BY ETHEL C. PIXKERTON. 149 



measured the percentage of carbon dioxide in the alveolar air by 

 a method worked out in the laboratory of the Finsen Institute 

 at Copenhagen. He took a series of samples at the end of a 

 number of respirations, and analysed the mixed sample. Lind- 

 hard pointed out that Haldane's method presupposed that the 

 last air expired, had the same content in carbon dioxide as the 

 alveolar air at the end of an expiration. This, he considered, 

 was n()t the case, as the last air to leave the alveoli remained in 

 the upper air-passages in what is known as the "dead space". 

 The error produced in this way always tended in one directi<jn. 

 The percentage of carbon dioxide in the expired portion would 

 be smaller than that in the alveoli. The value of the error 

 would vary with the length of the expiration. Not only so, but 

 the "form" of a respiration varied with its depth. 



Krogh(8) pointed out that the term "average alveolar air", or 

 simply "alveolar air ', had two distinct meanings. It might 

 refer to the air in the pulmonary air-cells, or it might refer to 

 the last air expired during expiration. He proposed, therefore, 

 to designate the latter air as "the alveolar expired air". He 

 concluded that the average alveolar tension of carbon dioxide 

 could not be determined with certainty by any method hitherto 

 employed. During rest, Haldane and Priestley's method yielded 

 the nearest approximation, but, during work, the results obtained 

 by this method were nuich too high. The percentage of carbon 

 dioxide in the "alveolar expired air" was not identical with the 

 average tension of carbon dioxide in the alveoli, but generally 

 lower. Krogh and Lindhard(9), using mechanical methods of 

 sampling the expired air, made a careful study of the distribution 

 of carbon dioxide in expired air under conditions of work and 

 rest. They showed that, during work, the carbon dioxide in- 

 creases directly with the time at which the sample of expired air 

 is taken for analysis. Each successive portion of expired air 

 contained more carbon dioxide than the portion which preceded 

 it, and less than the portion which followed it. With the l)ody 

 at rest, they found that the percentage of carl)on dioxide in the 

 expired air increased rapidly at first, and later more slowly, the 

 curve showing a marked tendency to become asymptotic. 



