632 NUTRITION OF FARM ANIMALS 



referred to in Part III are rendering it increasingly evident that, 

 quite aside from its value as a supply of structural material and 

 of energy, the nature of the feed may profoundly influence 

 the course and intensity of the metabolic processes. In par- 

 ticular it appears that the absence of certain as yet ill-defined 

 substances may constitute a limiting factor, particularly in 

 growth, or may lead to the development of specific diseases, 

 while, on the other hand, McCollum's observations on the ex- 

 clusive use of wheat products (499) seem to indicate that similar 

 effects of a more or less toxic character may follow the exces- 

 sive consumption of feeding stuffs ordinarily regarded as health- 

 ful. It is important, therefore, to secure as definite a concep- 

 tion as possible of the significance of these new facts in their 

 relation to the older conceptions of production values. 



738. Significance of " accessory ingredients." It is clear 

 that the " accessory ingredients " (using this simply as a con- 

 venient summary term for the various classes of substances 

 indicated in the last paragraph) influence the nutritive value 

 of a feeding stuff in an essentially different fashion than does 

 the quantity of available ash, protein, and energy which it 

 supplies. The latter limits the amount of production which the 

 feeding stuff can support ; the presence or absence of the former 

 may determine the extent to which this potential value is actu- 

 ally realized. Thus in Chapter XI, experiments by Osborne and 

 Mendel and by Hart and McCollum (498, 499) were described 

 which show that a mixture of pure nutrients may be prepared 

 which shall contain an abundant supply of complete proteins, 

 of ash and of energy but upon which young animals (rats) fail'to 

 grow, while the addition to such a mixture of minute amounts 

 of substances associated with certain fats enables the rations to 

 support normal growth. In some aspects of the matter, these 

 " accessory ingredients " might be crudely compared with the 

 lubricants of a machine, which of themselves furnish neither 

 power nor material, but which enable power derived from the 

 consumption of fuel to be more efficiently used and therefore 

 conduce to the production of a larger output. 



A lack of lubricants in the case just supposed might conceiv- 

 ably affect the output of a machine in one or both of two ways. 

 The undue friction might slow down the machine as a whole 

 so that less raw material would pass through it in a given time, 



