EXPERIMENTAL OBSERVATIONS 415 



glomerular epithelium, for it now excretes albumin, which it did not previ- 

 ously let pass. Therefore, it is not pressure merely that favors the secretion, 

 there must also be an efficient flow of blood. The secretion is influenced 

 especially by the amount of blood flowing through the kidney in a given time. 

 In the frog the kidney has a double blood supply. The renal artery 

 supplies the glomeruli, while a branch of the renal-portal vein supplies the 

 tubules. Nussbaum ligated the renal artery in one kidney of the frog, while 

 leaving the circulation of the other kidney undisturbed. He found that the 

 operated kidney secreted little or no urine, but that it could be made to secrete 

 by injections of urea, but not by injections of albumin or sugar as in the nor- 



FIG. 302. Roy's Onkograph, or Apparatus for Recording Alterations in the Volume 

 of the Kidney, etc., as shown by the onkometer. a, Upright, supporting recording lever /, 

 which is raised or lowered by the needle b, which works through/, and which is attached 

 to the piston e, working in the chamber d, with which the tube from the onkometer com- 

 municates. The oil is prevented from being squeezed out as the piston descends, by a 

 membrane, which is clamped between the ring-shaped surfaces of the cylinder by the screw 

 * working upward; the tube h is for filling the instrument. 



mal kidney. Ligation of the renal-portal vein, which supplies the tubules 

 in the frog, caused a decrease in the quantity of the secretion, whereas, ac- 

 cording to Ludwig's view, it ought to have increased the quantity, since 

 obviously resorption could not take place with any degree of efficiency. In 

 the main, the evidence is in favor of the view that even the glomerular epi- 

 thelium does not filter merely, but that it, as living protoplasm, regulates 

 and controls the quantity and kind of material passing through it. 



Microchemical observations have been enlisted to demonstrate more fully, 

 if possible, the activity of the different parts of the epithelial tubule. Heiden- 

 hain, by injections of indigo- blue into the blood stream, followed by rapid 

 fixation of the kidney in alcohol at the proper stage of elimination, has 

 demonstrated crystals of the pigment in the renal epithelial cells and in the 

 lumen of the tubule. He concluded that these cells were actively eliminating 

 the pigment by a secretory process. This observation has been questioned. 



