CIRCULATION OF BLOOD IN THE FETUS 791 



superior vena cava the less pure fluid of the two passes almost exclusively 

 into the right ventricle, through the auriculo- ventricular opening, just as it 

 does in the adult. The blood of the inferior vena cava is directed by a fold 

 of the lining membrane of the heart, called the Eustachian valve, through the 

 foramen ovale into the left auricle and into the left ventricle, and out of this 

 into the aorta, and thence to all the body, but chiefly to the head and neck. 

 The blood of the right ventricle is sent out in small amount through the pul- 

 monary artery to the lungs, and thence to the left auricle, as in the adult, 



FIG. 506. Diagram of the Fetal Circulation. 



but the greater part by far passes through a canal, the ductus arteriosus, 

 leading from the pulmonary artery into the aorta just below the origin of the 

 three great vessels which supply the upper parts of the body, and is distributed 



