154 THE VASCULAR SYSTEM 



fluid at another pressure in another system. If the capillary- 

 venous pressure were higher than the pressure in the tubules, the 

 latter would be obliterated until the pressures became the same, 

 likewise if the pressure in the tubules were higher than that in 

 the veins. If the ureter be obstructed so that the pressure rises 

 within it to say 40 mm. Hg, the capillary-venous pressure and 

 tension of the whole kidney rises to this amount, and it takes a 

 pressure of 40 mm. Hg to drive water through a hollow needle 

 into the kidney substance. If the renal vein be obstructed till 

 the pressure in the renal venules rises to 40 mm. Hg, the same must 

 hold good, and the pressure of the urine in the renal tubules be- 

 come the same. If the arterioles of the kidney be dilated the 

 whole organ swells in its capsule, and the tension of the whole 

 rises, the vascular system and the tubular system together ap- 

 proximating towards the rigid condition, but both at the same 

 pressure, viz. the capillary-venous pressure. Under these con- 

 ditions the velocity of blood flow is greatly increased, and if the 

 kidney be enclosed in plaster of Paris, so that it cannot expand, 

 it makes no jot of difference, because the arteries dilate at the 

 expense of the veins, which are narrowed until a rigid system with 

 a rigid flow is produced. When the blood reaches the glomerular 

 capillaries, three courses are open to the fluid part, the capillary 

 wall retaining the corpuscles. It may pass on by the efferent 

 venules or by the lymphatics, or into the capsular ends of the 

 renal tubules. Which course it takes depends on the play of 

 such forces as surface tension, adsorption, and osmosis. Filtration 

 has nothing to do with it, for the fluid pressure must be the same 

 on either side of the wet films engaged, which are of a tenuity 

 comparable to that of the bubbles of a soap lather. 



Confirmation of the above views may be drawn from the per- 

 fusion experiments carried out by Sollmann on excised kidneys 

 in spite of the fact that the flow he obtained from the ureter was 

 attributed by him to filtration. He perfused 1 per cent, salt 

 solution at arterial pressure through the renal artery, and collected 

 and measured the outflow both from the renal vein and the ureter. 

 He found the venous flow reached its maximum in fifteen minutes, 

 while the ureter flow continued to increase for one to two hours, 

 thus rising slowly to a maximum. As the ureter flow increased 

 the venous flow declined slowly, and then the two flows ran a 

 parallel course. Subsequently undergoing minor oscillations the 



