EXCRETION 



so that the general effect of stimulation of the nerve-roots, the splanch- 

 nics, or the renal nerves is shrinking of the kidney, with diminution or 

 cessation of the secretion of urine. But slow rhythmical stimulation 

 of the roots causes increase of volume, the scanty dilators being by this 

 method excited in preference to the constrictors. 



The renal nerves, entering at the hilum, branch repeatedly, so as to 

 form a wide-meshed plexus around the arteries, and accompany them 

 even to their finest ramifications in the cortex. Coming off from the 

 nerves surrounding the arteries are fine fibres which are distributed to 

 the convoluted tubules. Some of them terminate in globular ends, 

 others in fine threads that pass through the membrana propria (Berkeley) . 



Section of the renal nerves is followed by relaxation of the small 

 arteries in the kidney, and consequent swelling of the organ. The 

 flow of urine is greatly increased, and sometimes albumin appears 

 in it, the excessive pressure in the capillaries (particularly in those 



of the glomeruli) being 

 supposed to favour the 

 escape of substances to 

 which a passage is refused 

 under normal conditions. 

 An experiment which is 

 sometimes quoted as a de- 

 cisive test of the relative 

 importance of changes in 

 the rate of flow, and in 

 the pressure of the blood 

 within the glomeruli, is 

 that of tying the renal 

 vein. This undoubtedly 

 does not lower the intra- 

 glomerular pressure on 

 the contrary, it must in- 

 crease it but the secretion of urine stops. If the venous outflow 

 from the kidney is only partially interfered with, the flow of urine is 

 immediately diminished, but the administration of a diuretic like 

 potassium nitrate causes an increase. It is more than likely that 

 in these experiments the secretion stops or slackens not because a 

 high blood- pressure, but because an active circulation is its necessary 

 condition. When the blood stagnates in the kidney the natural 

 stimulus to the renal apparatus speedily disappears owing to the 

 elimination of the urinary constituents to the neutral or indifferent 

 point (p. 496). The experiment, however, is not perfectly conclu- 

 sive. For few glands can go on performing their function after the 

 circulation has ceased. The kidney must be able to feed itself in 

 order to continue its work. Above all, it needs oxygen; 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 ligation of the renal vein. 



Fig. 192. Diagram of Organ-Piethysmograph or 

 Oncometer. B, metal box in two halves open- 

 ing on the hinge H ; M, thin membrane ; A, space 

 filled with oil; O, organ enclosed in oncometer; 

 V, vessels of organ ; t, tube for filling instrument 

 with oil; T, tube connected with D, which opens 

 into cylinder C; C is also filled with oil; P, pis- 

 ton attached by E to a writing lever. 



