CHAP, iv.] RENAL SECRETION. 401 



arteries, as shewn by the shrinking of the kidney. Stimulation of 

 the medulla oblongata causes a very marked shrinking of the 

 kidney, indicating powerful constriction of its arteries, as does also 

 stimulation of the splanchnic nerve; the effect when the splanchnic 

 on one side is stimulated frequently affects the kidney on the 

 opposite side as much as that on the same side. Stimulation of 

 a sensory nerve causes shrinking of the kidney, in spite of a rise 

 of mean pressure, which in itself would tend to swell the kidney, 

 taking place at the same time ; this is an instance of reflex con- 

 striction as that of stimulation of the splanchnic nerve is of direct 

 constriction. A direct constriction may also be brought about by 

 stimulation of the renal nerves. When all the renal nerves 

 are divided (an operation by no means easy), stimulation of nerves 

 in other parts of the body does not cause a constriction but an 

 expansion of the kidney, since it gives rise to an increase of 

 blood-pressure, through which the renal vessels are passively filled 

 to a greater extent. 



The same method further shews that the vaso-motor mechanism 

 of the kidney is remarkably sensitive to changes in the chemical 

 constitution of the blood. The injection into the blood of even 

 a small quantity of water causes a shrinking of the kidney followed 

 by a more lasting expansion. The injection of urea and some 

 other diuretics produces the same effect to a more marked degree, 

 while the injection of normal saline solution and especially 

 of such diuretics as sodium acetate causes an expansion from the 

 very first, the primary shrinking being absent. It is moreover 

 worthy of note that these effects of diuretics and of chemical 

 changes in blood appear even after all the renal nerves have 

 apparently been completely severed, indicating that these bodies 

 induce vascular changes by acting either upon some peripheral 

 vaso-motor mechanism, or, even more directly, on the blood-vessels 

 themselves. It may be added that they will produce considerable 

 effects in the kidney itself without appreciably modifying the 

 general blood-pressure. 



As yet this method has not disclosed any distinct vaso-dilator 

 fibres passing to the kidney from other parts, positive dilation 

 having been observed only as the result of chemical agents. But, 

 even if these prove eventually to be really absent, enough has been 

 said to shew that the kidney has an ample and well-developed 

 vaso-motor supply. In many of the observations referred to above, 

 the flow of urine was determined at the same time as the volume 

 of the kidney, by measuring the escape from the ureter of the kidney 

 experimented upon through a cannula tied into it. And it was 

 found that, unless special causes intervened, expansion of the 

 kidney was accompanied by an increase and contraction by a 

 decrease in the flow of urine. But before we attempt to illustrate 

 the working of the vaso-motor mechanism just described, it will be 

 as well to call attention to the fact that, as far as filtration is 



v. 23 



