596 GAS INTERCHANGE IN LUNGS 



particularly those of Krogh, 40 a pupil. of Bohr's, all agree 

 that both the absorption of oxygen and the separation of 

 carbonic acid in the lungs normally takes place exclusively 

 by diffusion, and that the assumption of special vital forces 

 regulating the gas exchange has become superfluous. Hal- 

 dane and Douglas alone 41 believe that if (as in carbon 

 monoxide poisoning, in violent muscular exercise, and in 

 case of oxygen deficiency in the respired air) there occurs 

 a deficiency of oxygen in the tissues, through a regulatory 

 process the oxygen is secreted by active forces ; that in this 

 way the oxygen tension in the arterial blood is apparently 

 distinctly raised above that of the air in the pulmonary 

 alveoli, which could not be reconciled with ideas of processes 

 of simple diffusion. E. duBois-Reymond, however, notes 

 in answer to this that if normally the gas exchange is the 

 result of simple diffusion, it would not be easy to see how 

 the pulmonary epithelial cells may be supposed to suddenly 

 acquire a power of gas secretion; that aside from this the 

 histological appearances give no basis for ascribing to these 

 cells any such activity. It may therefore be held that we 

 are fully justified in dropping the hypothesis of gas secre- 

 tion by the pulmonary epithelium. 



Secretion of Oxygen in the Swim-bladder of Fish. We 

 know from comparative physiology of an unmistakable 

 example of undoubted oxygen secretion actually performed 

 by an organ in nature; this is the swim-bladder of fishes. 

 Long ago Biot was struck by the fact that the gas filling the 

 swim-bladder (an organ apparently primarily serving hy- 

 drostatic purposes) may consist largely of oxygen; and this 

 is true particularly of fishes which come from the deeper 

 sea levels. When it is recalled that the partial pressure 

 of oxygen within the swim-bladder in marked depths may 



40 A. Krogh, Skandin. Arch. f. Physiol., 23, 248, 1910; cf. also P. Trendelen- 

 burg (Zool. Station, Naples), Zeitschr. f. Biol., 57, 495, 1912. 



41 C. G. Douglas and J. S. Haldane (Physiol. Lab., Oxford), Jour, of 

 Physiol., 44, 305, 1912, and earlier contributions. 



