THE EXCRETION OF URINE 



515 



pressure equal to that found in the aorta, a considerable flow of fluid 

 may be secured from the ureters, but no fluid is obtained when the renal 

 portal vein is perfused at a pressure equal to that normally present in 

 this vein. Rowntree and Geraghty 11 found that phenolsulphonephthalein 

 added to the perfusion fluid passed through the renal portal vein, did not 

 cause secretion, but when urea was added to the perfusate, fluid con- 

 taining the dye was obtained from the ureter. Unfortunately the pres- 

 sure employed in these experiments may have allowed some fluid to be 

 forced backward into the glomeruhis, so that the results may be due to 

 filtration through the capsule. 



Renal 

 artery 



jMalpighian 

 corpuscle 



Renal-portal vein 



Fig. 172. Diagram of blood supply of Malpighian corpuscle and of convoluted tubules in amphibian 



kidney. (Redrawn from Cushny.) 



It is generally accepted that the proof that the capsule acts as a filter 

 is fairly complete. Unfortunately such decisive experimental facts can 

 not be offered to prove the assumption that the epithelium of the tubules 

 reabsorbs the excess of water and salts which are filtered off through 

 the capsule. If the modern theory of urine excretion is correct, the cells 

 of the tubules must not only absorb large amounts of water, but they 

 must also allow for the reentrance into the blood, either completely or 

 partially, of certain salts, while they must reject others entirely. 



"We have called attention above to the fact that the glomerular filtrate is 

 very different from the urine that is finally passed. The urine contains a 

 very high percentage of small molecules, and the proportion in which they 



