THE THORAX 997 



onl}' flow from the right auricle into the left, its return being pre- 

 vented by this valve. 



The Eustachian valve is of large size during foetal life, and is 

 associated with the orifice of the inferior vena cava at the postero- 

 inferior angle of the right auricle. It is a crescentic fold of the 

 endocardium, which extends between the anterior margin of the 

 orifice of the inferior vena cava and the anterior horn of the annulus 

 ovalis. Its situation is such as to direct the blood entering the 

 right auricle by the inferior vena cava to the foramen ovale. 



The ductus arteriosus connects the trunk of the pulmonary artery 

 to the arch of the aorta at a point immediately beyond the origin 

 of the left subclavian arter\', and it conveys the most of the blood 

 from the right ventricle to the descending thoracic aorta. 



The hypogastric arteries, right and left, are prolonged from the 

 common iliac arteries to the umbilicus, through which they leave 

 the body of the foetus, and, under the name of the umbilical arteries, 

 they pass along the umbilical cord to be distributed to the foetal 

 part of the placenta. They convey the impure blood of the foetus 

 to the placenta. 



The umbilical vein extends from the foetal part of the placenta 

 through the umbilicus to the inferior surface of the liver, where it 

 traverses the umbilical fissure, and gives off right and left branches 

 to the right and left lobes, which join the branches of the vena 

 portae. After parting with its right and left branches the mnbilical 

 vein is prolonged, under the name of the ductus venosus, to the 

 inferior vena cava, which it joins at the caval fossa of the liver. 

 The umbilical vein conveys pure blood from the placenta into the 

 body of the foetus. 



Foetal Circulation. — ^The right auricle receives blood from the 

 superior and inferior venae cavse, and from the coronary sinus. 

 The superior vena cava returns the venous blood from the head 

 and neck, the upper limbs, and the thoracic portion of the trunk ; 

 and the inferior vena cava returns the blood from the lower limbs, 

 the abdominal portion of the trunk, and the placenta. The 

 placental blood is pure, and reaches the inferior vena cava in two 

 ways : (i) a large quantity of it is conveyed directly into the 

 inferior vena cava by the ductus venosus ; and (2) some of it cir- 

 culates through the liver in the branches of the vena portae, and 

 is thereafter conveyed to the inferior vena cava by the hepatic 

 veins. The blood entering the right auricle by the inferior x^ena 

 cava is necessarily of a mixed nature, being partly placental and 

 partly venous. 



The impure blood which enters the right auricle by the superior 

 vena cava, having received a small quantity of the mixed blood 

 which enters by the inferior vena cava, passes through the right 

 auriculo-ventricular orifice into the right ventricle, and from thence 

 is driven into the trunk of the pulmonary artery. A small 

 quantity of it is conveyed to the lungs by the right and left pul- 

 monary arteries, which at this period are of small size, and it is 



