THE SECRETION OF THE URINE 489 



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. 



The Question of Reabsorption from the Tubules. 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 strychnine 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 phlorhizin 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 im- 

 portant 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 

 control 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 men- 

 tioned on p. 649 suggest a different interpretation of these observa- 

 tions. 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 advantage 

 of the anatomical fact that the kidney of batrachians, and, indeed, 

 that of fishes and ophidia as well, has a double blood-supply. The 

 renal artery gives off afferent vessels to the glomeruli; the vena 

 advehens, or renal portal vein, breaks up, like the portal vein in the 

 liver, into a plexus of capillaries surrounding the tubules, and there 

 seems to be no communication between the two vascular systems. 



