{58} The sac is usually white and glistening, thinner at some places than at others, and lined with a smooth, pale, and atrophied mucous membrane. The muscular layer has degenerated, and perhaps partly disappeared. The liquid contained in the sac, supposing no inflammatory products to have been mingled therewith, is at first nearly identical with urine, and always contains urea. Afterward its character changes from the absorption of the urinary salts and the secretion of mucus. The contents may be dark-colored from hemorrhage or somewhat gelatinous. At a later period again they become serous and may contain cholesterin. The description just given, as well as that of the symptoms, applies to simple hydro-nephrosis. When the sac has become inflamed we have the very common combination with pyelitis, and the affection is called pyo-nephrosis. The progress of a case of hydro-nephrosis may be in rare cases to recovery by spontaneous re-establishment of the permeability of the ureter. In others it persists a long time without giving rise to trouble. If inflammation supervene, it is obvious that fever, either simply irritative or of pyæmic character, may be a severe or even a fatal concomitant, or that in this condition a perforation may take place. When the tumor is large it may from its bulk alone produce disturbance of the circulation, dyspnoea, palpitation, and oedema of the lower limbs. As regards the influence of this lesion on the secretion of urine, everything must depend on the amount of renal atrophy. A single kidney may undoubtedly be completely atrophied by this as by any other lesion without producing serious symptoms, since, as has been repeatedly demonstrated, the other is sufficient to carry on the work under ordinary circumstances; but if, as very frequently happens, both kidneys are involved, there must come a time when the renal substance no longer suffices, and the usual results of suppression of urine follow. It is possible, however, for extensive changes to take place in both kidneys before symptoms of insufficient secretion arise.