BY H. S. HALCRO WAHDLAW. 795 



the rate, at which the tension of oxygen falls, is very much 

 greater than the rate at which the tension of carbon dioxide 

 rises. In 35 seconds, the tension of oxygen falls from 116-5 to 

 81 "0, 35-5 mm. Hg, or more than three times as much as the 

 tension of carbon dioxide rises. It will be noticed also that, 

 although the rate of change of the alveolar tension of oxygen 

 slows down with time, this slowing down is much less marked 

 than in the case of the tension of carbon dioxide. During the 

 first 5 seconds of the experiment, the tension falls 7*5 mm Hg; 

 during the last 5 seconds, 3-5mm.Hg or at about one-half the 

 initial rate. The tremors of the respiratory muscles, which 

 make their appearance towards the end of the experiment, 

 apparently do not become of sufficient intensity to afiect notice- 

 ably the rate of absorption of oxygen in the lungs. 



When the breath is held for a long enough period, therefore, 

 the tension of carbon dioxide gives indications of attaining a 

 certain fixed value The alveolar tension of oxygen, on the 

 other hand, falls rapidly during the whole period for which the 

 breath can be held. 



Effect of rebkeathinc, the same Air. 



In the following Table are given the alveolar tensions and 

 percentages of carbon dioxide and of oxygen after the air in the 

 lungs at the end of a normal inspiration, instead of being 

 held there for a certain period, is breathed into and out of a 

 closed, empty bag. Under these circumstances, the air of the 

 lungs does not remain stagnant, but is mixed together by the 

 movements of breathing. The contents of the lungs are also 

 mixed with the air in the mouth-piece, and in the bag, which 

 cannot be emptied completely. The volume of this air is not 

 more than 100 cc. The average volume of the deepest expira- 

 tion which the present subject can make, after taking a normal 

 inspiration, is 2200 cc. As the volume of the residual air may 

 be taken as 800-1000 cc, the total volume of the air in the lungs, 

 at the end of a normal inspiration, amounts to approximately 

 3000 cc. The contents of the lungs are mixed, therefore, with 

 about 3% of their volume of air by breathing into and out of the 



