GENERAL METABOLISM. 623 



The income is to be contrasted with the outgo. There are three prin- 

 cipal ways in which the products of metabolism leave the organism, 

 through the kidneys, the intestine, and the lungs. The faeces contain 

 not only products of metabolism, but also the unabsorbed food. The 

 determination of the nature of the faeces is highly important. It gives us 

 an idea how completely the nourishment has been utilized. In the urine, 

 besides the inorganic salts, we have to consider the nitrogen, sulphur, 

 phosphorus, carbon and hydrogen content. The first three elements give 

 us information concerning the decomposition of the protein. Usually the 

 nitrogen is alone determined and the amount of consumed protein is 

 obtained by multiplying the nitrogen value by 6.25. In many cases the 

 decomposition of the protein is also traced qualitatively, and it is deter- 

 mined how much nitrogen is present as urea, and how much in the form 

 of other compounds. In order to get an idea of the energy economy, the 

 heat of combustion of the entire excreta is determined, further the heat 

 given off by the body, and the amount of heat equivalent to the muscular 

 work performed. 



The gas metabolism is studied with the help of apparatus of special 

 construction, 1 by means of which the amounts of carbon dioxide and 

 water vapor eliminated are determined. From the amount of carbon 

 dioxide the equivalent weight of carbon may be computed. This, together 

 with the amount of carbon contained in all the remaining excreta, gives us, 

 first of all, an idea as to the utilization of the carbon in the food by the 

 organism. The question then arises how can we tell from the total amount 

 of carbon the amount that has resulted from the separate foodstuffs, 

 albumin, carbohydrate, and fat. To estimate this we start with the total 

 amount of nitrogen eliminated, and from that compute the amount of 

 albumin decomposed. Since the average carbon content of protein is 

 known, it is easy to compute how much of the carbon came from proteins. 

 This is naturally on the assumption that the combustion of the remaining 

 protein molecule takes place simultaneously with the elimination of the 

 nitrogen. As a rule, the relation of nitrogen (16 per cent) to carbon (53 

 per cent) in protein is as 1 : 3.3. If, then, we multiply the nitrogen value 

 by 3.3 we obtain the amount of carbon which was obtained from protein; 

 and by deducting the product from the total amount of carbon in the 

 egesta, the amount obtained from nitrogen-free food is given. If there 

 is no remainder, then only protein was consumed. By comparing the total 

 amount of carbon and the remainder after deducting the carbon from 



1 Descriptions of such apparatus may be found as follows: Regnault and Reiset: 

 Annal. 73, 92, 129, 257 (1850), and Ann. chim. et phys. (3) 26 (1849). Hoppe-Seyler: 

 Z. physiol. Chem. 19, 574 (1894). Pettenkofer: Annal. II, Suppl.-Band, p. 1 (1862). 

 Voit: Z. Biol. 11, 541 (1875). Sonden and Tigerstedt: Skand. Arch. Physiol. 6 (1895); 

 and Atwater: Ergeb. Physiol. (Asher and Spiro) Jg. 3, 498 (1904). 



