CHAP, in.] ELIMINATION OF WASTE PRODUCTS. 693 



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 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 a less 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 counterbalancing the 

 general ones. 



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

 lesser flow through the renal vessels and a shrinking of the kidney 

 unless it be accompanied by a dilation of the renal vessels out of 

 proportion to the general fall. Thus when the spinal cord is 

 divided below the spinal bulb the fall of general blood-pressure is, 

 as we have seen ( 173), 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 pressure 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 

 remaining the same, the flow through the kidney will at once be 

 on the one hand increased by dilation 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 remaining 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. 



412. 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 vaso-motor impulses to the kidney have been 

 approximately made out. Vaso-constrictor fibres for the kidney 

 are supplied from what we have previously ( 169 and elsewhere) 



