THE HEART 



573 



descending aorta. From the head and upper extremities the blood Js returned 

 by the tributaries of the superior vena cava (precava) to the right auricle, where 

 it becomes mixed with a small portion of the blood from the inferior vena cava. 

 From the right auricle it descends over the Eustachian valve into the right ventricle. 



Ductus arteriosus. 



Internal iliac artery. 



FIG. 42fi. Plan of the fetal circulation. In this plan the figured arrows represent the kind of blood, as well 



5 the direction which it takes in the vessels. Thus, arterial blood is figured -->', venous blood, 



>; mixed (arterial and venous) blood, >. 



and from the right ventricle passes into the pulmonary artery. The lungs of the 

 fetus being inactive, only a small quantity of the blood of the pulmonary artery is 

 distributed to them by the right and left pulmonary arteries, and is returned by the 

 pulmonary veins to the left auricle: the greater part passes through the ductus 



