THE SECRETION OF THE URINE 599 



oblongata increases the urinary secretion, because now the rise of 

 general blood-pressure is no longer counterbalanced by constriction 

 of the renal vessels. An increase in the urinary flow can be pro- 

 duced 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 stimu- 

 lated, 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 pressure, 

 which is, however (in animals like the dog, in which compensation 

 soon takes place), more than balanced by the simultaneous dilata- 

 tion 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 con- 

 striction 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. The 

 accompanying diuresis, seen practically only in cardiac disease with 

 dropsical effusions, is due to the improvement of the renal circulation 

 and the absorption of the oedema fluid. Caffein, when injected into the 

 blood, affects the pressure but little. It causes dilatation of the renal 

 vessels after a passing constriction, and an increase in the flow of urine 

 after a temporary diminution. The vascular dilatation is not the 

 chief reason for the diuretic effect, for the latter is still obtained when the 

 vaso-motor mechanism has been paralyzed by chloral hydrate, and 

 even after the secretion of urine has been stopped by the fall of pressure 

 consequent on section of the spinal cord. Caffein, therefore, acts 

 directly on the renal epithelium. When these cells are asphyxiated by 

 temporary clamping of the renal artery, caffein after removal of the 

 clamp causes no diuresis, while the injection of Ringer's fluid still 

 causes a secretion of urine possessing the same crystalloid composition 

 as the plasma, just as it would have done if perfused through the excised 

 kidney. The action of urea, potassium nitrate, and saline diuretics 

 such as sodium sulphate is probably also a direct action on the secret- 

 ing structures, although some have supposed that their primary effect 

 is to cause vaso-dilatation in the kidney. 



Summary. Our knowledge of renal secretion may be thus 

 summed up: The water and salts of the urine are chiefly separated 

 by the glomeruli ; the process is not a mere physical filtration, but 

 a true secretion. Substances like sugar, peptone, egg-albumin, and 

 hemoglobin, when injected into the blood, are probably excreted mainly 

 by the glomeruli ; and so is the sugar of diabetes. Urea, uric acid, 

 and presumably the other organic constituents of normal urine, with 



