156 HUMAN PHYSIOLOGY 



that they do not need to be considered or are excluded by 

 the condition of the experiment. 



The ideal experiment in metabolism would be to estimate 

 quantitatively each individual constituent of the incomings 

 and outgoings and to use the results in striking the balance 

 of nutrition. But this is impossible because of unavoidable 

 difficulties in the methods of investigation. But to form a 

 correct conception of the extent and nature of the metabol- 

 ism, it is sufficient to know the amount of some of the con- 

 stituents or even some of the elements of the income and 

 outgo. Of these elements the most important are the 

 carbon, nitrogen, the oxygen of inhaled air, and frequently 

 the sulphur and phosphorus. 



The nitrogen both of the income and outgo can be 

 directly determined by Kjeldahl's method, by which the 

 nitrogen of the substance to be analyzed is changed, by 

 boiling with concentrated sulphuric acid and mercury, into 

 ammonia, and as such it may be measured. 



The carbon of the income and of the urine and faeces is 

 determined by analysis. The expired carbon is calculated 

 from the amount of carbon dioxide exhaled. 



The inhaled oxygen is either determined directly from the 

 amount of oxygen taken from the respired air or calculated 

 from the other data of the balance of nutrition. 



The taking up of oxygen and the giving off of carbon 

 dioxide is called ' ' respiratory metabolism. ' ' 



For investigating the respiratory metabolism, we may use 

 the apparatus of: (i) Pettenkofer-Voit, (2) Regnault-Reiset, 

 or (3) Geppert-Zuntz. 



By the Pettenkofer-Voit apparatus the gaseous outgo of 

 carbon dioxide and water vapor is determined directly. This is 

 done as follows: A person breathes in a hermetically sealed 

 chamber. The percentage of carbon dioxide and water vapor of 

 the air inhaled is known, and the amount of respiration is measured 

 by a gasometer. The increase in carbon dioxide and water vapor 

 in the expired air is determined by taking an accurately measured 

 quantity of air from the chamber and passing it through a weighed 

 quantity of sulphuric acid and through potassium hydrate, the one 



