_The placenta_ is a highly vascular structure spread out or scattered over the surface of the chorion and the mucous membrane of the uterus, that attaches the foetus and its envelopes to the womb (Fig. 19). It is by means of this vascular apparatus that the foetus is furnished with nourishment. The fetal and maternal placentas are made up of vascular villi and depressions that are separated only by the thin walls of capillaries, and a layer of epithelial cells. This permits a change of material between the fetal and maternal circulation. The arrangement of the placenta differs in the different species. In the mare and sow, the villi are diffused. In ruminants, the villi are grouped at certain points. These vascular masses are termed cotyledons. The maternal cotyledons or "buttons" form appendages or thickened points that become greatly enlarged in the pregnant animal. Toward the end of the pregnant period, the attachments between the fetal and maternal placentulae undergo a fatty degeneration and finally separate. This results in contractions of the muscular wall of the uterus, and the expulsion of the foetus and its envelopes. In the mare, it is not uncommon for the colt to be born with the covering intact. This does not occur in the cow. Usually the envelopes are not expelled until a short time after birth in all animals, and it is not uncommon for them to be retained. This complication is most commonly met with in the cow. In the mare the _retention of the fetal envelopes_ or "afterbirth" is commonly due to the muscles of the womb not contracting properly following birth. Abortion, especially the infectious form, is commonly complicated by a retention of the fetal membranes. Any condition that may produce an inflammation of the lining membrane of the womb may result in retention of the "after-birth." Injuries to the uterus resulting from the animal slipping, fighting and becoming crowded are, no doubt, common causes of failure to "clean" in cows. _The symptoms_ are so marked that a mistaken diagnosis is seldom made. A portion of the membranes is usually seen hanging from the vulva, and the tail and hind parts may be more or less soiled. The latter symptom is especially prominent if the membranes have been retained for several days, and decomposition has begun. In such case, the discharge from the vulva is dark in color, contains small pieces of the decomposed membrane and has a very disagreeable odor. In the mare, acute inflammation of the womb may result if the removal of the "after-birth" is neglected. Loss of appetite, abnormal body temperature, weakness and diarrhoea may follow. Such cases usually terminate in death. Retention of the fetal membranes is a very common cause of leucorrhoea.