240 SCIENCE PROGRESS. 



normal quiet inspiration being about 350 cc. The expired 

 air thus consists of a mixture of about two parts of 

 alveolar air, with one part of pure air ; consequently it 

 is necessary to increase by about one half the carbonic 

 tension in expired air to obtain the alveolar carbonic acid 

 tension. The alveolar carbonic acid will therefore be about 

 5 per cent, of an atmosphere in man. This tension will 

 vary only slightly during the different phases of a respira- 

 tion, since the volume of pure air introduced into the alveoli 

 at each inspiration is only about a tenth of the total volume 

 of air in the lungs. It is to be regretted that in the experi- 

 ments from Pfliiger's laboratory on the arterial carbonic 

 acid tension in dogs no attempt was made to estimate the 

 carbonic-acid tension of the alveolar air, or even of the 

 expired air, during the experiments. Three analyses are, 

 however, quoted by Wolffberg of the expired air of the 

 dog. These gave a mean of 2 '8 per cent. If we increase 

 this value by one half to obtain the alveolar tension, as in 

 the case of man, we get about 4 per cent. This is a good 

 deal above the mean carbonic acid tension (2*8 per cent.) 

 found by Strassburg in the arterial blood, and the latter 

 value is surprisingly low on the diffusion theory. It may 

 have been, however, that the animals on which Strassburg 

 made his experiments, though they were not tracheotomised, 

 were breathing so freely that the carbonic-acid tension in 

 the alveoli was reduced to the values which he found for 

 arterial blood. 



Much more satisfactory evidence in favour of the diffu- 

 sion theory was obtained in another way. The tension of 

 carbonic acid in venous blood from the right side of the 

 heart was measured by the aerotonometer, and found to be 

 on an average 3-81 per cent. In the same animals, which 

 were of course tracheotomised, the air supply to a portion 

 of one lung was entirely cut off, and a sample of the gas 

 withdrawn after a few minutes by means of an instrument 

 known as the lung-catheter. On the diffusion theory the 

 tension of carbonic acid in the gas contained in this blocked- 

 off portion of lung ought evidently to become equal after a 

 short interval of time to the tension of the same gases in 



