NUTRITION 345 



The nitrogen is excreted almost wholly by the urine, excepting 

 in the horse, where there is a loss by the skin. It is usual to 

 regard the urine- nitrogen as a measure of the protein changes 

 in the system, and this passes away mainly as urea, and in smaller 

 proportion as uric and hippuric acids and other nitrogen com- 

 pounds. The hydrogen is excreted as water by the lungs, skin, 

 and urine, and in all herbivora by the bowels and respiratory 

 passages as marsh-gas. 



In order to arrive at the processes involved in nutrition, tables 

 of income and expenditure of the body for twenty-four hours 

 have been drawn up. For this purpose a Respiration Chamber 

 is employed, in which the animal under observation lives. The 

 air supply to the chamber is dried and freed from carbon dioxide. 

 Ventilation is carried out by means of a pump, and from time to 

 time samples of the air are analysed, in order to determine the 

 amount of 2 absorbed and C0 2 given off. The composition of 

 the food is known, and the total excreta analysed, in order to 

 ascertain to what extent this differs from the food entering the 

 body. In this way the chemical elements entering the body as 

 food and leaving it as excreta are known. When these are found 

 to be practically identical, the animal is in metabolic balance. 

 There is probably no class of experiment so tedious as this. 

 The work is continuous for twenty-four or forty-eight hours, and 

 a very small error in the technique renders the whole observation 

 useless. In the latest form of respiration chamber for men 

 means have been introduced to enable a definite amount of work 

 to be performed, and in this way the metabolism of the body at 

 work as well as at rest can be ascertained. The most advanced 

 method of inquiry is the Respiration Calorimeter, in which not 

 only the nitrogen and carbon balance can be ascertained, but 

 also the heat given out by the body. By means of the respira- 

 tion calorimeter not only is the proportion of the digestible 

 matter in the food ascertained, but also the potential energy 

 contained in it, and the use the body makes of the material 

 supplied. Recently such an appliance for large animals has 

 been erected at the Institute of Animal Nutrition of the Penn- 

 sylvania State College.* It is extremely costly, and so elaborate 

 that it takes seven men to work it, exclusive of those in attendance 

 on the animal. The ordinary calorimeter employed in physio- 

 logical research is figured in the chapter on Animal Heat. It 

 is a double-walled chamber containing water. As the tempera- 

 ture of the chamber rises, the heat is conveyed to the water in 

 its walls, and its temperature measured. The respiration 

 calorimeter used for animals is copied from the one employed 

 by Atwater in his classical experiments on nutrition in men. 

 * See Bureau of Animal Industry, Twenty-Third Report, p. 263. 



