THE URINARY SYSTEM. 393 



urine ceases entirely. The reason is that the flow of blood through the 

 glomeruli ceases, and their contents soon consist of little more than 

 a mass of blood corpuscles, rendering nitration impossible. 



The more rapidly urine is formed by the kidney, the more nearly 

 does its composition approximate to that of blood plasma ; and when 

 the blood is greatly diluted, for example by repeated injections of 

 Ringer's solution, the percentage of sodium chloride and urea in the 

 plasma and the urine may be identical. 



ANALYSIS OF PLASMA AND URINE IN HYDR^MIA. (BARCROFT.) 



Similar results have been .obtained in animals, in which the renal 

 tubules were poisoned with corrosive sublimate, or other drugs, so as 

 to eliminate their functions. The fluid formed by the glomeruli appears, 

 therefore, to be isotonic with the blood plasma, a fact which is intelligible 

 if it is formed by filtration, but which does not fall into line with our 

 knowledge of secretion. 



FUNCTIONS OF THE TUBULES. 



There is no doubt that the renal tubules form urine by a process 

 of secretion, which in its essential features is strictly comparable with 

 that occurring in other secretory glands. In the first place, the com- 

 position of the urine formed by the tubules differs greatly from that of 

 the blood. Secondly, increased secretory activity of the tubules is 

 accompanied by a larger consumption of oxygen, and may take place 

 without any alteration in the rate of blood flow through the kidney. 



The secretory function of the tubules has been most clearly proved 

 in the frog, in which, owing to the arrangement of the blood supply to 

 the kidneys, the functions of the tubules and glomeruli can be studied 

 separately. The glomeruli are supplied with blood solely through the 

 renal artery, whereas the tubules have a double supply. On the one 

 hand, the efferent vessels from the glomeruli enter the network of 

 vessels round the tubules ; on the other hand, the tubules also receive 

 blood from the renal portal vein, which arises from the femoral vein 

 (fig. 132). 



When the renal arteries are ligatured in the frog, the circulation 



