THE EXTERNAL ENVIRONMENT 245 



The variations of pressure modify the amount of oxygen in the 

 air. Normally there are 300 grammes of oxygen per cubic metre 

 of air at a pressure of 760 millimetres and at 0C. If the pressure 

 falls by half to 380 millimetres there will only be about 150 

 grammes of oxygen. Has this any physiological effect on man ? 



Compressed air, on the contrary, is richer in oxygen than free 

 air. What is its effect on the organism ? These questions will 

 be answered briefly. 



180. Effects of Atmospheric Pressure. Take first of all the 

 case of barometric depression. It causes a decrease in the normal 

 proportion of oxygen. Up to 115 grammes per cubic metre the 

 respiration is in no way troubled. This is true whether the 

 pressure is maintained constant at 760 and the proportion of 

 oxygen reduced, ( ! ) or if both the pressure and the proportion of 

 this gas fall at the same time.f 2 ) 



The limit of 115 grammes corresponds to a pressure of about 

 290 millimetres, or an average altitude of 7,680 metres. Up to 

 this point the red corpuscles of the blood fulfil their normal 

 function as vehicles of oxygen, and the inter-organic combustions 

 preserve their character of regularity. 



At altitudes of 1,500 metres and upwards the number of red 

 corpuscles increases, ( 3 ) and by this increase of their total surface, 

 they make up the deficit created by the fall in the proportion 

 of oxygen. For instance, Mercier( 4 ) noticed at Zurich (Z) at an 

 altitude of 412 metres, that a cubic millimetre of his blood con- 

 tained 5-4 millions of red corpuscles approximately ; on Mount 

 Arosa (A), at an elevation of 1,800 metres, the number rose to 

 7 millions and more ; at Bale (B), where the altitude is 266 

 metres, it fell to 6 millions, still feeling the effects of the stay on 

 Mount Arosa, which had lasted ten months (see diagram, fig. 146). 

 The stay must needs have a certain duration, ( 5 ) at least if the 

 altitude~is not more than 2,800 metres. ( 6 ) 



By making good the deficit in oxygen consequent on the rare- 

 fication of the atmosphere, life can be sustained at a height of 

 11,650 metres without difficulty. The experiment consists ( 7 ) 

 in breathing from a pneumatic receiver air rich in oxygen, while 

 the barometric pressure is reduced to 192 millimetres, which cor- 

 responds to an altitude of 11,690 metres. 



(!) Paul Bert, La Pression Atmosphevique, 1878, p. 654; Speck, Physiol. 

 d. Menschl. Athmens, Leipzig, 1892, p. 123 ; J. Tissot (Comptes Rendus 

 Biologie, 1901, pp. 876, 941). 



( 2 ) J. Tissot (Ibid., 1902, pp. 682, 688). 



( s ) Viault (Comptes Rendus Acad. Sciences, 1890, vol. cxi., p. 917). 



( 4 ) A. Mercier (Arch, de Physiol., 1880, p. 769). 



( 5 ) Giacosa (Rend. R. Instit. Lomb. d. Sc. and Lett., 1897, p. 410). 



( 6 ) Kronecker and Marti (Arch. Ital. BioL, vol. xxvii., p. 333, 1897). 



( 7 ) A. Mosso, Fisiologia del Homo sulle Alpi, Milan, 1897. 



