LIGATURE OF THE URETER. 649 
of the expansion, the urinary flow recommences and becomes much more 
capid than it was previously to the injection of the drug. On the general 
blood pressure the injection of caffein causes an initial sHghl fall, followed 
by a return to normal or a little above normal. In this case we seem to 
be dealing with a drug, the most important action of which is on the 
renal vessels, and it is probable that the increased pressure in and now 
through the glomerular capillaries induced by the drug is largely 
responsible for the augmented flow of urine. According to von 
Schroder, 1 it is possible, by the administration of chloral, to abolish the 
vaso-dilator effect of caffein in rabbits without destroying the diuretic 
action of the drug; but too much reliance cannot be placed on this 
statement, since the volume of the kidney was not measured in this 
observer's experiments. 
The effect of digitalis is rather more complex. It slows and 
strengthens the cardiac beat, and at the same time constricts the smaller 
arteries of the body, so that the arterial pressure is raised. In heart 
disease the result of the improved working of the cardiac pump is to 
relieve the venous pressure, increase the arterial pressure, and so bring 
about an improved blood flow through the kidney. In such cases, there- 
fore, digitalis acts as a powerful diuretic. In the healthy animal the 
effect of this drug is more doubtful. It causes a constriction of the 
renal vessels and therefore a shrinking of the kidney. Under certain 
circumstances, however, it does exert an appreciable influence in causing 
diuresis, winch we may either explain, with Bradford and Phillips, 2 
as due to a direct action of the drug on the renal epithelium, or to the 
fact that the rise of blood pressure more than counteracts the renal 
constriction, so that there is an increased blood flow and pressure in the 
glomerular capillaries. 
Effects of ligature of the ureter. — If we are to look upon urine as a 
nitrate, the amount of it must vary as P — p, where P represents the 
pressure in the glomerular capillaries, while p represents the pressure at 
the beginning of the urinary tubule. So far, we have only considered 
the effects of altering P, and have seen that, in the majority of cases at 
any rate, the secretion of urine rises and falls with this pressure. Under 
normal circumstances p is so small that it may be neglected, but we 
ought to be able to diminish the flow of urine by increasing p. If the 
ureter be obstructed by connecting it with a mercurial manometer, it 
will lie found that the mercury in the manometer rises quickly to 10 or 
20 mm. Hg, and then more slowly until, in the dog, it may attain the 
height of 50 or 60 mm. Hg, at which pressure the mercury column 
remains stationary. The pelvis of the kidney and the ureter above the 
ligature are now strongly distended ; the kidney is swollen, and a marked 
oedema is soon observed extending to the perinephritic tissues, while 
the lymphatics of the hilus are distended with clear fluid. Some hours 
later, haemorrhages are found in the fatty capsule and in the pelvis and 
ureter. Ludwig interpreted these results as determining the conclusions 
he had already drawn from the effects of section of the spinal cord, i.e., 
that, for the production of urine, a certain minimum difference of 
pressure P — p is necessary, and that the difference might be reduced 
below this limit either by diminution of P or by augmentation of p. 
1 Arch. f. c:rper. Path. u. Pharmakol., Leipzig, 1887, Bd. xxii. S. 39; 1888, Bd. 
xxiv. S. 85. 
- Journ. Physiol., Cambridge and London, 1887, vol. viii. p. 117. 
