106 



THE HEART AND CIRCULATION. 



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. 



Circulation in Fetus. In the fetus there is direct com- 

 munication between the two auricles through the 

 foramen ovale, which normally 

 closes at birth, though occa- 

 sionally it remains open. There 

 is also communication between 



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 in- 

 ferior 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 



FIG. 43. The fetal circulation. 



