378 HANDBOOK OF PHYSIOLOGY. 



substances such as urea, whereas from the glomeruli is separated the 

 water and the inorganic salts. Another theory suggested by Ludwig 

 (1844) is that in the glomeruli are filtered off from the blood all the con- 

 stituents of the urine in a very diluted condition. When this passes 

 along the tortuous uriniferous tube, part of the water is reabsorbed into 

 the vessels surrounding them, leaving the urine in a more concentrated 

 condition retaining all its proper constituents. This osmosis is pro- 

 moted by the high specific gravity of the blood in the capillaries sur- 

 rounding the convoluted tubes, but the return of the urea and similar 

 substances is prevented by the secretory epithelium of the tubules. Lud- 

 wig'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 

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

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

 certain substances, e. g., peptones and 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 pig- 

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

 circumstances swept away from them by the flushing of them which ordi- 

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

 uli. 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 for believing that part of the increase of the secretion is 



