CAUSES OF THE EXCHANGE OP GASES. 779 



it correctly. Some of the results obtained by Haldane and Lorrain Smith in 

 their examination of these sources of fallacy are opposed to those obtained by 

 Hiifner l and Saint-Martin. 2 



It is impossible to pass a verdict upon such discordant evidence, 

 especially since further investigation is necessary to test the soundness 

 of many of the experiments and of the conclusions based upon the results. 

 It is permissible, however, to accept the provisional conclusion that the 

 exchange of gases between the blood and the air in the lungs is effected 

 by physical and chemical means, of which the most important is 

 diffusion. 



According to the calculations made by Zuntz, 3 the surface of the 

 human lungs is 90 square metres, and through this there diffuse during 

 quiet breathing about 300 c.c. of carbon dioxide and about the same 

 quantity of oxygen in a minute. Through the square centimetre of 

 surface there would pass only the small quantity of 0*0003 c.c. of gas. 

 Now Exner's 4 experiments show that through the square centimetre of 

 a soap film 0*6 c.c. of air diffuse into an indifferent gas during one 

 minute. The velocity of diffusion is proportional to the density of the 

 gas, therefore a difference in tension of 20 1 of an atmosphere, or 0'3 

 mm. of mercury, would be sufficient to make 0'0003 c.c. of oxygen pass 

 through such a film in a minute. Further, the velocity of diffusion is 

 proportional to the coefficient of absorption of the gas in the fluid in 

 question, and inversely proportional to the square root of its density ; 

 therefore the velocity for carbon dioxide is about thirty times greater 

 than that of oxygen, and there is needed for carbon dioxide an even less 

 difference of tension to cause a diffusion of gas from the blood into the 

 alveoli. These considerations Zuntz supports by the following experi- 

 ment. The bronchus of a frog's lung is ligatured, and the lung is placed 

 in carbon dioxide ; within a minute the lung is distended, owing to the 

 diffusion of carbon dioxide being, on account of its high coefficient of 

 absorption, about forty-five times greater than that of air. If a tube be 

 placed in the bronchus, the diffused gas can be collected and measured. 



Diffusion appears to be sufficient to account for the phenomena of gaseous 

 exchange in the lungs. Other conditions possibly assist in the process. It 

 has been shown that oxygen in combination with haemoglobin appears to have 

 the property of driving out carbon dioxide. 5 



Fleischl von Marxow 6 supposes that the sudden percussion given by the 

 contraction of the ventricles to the blood assists in the liberation of the 

 carbon dioxide in the lungs, and of oxygen in the arterioles supplying the 

 tissues of the body. This theory, however, after the criticisms brought forward 

 by Zuntz, 7 appears to be untenable. 



1 Arch.f. Physiol., Leipzig, 1895, S. 213. 



2 Compt. rend. Acad. d. sc., Paris, 1891, tome cxii. p. 1232 ; 1892, tome cxv. p. 835. 

 G Arch.f. d. ges. Physiol. , Bonn, 1888, Bd. xlii. S. 408. 



4 Ann. d. Phys. u. Ohcm., Leipzig, 1875, Bd. civ. S. 321, 443. 



5 This article, p. 771. See also Holmgren, Sitzungsb. d. k. Akad. d. Wisscnsch. 

 Math.-naturw. 01., Wien, Bd. xlviii.; Werigo, Arch. f. d. ges. Physiol., Bonn, 1892, 

 Bd. li. S. 321 ; 1892, Bd. Hi. S. 194 ; Zuntz, ibid., Bd. Hi. S. 191, 198. 



"Die Bedeutung des Herzschlages f. d. Athmung, eine neue Theorie des Respiration," 

 Stuttgart 1887 ; Centralbl. f. Physiol., Leipzig u. Wien, 1887, S. 231, 662. 

 7 Arch.f. d. ges. Physiol., Bonn, 1888, Bd. xlii. S. 408. 



