Lobstein has mentioned a mode of illustrating this fact (_Essai sur la Nutrition du Foetus_,) which is both simple and striking. Upon examining the uterine surface of a placenta which has been expelled at the full term, it presents the appearance of a spongy mass gorged with blood, which may be removed by washing or maceration, and if a placenta thus prepared be injected, the fluids will pass with the greatest facility from the umbilical arteries into the umbilical vein, but not one drop into its cellular structure; it is evident, therefore, that the blood which had filled the intervals between the vessels, and which had been removed by washing and maceration, could not have belonged to the foetus, but must have come from the mother; for if any of the vessels had been ruptured the injection would not have succeeded. In concluding these observations upon the placenta, we may briefly state, that there is the same relation between the umbilical vessels and the maternal blood, which fills the placental cells, as there is between the branches of the pulmonary artery, and the air which fills the bronchial cell.[23] _Umbilical cord._ The umbilical cord, funis, or navel string, is a vascular rope extending between the foetus and placenta, by which they are connected together. It usually arises, as we have before observed, from about the middle of the placenta, and terminates at the umbilical ring of the foetus; it consists of two umbilical arteries and one umbilical vein; the former conveying the blood from the common iliac arteries of the foetus to the cotyledons of the placenta; the latter formed by the union of the collected umbilical veins, on the inner surface of the placenta, and returning this blood to the foetus. In the early periods of pregnancy it also consists of the duct and vessels of the vesicula umbilicalis, the urachus, and more or less of the intestinal canal. The umbilical cord does not present the same form or appearance at every period of gestation; the younger the embryo, the shorter and thicker is the cord; in fact, there are no traces whatever of a cord at first, the embryo adhering, by its lower or caudal extremity, directly to the membranes. By the fifth or sixth week it becomes visible; at this early period the vessels of which it is composed pass from the foetus in a straight direction, but as pregnancy advances they become more or less spiral, winding round each other, and usually from left to right: according to Meckel, they take the opposite direction much less frequently, viz. in the proportion of one to nine.