THE SECRETION OF THE URINE 501 



According to Ludwig, indeed, the flow of urine stops, in spite 

 of continued nitration through the glomeruli, because the swelling 

 of the veins in the boundary layer compresses the tubules, and may 

 even obliterate their lumen. There is no conclusive experimental 

 evidence, however, and no a priori probability, that the obstruction 

 so produced is sufficiently sudden or sufficiently complete to cause 

 instant and total cessation of the flow. It is even less justifiable 

 to conclude from the experiment that the liquid part of the urine 

 is, at any rate, not separated by the epithelium of the tubules, since 

 the blood-pressure in the capillaries around the tubules must rise 

 very greatly after ligature of the vein, and yet secretion is stopped. 

 It might equally well be argued that the renal epithelium normally 

 secretes water under a low blood-pressure, but is disorganized under 

 the excessive and entirely unaccustomed pressure which follows the 

 closure of the vein. 



It is not only through nerves directly governing the calibre of the 

 vessels of the kidney that the rate of urinary secretion can be 

 affected. Any change in the general blood-pressure, if not counter- 

 acted by, still more if conspiring with, simultaneous local changes 

 in the renal vessels, may be followed by an increased or diminished 

 flow of urine ; and the law which explains all such variations, or at 

 least serves to sum them up, is that in general an increase in the 

 rate of the blood- flow through the kidney is followed by an increase in 

 the rate of secretion. It will be remarked that this is the converse 

 of the great law, of which we have already seen so many illustra- 

 tions, that functional activity increases blood-flow. It is 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 



