398 MANUAL OF PHYSIOLOGY. 



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 blood vessels. 



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. 



The amount of liquid passing out at the kidneys is in direct 

 proportion to the blood pressure, whereas the excretion of the 

 specific constituents of urine is independent of the pressure, but 

 is related to the amount existing in the blood, and the condition 

 of the epithelium. This is shown by the increased elimination of 

 urea when that substance is artificially introduced into the circu- 

 lation, even after the flow of the fluid has been checked by section 

 of the spinal cord. 



Another view has been put forward, which, with some modifi- 



