CHAP, in.] ELIMINATION OF WASTE PRODUCTS. 533 



Secretion by the Renal Epithelium. 



336. The glomerular mechanism is after all a small por- 

 tion only of the whole kidney, and the epithelium over a large 

 part of the course of the tubuli uriniferi bears most distinctly 

 the characters of an active secreting epithelium. These facts 

 would lead us a priori to suppose that the flow of urine is in 

 part the result of an active secretion comparable to that of the 

 salivary or other glands which we have already studied. And 

 we have experimental and other evidence that such is the case. 



In the first place a flow of urine may be artificially excited 

 even when the natural flow has been arrested by diminution of 

 blood-pressure. Thus if, when the urine has ceased to flow in 

 consequence of a section of the spinal bulb, 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 334) that the injection of 

 these substances leads to an expansion of the kidney, to a 

 fuller flow of blood through it; but this is the effect rather 

 than the cause of the secretory activity. We may infer 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 sup- 

 ported by the following remarkable 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 



