Besides, the fact that the upper half of the hetal body receives blood richer in oxygen than the lower half is settled. The descend ing aorta contains venous blood carried into it by the ductus Botalli, so that the arteries which leave it after it has united with that vessel carry blood which is not so pure as that found in the arteries supply ing the upper half of the body, which arise from the aorta above the ductus 13otalli.
Complete separation between the systemic and the pulmonary circulations is first brought about by birth. But preparations have been made for this toward the end of pregnancy; the foramen ovale and the ductus Botalli become smaller in lumen during the last months of pregnancy, and second, the growing lungs demand more blood toward. the termination of pregnancy than in the earlier months. Therefore, toward the end of pregnancy, a decreased amount of blood is carried into the left auricle by the inferior vena nava, and a smaller quantity of blood is sent through the ductus Botalli into the descending aorta.
At the moment of birth the placental circulation is interrupted and in its place pulmonary respiration occurs in the child independently.
The umbilical vein no longer carries blood into the right auricle; be sides, the right ventricle must supply more blood to the unfolding lungs. As a result, a decided diminution in blood pressure oeeurs in the right side of tbe heart, which is also a cause for the closure of the ductus Botalli. Now, however, the lungs have withdrawn almost all the blood from the right side of the heart and have returned it, arterialized, into the left auricle. IIere, in opposition to the right auricle, in which blood pressure has become less, the blood pressure niust increase. These changes in the circulation of the blood, occurring independently, pre pare for the obliteration of the foranien ovale; the valves of the foramen ovale lie with their edges close to the upper free surface of the foramen and gradually fuse with it. Complete separation of the systemic and pulmonary circulations is made perfect after birth by the interruption of tbe blood current through the cluctus Botalli and ductus Arantii on the one hand, and through the foramen ovale on the otber.