DIFFUSION OF AIR IN THE LUNGS. 139 



The interchange between the fresh air in the upper portions of the respiratory appa- 

 ratus and the air in the deeper parts of the lungs is constantly going on, in obedience to 

 the well-known law of the diffusion of gases, aided by the active currents or impulses 

 produced by the alternate movements of the chest. When two gases, or mixtures of 

 gases, of different densities are brought in contact with each other, they diffuse or mingle 

 with great rapidity, until, if undisturbed, the whole mass has a uniform density and com- 

 position. 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 reservoirs are connected by a 

 small tube many feet in length, though then it proceeds quite slowly. In the respiratory 

 apparatus, at the termination of inspiration, the atmospheric air, composed of a mixture 

 of oxygen and nitrogen, is introduced 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 carbonic 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 finds 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 in- 

 versely proportionate to the square root of their densities, the penetration of atmos- 

 pheric 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 

 eighty-one parts of carbonic acid should be replaced by ninety-five of oxygen. It is 

 found, indeed, 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 pul- 

 monary vesicles, where the interchange of gases with the blood takes place, maintains a 

 pretty uniform composition. The process of aeration of the blood, therefore, has none 

 of that intermittent character which attends the muscular movements of respiration, 

 which would undoubtedly occur if the entire gaseous contents of the lungs were changed 

 with every respiratory act. 



CHAPTER V. 



CHANGES WHICH THE AIR AND THE BLOOD UNDERGO IN RESPIRATION. 



Composition of the air Consumption of oxygen Exhalation of carbonic acid Influence of age Relations between 

 the quantity of oxygen consumed and the quantity of carbonic acid exhaled Exhalation of watery vapor Ex- 

 halation of ammonia Exhalation of organic matter Exhalation of nitrogen Changes of the blood in respira- 

 tion (haematosis) Difference in color between arterial and venous blood Comparison of the gases in venous 

 and arterial blood Analysis of the blood for gases Relative quantities of oxygen and carbonic acid in venous 

 and arterial blood Nitrogen of the blood Condition of the gases in the blood Mechanism of the interchange 

 of gases between the blood and the air in the lungs Relations of respiration to nutrition, etc. Views of physi- 

 ologists anterior to the time of Lavoisier Relations of the consumption of oxygen to nutrition Relations of 

 the exhalation of carbonic acid to nutrition Essential processes of respiration The respiratory sense, or want 

 on the part of the system which induces the respiratory movements Respiratory efforts before birth Cuta- 

 neous respiration Asphyxia. 



FROM the allusions which we have already made to the general process of respiration, 

 it is apparent that, before the discovery of the nature of the gases which compose the 

 air and those which are exhaled from the lungs, it was impossible for physiologists to 

 have any correct ideas of the nature of this important function. It is not surprising that 

 the ancients, observing the regular introduction of air into the lungs and noting the fact 

 that the air is generally much cooler than the body, supposed the great object of respi- 

 ration to be the cooling of the blood. It is also evident that no definite knowledge of 

 any of the processes of respiration could exist prior to the discovery of the circulation 



