RESPIRATION 31 



pressure of CO 2 in the alveolar air remained about constant, while 

 the percentage varied. A more conclusive experiment was made in 

 a large steel pressure chamber, employed at the Brompton Hospi- 

 tal, London, for the treatment of asthma. In this chamber the 

 only one then existing in England of the kind we compared 

 our alveolar air at normal atmospheric pressure, and at the highest 

 pressure which the chamber would stand. The mean results were 

 as follows : 



It is quite clear from these results that it is the pressure of CO 2 

 in the alveolar air, and not its mere percentage, which regulates 

 the breathing. It is also as evident from these experiments as 

 from those already mentioned in which the oxygen percentage 

 was varied, that the oxygen pressure in the alveolar air may be 

 increased very greatly without at the time affecting the regula- 

 tion of the CO 2 pressure. The actual alveolar oxygen pressure 

 was 13.0 per cent of an atmosphere in the observations at ordinary 

 pressure, and 26.8 per cent in those at the high pressure. 



Still more striking results were obtained by Leonard Hill and 

 Greenwood, 15 and by Boycott 16 in steel chambers erected later 

 for the investigation of the effects of high atmospheric pressures. 

 Hill and Greenwood obtained the following results. 



They considered at the time that their results showed that the 

 production of CO 2 remained unaltered during the experiments; 

 and it is evident that had the volume of air breathed and the mass 

 of CO 2 produced remained the same the results would have been 

 as they found. But the constancy of the partial pressure of CO 2 

 was certainly due, not to the cause which they suggested, but to 

 the fact that the breathing was regulated so as to keep the partial 

 pressure of CO 2 steady. 



"Hill and Greenwood, Proc. Roy. Soc., 1906, B, LXXVII, p. 442, 1906. 

 18 Boycott and Haldane, Journ. of Physiol., XXXVII, p. 365, 1908. 



