CHAP, in.] ELIMINATION OF WASTE PRODUCTS. 667 



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 substances 

 therefore are excreted by the glomeruli. 



The validity of this experiment, which may be accepted as 

 indicating a marked difference between glomerular 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 anatomoses 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 appropriate times after 

 the injection of the material, and examining the kidneys micro- 

 scopically and otherwise, it may be ascertained that the pigment 

 so injected passes from the blood into the renal epithelium, and 

 from thence into the channels of the tubules. There being no 

 stream of fluid through the tubules, owing to the arrest of urinary 

 flow by means of the preliminary operation, the pigment travels 

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

 much where it was cast out by the epithelium cells. There are no 

 traces whatever of the pigment having passed by the glomeruli ; 

 and the cells which appear most distinctly to take up and eject 

 it, are those lining such portions of the tubules (viz. the first and 

 second convoluted tubules, zigzag tubules and ascending limbs of 

 the loops of Henle) as from their microscopic features have been 

 supposed to be the actively secreting portions of the entire tubules. 

 The following observation which has been made is of a peculiarly 

 interesting character. After injecting a certain quantity of pig- 

 ment, and allowing such a time to elapse as might be judged from 

 previous experiments would suffice for the passage of the material 

 through the epithelium to be pretty well completed, a second 



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

 to a crude impure preparation of potassium sulphindigotate. 



