CH. xxxvi.] NERVES OF THE KIDNEY. 539 



of the kidney, as in cold weather, when the cutaneous capillaries 

 are constricted.* 



(3.) By increase in the total contents of the vascular system, 

 as after drinking large quantities of fluid. 



The blood-pressure in the renal capillaries may also be 

 increased locally by anything which leads to relaxation of the 

 renal arterioles. 



Decrease in the quantity of urine is produced by the opposites 

 in each case. 



If the renal nerves are divided, the renal arterioles are relaxed, 

 and pressure in the renal capillaries is raised, so there is an 

 increased flow of urine. This is accompanied by an increase in 

 the volume of the kidney, as can be seen by the oncometer. 



Stimulation of the divided nerves produces a diminution in 

 the amount of urine, and a shrinkage of the kidney due to a 

 constriction of its blood-vessels, t 



If the splanchnic nerves are experimented with instead of the 

 renal, the effects are not so marked, as these nerves have a wide 

 distribution, and section leads to vascular dilatation in the whole 

 splanchnic area ; hence the increase in pressure in the renal 

 capillaries is not so noticeable. 



Puncture of the floor of the fourth ventricle in the neighbour- 

 hood of the vaso-motor centre (close to the spot, puncture of 

 which produces glycosuria) leads to a relaxation of the renal 

 arterioles and a consequent large increase of urine (polyuria). 



Section of the spinal cord just below the medulla causes a 

 cessation of secretion of urine, because of the great fall of general 

 blood-pressure which occurs. If the animal is kept alive, 

 however, blood-pressure goes up after a time, owing to the action 

 of subsidiary vaso-motor centres in the cord. When this has 

 occurred stimulation of the peripheral end of the cut spinal cord 

 a^ain causes urinary secretion to stop, because the renal artery 

 (like the other arteries of the body) is so constricted that the 

 pressure in the renal capillaries becomes too low for secretion to 

 occur. 



We thus see that the amount of urine varies with blood- 

 pressure. But such a statement does not give the whole truth. 

 Increase of blood-pressure and an increased amount of blood 

 flowing through the kidney go together when the blood is 

 circulating normally, and it is really the increase in the amount of 



* The reciprocal action between skin and kidneys will be discussed more 

 fully in the chapter on the skin. 



t The nerves also contain vaso-dilatator fibres, which are excited when a 

 low rate of stimulation is used (see p. 309). 



