634 SECRETION BY KENAL EPITHELIUM. [BOOK n. 



system. Now the glomeruli, in some species at least of these 

 animals, are supplied exclusively by the branches of the renal 

 artery, the renal vena portae only serving to form the capillary 

 plexus around the tubuli uriniferi, which is also supplied by the 

 efferent vessels of the glomeruli. From this it is obvious that 

 if the renal artery be tied, the blood is shut off entirely from 

 the glomeruli ; and actual observation of the kidney has, in the 

 animals in question, shewn that under these circumstances there 

 is no reflux from the capillary network surrounding the tubules 

 back to the glomeruli; thus the kidney by this simple operation 

 is transformed into an ordinary secreting gland devoid of any 

 special filtering mechanism. Such a kidney may be used to 

 ascertain what substances are excreted by the glomeruli, and 

 what by the tubules in some other part of their course. It is 

 found that urea injected into the blood gives rise to a secre- 

 tion of urine when the renal arteries are tied ; this substance 

 therefore is secreted by the epithelium of the tubules, and in 

 being so secreted gives rise at the same time to a flow of water 

 through the cells into the interior of the tubules. Sugar and 

 peptones, on the other hand, which injected into the blood 

 readily pass through the untouched kidney and appear in the 

 urine, do not pass through a kidney the renal arteries of which 

 have been tied, even when a diuretic such as urea is given at 

 the same time in order to secure a flow of urine. These sub- 

 stances therefore are excreted by the glomeruli. 



The validity of this experiment, which may be accepted as 

 indicating a marked difference between glomfcrular secretion on 

 the one hand and epithelial or tubular secretion on the other, 

 depends on the absence of any collateral circulation whereby the 

 glomeruli may be supplied with blood after ligature of the renal 

 artery. In these animals anastomoses occur between the renal 

 arteries and the arteries of the generative organs ; and unless the 

 renal artery be so tied as to avoid these collateral communications 

 the results of the experiment are different. 



Additional evidence in favour of the secretory activity of the 

 epithelium cells is afforded by the following observation. Into 

 the veins of animals in which the urinary flow had been arrested 

 by section of the spinal cord below the medulla a quantity of 

 the blue colouring material known as sodium sulphindigotate 1 

 is injected. This substance is rapidly excreted on the one hand 

 by the liver in the bile, and on the other hand by the kidney. 

 By varying the quantity injected, killing the animals at appro- 

 priate times after the injection of the material, and examining 

 the kidneys microscopically and otherwise, it may be ascertained 

 that the pigment so injected passes from the blood into the renal 



1 Sometimes called indigo-carmine, though this name is more properly applied 

 to a crude impure preparation of potassium sulphindigotate. 



