THE SECRETION OF THE URINE 499 



high blood-pressure, or because there would be a danger of substances 

 which ought to be retained being cast out into the urine. In this con- 

 nection it is interesting to note that the specific constituents of urine 

 are separated by epithelium surrounded by capillaries of the second 

 order, and therefore with a smaller blood-pressure than exists in the 

 capillaries of most glands, while the same is true of bile, another 

 (practically) protein-free secretion. 



The maximum secretory pressure in the kidney, as shown by a 

 manometer tied into the divided ureter, is about 60 mm. of mercury 

 in the dog, or less than half that of saliva. If the escape of the 

 urine is opposed by a greater pressure than this, or if the ureter is 

 tied, the kidney becomes cedematous. Whether the oedema is due 

 to reabsorption of urine or to the pouring out of lymph owing to 

 the pressure of the dilated tubules on the veins has not been de- 

 finitely settled. It has been already pointed out that there is no 

 necessary relation between the blood-pressure in the capillaries of 

 a gland and its secretory pressure; and, so far as this goes, water 

 might just as well be secreted at a pressure of 60 mm. of mercury 

 from the low-pressure blood of the second set of renal capillaries 

 as from the high-pressure blood of the glomeruli. By obstruction 

 the molecular concentration of the urine is diminished to half or 

 three-quarters of the normal. 



The Influence of the Circulation on the Secretion of Urine. 

 Although the activity of no organ in the body is governed more 

 by the indirect effects of nervous action than that of the kidney, 

 no proof has been given of the existence of secretory fibres for it 

 comparable to those of the salivary glands. All the changes in the 

 rate of renal secretion which follow the section or stimulation of 

 nerves can be explained as the consequences of the rise or fall of 

 local or general blood-pressure, and of the corresponding variations 

 in the velocity of the blood in the renal vessels. 



The best way to study variations in the calibre of the renal vessels is 

 the pie thysmo graphic method, and the oncometer of Roy is a plethysmo- 

 graph adapted to the kidney (Fig. 192). It consists of a metal capsule 

 lined with loose membrane, between which and the metal there is a 

 space filled with oil. The two halves of the capsule open and shut on a 

 hinge; and the kidney, when introduced into it, is surrounded on all 

 sides by the membrane, the vessels and ureter passing out through an 

 opening. The oil-space is connected with a cylinder also filled with oil, 

 above which a piston, attached to a lever, moves. The lever registers 

 on a drum the changes in the volume of the kidney i.e., practically the 

 changes in the quantity of blood in it, and therefore in the calibre of 

 its vessels. A still better oncometer is that of Schafer, in which air is 

 employed instead of oil. 



Nerves of the Kidney. Both vaso-constrictor and vaso-dilator fibres 

 for the renal vessels, but most clearly the former, have been shown 

 to leave the cord (in the dog) by the anterior roots of the sixth thoracic 

 to second lumbar nerves, and especially of the last three thoracic. 

 They run in the splanchnics, and then through the renal plexus around 

 the renal artery into the kidney. The vaso-constrictors predominate, 



