4 i2 A MANUAL OF PHYSIOLOGY 



itself in order to continue its work ; and it might be urged 

 that if the blood in the glomeruli could be kept at the normal 

 standard of arterial blood, secretion might still go on after 

 ligature of the renal vein. 



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

 spite of continued filtration 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 epi- 

 thelium normally secretes water under a low blood-pressure, 

 but is disorganized under the excessive and entirely un- 

 accustomed pressure which follows the closure of the vein. 

 But the whole discussion is an illustration and this is the 

 reason we have gone into it so fully of the complexity, the 

 many-sidedness of physiological phenomena, even when 

 reduced by well-planned experiments to their simplest terms, 

 and the unconscious bias which theory sometimes gives to 

 even the most acute and original minds in interpreting the 

 results of observation. 



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 counteracted by, still more if conspiring with, simul- 

 taneous 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 illus- 

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



