=Circulation.=--The blood, after it has given off its oxygen and collected carbon dioxide, returns to the heart through two main channels, the _superior_ and _inferior venæ cavæ_, the former bringing the blood from the upper part of the body, including the head, neck, and arms, and the latter from the lower part below the diaphragm. The two vessels empty along with the _coronary sinus_, which is guarded by the _coronary valve_, into the right auricle. At the same time that they empty into this auricle the four _pulmonary veins_, the only veins that carry arterial or oxygenated blood, are emptying the fresh blood from the lungs into the left auricle. When both auricles are full, 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. [Illustration: 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 to the head and upper extremities. Hence their large size and perfect development at birth. Returned from the upper extremities by the superior vena cava, the blood enters the right auricle again and, passing over the Eustachian valve this time, descends to the right ventricle, from which the greater part passes by the pulmonary artery and the ductus arteriosus to the descending aorta, though a small amount keeps on through the pulmonary artery to the lungs. In the aorta