THE SECRETION OF URINE 697 



inhibited sufficiently to produce a fall in the arterial blood pressure, 

 the kidney volume shrinks and the flow of urine is diminished. 

 The vagus probably has no direct action upon the secretion of 

 urine, as is shown by the fact that stimulation of the vagus below 

 the diaphragm does not, in the hands of most observers, influence 

 the secretion of urine. The necessary fall of general arterial 

 blood pressure may also be brought about by widespread vaso- 

 dilatation. Thus, section of the splanchnics in the rabbit may 

 reduce the general arterial blood pressure sufficiently to counter- 

 balance the effect of the dilatation of the renal arterioles, and may 

 so prevent any increase in the urinary excretion. But section of 

 the spinal cord in the cervical region produces such an enormous 

 fall of arterial pressure that the kidney volume shrinks in spite of 

 the renal vaso-dilatation, and the secretion of urine is greatly 

 decreased or abolished. It has frequently been found that when 

 the aortic blood pressure falls below about 40 mm. Hg, the flow 

 of urine stops. (2) Constriction of the renal arterioles, the general 

 arterial pressure remaining unaltered. This condition may be 

 produced by stimulating the renal nerves, which causes a shrinkage 

 of the kidney volume and a decreased flow of urine. The same 

 result may be brought about by mechanically constricting the 

 renal artery. It is then found that the flow of urine varies, after 

 a certain point, with the degree of the constriction, and that when 

 the artery is completely occluded, or even before, the flow of urine 

 stops abruptly. It has been found that when the constriction is 

 relieved after having lasted only a few minutes, there may be an 

 interval of forty-five minutes before the secretion of urine begins 

 again. At first sight this is a surprising result, and the explana- 

 tion of it is by no means certain. It is possible that the temporary 

 suppression of urine is in part due to an effect produced upon the 

 cells of Bowman's capsule, and that they are affected seems to be 

 shown by the albuminuria which accompanies the secretion. It 

 would seem more probable that the explanation is to be found in 

 the following series of events, namely, that arterial ischaemia is 

 followed by arterial engorgement, which may be so severe and 

 lead to such an increased formation of extravascular fluid that 

 strangulation of the veins and cessation of the blood-flow through 

 the part takes place. 



Obstruction of the Renal Vein is the exception referred to above, 

 for, while it raises the capillary blood pressure in the kidney, it 



