THE VEINS. 351 



the mid-ventral line, and arches over to the 

 dorsal surface of the left auricle, where it 

 divides into the right and left branches, sup- 

 plying the two lungs. 



c. Opening into the left auricle. 



The pulmonary veins return the blood from the 

 lungs to the heart. There are two main 

 veins on each side, which converge to open 

 into the dorsal surface of the left auricle. 



d. Arising from the left ventricle. 



The aorta conveys arterial blood from the heart 

 to all parts of the body. It arises from the 

 middle of the base of the heart, dorsal to 

 the root of the pulmonary artery, runs for- 

 wards about half an inch, and then arches 

 over to the left side, and runs back along the 

 left side of the vertebral column. Its thoracic 

 portion is readily seen on turning aside the 

 left lung. 



The ductus arteriosus is a ligamentous 

 band, connecting the aorta with the pulmonary 

 artery, just before the division of the latter 

 into right and left branches. During em- 

 bryonic life it forms a tubular connection 

 between the two vessels. 



In cleaning the ductus arteriosus, take care not to injure 

 the left recurrent laryngeal nerve which loops round it (see 

 fig. 65, p. 374). 



B. The Veins. 



The veins form three distinct systems, communicating 

 with each other only through capillaries, and containing 

 different kinds of blood. 



(1) The first of these systems consists of the three venae 

 cavse, with the veins opening into them : these bring to the 

 right auricle venous blood from the body generally, and from 

 the liver. 



