CHAPTER XIII 



Effects of Low Atmospheric Pressures. 



Very low atmospheric pressures are met with on mountains or 

 high plateaus and in ascents by balloons or aeroplanes to great 

 altitudes. Mountain sickness, one of the characteristic effects of 

 low atmospheric pressures, was known long before atmospheric 

 pressure and the composition of the atmosphere were understood. 

 It was commonly attributed to poisonous emanations. A good 

 account of earlier records of it is given by Paul Bert. His experi- 

 ments on animals and men showed clearly that the physiological 

 effects produced by low atmospheric pressure are simply the re- 

 sult of the diminished partial pressure of oxygen. The nature of 

 these effects and the manner in which they are produced have been 

 described generally in Chapters VI and VII in connection with 

 the symptoms and causes of anoxaemia. It remains, however, to 

 discuss the subject in detail. 



Although Paul Bert's very important conclusion that the physi- 

 ological actions of oxygen and other gases depend on their partial 

 pressures has often been referred to in preceding chapters, no 

 very definite account has been given of his experiments. It will 

 be convenient to summarize them here, and at the same time refer 

 to certain points on which later investigation has thrown new 

 light. 



By studying the conditions producing death in animals (chiefly 

 sparrows) confined in a closed vessel at varying atmospheric pres- 

 sures and with varying compositions of the initial air breathed, 

 Paul Bert proved that if the pressure of oxygen was not sufficiently 

 high to produce oxygen poisoning, death was due either to in- 

 creased pressure of CO2 or to diminished pressure of oxygen. At 

 ordinary barometric pressure, and with ordinary air inclosed in 

 the vessel, death occurred when the oxygen percentage fell to 

 about 3.5. At half the ordinary pressure 7.0 was the fatal oxygen 

 percentage, so that the partial pressure of oxygen was the same; 

 and so on down to pressures of a third or even a fourth of an 

 atmosphere. If the vessel was filled with air highly enriched with 

 oxygen and the pressure was reduced to a fourth, or even a tenth, 

 the result was the same as regards the fatal partial pressure of 



