418" THE ELIMINATION OF WASTE PRODUCTS. 



if, when the urine has ceased to flow in consequence of a section of the 

 medulla oblongata, certain substances, such as urea, urates, sodium acetate, 

 and the like, be injected into the blood, a more or less copious secretion is at 

 once set up. This secretion is, or at least may be, unaccompanied by any 

 rise of general blood-pressure sufficient to account for the increased secretion 

 as the mere result of an increased flow of blood. It is true (as we have 

 seen, 350) that the injection of these substances leads to an expansion of 

 the kidney, an expansion which is probably due to a local dilatation of the 

 small renal arteries ; but the flow of urine which is observed in these cases 

 is too great to be accounted for by any increase of flow of blood which the 

 local dilatation may bring about ; and hence we conclude that the increase 

 of secretion is of a different kind from that which follows upon mere increase 

 of blood-flow. It seems much more reasonable to suppose that the presence 

 of the above substances in the blood excites the renal epithelial cells to an 

 unwonted activity, causing them to pour into the interior of the tubules a 

 copious secretion, just as the presence of pilocarpine in the blood will cause 

 the salivary cells to pour forth their secretion into the lumen of their ducts ; 

 and that this activity of the epithelial cells is accompanied, also as in the 

 case of the submaxillary and other glands, by a vascular dilatation, which, 

 though adjuvant and beneficial, is not the distinct cause of the activity. 

 This view is further supported by the following remarkable experiment, 

 which goes far to show that of the various substances which, having found 

 their way into the blood, are thrown out by the kidney, some pass into the 

 urine through the glomeruli, while others are distinctly secreted by the 

 tubuli uriniferi, the discharge of the latter being accompanied by a general 

 activity of the secreting cells, as shown by the flow of water taking place at 

 the same time. 



In the amphibia the kidney has a double vascular supply ; it receives 

 arterial blood from the renal artery, but there is also poured into it venous 

 blood from another source. The femoral vein divides at the top of the thigh 

 into two branches, one of which runs along the front of the abdomen to 

 meet its fellow in the middle line and form the anterior abdominal vein, 

 while the other passes to the outer border of the kidney and branches in the 

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

 the glomeruli, in some species at least of these animals, are supplied exclu- 

 sively by the branches of the renal artery, the renal vena porta3 only serv- 

 ing 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 ques- 

 tion, shown that under these circumstance there is no reflux from the capil- 

 lary 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 secretion of urine when the renal arteries 

 are tied; this substance, therefore, is secreted by the epithelium of the tu- 

 bules, 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 un- 

 touched 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. 



