RESPIRATION 221 



gases contained in venous and arterial blood, so that these pres- 

 sures could be compared with one another and with the corre- 

 sponding pressures in the air of the lungs. The aerotonometer 

 consisted of two tubes immersed in a water bath at body tempera- 

 ture, and closed below by a mercury seal. In one tube was placed 

 a mixture containing a smaller percentage of CO 2 and greater 

 percentage of oxygen than corresponded to the partial pressures 

 expected in the blood ; and in the other tube a mixture containing 

 a higher percentage of CO 2 and a lower percentage of oxygen. 

 The blood from the animal was then allowed to trickle down the 

 inside of the tubes, so that it should as far as possible equalize its 

 gas tensions with those in the tubes, either by taking up or giving 

 off CO 2 or oxygen. In a successful experiment the blood gave off 

 CO 2 and absorbed oxygen in one tube, and vice versa in the other, 

 so that the gas pressures of the blood were defined within narrow 

 limits on the analyses of the gases in the two tubes. The sample of 

 lung air was obtained by another ingenious instrument, the "lung 

 catheter," by means of which a bronchus could be blocked off and 

 a sample of the gas in the lungs drawn off as soon as the air thus 

 confined had reached a constant composition. 



The conclusion drawn from the actual experiments by Pfliiger 

 and his pupils was that there was no average difference in gas 

 pressures between the venous blood and the air inclosed beyond 

 the blocked bronchus; and therefore no evidence of any giving 

 off of CO 2 or absorption of oxygen except by simple diffusion. 11 



The question was taken up again by the late Professor Bohr of 

 Copenhagen, one of Ludwig's pupils. 12 Bohr improved the aeroto- 

 nometer, so that a large stream of arterial blood could be run 

 through it and back to the animal, the blood of which had first 

 been rendered incoagulable by injecting peptone or leech extract. 

 He obtained the result that while usually the CO 2 pressure in the 

 arterial blood is not less than in the alveolar air, and the oxygen 

 pressure not greater, yet sometimes this relation is reversed. From 

 these results he concluded that active secretion of oxygen from 

 the lung air into the blood, and of CO 2 from the blood into the 

 lung air, may both occur. Owing to the many possibilities of 

 error the results were not very convincing, however; and Fred- 

 ericq 13 of Liege soon afterwards made a further series of experi- 



11 Pfli'tger's Archiv, IV, p. 465 ; VI, p. 65 ; VII, p. 23, 1871-1873. 



12 Bohr, Skand. Arch, of Physiol., p. 236, 1891. 

 18 Fredericq, Arch, de Biol., XIV, p. 105, 1896. 



