SECRETION AND ABSORPTION OF GAS, ETC. 241 



the venous blood. It was found by Wolff berg and Nuss- 

 baum that on an average this was the case, although in 

 individual determinations the tension found was sometimes 

 a good deal higher in the lung and sometimes in the blood. 

 The average carbonic-acid tension in the blocked-off lung 

 was 3*84 per cent., as compared with 3 - 8i per cent, for the 

 venous blood. 



These results evidently afford very strong support to 

 the diffusion theory. It maybe pointed out, however, that, 

 apart from a probable source of fallacy due to the bronchial 

 circulation, there appears to be clear evidence that the 

 venous blood coming to the lungs was not normal venous 

 blood, but blood in a more or less arterialised state, the 

 difference being due to the disturbance produced by the 

 tracheotomy and the introduction of the lung catheter. In 

 animals which had not been tracheotomised Strassburg found 

 that the carbonic-acid tension in the venous blood varied 

 from about 5 to 6*5 per cent., and was thus always much 

 higher than in the experiments just mentioned. On the 

 hypothesis that the lung epithelium acts like that of other 

 excretory glands, it is reasonable enough to suppose that 

 the excretory activity would not come into play with a low 

 carbonic-acid tension in the venous blood, or even that the 

 epithelium might oppose the tendency of carbonic acid to 

 escape by diffusion. The kidney epithelium, for instance, 

 seems to regulate in the most delicate manner the per- 

 centage of water, chlorides, sugar, etc., in the blood. Thus 

 in the case of chlorides the kidney epithelium either actively 

 opposes their natural tendency to escape by diffusion and 

 filtration or actively excretes them, according to their scarcity 

 or excess in the food taken in. The normal high percentage 

 of chlorides in the blood is thus kept very steady. A certain 

 tension of carbonic acid in the tissues of the body may be 

 as requisite to health as a certain tension of chlorides ; 

 hence the fact that under some conditions the evidence is 

 distinctly against an active excretion of carbonic acid by the 

 lungs does not exclude the hypothesis that they act towards 

 carbonic acid like true excretory glands. 



In the experiments of Pfliiger and his pupils very little 



