Chemical Changes in Animal Organism. 273 



comparing these with the heat- values of the foods ingested. 

 The larger the relative heat value of the excreta as com- 

 pared with that of the foods, the smaller the utilization of 

 the latter. The energy-yielding value of any given diet 

 can be ascertained in this way by a direct method. It is 

 also possible to measure directly the heat given out by an 

 animal by placing it in a specially constructed calorimeter. 

 Experiments of this description have been carried out by 

 means of a large scale apparatus. If the subject of the 

 experiment is allowed to do some measurable amount of 

 work, such as the riding of a stationary bicycle connected 

 with an ergometer which measures the amount of work, 

 his efficiency as a machine can be measured when the 

 caloric value of the food and excreta are also measured. 



From the point of view of the chemist, however, the 

 metabolism of matter is of greater interest, although the 

 determination of the caloric values of the foods and excreta 

 forms a parallel method of investigation. 



In order to investigate the total metabolism of an 

 animal, it is essential to be able to measure accurately the 

 amounts of the various constituents of the ingesta and 

 egesta. The following series of determinations must be 

 made: (1) The analysis of the amounts of the gaseous 

 products of metabolism namely, the intake product, 

 oxygen, which is inspired, and the output product, carbon 

 dioxide, which is expired by the lungs. (2) The analysis 

 of the foods ingested. (3) The analysis of the urine and 

 faeces. (4) In addition to the determination of these 

 factors, the weight of the subject of the experiment must 

 be carefully ascertained both at the beginning and the end 

 of the experiment, for when the food ingested is in- 

 sufficient for the supply of the needs of the organism, 

 the latter possesses the capacity of making up the deficiency 

 from its own body material. In such a case it loses weight 



(1,99V) 18 



