THE REACTION OF THE URINE 727 



simply by the greater absorption of water on the obstructed side. 

 But he showed that on the obstructed side the percentage acidity 

 to litmus rose out of all proportion to the percentage increase in 

 phosphates, and, as the absolute quantity of phosphate was re- 

 duced on the obstructed side, there must have been an even 

 greater reduction in the sodium present. In other words, the 

 effect of obstruction was to decrease the absolute excretion both 

 of phosphate and sodium, but the decrease in the excretion of 

 sodium was the greater. From these results he concludes that 

 the greater acidity on the obstructed side is due not to an in- 

 creased excretion of acids, but to an increased absorption of base 

 as compared to the phosphate. He therefore considers that the 

 normal difference between the reaction of blood and urine is 

 brought about, not by the addition of = HP0 4 anions, but to the 

 absorption of Na cations in combination with OH or HC0 3 . 



Although these experiments by Cushny, like others of his 

 already considered, are suggestive of absorption by the tubules, 

 they can hardly be considered to prove it. For, the essence of 

 the experiments is the differences in the urine from an obstructed 

 and free ureter. It is quite possible that partial obstruction does 

 favour absorption by the tubules, but it is conceivable that it 

 might affect and alter their excretory powers. 



Conclusions. We have seen that there are as yet no experi- 

 ments which prove how the kidney produces urine from the 

 blood. We are therefore reduced to weigh the evidence for and 

 against the various theories in order to find a working hypothesis. 

 And in doing so, it must be remembered that if we knew the 

 function either of the tubule or of the glomerulus, we could 

 deduce roughly the function of the other. For, if the tubules 

 were shown to excrete, it would follow that the glomerulus could 

 not act as the mechanical filter supposed on Ludwig's view ; and 

 conversely, if the glomerulus is a mechanical filter, the tubules 

 must absorb. But, on the other hand, if the tubules were 

 shown to absorb, it would still be possible for them also to 

 excrete, because, on Heidenhain's view of the function of the 

 glomerulus, there is room for absorption as well as for excretion 

 by the tubule. We have seen that the evidence that the tubule 

 can excrete is certainly stronger than that it can absorb ; it follows 

 that the glomerulus does not act as a mechanical filter, and this 

 again coincides with the balance of the experimental evidence on 

 this point. But when we come to the question whether the 



