PROBLEMS IN ANIMAL NUTRITION. 513 



that of the dio-ostible matter of grains, while Zuntz actually computes 

 a negative result tor the nutritive value of sti'aw foi- the horse. 



In the face of such dilferences as these, with what justification can 

 we calmly continue to publish the old figures for digestible nutrients 

 and to teach our students the conventional computation of rations? 



Nor does it better matters much to add to our tables figures for the 

 so-called '" fuel values "' of feeding stuffs. 



In the fii'st place, the figures connnonly given are incorrect. They 

 are based in most cases on Hubner's or Atwater's factors for hinuaii 

 dietaries, and while these factors have been shown to be substantially 

 accurate foi- the purpose for which they were intended, they have also 

 been shown to be grossly inaccurate when ai^plied to the digestil)le 

 nutrients of stock feeds, the results being too high, in some cases b}' 

 as much as twenty-five per cent. 



In the second place, even if these energy values were correct, they 

 do not help us much. We can not rejuvenate the old tables in this 

 way. The writer is fully convinced of the advantages for many 

 purposes of regarding nutrition problems from the standpoint of 

 energetics, but it needs to be emphasized that the advantage of the 

 calorie over the i^ound is simply as a measure. It is a better measur- 

 ing stick than the pound, with a wider range of uses, but it is still 

 a measuring stick — an implement — and the value of the results de- 

 pends on what and how we measure. The prime failure of Plenne- 

 berg's successors was not in the units which they used, but that they 

 failed to measure the real nutritive effect of their feeds and rations, 

 and we but repeat their failure when we seek to make j^rogress by 

 substituting these energy values for the equivalent amounts of mat- 

 ter. It is doubtless very desirable to know the amounts of digestible 

 matter in feeding stuff's and the corresponding quantities of energy 

 more accurately than we do at i3resent, l)ut neither the one nor the 

 other by itself gives us any definite infornuition as to the use made 

 by the organism of tlie matter or energy sujiplied in the feed. It 

 is only as we determine by the use of the resj)irati()n aj)iiaratus or 

 calorimeter (or possibly by the comj)arative slaughter test) the actual 

 changes ])rought about by the feed in the store of matter or of poten- 

 tial energy contained in the body that we can reach a scientifically 

 accurate determination of the nutritive value of that feed. Unless 

 we do this, no matter how accurately we analyze the feeding stuffs 

 sujiplied or determine their energy, the second member of the equa- 

 tion is lacking. We stand in urgent need of actual determinations 

 by modern methods of the nutritive values of feeding stuff's for dif- 

 ferent purposes, the results of which we may substitute for the 

 assumptions on which we are now basing our teachings. 



I may instance in particular the importance of determinations of 



