THE SECRETION OF URINE. 415 



the renal vessels. So also when the medulla or spinal cord is directly stimu- 

 lated by induction shocks (the animal being under urari so as to eliminate 

 the complications due to contractions of the skeletal muscles) the renal ves- 

 sels share so fully in the arterial constriction which results that, in spite of 

 the great rise of mean pressure which is induced, less blood than normal 

 passes through the renal vessels, and the kidney shrinks. Or if the abdom- 

 inal splanchnic nerves be stimulated, since, as we shall see, these carry vaso- 

 constrictor fibres for the kidney, in spite of the rise of blood-pressure which 

 follows, the kidney shrinks on account of the great constriction of the renal 

 vessels. 



On the other hand, if a rise of blood-pressure be for any reason not ac- 

 companied by a compensating constriction of the renal arteries, that rise, 

 whether it be brought about by general constriction of arteries other than 

 the renal or by an increase of the cardiac delivery, causes the kidney to 

 swell, showing a greater flow of blood. Such a condition of things may be 

 induced by section of the nerves of the renal plexus, whereby the paths of 

 all vaso-constrictor impulses to the kidney are blocked. After this has been 

 done, a rise of general pressure, whether by dyspnoea, or by direct stimulation 

 of the spinal cord, or by stimulation of the abdominal splanchnic nerves, 

 leads to a greater flow through the renal vessels and an increased expansion 

 of the kidney. 



A rise of general blood-pressure, then, may be accompanied by either a 

 shrinking or a swelling of the kidney, by either a greater or lesser flow of 

 blood through the kidney, according to the concomitant condition of the 

 renal vessels ; or, indeed, may under certain circumstances be accompanied 

 by no change at all in the renal circulation, the local effects exactly counter- 

 balancing the general ones. 



Conversely, in a similar way, a fall of blood- pressure leads to a lesser 

 flow through the renal vessels and a shrinkage of the kidney unless it be 

 accompanied by a dilatation of the renal vessels out of proportion to the 

 general fall. Thus when the spinal cord is divided below the medulla the 

 fall of general blood-pressure is, as we have seen ( 159), very marked, 

 being due to an abolition for the time being of wonted constrictor impulses. 

 The pressure in the aorta falls rapidly, and at the same time, owing to the 

 more open pathway through the region of peripheral resistance in the body 

 generally, the pressure in the vena cava is increased ; the difference of pres- 

 sure between the mouth of the renal artery in the aorta and the mouth of 

 the renal vein in the vena cava is so largely reduced that in spite of the 

 concomitant relaxed condition of the renal vessels themselves the flow of 

 blood through the kidney is largely diminished. 



It will of course be understood that, the general blood-pressure remain- 

 ing the same, the flow through the kidney will at once be, on the one hand, 

 increased by dilatation and, on the other, decreased by constriction of the 

 renal vessels themselves. The constricted or dilated condition of the renal 

 vessels can by themselves produce but little effect on the pressure either in 

 the aorta or in the vena cava ; and the difference between the pressure at 

 the mouth of the renal artery and that at the mouth of the renal vein re- 

 maining the same, the more open passages of the dilated renal vessels must 

 lead to a fuller, and the narrower passages of the constricted renal vessels to 

 a scantier, flow through the kidney. 



348. By means of the oncometer, watching the shrinking and swelling 

 of the kidney and thus judging of the flow of blood through it, the results 

 being always interpreted with reference to the general blood-pressure on the 

 lines of the above discussion, the paths of vasomotor impulses to the kidney 

 have been approximately made out. Vaso-coustrictor fibres for the kidney 



