CHAP, in.] ELIMINATION OF WASTE PRODUCTS. 697 



blood. It is true (as we have seen 414) that the injection of 

 these substances leads to an expansion of the kidney, an 

 expansion which is probably due to a local dilation 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 dilation 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 epithelium cells to an 

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

 tubules a copious secretion, just as the presence of pilocarpin 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 

 epithelium cells is accompanied, also as in the case of the 

 submaxillary and other glands, by a vascular dilation, which, 

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

 activity. This view is further supported by the following 

 experiment, which goes far to shew 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 shewn 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 exclusively by the branches of the renal artery, the renal 

 vena portse 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 secretion of urine when the renal arteries are tied ; this 



F. ii. 45 



