PLACENTA (Lat.. flat cake). The after birth—a spongy vascular mass attached to the uterus before the birth of a young animal and expelled after labor is completed. It is the structure that unites the foetus to the wall of the maternal uterus. It occurs in all mammals ex cept the egg-laying Ornithorhyncus and Echidna, though only rudimentary in the marsupials. It presents a variety of forms among the different mammals. Thin in the pig, mare. and the Cetacea it is diffused over the whole interior of the uterus and is termed diffuse; in ruminants it is attached in scattered segments over the uterine wall. cotyledonary; in certain of the Edentata, the elephant, and Carnivora it occurs as a zone around the uterine surface, zononj; in most of the Edentata. the Insectivora. and Rodentia it is found as a circular disk, discoidal; while in the monkeys and man. being first ar ranged in scattered patches and later as a disk. it is known as meta-discoidal. The placenta is formed in its greater part from hypertrophy and other changes in those chorionic villi which chance to be•in contact with the uterine surface when fatation begins. The development of these adherent villi produces what is known as the fatal portion of the placenta, while the re mainder, the decidua serotina, furnishes the ma ternal portion. Around the placental villi are de veloped vascular spaces in which certain arteries and veins from the uterine wall communicate freely. The villi being thus immersed in the maternal blood. osmotic interchange takes place between the foetal and maternal circulations. On gross inspection the placenta presents itself as a round or slightly oval disk from six to eight inches in diameter and from three-quarters to an inch in thickness. Its weight is about a pound. The fatal surface is smooth and covered
by the amniotic membrane. which is reflected from the cord on to the placenta at its centre and again from the placental margins to the uterine wall. The maternal surface is deep red, rough, and irregular. and divided by numerous grooves or sulci. A close inspection shows this surface to he covered by a fine delicate mem brane which dips down into the sulci and which is the stripped-off cellular layer of the decidua serotina. Attached to the fatal surface of the placenta, usually at its centre, but occasionally at or near its margin, is the uwhn17l.ai cord. The cord is a whitish senni-trauspau•ent structure of an al crage length of about twenty inches and a diameter about Got of the little linger. It con sists of two arteries and a vein tWisted one another and surrounded by a peculiar substance called from its consi-terc•e the jell• of Wharton, gives the cord its characteristic appear ance.
The function t.f the placenta is both respira tory and alimentary. It aerates the fu-tal blood by gaseous interchange of oxygen for carbon dioxide and other gaseous waste products. (Sec OactiAtiox: and PLOOD.) ID its nutritional function it supplies material for the rapid growth of the twills, at the same time remov ing the products of tissue decomposition. The placenta is subject to several pathological ab normalities in form and attachment, to inflamma tion, and to degenerative changes. For their detailed deseription, and for the treatment of that dangerous form of misplacement known as placenta previa, consult: Jewett, The Prac tice of Obstetrics (New York, 1909) ; A merieen systems of and Obstet ries I899),