THE TOTAL EXCHANGES OF THE BODY 691 



respiratory movements can thus proceed without altering appreciably 

 the pressure within the closed system of tubes. By the admission of 

 oxygen the supply of oxygen is adjusted so as to keep the bag from 

 becoming either too much distended or too much flattened. As 

 the air leaves the lungs and passes into the constantly moving current 

 of air, it is carried along by the pump and flows through two Wolff's 

 bottles containing strong sulphuric acid and pumice for the removal 

 of water vapour. It then passes 

 through a brass cylinder, c, filled 

 with soda lime for the absorp- 

 tion of carbon dioxide. From 

 here it passes again through 

 sulphuric acid in a Kipp gener- 

 ator for the absorption of water 

 given off by the soda lime. 

 Since the air so deprived of 

 moisture would be uncomfort- 

 able to breathe, it is then 

 carried through another Kipp 

 generator containing water with 

 a trace of sodium carbonate for 

 the neutralisation of any acid 

 fumes which may be given off 

 by the sulphuric acid. It then 

 passes back to the tube from 

 which the subject is breathing. 

 In this way it is possible to 

 determine very accurately the 

 amount of oxygen used up and 

 the amount of carbon dioxide 

 given off in the course of an 



experiment lasting one to three hours or longer. The oxygen con- 

 sumption is measured by weighing the cylinder of this gas, chosen 

 small for this purpose, before and after the experiment. 



III. PETTENKOFER'S METHOD. In the apparatus designed by 

 Pettenkofer the animal or man was placed in a chamber through 

 which a constant current of fresh air was passed. The amount of air 

 passing through the chamber was measured by means of a meter. 

 Throughout the experiment continuous samples both of the air enter- 

 ing the chamber and of the air leaving the chamber were taken. The 

 analyses of these samples served to show the composition of the whole 

 air entering and leaving the chamber, and therefore the changes in 

 the air caused by the presence of the animal. The advantage of this 

 apparatus is that an adequate ventilation can be kept up, and the 



FIG. 316. Arrangement of apparatus in 

 Benedict's method for determination of 

 respiratory exchange. 

 N, tubes inserted into nostrils of patient ; 

 A, tension equaliser ; c, cylinder contain- 

 ing soda lime for absorbing C0 2 . 



