EXCRETION 413 



probable that this law holds for the kidney as well as for 

 other organs, but that the influence of activity on blood- 

 supply is subordinated to that of blood-supply on activity, 

 while in most tissues, as in the muscles, the opposite is the 

 case. It is evident that an increase in the blood-flow would 

 favour the secretory activity of the renal cells, since the 

 average concentration of the blood presented to them as 

 regards those constituents which they select would remain 

 relatively high in its circuit through the kidney. The 

 ' stimulus ' to secretion would, therefore, be relatively 

 intense. 



Destruction of the medulla oblongata (i.e., of the vaso- 

 motor centre), or section of the cord below it, diminishes 

 the secretion of urine, because the arterial pressure is 

 lowered so much as to over-compensate the dilatation of the 

 renal vessels, which the operation also brings about. If the 

 blood-pressure falls below 40 mm. of mercury, the secretion 

 is abolished. Stimulation of the medulla or cord also 

 lessens the flow of urine by constricting the arterioles of the 

 kidney so much as to over-compensate the rise of general 

 blood-pressure, caused by the constriction of small vessels 

 throughout the body. 



If the renal nerves have been cut, stimulation of the 

 medulla oblongata increases the urinary secretion, because 

 now the rise of general blood-pressure is no longer counter- 

 balanced by constriction of the renal vessels. Puncture of a 

 certain part of the floor of the fourth ventricle may produce 

 a copious flow of urine, perhaps by destroying the portion 

 of the vaso-motor centre governing the renal nerves, while 

 the rest remains uninjured and keeps up the general blood- 

 pressure, but possibly by stimulating a secretory * centre.' 



Section of the splanchnic nerves causes a fall of arterial 

 pressure, which is, however (in animals .like the dog, in 

 which compensation soon takes place), more than balanced 

 by the simultaneous dilatation of the renal vessels, and 

 therefore for some time the flow of urine is increased, but 

 not so much as when the renal nerves alone are cut. In the 

 rabbit there is no increase. On the other hand, stimulation 

 of the splanchnics stops the urinary secretion, because the 



