THE KIDNEYS AND URINE. 369 



two parts of the tubules,, is that the cells of the convoluted tubes, by a 

 process of true secretion, separate from the blood substances such as 

 urea, whereas from the glomeruli are separated the water and the inor- 

 ganic salts.. Another theory suggested by Ludwig (1844) is that in the 

 glomeruli is filtered off from the blood all the constituents of the urine in 

 a very diluted condition. When this passes along the tortuous uriniferous 

 tube, part of the water is re-absorbed into the vessels surrounding them, 

 leaving the urine in a more concentrated condition retaining all its 

 proper constituents. This osmosis is promoted by the high specific gravity 

 of the blood in the capillaries surrounding the convoluted tubes, but the 

 return of the urea and similar substances is prevented by the secretory 

 epithelium of the tubules. Ludwig's theory, however plausible, must, 

 we think, give way to the first theory, which is more strongly supported 

 by direct experiment. 



By using the kidney of the newt, which has two distinct vascular sup- 

 plies, one from the renal artery to the glomeruli, and the other from the 

 renal portal vein to the convoluted tubes, Nussbaum has shown that cer- 

 tain substances, e.g., peptones, sugar, when injected into the blood, are 

 eliminated by the glomeruli, and so are not got rid of when the renal 

 arteries are tied; whereas certain other substances, e.g., urea, when injected 

 into the blood, are eliminated by the convoluted tubes, even when the 

 renal arteries have been tied. This evidence is very direct that urea is 

 excreted by the convoluted tubes. 



Heidenhain also has shown by experiment that if a substance (sodium 

 sulphindigotate), which ordinarily produces blue urine, be injected into 

 the blood after section of the medulla which causes lowering of the blood- 

 pressure in the renal glomeruli, that when the kidney is examined, the 

 cells of the convoluted tubules (and of these alone) are stained with the 

 substance, which is also found in the lumen of the tubules. This appears 

 to show that under ordinary circumstances the pigment at any rate is 

 eliminated by the cells of the convoluted tubules, and that when by 

 diminishing the blood-pressure, the filtration of urine ceases, the pigment 

 remains in the convoluted tubes, and is not, as it is under ordinary cir- 

 cumstances, swept away from them by the flushing of them which ordi- 

 narily takes place with the watery part of urine derived from the glom- 

 eruli. It therefore is probable that the cells, if they excrete the pigment, 

 excrete urea and other substances also. But urea acts somewhat differ- 

 ently to the pigment, as when it is injected into the blood of an animal in 

 which the medulla has been divided and the secretion of urine stopped, a 

 copious secretion of urine results, which is not the case when the pigment 

 is used instead under similar conditions. The flow of urine, independent 

 of the general blood-pressure, might be supposed to be due to the action 

 of the altered blood upon some local vaso-motor mechanism; and, indeed, 

 the local blood-pressure is directly affected in this way, but there is reason 

 VOL. I. 24. 



