RESPIRATION 359 



oxygen. On the other hand if the vessel was filled with the en- 

 riched air and left at ordinary barometric pressure, death oc- 

 curred when the percentage of CO2 reached about 26, although 

 the oxygen pressure was far above the danger point ; and similarly 

 if the vessel was filled with compressed air at a pressure not suffi- 

 cient to cause oxygen poisoning. The cause of death depended 

 simply on whether the partial pressure of 3.5 per cent of an 

 atmosphere of oxygen or 26 per cent of an atmosphere of COg 

 was reached first. The mere mechanical pressure had no influence. 

 When, however, the partial pressure of oxygen was raised to the 

 dangerous limits referred to in Chapter XII, death was due to 

 oxygen poisoning, or hastened by it ; and the results suggest that 

 increase of the circulation rate, owing to the presence of CO2, with 

 consequent increase of the partial pressure of oxygen in the tissues, 

 increased the poisonous action of the oxygen, though Paul Bert 

 was unaware of the action of CO2 on the circulation. 



Figure 93 shows an apparatus used by Paul Bert for showing 

 that it is the diminished pressure of oxygen, and not simply the 

 diminished barometric pressure, that affects an animal. The fol- 

 lowing are the notes of an experiment on a sparrow. 



"At 3.20 pressure reduced to 250 mm. in a few minutes. On 

 further reduction to 210 mm. the animal turned round and round, 

 fell down, and was at the point of death. I restored the normal 

 pressure by letting in air enriched with oxygen; the animal re- 

 covered immediately and appeared lively and well. The air in the 

 bell jar now contained 35 per cent of oxygen. At 3.30 pressure 

 reduced to 180 mm. when the animal again became very ill. Pres- 

 sure again restored to normal by letting in oxygen, when the ani- 

 mal recovered at once. The air now contained 77.2 per cent of 

 oxygen. On again reducing the pressure the animal did not fall 

 over till 100 mm. pressure was reached. Immediate recovery on 

 restoring the pressure by letting in oxygen. The air now contained 

 87.2 per cent of oxygen. On reducing the pressure to 100 mm. at 

 3.50 the animal did not seem at all in danger; but at 80 mm. it fell 

 over in a dying condition. It recovered at once on letting in oxy- 

 gen. The air now contained 91.8 per cent of oxygen, and at 4.05 

 the pressure was reduced to 75 mm., when the animal again be- 

 came very ill, so that there was only just time to open the taps and 

 let it recover." This experiment shows very clearly that in air 

 greatly enriched with oxygen the barometric pressure could be 

 reduced to about a third of what was possible in ordinary air. 



It was evident that oxygen could be used to avert the very 



