THE SECRETION OF URINE. 419 



The validity of this experiment, which may be accepted as indicating a 

 marked difference between glornerular secretion on the one hand and epithe- 

 lial 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 favor of the secretory activity of the epithelial 

 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 coloring 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 microscopically 

 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 pig- 

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

 much where it was cast out by the epithelial 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 micro- 

 scopic 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 epithe- 

 lium to be pretty well completed, a second quantity was injected. It was 

 found that the excretion of this second quantity was most incomplete and 

 imperfect. It seems as if the cells were exhausted by their previous efforts, 

 just as a muscle which has been severely tetanized will not respond to a 

 renewed stimulation. 



The above observation may be objected to on the ground that this color- 

 ing matter does not occur as a constituent of the blood either in health or 

 disease, and especially that the absence of any concomitant discharge of 

 fluid from the cells excites suspicion that the process observed was not really 

 one of secretion ; for the injection of such substances as urea or urates into 

 the blood does cause a copious flow of fluid, and indeed thus prevents the 

 microscopic tracking out of their passage, which in the case of urates might 

 otherwise be done much in the same way as with the sodium sulphindigotate. 

 Moreover, other observers have maintained that the sodium sulphindigotate 

 does like ordinary carmine pass through the glomeruli. But their results 

 may probably be explained by the glomeruli having been damaged by a too 

 rapid or too abundant injection ; and in the case of the amphibian kidney, 

 when sodium sulphindigotate is injected after ligature of renal arteries, no 

 urine is found in the bladder, but the pigment can be traced through the 

 epithelium of the secreting portions of the tubules. Without insisting too 

 much on the value of the sodium sulphindigotate experiments, they may be 

 taken as fairly supporting the view which we are considering. We may add 



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

 crude, impure preparation of potassium sulphiudigotate. 



