THE THORAX 957 



covers the lower part of the vessel except over about its posterior 

 fourth. 



The superior vena cava is destitute of valves. 



Tributaries. — ^The chief tributary is the right azygos vein, after 

 it has arched forwards over the right bronchus. It opens into 

 the superior vena cava immediately before that vessel pierces the 

 fibrous portion of the pericardimn. Other minute tributaries are 

 pericardial and mediastinal veins. 



Development. — The portion of the superior vena cava above the right 

 azygos vein is developed from that part of the right primitive jugular vein 

 which lies below the point where it is joined by the transverse jugular vein ; 

 and the portion below the right azygos vein is developed from the right duct 

 of Cuvier. 



Inferior Vena Cava. — ^This vessel enters the thorax through the 

 foramen quadratum in the central tendon of the diaphragm, and 

 immediately thereafter it is received within the fibrous portion of 

 the pericardium. Its course in the thorax is practically nil, as 

 it may be said to open at once into the postero-inferior angle of 

 the right auricle of the heart. 



Thoracic Aorta. — The thoracic aorta extends from the base of 

 the left ventricle of the heart to the level of the lower border of 

 the body of the twelfth thoracic vertebra. At this point it passes 

 through the aortic opening of the diaphragm, and thereafter enters 

 upon the abdominal part of its course. It passes at first upwards 

 and to the right ;. thereafter it sweeps, in an arched manner, up- 

 wards, backwards, and to the left, over the root of the left lung ; 

 and finally it descends in close contact with the vertebral column, 

 lying at first upon its left side, but subsequently in front of it. It 

 is therefore conveniently divided into three parts, namely, the 

 ascending aorta, the arch of the aorta, and the descending aorta. 



Ascending Aorta. — ^The ascending aorta commences at the base 

 of the left ventricle of the heart, behind the left border of the 

 sternum, on a level with the lower margin of the third left costal 

 cartilage, and it terminates at a point behind the right border of 

 the sternum on a level with the upper margin of the second right 

 costal cartilage. Its course is upwards and to the right, with an 

 inclination forwards. It lies in the middle mediastinum, and 

 within the fibrous pericardimn, which ensheathes it, and, for about 

 the first i| inches of its course it is enclosed, along with the ad- 

 jacent portion of the pulmonary arterj^ in a sheath formed by the 

 serous part of the pericardium. The ascending aorta measures 

 about 2 inches in length. At its commencement it presents three 

 bulgings, which correspond to the sinuses of Valsalva in the interior 

 and lie opposite the segments of the aortic valve. The bulgings 

 are disposed as right antero-lateral, left antero-lateral, and posterior 

 respectively. Along the right side of the vessel there is another 

 somewhat extensive dilatation, called the great sinus of the aorta. 



Relations — Anterior. — ^The root of the pulmonary artery and the 

 right auricular appendix, at first, and subsequently the first piece 



