Pyelitis is divided by some foreign writers into catarrhal and diphtheritic--a distinction rather of degree than of kind. The mucous membrane of the pelvis may, like other mucous membranes rarely, and like serous membranes often, throw out a fibrinous exudation which takes the form of false membrane. This indicates intensity of inflammation, but has no necessary connection with diphtheria. A true diphtheritic pyelitis, that is, connected with the general disease known as diphtheria, is of course a conceivable lesion, but certainly not a common one. The renal symptoms--especially true albuminuria, so common and of such grave import in this disease--are due to lesions of the secreting substance, and not of the pelvis. It is important, but not always easy, to decide whether there is more albumen present than is to be accounted for by the pus. The pyelitis may be acute or chronic, being {48} characterized by the intensity of the attack and the rapidity with which the symptoms subside. The prospect of a given attack being acute is decided largely by the supposed cause: a small calculus passing into the ureter undoubtedly gives rise in most instances to a localized pyelitis, which subsides after the cause of irritation has disappeared. An inflammation from a larger one remaining is naturally of slower development, but may be more acute while the calculus remains rough and irritating, and partially subside when it becomes covered with a smoother coating of phosphates. The mucous membrane, however, is not likely to regain a completely healthy condition. The mucous membrane in severe pyelitis may be deeply eroded, and even perforated, so that the contents of the pelvis escape and give rise to abscess in the perinephritic or prevertebral cellular tissue, which may be discharged through the loins with resulting cure, or the establishment of a fistula, from which issues pus and at times calculi. Among the rarer results of perforation may be mentioned gastro-nephric and duodeno-nephric fistulæ. These might be diagnosticated by the presence of food and other intestinal contents in the urine, provided that the ureter were still pervious. Vomiting of calculi and urine has been reported by the older writers. The writer is indebted to J. R. Chadwick for references to two modern cases--one where such a fistula was diagnosticated during life;[16] and another where a gastro-nephric fistula was found after death.[17] In the latter case a diagnosis would have been impossible, as the kidney was disorganized and the ureter occluded. The extent to which the renal secreting substance suffers in calculous pyelitis varies considerably, and is very probably connected with the amount of pressure exercised either by the calculus itself when it attains a large size or by the urine in cases of obstruction. It is rare for either pyelitis or hydro-nephrosis to exist entirely independently. [Footnote 16: _Giornale di Anat. e Fis. path._, iii. p. 370.]