THE HEART AND VASCULAR SYSTEM. 177 



THE CIRCULATION OF THE BLOOD. 



Festal Circulation. The fresh arterial blood returning from 

 the placenta through the umbilical vein enters the foetus at the 

 umbilicus,, passes along the suspensory ligament to the under 

 surface of the liver, where a portion passes directly into the 

 inferior vena cava by the junction of the ductus venosus with 

 the left hepatic vein, a portion enters the liver and reaches the 

 inferior vena cava through the hepatic veins, and the larger por- 

 tion mixes with the portal venous blood before reaching the vena 

 cava through the hepatic veins. It ascends in the inferior 

 vena cava along with the venous blood from the trunk and lower 

 extremities to enter the right auricle of the heart (Fig. 105). 

 Here it is directed by the Eustachian valve through the foramen 

 ovale into the left auricle, into the left ventricle, and so through 

 the aorta, chiefly to the head and upper extremities, a small por- 

 tion only entering the descending aorta. It is returned by the 

 superior vena cava to the right auricle, passes over the Eustachian 

 valve to the right ventricle, and so tjirough the pulmonary artery 

 toward the lungs, but (the lungs being almost impervious) most 

 of it passes through the ductus arteriosus into the descending 

 aorta (mixing with the small quantity from the left ventricle 

 before mentioned) to supply the lower extremities, abdominal 

 and pelvic viscera, and as venous blood to return through the 

 umbilical arteries to the placenta. 



Circulation After Birth and in Adult. The dark venous 

 blood from the entire body is received through the superior and 

 inferior venae cavaa and coronary sinus into the right auricle, 

 from whence it passes into the right ventricle, to be sent through 

 the pulmonary artery into the lungs. Here it becomes oxy- 

 genated, and as bright arterial blood it returns to the left auricle 

 by the pulmonary veins, and passes into the left ventricle, from 

 whence it is distributed through the aorta and its branches to 

 the entire body. 



AORTA. 



This is the main trunk of the systemic arteries. Commenc- 

 ing at the upper part of the left ventricle, it ascends for a short 

 distance, arches backward over the right pulmonary artery, the 

 root of the left lung, to the left side of the body of the fourth 

 dorsal vertebra, from where it passes downward through the dia- 

 phragm and becomes the abdominal aorta. It then descends to 

 the lower border of the fourth lumbar vertebra, where it termi- 

 nates by dividing into the left and right common iliac arteries. 



