PLACENTA

(Gr. ir\a.xovs, a cake.) The afterbirth. In the human subject it is a single sub circular, flattened, and lobulated organ, composed of the capillary extremities of the foetal hypogastric arteries and umbilical vein, and of a fine cellular structure, which receives the maternal blood from the tortuous uterine or decidual arteries. The placenta forms a single lobe in the New World monkeys, the bats, the Insect i vor a, and the Rodent i a. It surrounds the foetus like a broad hoop in the Carnivora. It is bilobed in the Old World monkeys; and subdivided into many separate lobes, called cotyledons, in the true Ruminant i a. The placenta is replaced by a diffused vascular villosity of the chorion in the Camellidce, the ordinary Pachyderms, and the Cetacea. The placenta is absent and the chorion ceases to be vascular in the Marsupial i a. [s. 944]