Another difficulty in midwifery practice, but far less serious than the preceding, is undue retention of the placenta. In ordinary cases, the average interval between the birth of the child and the expulsion of the after-birth is a quarter of an hour. When the expulsion does not take place within an hour or an hour and a half, the ease is regarded as coming under the head of "retained placenta." It occurs in about 1 case in 400, and in these cases is fatal to about one mother in five; the cause of death being hemorrhage. The principal causes of retention are either imperfect and insufficient, or irregular contraction of the womb after the birth of the child. In the first of these eases, if the uterus cannot be excited to sufficient action, the placenta must be itvith drawn by steady traction of the umbilical cord, and if it fail, extraction by the intro duction of the hand (an operation always to be avoided if possible) must be .resorted to; in the letter case, manual extraction is commonly necessary. Sometimes, in conse quence of inflammatory or other affections of the placenta, there may be adhesion between its outer surface and the inner surface of the womb. This is the most danger ous form of retention, there being usually excessive flooding, and additionaliy the peril arising from the decomposition of any portion that cannot be removed without undue violence.
The placenta acquires its proper character, in the human subject, during the third' month, and it subsequently goes on increasing to the full period of gestation. At about the fourth mouth, the blood, moving through the enlarged uterine vessels, produces a peculiar murmur, which is known as the placental bruit, resembling the sound made by blowing gently over the lip of a wide-mouthed phial, and increasing in intensity and strength as pregnancy (of which it is one of the characteristic signs) advances.
In animals exhibiting the second type of placental structure—as, for example, the pi the placenta is comparatively simple in its structure. "No decidua is developed; the elevations and depressions of the unimpregnated uterus simply seqnire size and vascularity during: pregnancy. and cohere close!! with the choriome which do not become restricted to one spot, but are developed from all parts of the chorion, except its poles, and remain persistent. in the broad zone thus formed through out fetal life. The cohesion of the fetal and maternal ph,ceutte. however, is overcome by slight inaceratiou or post-mortem change; and at parturition, the fetal villi are snit- • ply drawn out like fingers from a glove, no vascular substance of the mother being threwn off." Prof. Huxley, from whose Elements qf Compatatere Anatomy (1864. p.
103) the preceding extract is borrowed, follows the opinion adopted by De Blainville, Von Baer, Eschneht, Milne-Edwards, Gervais, and Vogt in regarding "the features of the placenta as affording the best characters which have yet been proposed for classify ing the monodelphons [or placental] mammals." Ile proposes to apply the term dead n ate to those animals whose placenta presents the human type, and which throw off a decidua; and to term those animals nomrlecidaate in which the placenta is const•ueted on the same plan as that of the pig. • "Thus," he observes, "man; the apes, or so-called quadry maim; the inseetirora; the eheiroptow; the rodentia, to which the lowest apes present so many remarkable approximations; and the earnicora, are all as closely con nected by their placental structure as they are by their general affinities. With the pig, on the other hand, the ungulate quadrupeds, and the eetaeca which have been sind ied, agree in developing 110 dc-china, or, in other words, in the fact that no vascular maternal parts are thrown off during parturition. But considerable differences are observed in the details of the disposition of the funal villi, and of the parts of the uterus which receive them. Thus, in the horse, camel, and eetacen, the villi are scattered as in the pig, and the placenta is said to he dejftette; while in almost all true ruminants the Retal villi are gathered into bundles or cotyledons, which in the sheep are convex, and are received into cups of the mucous membrane of the uterus; while in the cow, on the contrary, they are concave, and fit upon corresponding convexities of the uterus." The remarks which have been made on the functions of the human placenta, are equally applicable to all placental mammals g6nerally.
The diseases of the human placenta had not been studied with any accuracy, until the subject was taken up by Simpson. This distinguished physician and subsequent observers have ascertained that the placenta is liable to (1) congestion. ending in the effusion of blood into the substance of the organ upon its surfaces, or between the mem branes; (2) inflainmat ion, giving rise to adhesions, or terminating in suppuration, which may occasion very serious constitutional disturbances; (3) partial or entire hypertrophy or atrophy; and (4), fatty degeneration, affecting its 5101111 vessels. Whatever be the form of disease by which the placenta is attacked, the result is usually fatal to the fetus.