EXHALATION OF CARBONIC ACID. 425 



were experienced in forming an estimate of the proportion 

 of this gas consumed. As we assumed, after a comparison 

 of the results obtained by different observers, that the vol- 

 ume of oxygen consumed is about five per cent, of the entire 

 volume of air, it may be stated as an approximation, that in 

 the intervals of digestion, in repose, and under normal con- 

 ditions as regards the frequency of the pulse and respiration, 

 the volume of carbonic acid exhaled is about four per cent, 

 of the volume of the expired air. 1 As the volume of the oxy- 

 gen which enters into the composition of a definite quantity of 

 carbonic acid is precisely equal to the volume of the car- 

 bonic acid, it is seen that a certain quantity of oxygen disap- 

 pears in respiration, and is not represented in the carbonic 

 acid exhaled. 



There are great differences in the proportion of carbonic 

 acid in the expired air, depending upon the time during 

 which the air has remained in the lungs. This interesting 

 point has been studied by Yierordt, in a series of 94 experi- 

 ments made upon his own person, with the following results : 2 



" When the respirations are frequent, the quantity of car- 

 bonic acid expelled at each expiration is much less than in a 

 slow expiration ; but the quantity of carbonic acid produced 

 during a given time by frequent respirations is greater than 

 that which is thrown off by slow expirations." ; 



The air which escapes during the first period of an expi- 

 ration is naturally less rich in carbonic acid than that which 

 is last expelled and comes directly from the deeper portions 

 of the lungs. Dividing, as nearly as possible, the expiration 

 into two equal parts, Vierordt found, as the mean of twenty- 



1 MILNE-EDWARDS, Physiologic, tome ii., p. 507. This approximation is taken 

 from the observations of Valentin and Brunner, Dalton, Prout, Apjohn, Coathupe, 

 Horn, and Yierordt. The experiments of Vierordt are, perhaps, entitled to the 

 most credit, as he has studied very carefully the influence of the frequency of res- 

 piration upon the quantity of carbonic acid exhaled. 



a Cited in MILNE-EDWARDS, Physiologic, tome ii., p. 5'74, and BERARD, Cours 

 de Physiologie, tome iii., p. 349. 



8 BERARD, loc. cit. 



