SECBETIOX OF THE URINE. 395 



From the glomerulus the watery parts of the fluid are filtered, 

 while the glandular epithelium selects the important solid constit- 

 uents which it is necessary to remove from the blood. 



Ludwig takes a different view. He believes that the watery 

 part of the plasma, bearing with it the salts, etc., is filtered from 

 the glomerulus. As this fluid passes through the tortuous urinary 

 tubules, a large portion of the water is reabsorbed into the capil- 

 lary networks surrounding them. This reabsorption is assisted 

 by the high specific gravity of the blood and the low pressure in 

 these capillaries as compared with the glomeruli, where the fil- 

 tration of the liquid occurs. The role of the epithelium is not 

 then selection from the blood of specific materials, but possibly 

 the prevention of the return of the solids with the water back to 

 the bloodvessels. 



Heidenhain attempted to settle the question as to the function 

 of the renal epithelium, by introducing into the blood a blue 

 coloring matter pure sodium sulphindigotate which he found 

 to be eliminated by the kidneys, giving rise to blue urine. On 

 examining the organ with the microscope at a suitable time after 

 the injection of the color into the blood, the tubules are found to 

 be filled with the pigment, and in some cases the peculiar epi- 

 thelium of the convoluted tubules is stained with the blue sub- 

 stance, while the glomerulus and capsule are entirely free from 

 the color. If the stream of fluid from the glomerulus be stopped 

 in any way tying the ureter, section of the spinal cord, or local 

 destruction of the glomeruli the blue color is only to be found 

 in the convoluted tubes and their epithelium, and hence it has 

 been concluded that its presence in the looped and collecting tubes 

 of the kidneys and urinary bladder, depends upon its being washed 

 out of the convoluted tubes by the stream of fluid filtered from 

 the blood at the glomerulus. 



The following facts may also be adduced in further support of 

 the view .that the glandular epithelium bears no mean share in 

 the removal of the more important solid constituents of the urine. 



The epithelium in the tubules of the kidney of birds is found 

 impregnated with acid urate of potassium, which insoluble sub- 

 stance forms the chief constituent of the solid urine of birds. 



