CH. xxxix.] EXCHANGE OF MATERIAL. 583 



The construction of balance-sheets for the human and animal 

 body may be summed up in the German word Stoffwechsel, or 

 "exchange of material." A large number of investigators have 

 applied themselves to this task, and from the large mass of 

 material published, it is only possible to select a few typical 

 examples. The subject has been worked out specially by the 

 Munich school, under the lead of Pettenkofer and Voit. 



The necessary data for the construction of such tables are : 



(1) The weight of the animal before, during, and after the 

 experiment. 



(2) The quantity and composition of its food. 



(3) The amount of oxygen absorbed during respiration. 



(4) The quantity and composition of urine, faeces, sweat, and 

 expired air. 



(5) The amount of work done, and the amount of heat 

 developed. (The subject of animal heat will be considered in the 

 next chapter.) 



Water is determined by subtracting the amount of water 

 ingested as food from the quantity lost by bowels, urine, lungs, 

 and skin. The difference is a measure of the katabolism of 

 hydrogen. 



Nitrogen. The nitrogen is derived from proteids and albumi- 

 noids, and appears chiefly in the urine as urea and uric acid. 

 Minute quantities are eliminated as similar compounds in sweat 

 and faeces. From the amount of nitrogen so found, the amount 

 of proteids which have undergone katabolism is calculated. 

 Proteids contain, roughly, 1 6 per cent, of nitrogen ; so i part of 

 nitrogen is equivalent to 6 '3 parts of proteid ; or i gramme of 

 nitrogen to 30 grammes of flesh. 



Fat and carbohydrate. Subtract the carbon in the metabolised 

 proteid (proteid contains 54 per cent, of carbon) from the total 

 carbon eliminated by lungs, skin, bowels, and kidneys, and the 

 difference represents fat and carbohydrate that have undergone 

 metabolism. 



The Discharge of Carbon. 



The influence of food on the rate of discharge of carbonic acid 

 is immediate. The increase after each meal, which may amount 

 to 20 per cent., reaches its maximum in about one or two hours. 

 This effect is most marked when the diet consists largely of 

 carbohydrates. 



About 95 per cent, of the carbon discharged leaves the organism 



