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they contract and send the blood into the ventricles, the auricular systole. As the blood comes through into the ventricles it probably comes around by the walls and closes the auriculo-ventricular valves, though just how the valves close is not certain. When the two ventricles are full they in turn contract, the ventricular systole, and the blood is forced out, that in the right ventricle passing to the lungs for its new supply of oxygen through the pulmonary artery, the only artery to carry venous blood, and that from the left ventricle entering the aorta for general distribution through the body. Following the systole is a pause, the diastole, while the heart fills again.

Fig. 43.—The fetal circulation.

Circulation in Fetus.—In the fetus there is direct communication between the two auricles through the foramen ovale, which normally closes at birth, though occasionally it remains open. There is also communication between the pulmonary artery and the arch of the aorta through the ductus arteriosus. The freshly oxidized blood comes to the fetus through the placenta, from which it is brought along the umbilical cord in the umbilical vein to the liver and thence to the inferior vena cava, where it mixes with the blood from the lower extremities. By the inferior vena cava it is carried to the right auricle, where the Eustachian valve—a valve between the inferior vena cava and the auriculo-ventricular opening, larger in the fetus than in later life where it serves no special purpose—guides it across the auricle and through the foramen ovale to the left auricle. From this auricle, together with a small amount of blood from the lungs, it goes to the left ventricle and is distributed by the aorta almost entirely