104r> EXPERIMENT STATION RECORD. 



expoi'iinont, to determine with a fair degree of accurac}^ the so-called 

 '■'' digestible nutrients''' consumed in the several periods of a feeding 

 (wperiment and thus to secure a more rational basis of comparison. 

 To the (•()n\(Mition:il d(>tei'minations it is of course easy to add others, 

 such as that of aniids, pentosans, etc., and particularly the heat of 

 combustion. 



A knowledge of the elements of the food consumed, however, is the 

 smaUer half of the problem. It is necessary to secure some definite 

 and accurate measure of its effects upon the animal consuming it. 

 This has l)cen the weak side of investigation in stock feeding. While 

 much labor has been expended in determining the composition and 

 digestibilitv of feeding stuffs with scientific accuracy, too many exper- 

 imenters have been, of choice or necessity', content to limit their deter- 

 minations of nutritive effect to simple weighing of the product. As 

 a consequence, the expenditure in the examination of the feeding 

 stuff's has, as a whole, failed of its due reward through lack of the 

 other term of the comparison. This is less true, of course, of experi- 

 ments on milk production, since in these the material product ma}^ be 

 subjected to chemical and phj'sical examination, but even here half or 

 two-thirds of the food may serve to support those nutritive functions 

 of the body whose net result is expressed in the term "maintenance," 

 but of w^hose amount and character the live-weight experiment fur- 

 nishes no exact measure. 



The problem is to determine the changes in the make-up of the body 

 during an experiment. Two general methods have been applied to its 

 solution. 



The first is the method of comparative slaughter tests. Of two 

 animals or lots, selected for their apparent identity as to weight and 

 condition, one is killed and analyzed at the beginning of the experi- 

 ment and the other at its close, and the difference in the amounts of 

 the several ingredients found is regarded as representing the gain 

 made by the second animal or lot. The weak point of the method, 

 of course, aside from its laboriousness, is the impossibility of proving 

 the fundamental assumption of identity of composition of the two ani- 

 mals at the beginning of the experiment. 



The second method, wdiich especially interests us here, dispenses 

 with an}^ knowledge of the initial composition of the animal and 

 attempts to determine directl}^ the increment or decrement of each 

 import::::!: ingredient of the bod}^ during the experiment. The basis 

 of the method is Henneberg's conception of the schematic bod3\ This 

 is, in brief, that for this particular purpose the animal body may be 

 regarded as composed of water, ash, protein, and fat, each of practi- 

 cally invariable elementar}^ composition. The writer has discussed 

 this conception at some length elsewhere,^' and it seems sufficient here 



« Principles of Animal Nutrition, pp. 60-66. 



