NUTRITIVE VALUE OF FEEDING STUFFS 39 



paunch and intestines which result in the evoluton of incompletely 

 oxidized combustible gases that escape from the alimentary tract 

 (p. 30). 



Third, incompletely oxidized protein substances are excreted as 

 urea, and the energy which they represent is, therefore, of no value 

 to animals. The total energy less that lost through these three 

 sources furnish the available or so-called metabolizable energy of 

 the feed.', i.e., the energy capable of being transformed (metabolized) 

 in the body. This may be determined by means of the respiration 

 apparatus, or its improved form, the respiration calorimeter. 



The Respiration Apparatus. The first apparatus of this kind 

 was constructed by Pettenkofer, the great Munich chemist. It 

 consists of a large air-tight chamber, through which a measured 

 current of air is maintained. The animal experimented with is 

 kept in this chamber for a given period, 24 hours or longer. By 

 weighing and analyses of the feed, water, and air taken in by the 

 animal, as well as of the gaseous and solid excreta given off, the 

 intake and outgo of carbon, nitrogen, and other elements from the 

 body can be determined with great accuracy. By determining the 

 chemical energy of the feed eaten and of the excretory products in 

 a calorimeter the " energy balance " of the body may be studied and 

 thus important information bearing on nutrition problems secured. 

 The effect of a given ration on the nutritive processes in the 

 animal body is shown by the data secured in respiration experi- 

 ments, viz., whether the animal lost or gained in flesh or body fat 

 on the ration fed, and also the exact amount of the gain or loss. 



An example will illustrate how this information is obtained. 



A steer received daily the following amounts of nitrogen and carbon in 

 the feed, water, and air: 0.44 pound nitrogen and 13.25 pounds carbon; he 

 excreted in the urine, dung, vapor, and gases given off during the 24 hours 

 0.35 pound nitrogen and 12.10 pounds carbon, or there remained in the 

 body 0.09 pound nitrogen and 1.15 pounds carbon. 



Pure muscular tissue (lean meat) contains, on the average, 16.67 per 

 cent nitrogen and 52.54 per cent carbon. The addition of 0.09 pound 

 nitrogen, therefore, -equals 0.09 multiplied by 100/16.67, or 0.54 pound 

 of dry lean meat; this amount contains 0.28 pound carbon (0.54 pound 

 multiplied by 52.54/100). The difference between this amount of carbon 

 and that remaining in the body is 0.87 pound. As only very small amounts 

 of other non-nitrogenous components than fat are found in the body, we 

 are safe in assuming that the excess of the carbon was used for the forma- 

 tion of body fat; since this contains, on the average, 67.5 per cent carbon, 

 the difference of 0.87 pound equals 1.14 pounds of fatty tissue which was 

 added during the day. The steer gained 0.54 pound of dry lean meat and 

 1.14 pounds body fat during the day. If the increase was 2.50 pounds a 

 day on the average throughout the experimental period, the difference, 

 amounting to 0.82 pound, was composed of water and a small amount of 

 mineral matter, both of which can be readily determined. 



The Respiration Calorimeter. The Pettenkofer respiration ap- 

 paratus was greatly improved by Atwater and Rosa by making the 



