PLACENTA, or AFTER-BIRTH, a temporary organ that is developed within the uterus during pregnancy. and is, as its popular name implies, expelled from the maternal organism shortly after the birth of the child or young animal. It is a spongy vascu:ar mass, existing in some form or other in all mammals, excepting the 31arsupialia and hionotremata, as an appendage to the fetal membrane called the chorion. In the human subject, it is of considerable size at the period of delivery, being-of a rounded or oval form, with a diameter of 6 or 8 and a thickness of somewhat more than an inch. Its outer surface, which, till the period of its detachment and expulsion, is attached to the walls of the uterus, is uniform and level (unless it has been morbidly adherent), being covered by a membrane, shortly to be noticed, called the decidua serotina; and on peel ing off this membrane, the various lobes of which the placenta is composed are apparent. The internal or free surface is smooth and shining, and gives attachment to the umbili cal cord or navel-string, which connects it with the fetus. To render the mode of for mation of the placenta clear, we must premise that the impregnated ovum, when it reaches the uterus, is invested with an outer membrane, the chorion, which forms a shut sac, externally covered with short villi. As the ovum advances in age, these villi dimin ish in number, until few remain, except at that part of the chorion which is in contact with the uterus; and here, about the second mouth (in the human subject), they divide into branches. While these changes are going on in the membrane of the ovum, the uterus is also undergoing modification; and it is on the nature and extent of these uterine changes that the character or type of the placenta depends. There are two such types, the first of which is best represented by the human placenta, and the latter by that of the pig.
In animals exhibiting the first type of placental structure, the mucous membrane lining the uterus undergoes a rapid growth and modification of texture, becoming con nected with the membrana decidua, which is so called from its being thrown off at each parturition. For brevity, it is usually termed the decidua. This decidua is from an early period separable into three portions—the decidua 28ra, or decidua uteri, which lines the general cavity of the uterus: the decidua reflexa, which immediately invests the ovum; and the decidua serotina, which is merely a special development of a part of the decidua Vera at the part where the villi of the chorion are becoming converted into the fetal portion of the placenta. At first, the villi of site chorion lie loosely in the corre sponding depressions of the decidua; but subsequently, the fetal and maternal structures (the villi and the decidua vera) become closely united, so as to form one inseparable mass, by the following means: the deeper substance of the uterine mucous membrane in the region of the placenta is traversed by vessels which enlarge into what, in the case of the veins, are termed sinuses, clip down between the villi, and at last swell round and between them, so that finally the villi are completely bound up or covered by the mem brane which constitutes the walls of the vessels, this membrane following the contour of all the and even passing, to a certain extent, over the branches and stems of the tufts."—Goodsir's Anatomical and Pathological Observations, p. 60.
The pure maternal blood is conveyed to the placenta by what are termed, from their tortuous course, "the curling arteries" of the uterus, and is returned by the large veins termed sinuses. `• The fetal vessels," says Dr. Carpenter, " being bathed in this blood, as the branchke of aquatic animals are in the water that surrounds them, not onlyenable the fetal blood to exchange its venous character for the arterial, by parting with its carbonic acid to the maternal blood, and receiving oxygen from it, but they also serve as rootlets, by which certain nutritious elements of the materual blood (probably those composing the liquor sanguinis) are taken into the system of the fetus. It is probable, too, that the placenta is to be regarded as an excretory organ, serving for the removal, through the maternal blood, of excrementitious matter, whose continued circulation through the blood of the fetus would be prejudicial to the latter."—Haman Physiology, 3d ed., pp. 1013, 1014. Moreover, the recent investigations of Bernard show that the placenta secretes, like the liver, the saccharine matter known as glycogen (q.v.), which probably takes part in keeping up the animal heat. The vascular connection between the fetus and the placenta is effected by the umbilical vein (containing arterial blood) and the two umbilical arteries (containing venous blood), all of which lie in the umbili cal cord which connects the fetus (q.v.) with the placenta. The placenta may be formed at any.point of the uterus, but is most commonly on the left side. Occasionally (in 11 cases out of 600, according to Naegele), it is situated partially or entirely over the mouth of the womb (as uteri), in which case dangerous flooding takes place previous to or at the period of labor. This condition is known as placenta pramia, and under ordi nary management, "one in three of the mothers are lost, and more than 65 per cent of the children."—Churchill, Theory and Practice of Midwifery, 3d ed., p. 473. By substi tuting the detachment and extraction of the placenta for the old method of turning the child in Were, prof. Simpson finds that the mortality sinks to 1 in 14 of the mothers, but slightly rises (to 69 per cent) in the case of the children.