roentgenography of the bladder after the latter has been filled with a contrast medium, which either descends into the bladder from the kidneys (excretory urography) or is injected into the bladder through the urethra (ascending urography). The medium used in cystography may be one of various contrast substances (such as Sergosin and Urotrast) or—in pneumocystography—a gas (oxygen or carbon dioxide); or it may be a combination of a gas and a liquid contrast substance. Sometimes several photographs of the bladder are taken on a single film (polycystography). If a photograph shows deformations in the contour of the bladder or defective filling (that is, the absence of the contrast medium in certain sections of the bladder), this may indicate the presence of calculi or a diseased condition in the bladder walls (tuberculosis or tumor) or the prostate.