_The placenta_ constitutes, from the third month of intra uterine life, the nutritive and respiratory organ of the foetus. The placenta consists of a maternal portion and a foetal portion. _The maternal portion_ is that portion of the placenta next to the uterine wall of the mother. In this are intervillus blood spaces, which may be regarded as derivations from the eroded maternal blood vessels. In the non-pregnant state the uterus is supplied with branches from the internal iliac artery, which end in capillaries in the wall of the uterus. In the pregnant state the numerous branches of the arteries supplying the uterus do not end as capillaries, but pierce the basal plate of the placenta, where the arterial vessels lose their muscular coat and open directly into the intervillus or intraplacental blood spaces. Maternal capillaries are wanting within the placenta, since they become early replaced by the intervillus spaces. The maternal blood is carried away from these spaces by wide venous channels, forming networks from which proceed the larger venous trunks. [Illustration: FIG. 44—Plan of the foetal circulation. (Gray)] _The foetal portion_ of the placenta is that portion next to the child. Here end the terminal loops of the foetal blood vessels, the blood being conveyed to and from the placenta along the umbilical cord, by the umbilical arteries and vein. Although coming into close relation, the blood streams of the mother and of the child never actually mingle, because of the delicate septum which intervenes. The delicate septum, however, allows the free interchange of gases necessary for the respiratory function as well as the passage of nutritive substances into the foetal circulation. _The umbilical cord_ connects the body of the foetus with the placenta, and conveys the foetal blood to and from the placenta to the child. This blood is carried by means of two umbilical arteries and one umbilical vein.