388 EXCRETION 



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- 

 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, accord- 

 ing 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 epithelium does 



FIG. 302. Roy's Onkograph, or Apparatus for Recording Altera^ns in the Volume of the Kid- 

 ney, 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 communicates. 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 instru- 

 ment. 



not filter merely, but that it, as living protoplasm, regulates and controls 

 the quantity and kind of material passing through it. 



Micro-chemical observations have been enlisted to demonstrate more fully, 

 if possible, the activity of the differem 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 de- 

 monstrated 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. 

 But Heidenhain's view is strengthened by Bowman's observation that in birds 

 crystals of uric acid are to be seen in the cells of the convoluted tubules, and 

 in the lumen adjacent. 



Only traces of the sugars and proteids of the blood are found in normal 

 urine, but when either cane sugar, peptone, or egg albumin is introduced into 

 the blood it is rapidly eliminated by the kidney. Egg albumin is not essen- 



