 to carry the analogy a step further, it must obtain from its feed 

 such amounts and proportions of the several ash ingredients 

 as will maintain the necessary working conditions of osmotic 

 pressure, ionic concentration and the like, somewhat as the 

 engine must be lubricated. 



277. Balance of income and expenditure. It is evident 

 from the foregoing considerations that the body exhibits two 

 sets of activities, those concerned in its actions as a prime 

 motor, tending to destroy it, and those of nutrition, tending to 

 build up and increase it. Whether the body gains, is main- 

 tained or falls away depends upon the balance between these 

 two sets of activities. 



In a broad general way, of course, this fact is perfectly obvious. 

 We do not need a physiologist to teach us that the horse or cow 

 cannot long continue to do work or to yield milk unless supplied 

 with sufficient feed to make good the resulting loss of body 

 material. Similarly, we are familiar with the fact that those 

 operations of the body which go on in a state of so-called rest 

 likewise require material for their support, so that the mere 

 maintenance of an animal calls for an expenditure of feed. 

 What is needed in a scientific study of nutrition is something 

 more than the mere general knowledge of these familiar facts ; 

 namely, a quantitative measure of the extent to which the 

 various feeding stuffs or their single ingredients contribute to 

 the nutritive functions of the body under varying conditions. 



2. METHODS OF INVESTIGATION 



278. Investigation of details of metabolism. One method 

 of attacking the problem just stated is by investigating the 

 details of the metabolic processes. In the study of metabolism 

 (including the chemical changes in digestion and resorption) 

 the attempt is made to follow the various ingredients of the feed 

 through the body and to trace in detail how, where and to what 

 extent they contribute to the maintenance or growth of tissue 

 or supply energy for the use of the organism. Such studies are 

 of fundamental importance. They reveal to us how the animal 

 mechanism operates. When carried to their ultimate con- 



