INVESTIGATIONS ON THE METABOLISM OF MILCH COWS. 807 



attempted in addition to that of nitrogen and carbon. Samples of food, 

 urine, and feces are burned in a Berthelot bomb in an atmosphere of 

 oxygen under pressure. In this way the total energy — i. e., heat of 

 combustion — may be determined. The fuel value of the food less the 

 fuel value of the feces gives the fuel value of the food digested. This 

 amount, less the fuel value of the urine, gives the fuel value available 

 for the body. If the income of energy is greater than the outgo, the 

 ration is more than sufficient for maintenance. Kellner published the 

 results of such experiments with cattle about 2 years ago. However, 

 as early as 1880 he called attention to the importance of ealorimetric 

 investigations of this nature. 



Another important factor upon which information is essential is the 

 amount of energy which is expended in digesting food. An idea of its 

 importance may be gathered from an experiment which was undertaken 

 by Zuntz and the writer with a horse. It was found that the horse 

 digested 391 gin. of fiber-free nutrients from a kilogram of hay and 615 

 gin. from a kilogram of oats. Therefore, according to the usual method 

 of comparison, hay : oats : : 391 : 615, or in round numbers, as 2 ; 3. 

 This deduction, however, is not warranted. A kilogram of hay requires 

 for chewing and digesting an expenditure of energy equivalent to 209 

 gm. of fiber-free nutrients, and a kilogram of oats an amount equal to 

 121 gm. This energy is eventually transformed into heat and supplies 

 wholly or in part the warmth required by the animal body, so that as 

 regards maintenance hay and oats may truly be said to stand in the 

 ratio of 2 : 3. But if the ration is in excess of the amount required for 

 maintenance, for instance, if work is to be performed, then the ratio 

 is as follows : Hay : oats : : 391 — 209 : 615—124, or 182 : 491, or in round 

 numbers 1 : 2.7. That is, 2.7 kg. instead of 1.5 kg. of hay is equal to 

 1 kg. of oats. 



EXPERIMENTAL METHODS. 



In metabolism experiments it is absolutely necessary to collect the 

 urine and feces and determine the amount and composition of each. 

 The nitrogen in the urine is always determined, and often the carbon, 

 hydrogen, and mineral matter. It is usually necessary to dry the feces 

 before analysis. The same determinations are then made as in the 

 urine, and, in addition, proximate analyses corresponding to the deter- 

 minations made in the food. In the case of large domestic animals, 

 milch cows, for instance, it would be very inconvenient to retain all the 

 feces for analysis, since they may amount to 20 or 30 kg. daily. There- 

 fore aliquot samples should be taken each day and dried and united 

 for analysis. The materials excreted each day in the feces can then be 

 calculated. In the case of steers, the collection of the urine is a com- 

 paratively simple matter, and special apparatus has been devised for 

 this purpose. In the case of cows, the usual plan is to watch the ani- 

 mals day and night and collect the urine directly in suitable vessels. 

 Sometimes animals are kept in a stall with a grating in the floor through 



