The manner in which the mouth of the Womb opens, and the time required for its dilatation, differ much in different cases. In females who have previously borne children, as before explained, the mouth is always considerably opened at the full term, while in a first pregnancy it is nearly closed, even till some time after the labor actually commences. Sometimes the dilatation takes place rapidly, and at others very slowly; it is especially liable to be delayed if the Membranes break too soon, because then the pressure of the bag of waters is lost, and that is an important agent in expanding the Os Uteri. In some cases the neck of the Womb is very hard and rigid, so that a long time is required to make it give way. When any other part than the head presents also, the opening of the mouth will not take place so soon, because no other part so completely fills up the passage. The breaking of the bag of waters will sometimes occur very early, almost as soon as it protrudes; while at other times it will be delayed till the whole Vagina is filled up by it, or even till it appears externally. The quantity of the water discharged at the time of the rupture is also variable; if the presenting part of the fœtus does not completely block up the passage, the whole may pass away when the rupture takes place; but if it does, as is usually the case when the head presents, only a part flows then, and the rest comes in gushes, as the head is raised, and when the child is born. The too early escape of the waters, as already explained, may retard the delivery, by delaying the expansion of the mouth of the Womb; and in this way unskilful accoucheurs have caused lingering labors, by breaking the membranes too soon. It is important to recollect also, as I explained before, that a portion of fluid sometimes exists between the amnion and chorion, which may pass first, and induce the belief that the true waters have escaped, when they have not. This is called the _false waters_, or _shows_, and is not connected with the true waters at all. The _general_ physiological phenomena of a natural delivery having thus been explained, we have now to state its duration, and then proceed to its conduct or management. DURATION OF NATURAL LABOR.