THE PULMONARY VEINS. 



853 



pulmonary vein of that side, those from the inferior lobe unite to form the in- 

 ferior pulmonary vein, while those from the middle lobe of the right lung unite 

 to form a single trunk which usually opens- into the right superior vein, although it 

 occasionally opens independently into the left auricle, forming what is then termed 

 the middle pulmonary vein. 



Each of the four pulmonary veins has a length of about 15 mm., and for about 

 one-third of its course is partially invested by the visceral layer of the pericardium 

 (page 715). The right superior vein is usually slightly the largest of the four, while 

 the left superior is the smallest, the right and left inferior veins being about the same 

 size. No valves occur either throughout the course or at the orifices of the pulmonary 

 veins. 



Relations. — The superior pulmonary veins have a course which is obliquely 

 downward and inward. In their extrapericardial portion they lie anterior to and 

 below the pulmonary arteries, and are separated by them from the bronchi; the 



Fig. 749. 



Right innominate vein > 



Innominate arterv 



Left common carotid artery 

 / Left subclavian artery 



Pulmonary 

 veins 



Left pulmonary artery 



Superior 

 puliiiunarj 



vein 



Stump of sup. 

 vena cava' 

 Right auricular 

 appendage 



Aorta, systemic 

 Left coronary artery 

 Right coronary vessels 



Left auric- 

 ular ap- 

 pendage 

 Con us 

 arteriosus 

 Interventricular 

 'branches of left 

 coronary vessels 



Left ventricle 



Right ventricle 



Injected heart and great vessels, viewed from before ; parts of superior vena cava and aorta 

 have been removed to show right pulmonary artery. 



vein of the right side is crossed from above downward by the phrenic nerve and by 

 the vena cava superior. In its intrapericardial portion the right superior vein Hes 

 behind the terminal portion of the superior vena cava and the left one behind the 

 pulmonary aorta (pulmonary artery), while posteriorly each is in relation with its 

 corresponding inferior vein. 



The inferior veins are more horizontal in position, but are directed forward 

 as well as inward. They lie in a plane considerably posterior to that of the 

 corresponding superior veins and are situated internally to and behind an anterior 

 descending branch of each bronchus. 



Anastomoses. — In addition to serving for the return flow of the blood carried 

 to the lungs by the pulmonary arteries, the pulmonary veins also receive a certain 

 amount of the blood carried by the bronchial arteries. Communications between 



