406 KESPIKATION. 



than that which is taken into the lungs. All the older ex- 

 perimenters, except Magendie, were agreed upon this point. 

 The loss was put by Davy at -fa, and by Cuvier at -^5- of the 

 amount of air introduced. 1 Observations on this point, to be 

 exact, must include a considerable number of respiratory 

 acts ; and from the difficulty of continuing respiration in a 

 perfectly regular and normal manner, when the attention is di- 

 rected to that function, the most accurate results may prob- 

 ably be obtained from experiments on animals. Despretz * 

 caused six young rabbits to respire for two hours in a con- 

 fined space containing 299 cubic inches of air, and ascertained 

 that the volume had diminished 61 cubic inches, or a little 

 more than one-fiftieth. "We may take the approximations of 

 Davy and Cuvier, as applied to the human subject, as nearly 

 correct, and assume that in the lungs, from T V to -^ of the 

 inspired air is lost. 



Diffusion of Air in the Lungs. When it is considered 

 that with each inspiration but about twenty cubic inches of 

 fresh air is introduced, sufficient only to fill the trachea and 

 larger bronchial tubes, it is evident that some forces must act 

 by which this fresh air finds its way into the air-cells, and the 

 vitiated air is brought into the larger tubes, to be expelled 

 with the succeeding expiration. The expired air may be- 

 come so charged with noxious gases, by holding the breath 

 for a few seconds, that when collected in a receiver under 

 water, it is incapable of supporting combustion. 



The interchange between the fresh air in the upper portions 

 of the respiratory apparatus 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 cur- 

 rents 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 



1 BERARD, Cours de Physiologic, Paris, 1851, tome iii., p. 338. 

 5 Idem. 



