664 



PHYSIOLOGY 



of carbon dioxide given off in the course of an experiment lasting one to three hours 

 or longer. The oxygen consumption 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 

 entering the chamber and of the air leaving the chamber were taken. The analyses 

 of these samples seryed 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 apparatus can be built of any size. In the apparatus of Tigerstedt built on this 

 plan the chamber had a capacity of 100-6 cubic metres, and was, in fact, a small room. 

 A small respiratory apparatus has been built by Atwater. 



IV. ZUNTZ AND GEPPERT'S 

 METHODS. For many purposes the 

 methods devised by Zuntz and Geppert 

 present many advantages, especially 

 when it is desired to take the respiratory 

 exchanges in man or any animal during 

 a limited period of time. The subject of 

 the experiment has his nostrils clamped 

 and breathes into and out of a face- 

 piece. This face-piece is provided with 

 valves either of aluminium or of animal 

 membrane, which serve to separate the 

 in-going from the out-going current 

 of air. In the course of the out-going 

 current is placed a very delicate gas 

 meter which presents practically no 

 resistance to the air current. A branch 

 from the efflux tube passes to a gas 

 analysis apparatus. By an ingenious 

 method it is arranged that an aliquot 

 part of the whole of the out-going air is 

 drawn off into this apparatus, so that 

 the experiment can be interrupted at 

 any time, and the analysis of this sample 



FIG. 329. 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 adsorbing C0 2 . 



will give the average composition of the 

 expired air, and therefore, on multipli- 

 cation by the total gas passing through 

 the gas meter, the total output of 

 carbon dioxide during the course of the 

 observation. One advantage of this method is that the apparatus is portable, and can 

 be applied to the investigation of the respiratory exchanges of patients in hospitals 

 or of man or animals while they are walking about. It has been used, for instance, 

 by Zunt/. and his pupils in an interesting series of researches on the gaseous metabolism 

 of men at high altitudes. 



V. THE DOUGLAS BAG. By far the most convenient method for estimating 



the respiratory exehangcs of man under varying conditions is the use of the Douglas 

 Bag. Jn this method the subject for experiment breathes through a mouthpiece 

 provided with valves into a bag of about 100 lit re capacity. The valves are so arranged 

 that he inspires from the external air and expires into the bag. After from two to 

 ten minutes the bag is removed, the time being accurately noted. The amount of 

 air expired during this time-is measured by emptying the bag through a gas meter. 

 A sample of its contents is analysed and the oxygen and CO., in it determined. Since 

 he composition of the external air is known, the analysis and measurement of the 



