THE ELIMINATION OF METABOLIC PRODUCTS. 587 



this nature made under quite different conditions that we may well 

 assume that an actual secretion of uric acid takes place, and its separa- 

 tion from the blood is probably the result of a selective action of the 

 epithelium of the above-mentioned portions of the uriniferous tube. We 

 will mention in addition that Hober and Konigsberg l have proved that 

 the epithelium of the uriniferous tube is not only able to take up colors 

 which are soluble in lipoids, but also those which are insoluble in lipoids. 

 Unfortunately, it has up to the present not been found possible to local- 

 ize th secretion of urea and other substances in the same way as in the 

 case of uric acid. 



We do not in any way intend to suggest that the secretion of the urine 

 is a perfectly simple and uniform process. Unquestionably a great num- 

 ber of different processes are taking place side by side, which mutually 

 assist one another. On one side constituents of the urine are given up 

 through the Malpighian bodies to the capsules of Bowman, and evi- 

 dently at this place the greater part of the water is sent out, while the 

 epithelium of the uriniferous tube is constantly removing definite con- 

 stituents from the blood, accumulating them and then giving them up 

 again inward to the lumen of the tube. If we consider in addition that 

 many observations indicate that there is more or less absorption in the 

 uriniferous tube of some of the substances which have previously been 

 secreted, of water especially, then we shall begin to understand that 

 diuretics can find a number of different points of attack, and cause, in a 

 number of different ways, disturbances in the secretion of the urine. 



We must come back once more to the fact that under normal condi- 

 tions there is no sugar in the urine. As we have said, it has been sug- 

 gested that this was because the glucose did not circulate as such in the 

 blood, but combined in some way with colloidal substances, although 

 direct experiments 2 have shown that this representation is in no way 

 justifiable. Sugar is present as such in the blood. Evidently the vas- 

 cular endothelia of the kidneys are adjusted to a certain definite sugar 

 content of the blood. When more than this is present the sugar passes 

 over into the urine. Now it seems probable that certain substances cause 

 sugar to pass into the urine even when there is no glucohemia. Such, 

 for example, is phloridzin, which, according to many observers, acts 

 directly upon the kidneys, or indeed at first upon the vascular endo- 

 thelia. 3 Recently Underbill and Closson 4 have stated that those forms 

 of glucosuria, which appear when common salt is introduced into the 

 circulation, are in some cases to be regarded as due to a direct influence 



1 Hober and Konigsberg: Pfluger's Arch. 108, 323 (1905). 



2 Leon Asher and R. Rosenfeld: Zentr. Physiol. 19, 449 (1905). See Lecture II, p. 30. 

 8 See Lecture V, p. 81. 



4 Am. J. Physiol. 15, 321 (1906). 



