EXHALATION OF CARBON DIOXIDE. 141 



The air which escapes during the first part of an expiration is less rich 

 in carbon dioxide than that which is last expelled and comes directly from 

 the deeper portions of the lungs. Dividing, as nearly as possible, the expira- 

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

 ments, a percentage of 3*72 in the first part of the expiration and 5-44 in the 

 second part. 



Temporary arrest of the respiratory movements has a marked influence 

 in increasing the proportion of carbon dioxide in the expired air, although 

 the absolute quantity exhaled in a given time is diminished. In a number 

 of experiments on his own person, Vierordt ascertained that the percentage 

 of carbon dioxide becomes uniform in all parts of the respiratory organs, 

 after holding the breath for forty seconds. Holding the breath after an 

 ordinary inspiration, for twenty seconds, the percentage of carbon dioxide in 

 the expired air was increased 1-73 above the normal standard; but the abso- 

 lute quantity exhaled was diminished by 2-642 cubic inches (43-3 c. c.) After 

 taking the deepest possible inspiration and holding the breath for one hun- 

 dred seconds, the percentage was increased 3*08 above the normal standard ; 

 but the absolute quantity was diminished more than fourteen cubic inches 

 (229-4 c. c.). Allen and Pepys noted that air which had passed nine or ten 

 times through the lungs contained 9*5 per cent, of carbon dioxide. 



Vierordt has given the following formula as representing the influence of 

 the frequency of the respirations on the production of carbon dioxide : 

 Taking 2*5 parts per hundred as representing the constant value of the gas 

 exhaled by the blood, the increase over this proportion in the expired air is 

 in exact ratio to the duration of the contact of the air and blood. 



The absolute quantity of carbon dioxide exhaled in a given time is a more 

 important subject of inquiry than the proportion contained in the expired 

 air ; for the latter varies with every modification in the number and extent 

 of the respiratory acts, and the volume of breathing air is subject to great 

 fluctuations and is very difficult of determination. 



Among the most reliable observations on the quantity of carbon dioxide 

 exhaled by the human subject in a definite time and the variations to which 

 it is subject, are those of Andral and Gavarret and of Edward Smith. The 

 observations of Lavoisier and Seguin, Prout, Davy,' Dumas, Allen and Pepys, 

 Scharling and others, do not seem to have fulfilled the necessary experimental 

 conditions so completely. The observations of Andral and Gavarret were 

 made on sixty-two persons of both sexes and different ages, and under identi- 

 cal conditions as regards digestion, time of the day, barometric pressure and 

 temperature ; and the observations on males between the ages of sixteen and 

 thirty, between 1 and 2 P. M., under identical conditions of the digestive and 

 muscular systems, each experiment lasting eight to thirteen minutes, showed 

 an exhalation of about 1,220 cubic inches (20 litres) of carbon dioxide per 

 hour. 



Edward Smith employed the following method for the estimation of 

 the carbon dioxide exhaled : He used a mask, fitting closely to the face, which 

 covered only the air-passages. The air was admitted after having been meas- 

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