The Separation of the Placenta.—The separation of the placenta does not include the membranes alone. We have seen that, together with the placenta, the uterine mucous membrane is shed as the decidua. This decidua, the maternal placenta, includes the entire superficial portion of the uterine mucous membrane. This shedding is not effected suddenly, but slowly and progressively, the efficient cause being the retractility and contractility of the uterus. The retractility first comes into play. Emptied of the fcetus, the uterus tends to reduction in size and in capac ity, and this can only happen through the retraction of its walls, from which results diminution in the size of its cavity. Now the placenta is a spongy, vascular, non-contractile body, and can only follow this retro grade movement of the uterus to a degree. The limit ones attained, it is compressed on itself, and, the retractile force continuing, the cellulo vascular adhesions, which unite it to the uterus, break, and the superfi cial portion of the mucous membrane yields at one point, exposing the uterine sinuses. From these sinuses escapes blood at the level of separa tion, and this blood, increasing in quantity, tends to increase the separa tion. The placenta thus becomes more and more a foreign body. Now the contractility of the uterus in turn plays its part, and, at the outset, completes the separation, later determines the expulsion, or at least the passage, of the placenta through the cervix. This process of separation does not always occur after the same manner, and it varies according to the point of placental insertion. It is generally granted that, when the point of insertion is at the fundus, the placenta begins to separate at its centre, because this centre is the thickest portion, and consequently less able to follow the retreat of the uterus. Separation then proceeds from the centre to the circumference, forming a cavity, as it were, in which the blood accumulates. The foetal surface of the placenta tends more and more to approach the internal orifice of the cervix, and when com plete separation has been effected, it will fall on this orifice by the same surface, carrying behind the membranes and the decidua, these latter being turned inside out like the finger of a glove, and making a pocket containing more or less blood and clots. If, on the other hand, the pla centa is inserted on one of the uterine walls, anterior or posterior, this separation, instead of beginning at the centre, begins at one or another edge, and, progressing after this manner, the placenta may appear at the cervix either by its foetal or maternal surface. If the separation proceeds from above downward, the placenta will present by its foetal surface, but in case of the separation beginning below, and extending upward, then remaining, as it will, adherent lastly at its upper border, the placenta will slide along the uterine wall, and will present either by its lower border or by the maternal surface. As a result, in the first instance, the blood will appear externally after the birth of the placenta, since it is retained in the pocket formed by the membranes; while in the second instance, the blood will appear externally as soon as separation begins.
The above process of separation, admitted by all authorities, has latterly been questioned by Matthews Duncan. According to him this inversion is due to tractions exercised on the cord, and, if we take as typical spon taneous delivery, we note that the portion of the placenta, which first presents at the cervix, and, in consequence, at the vagina, is not the amniotic or foetal strface, but the border or a portion at some distance from this border. " When the border does not exactly present, this is not
because the placenta is turned inside out, for only a small portion of the mass is in fact so turned. It is in reality the border which presents, but this border is slightly thickened, because it is turned up, and especially do the placental borders turn up in cases where they are thin. It is the still adherent membranes, which, making traction on the circumference of the placenta, determine the presentation, for the force necessary to detach them is greater than the resistance offered by the border of the lilaceuta. These points are confirmed by the researches of Ritgen and of Lemser. If the placenta is expelled after the manner described by Baudelocque and Schultze, there must needs occur considerable hemor rhage. The placenta, being in a measure a rigid body, it cannot turn on itself, take the shape of a cup, with the resulting cavity in which the blood accumulates. The force which is called into play to produce such an inversion tends at the same time to cause hemorrhage from the open uterine sinuses, which were applied against the surface of the placenta which has inverted. If the placenta detaches itself by presentation of its border, its uterine surface slides along the wall of the muscle, and there exists only a small space to receive the blood which flows from the uter ine sinuses. The muscular walls of the uterus remain close to the pla cental surface, the uterus contracts, pushes down the placenta, and soon it becomes almost spherical and empty. Hemorrhage, then, is no longer a consequence of the mechanism of labor." Devilliers is inclined to think that the point of insertion of the cord to the placenta has much to do with the mechanism of separation, which begins rather at the centre when the cord is inserted here, as is usually the case; while it begins near the placental border when the cord is in serted near the margin.
Expulsion and Passage of the Placenta through the Cervix.—Separation once accomplished, the placenta becomes a foreign body, against which the uterus tends to react. Contractility, hence, begins to play its part. This is manifested by the hardening of the uterus, which, changing its shape, projects through the abdominal wall; and this uterine contraction, being accompanied by pain, the woman complains, and the accoucheur knows that the third stage of labor is about to begin. Under the influence of these contractions, the cervix opens at the internal os, and the pla tends to engage within it. If, now, a vaginal examination be made, the finger touches a body, easily recognized as the placenta by the pro jection of the umbilical vessels, and insertion of the cord. If the third stage be left to the efforts of nature, the uterine contractions, in creasing in intensity, push this placental mass through the cervical canal, and contractility and retractility combining their forces, the placenta falls out of the cervix into the vagina, carrying with it the membranes which, in turn, detach themselves from the uterine wall.
Passage through the Vagina and through the Vulva. —Once in the vag ina, the placenta meets with no further resistance, and gentle traction suffices for delivery. If this delivery be left to nature, at the end of an in terval, which may. vary from ten minutes to many hours, the vagina in turn contracts and expels the fcetal annexes. But, remembering the ob jections to this method which we have already mentioned, it is customary to extract the placenta. This extraction, as is apparent, offers no diffi culties in a normal case, since the placenta is in part engaged in the cer vix, or has fallen into the vagina.