This occurs as a consequence of various kinds of irritation. Stone may produce it, and the extension of gonorrhoea, but by far the most frequent cause of it is alkaline decomposition of the urine.

The decomposition is due to the introduction of microbes and their propagation in the contents of the bladder. They are usually introduced by the catheter. In a healthy bladder such introduction may produce no effect, the growth of the microbes being inhibited by the normal mucous membrane. In cases of paralysis of the bladder, or of dilatation, especially when combined with a certain amount of inflam mation, the microbes multiply. This is particularly the case when there is stagnation of the urine, and the bladder has to be relieved at intervals by catheterization. The bacteria may find other means of entrance, as by fistulous openings. It is not impossible that in the case of the short female urethra they may propagate along that passage from the vagina.

The products of decomposition produce the usual inflammatory manifestations with various degrees of violence. In very acute cases there is great swelling of the mucous membrane, it may be with superficial or deep sloughing. In more chronic cases the mucous membrane gets thickened and very frequently becomes the seat of ulceration, so that with thickening and ulceration there is very great irregularity of the surface, sometimes with polypoid projections. The surface is occasionally incrusted with phosphates deposited by the alkaline urine. The muscular coat is often thickened, especially when there is at the same time obstruction to the passage of the urine, and there is the usual trabecular appearance, but obscured by the thickening of the mucous membrane. The bladder may undergo great contraction in consequence of chronic inflammation, the new-formed tissue in the mucous membrane shrinking.

The urine contains the inflammatory exudation as well as the products of decomposition. In acute cases there may be considerable quantities of pus. In more chronic cases the urine is thick and gelatinous like a mucous secretion. The toughness does not, however, depend on the presence of mucin, but is occasioned by the albumen of the inflammatory exudation being acted on by the alkaline salts in the urine. It is well known that the presence of pus in the urine may be detected by adding an alkali; the deposit assumes a gelatinous character. The pus and other inflammatory products in the bladder are similarly acted on when the urine becomes alkaline, and we have the viscid character mentioned. Under the microscope the urine presents abundant pus corpuscles and epithelium, with immense numbers of bacteria and crystals of triple phosphates.

Decomposition of the urine in the bladder is of consequence, not only in respect that it leads to Cystitis, but also that the decomposition is liable to extend upwards and produce still more serious effects. Thus the process may extend up the ureters so as to produce one of those serious septic conditions of the pelvis and kidney already described (see p. 963). These results sometimes follow the introduction of a catheter so rapidly as to have given origin to the term Catheter fever, a clinical designation which may include one or other of the conditions described under the designations, Pyelitis, Pyelonephritis, Suppurative Nephritis, and Pyonephrosis.