SOURCES OF CAKBONIC ACID EXHALED. 445 



These observations on animals have been confirmed in 

 the human subject by M. Doyere, who found a great varia- 

 tion in the relations of the two gases in respiration ; the vol- 

 ume of carbonic acid exhaled varying between 1-087 and 

 0*862 for 1 part of oxygen consumed. 1 



The destination of the oxygen which is not represented 

 in the carbonic acid exhaled is obscure. Some have thought 

 that it unites with hydrogen to form water; but there 

 is not sufficient evidence of the formation of water in the 

 economy, for researches have failed to show that there is 

 more thrown off from the body than is taken in with food 

 and drink. 



The variations in the relative volumes of oxygen con- 

 sumed and carbonic acid produced in respiration are not 

 favorable to the hypothesis that the carbonic acid is the re- 

 sult of a direct action of oxygen upon carbonaceous matters. 

 We should hardly expect a definite relation to exist between 

 these two gases in respiration, when we find carbonic acid 

 exhaled in the absence of oxygen, as has been shown by the 

 experiments of W. F. Edwards and Geo. Liebig. 



Sources of Carbonic Acid in the Expired Air. AH the 

 carbonic acid in the expired air comes from the venous blood, 

 where it exists in two forms : in a free state in simple solution, 

 or at least in a state of very feeble combination, and in union 

 with bases, forming the carbonates and bicarbonates. That 

 which exists in solution in the blood is simply displaced by 

 the oxygen of the air and exhaled. The alkaline carbonates 

 and bicarbonates of the blood, coming to the lungs, meet 

 witli pneumic acid (discovered by Yerdeil in 1851), and are 

 decomposed, giving rise to a farther evolution of gas. It is 

 pneumic acid which gives the constant acid reaction to the 

 tissue of the lungs. This principle is found in the pulmo- 

 nary parenchyma at all periods of life, from which it may 

 be extracted by the proper manipulations, and obtained 



1 MILNE-EDWARDS, Physiologic, tome ii., p. 694. 



