THE PRODUCTION VALUES OF FEEDING STUFFS 631 



for ascertaining this effect have been considered in Chap- 

 ter VI. It was there shown that neither the gain nor loss of 

 live weight or the gross weight of product is a sufficiently 

 accurate measure of it (281-283, 604) and that the attain- 

 ment of exact results requires the employment of some form 

 of the balance experiment (285), based on the conception of 

 the balance of nutrition. According to this conception, the 

 production values of a feeding stuff for various purposes are 

 measured, either by the extent to which it can prevent a 

 loss of protein, ash and fat from the body during maintenance 

 or work or can support the storage of these ingredients in the 

 body or the milk. It was also pointed out in the same chapter 

 that the investigations of the last thirty years have shown that 

 the problem may be advantageously studied from the stand- 

 point of energetics and that in this way the expression of the 

 results may be notably simplified and unified. From this stand- 

 point the feed is regarded as a supply of ash and protein (or 

 amino acids) on the one hand and of energy on the other and 

 its effect is similarly expressed by the gain or loss by the body 

 of protein and ash and of chemical energy respectively. We 

 may distinguish, therefore, between production values for pro- 

 tein, for ash, and for energy. 



737. Two aspects of feed supply. For a clear conception 

 of the nature and significance of the production values of feed- 

 ing stuffs, however, it is essential to distinguish between two 

 aspects or functions of the feed supply. 



In the past the feed has been regarded chiefly as the source of 

 the material necessary for the constructive processes going on 

 in the body and of the energy required to support its metabolic 

 activities. It supplies ash to maintain or increase the mineral 

 matter of the body, protein (or amino acids) to build up its 

 tissues or supply the protein of milk, energy to support the 

 vital activities of the various organs. This aspect of the mat- 

 ter has been the prominent one in the preceding chapters of 

 this work. 



Recent investigation, however, is bringing into prominence 

 another class of influences exerted by the feed upon the or- 

 ganism. The studies upon the " vitamins," " accessory in- 

 gredients," " growth substances," " stimulating substances," 

 " specific effects of feeds," etc., which have been several times 



