406 ACTIVITY OF THE EPITHELIUM, [BOOK n. 



substance of that organ, forming the so-called renal portal system. 

 Now the glomeruli are supplied exclusively by the branches of the 

 renal artery, the renal vena portse only serving to form the ca- 

 pillary plexus around the tubuli urmiferi which is also sup- 

 plied 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, actual observation of the kidney 

 of the newt having 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. We owe to Nussbaum the ingenious use of 

 such a kidney 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 sugar and peptones, 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. These substances therefore are excreted 

 by the glomeruli. Urea on the other hand, injected into the 

 blood, gives rise to a secretion 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. 



Additional evidence in favour of the activity of the epithelium 

 cells is afforded by an observation for which we are indebted to 

 Heidenhain. Into the veins of animals in which the urinary flow 

 had been arrested by section of the spinal cord below the medulla, 

 this observer injected a quantity of colouring material known as 

 sodium sulphindigotate 1 . By killing the animals at appropriate 

 times after the injection of the material and examining the 

 kidneys microscopically and otherwise, he was enabled to ascer- 

 tain that the pigment so injected passed from the blood into 

 the renal epithelium, and from thence into the channels of the 

 tubules, where it was precipitated in a solid form. There being 

 no stream of fluid through the tubules, owing to the arrest of 

 urinary flow by means of the preliminary operation, the pigment 

 travelled very little way down the interior of the tubules, and 

 remained very much where it was cast out by the epithelium cells. 

 There were no traces whatever of the pigment having passed 

 by the glomeruli; and the cells which could be seen distinctly 

 to take up and eject it, were those lining such portions of the 

 tubules (viz. the so-called secreting tubules, intercalated tubules 

 and portions of the loops of Henle) as from their microscopic 

 features have been supposed to be the actively secreting portions 

 of the entire tubules. By varying the quantity injected and the 

 time which was allowed to elapse between the injection and subse- 



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

 to a crude impure preparation of potassium sulphindigotate. 



