284 THE VASCULAR SYSTEMS 



The Superior Vena Cava and Innominate Veins 



The superior vena cava is a large trunk formed by the union 

 of the two venae innominatse, and returns the blood from the 

 head and neck, the thoracic walls, and the upper extremities. 

 It is about three inches long, and descends from the junction of 

 the first right costal cartilage with the sternum to its termin- 

 ation in the right auricle, opposite the upper border of the third 

 right cartilage. 



At first it is external to the innominate artery and internal 

 to the right phrenic nerve, partly covered by the pleura. It 

 then pierces the pericardium external to the ascending aorta, 

 having descended in front of the right division of the pulmonary 

 artery. It receives the azygos major and small pericardiac 

 and mediastinal veins. 



The innominate veins, formed by the union of the subclavian 

 and internal jugular of each side, behind the inner end of the 

 clavicle unite to form the superior vena cava. The right vein, 

 one inch long, descends vertically on the right side of the innomi- 

 nate artery, while the left, more than two inches in length, 

 descends slightly, running to the right, behind the sternohyoid 

 and thyroid muscles and upper part of the sternum. The 

 transverse aorta lies below it. 



Each receives the vertebral, inferior thyroid, and internal 

 mammary veins. The left vein also receives the superior inter- 

 costal and some small thymic, mediae-final, and pericardiac veins 

 and the thoracic duct, while the right is joined at its origin by 

 the right lymphatic duct. 



The vertebral vein descends with the artery of the same name 

 through the foramina in the transverse processes of the upper 

 six cervical vertebrae, crosses the subclavian artery, and opens 

 into the back part of the vena innominata. 



The inferior thyroid veins arise by tributaries from the lateral 

 lobes of the thyroid gland, and descend on the trachea beneath 

 the sternothyroid muscles. The left joins the innominate on 

 its own side, sometimes in common with the right. The latter 

 may empty into the junction of the two venae innominatae or 

 join the right vena innominata. 



The internal mammary veins are two on each side, and accom- 

 pany the artery, receiving corresponding tributaries, finally 



