470 A MANUAL OP PHYSIOLOGY 



ence of activity on blood-supply is subordinated to that of 

 blood-supply on activity, while in most tissues, as in the muscles, 

 the opposite is the case. It is evident that an increase in the 

 blood-flow would favour the secretory activity of the renal cells, 

 since the average concentration of the blood presented to them 

 as regards those constituents which they select would remain 

 relatively high in its circuit through the kidney. The ' stimulus ' 

 to secretion would, therefore, be relatively intense. 



Destruction of the medulla oblongata (i.e., of the vaso-motor 

 centre), or section of the cord below it, diminishes the secretion 

 of urine, because the arterial pressure is lowered so much as 

 to over-compensate the dilatation of the renal vessels, which the 

 operation also brings about. If the blood-pressure falls below 

 40 mm. of mercury, the secretion is abolished. Stimulation of 

 the medulla or cord also lessens the flow of urine by constricting 

 the arterioles of the kidney so much as to over-compensate the 

 rise of general blood-pressure, caused by the constriction of small 

 vessels throughout the body. 



If the renal nerves have been cut, stimulation of the medulla 

 oblongata increases the urinary secretion, because now the rise 

 of general blood-pressure is no longer counterbalanced by con- 

 striction of the renal vessels. An increase in the urinary flow 

 can be produced in the rabbit by a lesion in a part of the 

 funiculi teretes, which can be reached in the floor of the fourth 

 ventricle (Eckhard), perhaps by destroying the portion of the 

 vaso-motor centre governing the renal nerves, while the rest 

 remains uninjured, or is even stimulated, and thus keeps up or 

 even increases the general blood-pressure. There is either no 

 glycosuria, or it is very slight. 



Section of the splanchnic nerves causes a fall of arterial pres- 

 sure, which is, however (in animals like the dog, in which com- 

 pensation soon takes place), more than balanced by the simul- 

 taneous dilatation of the renal vessels, and therefore for some 

 time the flow of urine is increased, but not so much as when the 

 renal nerves alone are cut. In the rabbit there is no increase. 

 On the other hand, stimulation of the splanchnics stops the 

 urinary secretion, because the general rise of pressure is not 

 enough to make up for the constriction of the renal vessels. 



Diuretics are substances that increase the flow of urine. Some of 

 them act mainly on the circulation, 'as by increasing the general 

 blood-pressure, others mainly by a direct influence on the secreting 

 mechanism. Digitalis is a representative of the first class ; urea 

 and caffein belong to the second. The action of digitalis is to 

 strengthen the beat of the heart, which is at the same time somewhat 

 slowed, and to constrict the arterioles. Both effects contribute to 

 the increase of pressure. But the accompanying diuresis is due to 

 the cardiac factor, the vaso-constriction which involves the renal 

 vessels also, being overcompensated. The diuretic effect of digitalis 



