232 RESPIRATION 



depth of the individual breaths : the percentage could only re- 

 main absolutely constant if the depth of breathing was itself 

 constant. For the same reason the oxygen consumption should not 

 be determined over a shorter period than five minutes. 



One great advantage of this apparatus is that it is very easy 

 to subject oneself to atmospheres containing different percentages 

 of oxygen by means of it. To obtain an atmosphere poor in oxygen 

 all that is necessary is to uncouple the oxygen supply from the 

 valve M and breathe into the apparatus. Air now enters through 

 F instead of oxygen, and breathing is continued until analysis of 

 the inspired air shows that the required degree of oxygen de- 

 ficiency has been produced. If the oxygen supply is now reestab- 

 lished the artificial atmosphere produced will remain constant. 

 To obtain an atmosphere rich in oxygen, the gas may be blown in 

 through the orifice for the alveolar air sampling tube, leaving the 

 mouthpiece free for the escape of the displaced air from the re- 

 turn air pipe. 



The total volume of the air in our apparatus is about 15 liters, 

 and we may therefore presume that the whole of it goes through 

 the alveoli of a resting adult subject in three minutes. We have 

 on a number of occasions breathed into the apparatus for an 

 hour with the greatest comfort, the percentage of oxygen mean- 

 while varying only within the limits mentioned above. 



The time during which the subject breathed into the respiration 

 apparatus in our experiments has varied on different occasions 

 from twenty minutes to one hour. So far as we could ascertain the 

 shorter time was sufficient to establish equilibrium of concentra- 

 tion of the carbon monoxide in the blood and in the air breathed, 

 though we have as a rule adopted a period in excess of this as a 

 matter of precaution. In our earlier experiments we passed about 

 2 cc. of CO into the air in the respiration apparatus before begin- 

 ning to breathe into it, in order that the percentage of this gas 

 present at the start might approximate to its final value. As this 

 procedure had no influence on the result of the experiment we 

 gave it up, and the respiration apparatus thereafter always con- 

 tained air free from CO at the commencement of the experiment. 



Analyses of the inspired air were made several times during the 

 course of the experiment, as it was naturally important for our 

 purpose that the composition of the inspired air should show none 

 but minimal variations. Shortly before the close of the experi- 

 ment a sample of blood was withdrawn from the subject's finger 

 into a capsule, and defibrinated with a platinum wire. Five-hun- 



