EXCRETION 461 



secretion of urine is totally stopped, as Nussbaum found, but only 

 in three experiments out of eighteen was it possible to start the 

 secretion by injection of urea. The epithelium of the tubules 

 degenerated and desquamated after complete ligation of all the 

 renal arteries, showing that it requires some arterial blood as 

 well as the venous blood from the renal portal to maintain its 

 vitality. The degeneration of the epithelium can be prevented 

 by keeping the frogs in an atmosphere of oxygen after ligation of 

 the arteries. In six such frogs, in which the complete elimination 

 of the glomeruli was controlled by subsequent injection, secre- 

 tion of urine followed the injection of urea, alone or in combina- 

 tion with dextrose, phloridzin, or di-sodium hydrogen phosphate 

 (Na 2 HPO 4 ). In all the cases the urine contained urea, chlorides, 

 and sulphates, and was acid to phenolphthalein. In one case 

 after injection of urea and dextrose, and in another after urea and 

 phloridzin, the urine reduced Fehling's solution, and therefore 

 presumably contained dextrose (Beddard and Bainbridge). 

 When the frog's kidney is perfused in situ with oxygenated salt 

 solution a certain flow of urine takes place. Substitution of non- 

 oxygenated saline markedly slows the flow (Cullis). 



Apparently, then, the tubules have the capacity to secrete 

 practically all the constituents of urine, and when the flow of 

 urine is small, probably most of it comes from the tubules. When, 

 as in the diuresis produced by salt solutions, large quantities of 

 water and salts have to be rapidly excreted, the bulk of the liquid 

 comes from the glomeruli, but also by a process of secretion. 



Lindemann has endeavoured to exclude the glomeruli in 

 mammals by injecting oil through the renal artery. After a 

 short time, according to him, the oil emboli clear away from 

 practically all parts of the kidney except the glomeruli, which 

 remain plugged. If indigo-carmine be subsequently injected into 

 the blood, it is not only taken up from it by the embolized kidney 

 as well as by a normal one, but is excreted. The quantity of 

 urine is much diminished and its specific gravity increased, but 

 its composition is not essentially altered. He infers that the 

 tubules are in a high degree independent of the glomeruli as an 

 apparatus for the secretion of urine. 



As regards our first two questions we may conclude that 

 there is no good evidence that reabsorption of water or other con- 

 stituents of the urine in the renal tubules plays an important part 

 in the preparation of that secretion. Many facts favour the con- 

 clusion that the glomeruli and the renal epithelium act as separate 

 although, of course, mutually supplementary mechanisms, the 

 glomeruli separating the larger portion of the water and salts, the 

 epithelium the larger portion, if not the whole, of the characteristic 

 organic constituents. 



As regards the third question, it is now generally admitted, 



