THE HEART AND CIRCULATION. 107 



to the head and upper extremities. Hence their large size 

 and perfect development at birth. Returned from the 

 upper extremities by the superior vena cava, the blood 

 enters the right auricle again and, passing over the 

 Eustachian valve this time, descends to the right ven- 

 tricle, from which the greater part passes by the pul- 

 monary artery and the ductus arteriosus to the descend- 

 ing aorta, though a small amount keeps on through the 

 pulmonary artery to the lungs. In the aorta it mixes 

 with the blood from the left ventricle and part goes to 

 supply the lower extremities, though the greater part is 

 carried back to the placenta through the two umbilical 

 arteries. The fact that the greater part of the blood 

 traverses the liver accounts for its large size at birth, 

 while the lower extremities, which receive for the most 

 part blood that has already circulated through the upper 

 extremities, are of small size and imperfectly developed. 



Arteries. After birth the arterial blood for the gen- 

 eral circulation leaves the heart by the aorta, the main 

 distributing artery of the body. Through this and 

 its branches it is carried throughout the body in what, 

 with the return of the venous blood by the venae cavae 

 and other smaller veins, is known as the systemic circula- 

 tion. The aorta ascends from the left ventricle and 

 arches backward to the left over the root of the left 

 lung to descend along the spinal column at the left to 

 the fourth lumbar vertebra, about opposite the umbilicus, 

 where, considerably diminished in size by the branches 

 it has given off, it divides into the two common iliacs. 

 For convenience its different parts are named, according 

 to their position, the ascending aorta, the arch of the 

 aorta, and the descending aorta, the last being subdivided 

 into the thoracic and the abdominal aorta. 



From the ascending aorta come off the coronary 

 arteries which supply the heart muscle itself, as the cor- 

 onary sinuses carry off the venous blood from the 

 heart. . From the arch are given off the left common 

 carotid and left subclavian and the innominate, which 



