LIGATURE OF THE URETER. 649 



of the expansion, the urinary flow recommences and becomes much more 

 rapid than it was previously to the injection of the drug. On the general 

 blood pressure the injection of caffein causes an initial slight 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 flow 

 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, which 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 

 filtrate, 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 be 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. exper. Path. u. PharmakoL, Leipzig, 1887, Bd. xxii. S. 39; 1888, Bd. 

 xxiv. S. 85. 



2 Journ. PhysioL, Cambridge and London, 1887, vol. viii. p. 117. 



