THE EXCRETION OF URINE 517 



pens that the urine excreted has a concentration of salts less than that 

 present in the fluid of the blood. Some investigators believe that such a 

 condition is possible only on the assumption that water is actively ex- 

 creted, but a more plausible explanation based on the modern theory 

 is that the water that is absorbed from the alimentary tract reaches the 

 kidney as a dilute saline solution, and is rapidly filtered off in a form 

 somewhat more dilute than the optimal solution which blood plasma must 

 have for the well-being of the tissues. The tubules reabsorb the amounts 

 of water and of those salts, such as chlorides, uric acid, and sugar, nec- 

 essary to restore the plasma to the optimal concentration, but do not 

 absorb the nonthreshold substances, such as urea. 



It is impossible to analyze the forces that are responsible for such a 

 degree of absorption by the epithelium of the tubules. For the present 

 we must classify them, for want of a better term, as vital forces. The 

 questions that await immediate investigation are whether absorption 

 actually takes place, and, if it does so, what factors cause it to vary. 



Many attempts have been made, by destroying the capsules or the 

 tubules by means of poisons or by operation, to determine directly or 

 indirectly the question of the function of the tubules. 



In such experiments, however, the number of factors involved con- 

 fuse the issue and make the results practically valueless so far as de- 

 termining the normal function of the tubules. Other experimenters 

 have attempted to show absorption in the tubules by injecting diffusible 

 substances, such as chemicals and dyes, into the ureter under what they 

 deemed sufficient pressure to force the solution into the tubules, and by 

 an examination of the blood or the tissues to determine whether or not 

 the injected substances had been absorbed. The results obtained by 

 this method are not convincing, probably chiefly 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 ex- 

 crete dye substances injected intravenously. 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 experi- 

 ments with the dyes by assuming that the masses of dye substances 

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



