206 HUMAN ANATOMY. 



pulmonary artery is larger and longer than the left, and passes 

 behind the ascending aorta and superior vena cava to the root of 

 the right lung, where it divides into two branches. The left 

 pulmonary artery passes in front of the descending aorta and left 

 bronchus to the root of the left lung, where it divides into two 

 branches. 



THE VENOUS SYSTEM. 



The venous system, like the arterial, consists of two distinct 

 sets, the systemic and pulmonary. It is composed of seven (7) 

 main trunks and their branches : 



Systemic. Pulmonary. 



.1. Coronary vein; Four pulmonary veins. 



2. Superior vena cava; 



3. Inferior vena cava. 



The systemic veins return the venous blood from the body, 

 head, and extremities to the right auricle. 



The portal vein, with its branches and capillaries, is an 

 appendage to the systemic set, collecting the venous blood from 

 the organs of digestion and carrying it to the liver, where it 

 breaks into capillaries, and finally reaches the inferior vena cava 

 by means of the hepatic veins. 



The pulmonary veins are peculiar in carrying arterial 

 blood from the lungs to the left auricle. 



SYSTEMIC VEINS. The coronary sinus returns all the blood 

 from the substance of the heart, except that returned directly 

 from the the walls of the right auricle by the vena? Thebesii. It 

 is a dilatation of the great cardiac vein, about one inch in length, 

 situated in the posterior part of the left auriculo-ventricular 

 groove. It opens into the right auricle, its orifice being pro- 

 tected by the coronary valves, and receives the following : 



Great cardiac, or coronary vein; Posterior cardiac; 

 Anterior cardiac; Small cardiac; 



Middle cardiac; Oblique vein. 



The superior vena cava is a short trunk formed by the 

 union of the right and left innominate veins. It receives the 

 vena azygos major, has no valves, and is smaller in size than the 

 aorta. It ends in the right auricle, receiving the blood from the 

 whole upper half "of the body and the right lymphatic and 

 thoracic ducts. The left innominate passes to join the superior 

 cava in front of the great arteries of the arch. The inferior 

 thyroid, the internal mammary, and vertebral veins follow 

 closely the courses of the corresponding arteries, and terminate 

 in the innominate vein. 



