CAUSES OF THE EXCHANGE OF 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 bet ween 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. 
Xow Exner's i 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 ,-g 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," appears to be untenable. 
1 Arch.f. Physiol., Leipzig, 1895, S. 213. 
- Comtpt. rend. Acad. d. sc, Paris, 1891, tome cxii. p. 1232 ; 1892, tome cxv. p. 835. 
3 Arch. f. d. ges. Physiol., Bonn, 1888, Bd. xlii. S. 408. 
4 Ann. d. Phys. u. Chem., Leipzig, 1875, Bd. civ. S. 321, 443. 
5 This article, p. 771. See also Holmgren, Sitzungsb. d. k. Akad. d. JJ'isscnsch. 
Math.-natv/rv. CI., 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. 
6 "Die Bedeutungdes 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. rjes. Physiol., Bonn, 1888, Bd. xlii. S. 408. 
