410 MANUAL OF PHYSIOLOGY. 



NERVOUS MECHANISM OF THE URINARY SECRETION. 



With regard to the influence exerted by the nervous system on 

 the renal secretion, we have but little satisfactory information, 

 although there can be no doubt that here, as in other glands, the 

 process is under the control of the nerves. Many of the circum- 

 stances which cause greater activity of secretion, such as taking 

 large quantities of water, etc., have no effect on the general blood 

 pressure, so that, if the increased flow be brought about by the 

 vasomotor mechanisms, it must be by means of nervous channels 

 altering the blood flow in the special arteries of the glands. We 

 know, further, that emotional conditions, such as hysteria, exist 

 in which an unaccountably great quantity of urine of very low 

 specific gravity is evacuated. 



With regard to the effects of the vasomotor nerves, we know 

 that section of all the nervous twigs going to the kidneys causes 

 great congestion and an immense increase in the secretion, which 

 commonly contains albumin. This, no doubt, depends on the 

 sudden rise in pressure in the glomeruli, owing to the dilatation 

 of the arterioles. If the splanchuics,in which the renal vasomotor 

 nerves run, be cut, a great quantity of urine is produced from the 

 same cause vasomotor paralysis but, on account of the large 

 area of vessels injured, the general blood pressure falls, and, there- 

 fore, the effect is not so much marked. If the peripheral end of 

 the cut nerves be stimulated, the secretion is diminished, and, 

 owing to spasm of the renal arterioles and fall of blood pressure 

 in the glomerular capillaries, may be brought to a stand-still. 

 Section of the spinal cord at the seventh cervical vertebra stops 

 the flow, because it so reduces the general blood pressure that 

 the pressure in the renal vessels falls below that necessary for 

 the filtration of the urine. 



PASSAGE OF THE URINE TO THE BLADDER. 

 The pressure exerted by the blood in the glomerular capillaries 

 is quite sufficient to make the urine flow from the pelvis of the 

 kidneys into the bladder, because, when the ureters are tied, they 

 become distended above the ligature by the urine flowing from 

 the pelvis, where a pressure may be produced of some forty 



