Obstructions in the course of the ureters may exist at their opening into the bladder, which may be contracted by chronic cystitis; at a point immediately above this from compression by morbid growths, especially of the uterus, one of the most common causes of hydro-nephrosis, or even from retroflexion of the uterus when pregnant; at any point in its course by a twisting or sharp angle, as in movable kidney, although this is a much rarer accident than might be supposed; or at the brim of the pelvis, where it may be bound down by old peritoneal adhesions, and at its junction with the renal pelvis, which may be formed in such a manner as to constitute a valve, so that the urine escapes slowly or with great difficulty; or where it may be blocked by a calculus or other deposit in the cavity of the pelvis. Obstructions by a twist or angle or by a valvular opening may, it is obvious, be temporary or intermittent in their action, and probably some arrangement of this kind was present in the cases which have been reported of relief of hydro-nephrosis by gentle massage of the abdomen. Above the point of obstruction the ureter and pelvis are found dilated and the walls somewhat thinned. The kidney and its pelvis form a more or less irregular rounded pouch, with the tense cylindrical tube of the ureter attached to it below. The kidney itself becomes in various degrees atrophied. In some cases it retains nearly all its secreting structure, and is merely spread out upon the surface of the sac; in others, while the pelvis is but little dilated, the true kidney substance atrophies almost completely, and becomes a mere shell enclosing a cavity continuous with the pelvis and broken up by fibrous septa into subordinate cavities representing the original calices. A partial hydro-nephrosis is sometimes observed affecting only the calices. Whether the one or the other of these conditions shall result depends, as has already been remarked, upon the completeness and suddenness of the obstruction. If the ureter of a rabbit is ligatured, the second condition--that is, atrophy of the kidney with but little dilatation--is observed. The pressure of urine soon puts a stop to further secretion, and there is no time for a slow and gradual dilatation of the pelvis and ureter. When, as is much more frequently the case in the human subject, the obstruction is more gradual or incomplete, the back pressure is for a long time insufficient to completely stop the passage of fluid through the renal capillaries, so that the pelvis and ureter, though allowing their contents to pass out only under a considerable vis-a-tergo, have time to accommodate themselves to the change, and dilate gradually, attaining sometimes enormous dimensions. The size of a hydro-nephrotic sac varies greatly: 60 liters of contents is certainly a very extreme case.