THE EXCRETION OF URINE 



551 



because of the difficulty in reaching the tubules. Indeed, it is very questionable 

 whether it is possible to inject a substance into the tubules from the ureter. 



Years ago Heidenhain, the exponent of the vital theory of excretion, believed that 

 he had demonstrated the ability of the renal cells to excrete dye substances injected in- 

 travenously. Since he failed to find evidence of dye excretion in the capsule, but 

 found masses of dye in the tubules and stained granules in the cells of the tubules, 

 he concluded that the cells of the tubules had the power to excrete the dye, and from 

 analogy he believed that the tubules must likewise excrete the water and the various 

 urinary salts. Subsequent work, however, has failed to confirm his belief that the 

 capsule is not concerned in the excretion of the dye, and it is as reasonable to explain 

 the results of the experiments with the dyes by assuming that the masses of dye sub- 

 stances found in the tubules and in the cells are due to the reabsorption of water and 

 perhaps of some of the dye from the dilute glomerular filtrate, as to accept Heiden- 

 hain 's hypothesis. 



In the following table taken from Cushny the movements of the con- 

 stituents of the plasma may be followed through the kidney. The ulti- 

 mate destination of each is indicated in the enclosures. 



(From Cushny. 2 ) 



It will be noted that the dextrose alone is completely absorbed, and 

 that the urea and the sulphate are not absorbed at all from the glom- 

 erular nitrate. The other salts are partly absorbed. 



Although at present it is probably most useful for practical purposes 

 to accept Cushny 's hypothesis, it should be remembered that we are far 

 from being in a position to explain all the known facts of the renal 

 function by means of it. There can be no doubt that a process that is 

 closely analogous, if not identical Avith nitration plays an essential part 

 in the formation of urine, and that it is assisted by a more obscure 

 process depending on a selective action of the renal cells. But whether 

 this latter process is essentially one of reabsorption of certain molecules 

 from tubule to blood, or one of secretion from blood to tubule is problem- 

 atical. As G. N. Stewart points out, the reabsorption hypothesis is no 

 simpler to comprehend than the older hypothesis of Bowman and Heiden- 



