DIFFUSION OF AIR IN THE LUNGS. 407 



or mingle with great rapidity, until, if undisturbed, the whole 

 mass has a uniform density and composition. This has been 

 shown to take place between very light and very heavy gases 

 in opposition to the laws of gravity, and even when two res- 

 ervoirs are connected by a small tube many feet in length, 

 though then it proceeds quite slowly. In the respiratory ap- 

 paratus, at the termination of inspiration, the atmospheric 

 air, composed of a mixture of oxygen and nitrogen, is intro- 

 duced into the tubes with a considerable impetus, and is 

 brought into contact with the gas in the lungs, which is 

 much heavier, as it contains a considerable quantity of car- 

 bonic acid. Diffusion then takes place, aided by the elastic 

 lungs, which are gradually forcing the gaseous contents out 

 of the cells, until a certain portion of the air loaded with 

 carbonic acid linds its way to the larger tubes, to be thrown 

 off in expiration, its place being supplied by the fresh air. 



In obedience to the law established by Graham, that the 

 diffusibility of gases is inversely proportionate to the square 

 root of their densities, the penetration of atmospheric air, 

 which is the lighter gas, to the deep portions of the lungs 

 would take place with greater rapidity than the ascent of the 

 air charged with carbonic acid ; so that 81 parts of carbonic 

 acid should be replaced by 95 of oxygen. 1 It is found, in- 

 deed, that the volume of carbonic acid exhaled is always less 

 than the volume of oxygen absorbed. 



This diffusion is constantly going on, so that the air in 

 the pulmonary vesicles, where the interchange of gases with 

 the blood takes place, maintains a pretty uniform composi- 

 tion. The process of aeration of the blood, therefore, has 

 none of that intermittent character which attends the me- 

 chanical processes of respiration, which would undoubtedly 

 occur if the entire gaseous contents of the lungs were changed 

 with every act. 



There is no evidence sufficiently definite to show that the 

 muscular fibres in the bronchial tubes, which are of the un- 



3 Cyclopaedia of Anatomy and Physiology, vol. iv., part 1, p. 362. 



