THE PLACENTA is a spongy, vascular organ, at full term eight to ten inches in diameter, and two or three inches thick at center, thinning at the edges, weighing from three-fourths to one and one-fourth pounds. In appearance it is not unlike a piece of liver, only less solid. It is the proper vascular apparatus serving the combined purpose of fetal nutrition, respiration and excretion. At least, through its absorption all these functions are accomplished. This, with the membranes surrounding the fetus and umbilical cord, is called the _after-birth_. The placenta (Plates VIII and IX) lies in complete juxtaposition with the uterus, with an almost imperceptible membrane interposed. The fibers and bloodvessels of the uterus and placenta do not interlace, as some suppose; each has a distinct set of bloodvessels and capillaries, and a separate circulation. Nutrition and excretion are carried on by _exosmosis_, or transudation through this very attenuated membrane. THE FETAL CIRCULATION is an especially interesting phenomenon. Instead of the blood going to the lungs for oxygenation, the entire circuit is performed without this, the placenta serving the office of lungs as well as of the digestive organs. From the placenta oxidized blood is brought through the umbilical vein, a large portion of it passing to the liver, but all eventually enters the heart by the ascending vena cava. By the Eustachian valve it is directed through the foramen ovale to the left auricle, from this to the left ventricle, which conveys it to the aorta. Part of the blood, instead of taking this course, enters the right ventricle, and in place of going to the lungs through pulmonary arteries, passes at once to the aorta, through what is called the ductus arteriosus. After traveling the entire circuit, it is taken back to the placenta by two umbilical arteries, which are given off from the iliac arteries. At birth the _ductus arteriosus_ closes; the umbilical veins form the round ligament of the liver, and the umbilical arteries the round ligament of the uterus in the female, and the urachus, a ligament of the bladder, in the male. The _foramen ovale_ also closes, establishing a complete septum between the auricles of the heart. _A blue baby_ or _cyanosis neonatorum_ is the result should this valve fail to close. The venous blood commingles with the arterial blood, and death is the result sooner or later. THE UMBILICAL CORD is made up of two arteries and one vein. It is from two to four feet in length, attached at one extremity to the placenta, and at the other to the navel of the child. This is the medium of the circulation between the placenta and the fetus. The _membranes_ all unite before birth to form one thick, tenacious covering for the child, and also for the cord and fetal surface of the placenta.