Under normal conditions, during the intermenstrual period, a plug of clear viscid mucus, which is secreted by the glands of the cervical canal, blocks up that passage, but is washed away each month by the menstrual discharge.  Under ordinary conditions this obstruction must seriously interfere with the entrance of the spermatozoa into the cavity of the uterus, and renders the former theory, recently revived by Bossi, quite tenable, that impregnation is most likely to occur just after the menstrual epoch.

The vaginal secretion under certain pathologic conditions may become so acid that it induces sterility.  Women who suffer m severe vaginal catarrh are frequently sterile, the spermatozoa being found dead in the vagina some hours after copulation, although an examination a shorter time afterward revealed them still alive.  In cases where conception takes place in spite of a very acid condition of the vaginal secretion, it is probable that some of the spermatozoa enter the uterus before the secretion has had time to act on them, or possibly the spermatozoa being injected in a mass, the acid secretion is unable to penetrate and kill them all.

The reaction of the normal vaginal mucus is always acid, that of the cervix alkaline; but as the result of the inflammatory condition, the reaction of each is often intensified, especially that of the vagina, which has an exceedingly sour and penetrating odor.  This acid discharge, bathing the neck of the uterus, penetrates more or less into the cervical plug and causes coagulation of the alkaline mucus.

The chief constituent of the semen is albumin; agents which affect albuminous substances influence the functional activity of the spermatozoa—­ heat, concentrated acids, and probably concentrated alkalies.  In normal conditions the alkalinity of the seminal fiuid seems to be sufficient to neutralize the acidity of the vaginal secretions, so that the spermatozoa may remain seventeen days or more (Bossi) within the vaginal canal, even during a menstrual period, without having their vitality destroyed.

When hyperacidity of the vaginal secretion is present, it is probable that the fertilizing element is at once rendered inert; but should some of the spermatozoa succeed in reaching the interior of the cervical canal, the increased alkalinity of the secretion there would in all probability put an end to all further progress.

The conditions, then, which appear to prevent fecundation are:  First, the absence of the proper nidus for the ovum; second, the obstruction of the cervical canal by a mucus plug; third, increased alkalinity of the cervical secretion, often accompanied by the increased acidity of the vaginal secretion.  Three conditions must, then, be determined:  First, are there spermatozoa in the semen?  Second, do they get into the uterocervical canal?  Third, do the secretions in the canal poison the spermatozoa?