EXCRETION 457 



epithelium being the same, some quantitative or qualitative 

 difference in their activity may not exist, certain constituents, 

 e.g., passing mainly or exclusively through the one or the other. 



3. When these questions have been settled, we are in a position 

 to consider the nature of the process by which the urinary con- 

 stituents find their way from the blood into the lumen of the 

 capsules and the tubules, or, if there is reabsorption, out of the 

 tubules into the lymph and blood again, and to see whether or 

 no it can be entirely explained on mechanical and physico- 

 chemical principles. 



That some absorption can take place from the kidney when 

 the pressure in the ureter is abnormally raised need not be 

 doubted, and when substances like potassium iodide or strych- 

 nine are introduced into the ureter or the pelvis of the kidney 

 under these circumstances, they can speedily be detected in 

 the blood. When the ureter pressure (in dogs) is only slightly 

 increased, instead of evidence of reabsorption, we obtain evidence 

 of increased secretion. The volume of urine, the total quantity 

 of sulphate in the urine when sodium sulphate is injected into 

 the blood as a diuretic, and the total amount of reducing sugar 

 when phloridzin is injected, are all greater on the obstructed 

 than on the normal side. These facts are quite opposed to the 

 idea that filtration and reabsorption are important factors in the 

 preparation of normal urine (Brodie and Cullis). Changes in the 

 blood-flow through the kidney have nothing to do with the 

 results, since the small increase in pressure in the ureter was shown 

 not to affect the rate of flow of the blood. The attempt has been 

 made to decide whether absorption normally occurs by removing 

 as much of the tubules as possible, and seeing whether the 

 character of the urine is altered. In rabbits the whole or a large 

 portion of the medulla has been excised from one kidney and the 

 other then extirpated. From the mutilated kidney two or three 

 times as much urine was said to flow as was secreted by a con- 

 trol rabbit operated on in the same way, except for the removal 

 of the renal medulla (Ribbert). The conclusion was drawn that 

 the greater quantity of urine escaping was due to the smaller 

 opportunity for reabsorption of the water. But experiments 

 mentioned on p. 569 suggest a different interpretation of 

 these observations. And Boyd, who repeated Ribbert's work, 

 obtained quite different results after partial removal of the 

 medulla. He found it impossible to remove the whole. So that 

 hitherto the direct method of eliminating the tubules has left 

 the matter where it was. 



Some light has been thrown on this question, by taking ad- 

 vantage of the anatomical fact that the kidney of batrachians, 

 and, indeed, that of fishes and ophidia as well, has a double 



