544 THE URINARY APPARATUS [CH. XXXVI. 



no knowledge of true secretory nerves to the kidney ; the amount of 

 urine varies directly with the blood-pressure in its capillaries. 



Increase in the quantity of urine accompanies a rise of intra- 

 capillary pressure. This may be produced by increasing the general 

 blood-pressure ; and this in turn may be produced in the following 

 ways : 



(1.) By increase in the force or frequency of the heart-beat. 



(2.) By constriction of the arterioles of areas other than that 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.^ 



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 neighbourhood 

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

 duces 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 again 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. 



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

 in the chapter on the skin. 



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

 rate of stimulation is used (see p. 307). 



