CHAP, v.] NUTRITION. 829 



can be nothing else than the weight of the oxygen absorbed during the 

 period. This method in turn however is also open to objections, since 

 minute errors in the analyses of the small samples of air employed for 

 the determinations attain considerable dimensions when these are multi- 

 plied so as to give the changes in the whole mass of air passed through 

 the apparatus. It seems moreover undesirable to leave the quantity 

 used of so important an element as oxygen to be determined by indirect 

 calculations. 



In the method adopted by Zuntz and others the expired air is led 

 by means of easily working valves through a gasometer, a special mouth- 

 piece being used in experiments on man and a cannula introduced into 

 the trachea in experiments on animals. The gasometer measures the 

 quantity of air which is being breathed out, and in samples of this the 

 carbonic acid and the oxygen are, as required, determined. The 

 atmosphere breathed in being constant, the amount of oxygen consumed 

 and carbonic acid given out in a definite period is calculated from the 

 composition and quantity of the air breathed out in that period. 



Let us imagine, then, an experiment to have been completely 

 carried out by Pettenkofer and Voit's method, that the animal's 

 initial and terminal weights have been accurately determined, the 

 composition of the food satisfactorily known to consist of so much 

 proteid, fat, carbohydrates, salts, and water, and to contain so much 

 nitrogen and carbon, the weight of the faBces and the nitrogen they 

 contain ascertained, the nitrogen of the urine determined, the 

 carbonic acid and water given off by the whole body carefully 

 measured, and the amount of oxygen absorbed calculated what 

 interpretation can be placed on the results ? 



Let us suppose that the animal has gained w in weight during 

 the period. Of what does w consist ? Is it fat or proteid material 

 which has been laid on, or simply water which has been retained, 

 or some of one and some of the other ? Let us further suppose 

 that the nitrogen of the urine passed during the period is less, 

 say by x grammes, than the nitrogen in the food taken, after 

 deduction of course of the nitrogen in the faeces. This means 

 that x grammes of nitrogen have been retained in the body ; and 

 we may with reason infer that they have been retained in the 

 form of proteid material. We may even go farther and say that 

 they are retained in the form of flesh, i.e. of muscle. In this 

 inference we are going somewhat beyond our tether, for the 

 nitrogen might be stored up as some proteid constituent of the 

 hepatic cells or of some other tissue ; indeed it might be for the 

 while retained in the form of some nitrogenous crystalline body. 

 But this last event is unlikely ; and if we use the word ' flesh ' to 

 mean nitrogen (proteid) holding living substance of any kind, we 

 may without fear of any great error reckon the deficiency of x 

 grammes nitrogen as indicating the storing up of a grammes flesh. 

 There still remain w a grammes of increase to be accounted for. 

 Let us suppose that the total carbon of the egesta has been found 



