In the later stages, however, these organisms become inert, die, and disappear, so that in the majority of cases of chronic pyosalpinx the pus is found to be bacteriologically sterile. Observation on this subject made by a number of investigators shows that out of 133 cases of acute and chronic suppuration of the uterine appendages in which the pus was examined bacteriologically, no organisms whatever were found in 82 cases; in other words, the pus was sterile in about 61 per cent. of the cases. The pyosalpinx in time, therefore, becomes inert so far as any active inflammatory action is concerned, and resembles a chronic abscess in other parts of the body. Active inflammatory action may, however, be excited at any time, as in other chronic abscess, by a new infection, septic organisms entering the abscess by way of the uterine cavity, an adherent loop of intestine, or the bladder. The woman will then have an attack of acute septic inflammation in the old pyosalpinx, and will be exposed to the various dangers that were imminent during the primary acute stages of the disease. [Illustration: FIG. 149.--Hydrosalpinx, showing complete inversion of the fimbriæ.] It seems probable that if the woman survive the dangers to which she is exposed from a pyosalpinx, the tumor may in time become converted into a hydrosalpinx. The solid constituents of the fluid become absorbed or deposited upon the cyst-walls, and a clear watery fluid remains. In hydrosalpinx the recesses of the tube are often found to contain cheesy material and cholesterin--remnants of the old purulent accumulation. The tubo-ovarian cyst is formed in this way from a former tubo-ovarian abscess.