THE BALANCE OF NUTRITION 2OI 



be determined. The specific methods used for such comparisons 

 are described in the two following sections. 



A great advantage of the balance experiment is the comparatively 

 short time which it requires. A period sufficiently long for the deter- 

 mination of the digestibility of a ration (159) is in general suffi- 

 cient also for a balance experiment, while for the requisite determina- 

 tion of the respiratory products or of the heat produced twenty-four 

 to forty-eight hours suffice, and even this short period may be divided 

 into a number of subperiods of a few hours each. For this reason, 

 and also because the animal is not injured in the process, repeated 

 experiments may be made on the same subject, so that the effect of 

 various rations or conditions may be compared on the same individual, 

 while the method of comparative slaughter tests necessarily involves 

 comparisons between two different animals. 



On the other hand, the complete balance experiment requires elab- 

 orate and expensive apparatus, while opinions as to the relative 

 amount of labbr involved in the two classes of experiments would 

 perhaps depend largely upon the previous experience of the experi- 

 menter. Furthermore, the balance experiment shows only the amounts 

 of the constituent groups protein, fat, etc. gained or lost. It 

 affords no opportunity to subdivide these and determine the fate of 

 single chemical compounds nor does it give any clue to the particular 

 region of the body where the gains have been deposited. 



286. The balance of nutrition. The phrase " balance of nu- 

 trition " used as the title of this chapter refers in a general way to 

 the balance between income and outgo of matter and energy in the 

 body as determined by the methods of the balance experiment. 



Logically, of course, the comparative slaughter test, if com- 

 bined with determinations of the feed consumed, may also be 

 regarded as a balance experiment. In it the income of the body 

 and the resulting gain are determined, leaving the outgo to be 

 inferred, while in a balance experiment in the technical sense, 

 the income and outgo are determined and the gain is inferred. 

 Nevertheless, the latter type of experiment has played so large 

 a part in the study of the balance of nutrition, both for physio- 

 logical and for agricultural purposes, that a clear conception of 

 its methods and postulates is essential for a comprehension of 

 many of the results to be considered in subsequent chapters. 

 The subject may be conveniently considered under the two 

 heads of the balance of matter and the balance of energy. 



