The ascending vena cava, then, pours into the right auricle a mixture of arterial blood, which has come directly from the placenta, and venous blood returned from the liver, intestines and lower extremities. There is a difference of opinion concerning the course of the blood stream after reaching the right auricle. The general teaching, however, is that the eustachian valve, guarding the foramen ovale, deflects the current through this opening from the right into the left auricle. It then pours into the left ventricle, is pumped into the arch of the aorta, from which most of the blood is sent to the head and upper extremities, though a small part carries nourishment to other parts of the body. The descending, or superior, vena cava, carrying blood returning from the head and arms also empties into the right auricle; this stream presumably crosses the stream which is directed toward the foramen ovale, flows into the right ventricle by which it is pumped into the pulmonary artery. The circulation of blood through the lungs, however, is for their own nourishment, and not for aëration as with the adult. For this reason most of the contents of the fetal pulmonary artery empties into the aorta through the ductus venosus, one of the temporary fetal structures already referred to. From the aorta the stream is directed in part to the lower extremities and the pelvic and abdominal viscera, but most of it flows into the hypogastric arteries. These are also temporary arteries. They lead to the umbilical cord and, as the umbilical arteries, carry the venous or vitiated blood through the cord to the placenta where it is oxygenated, freed of its waste in the chorionic villi and returned to the fetus through the umbilical vein. As soon as the child is born and it is obliged to obtain its oxygen from the surrounding air, its pulmonary circulation of necessity becomes immediately more important and is greatly increased in volume. In fact, the entire fetal circulation is readjusted to meet the needs of the new and independent functions which the little body now assumes. The temporary structures are obliterated, since they are no longer needed, and the lungs and intestines become more active in compensation. [Illustration: FIG. 29.—Diagram showing circulation of the blood after birth, with hypogastric arteries, ductus venosus, ductus arteriosus and foramen ovale in process of obliteration and pulmonary circulation greatly increased. (From The American Textbook on Obstetrics.) ]