THE VENA CAVA. 611 



trachea, behind the Sterno-thyroid muscles. From this plexus a left vein descends 

 and joins the left innominate trunk, and a right vein passes obliquely downward 

 and outward across the innominate artery to open into the right innominate vein, 

 just at its junction with the superior vena cava. These veins receive oesophageal, 

 tracheal, and inferior laryngeal veins, and are provided with valves at their termi- 

 nation in the innominate veins. 



The Superior intercostal veins (right and left) drain the blood from two or three 

 intercostal spaces below the first. The right vein passes downward and inward and 

 opens into the vena azygos major; the left runs across the transverse aorta and 

 opens into the left innominate vein. It usually receives the left bronchial and 

 left superior phrenic vein, and communicates below with the vena azygos minor 

 superior. The highest intercostal vein, i. e., from the first space, opens directly into 

 the corresponding vertebral or innominate vein. 



The Superior Vena Cava receives the blood which is conveyed to the heart from 

 the whole of the upper half of the body. It is a short trunk, varying from two 

 inches and a half to three inches in length, formed by the junction of the two 

 innominate veins. It commences immediately below the cartilage of the first rib 

 close to the sternum on the right side, and, descending vertically, enters the peri- 

 cardium about an inch and a half above the heart, and terminates in the upper part 

 of the right auricle opposite the upper border of the third right costal cartilage. In 

 its course it describes a slight curve, the convexity of which is turned to the right 

 side. 



Relations. In front, with the pericardium and process of cervical fascia which 

 is continuous with it : this separates it from the thymus gland and from the ster- 

 num ; behind, with the root of the right lung ; on its right side, with the phrenic 

 nerve and right pleura ; on its left side, with the commencement of the innominate 

 artery and ascending part of the aorta. The portion contained within the peri- 

 cardium is covered by the serous layer of that membrane in its anterior three- 

 fourths. It receives the vena azygos major just before it enters the pericardium, 

 and several small veins from the pericardium and parts in the mediastinum. The 

 superior vena cava has no valves. 



The Azygos Veins connect together the superior and inferior venae cavae, taking 

 the place of those vessels in that part of the chest occupied by the heart. 



The larger, or right azygos vein (vena azygos major), commences opposite the 

 first or second lumbar vertebra by a branch from the right lumbar veins (the 

 ascending lumbar); sometimes by a branch from the right renal vein or from the 

 inferior vena cava. It enters the thorax through the aortic opening in the Dia- 

 phragm, and passes along the right side of the vertebral column to the fourth dorsal 

 vertebra, where it arches forward over the root of the right lung, and terminates 

 in the superior vena cava just before that vessel enters the pericardium. Whilst 

 passing through the aortic opening of the Diaphragm it lies with the thoracic 

 duct on the right side of the aorta, and in the thorax it lies upon the intercostal 

 arteries on the right side of the aorta and thoracic duct, and is partly covered by 

 pleura. 



Tributaries. It receives the lower ten intercostal veins of the right side, the 

 upper two or three of these opening first of all into the right superior intercostal 

 vein. It receives the azygos minor veins, several oesophageal, mediastinal, and 

 pericardial veins ; near its termination, the right bronchial vein ; and generally 

 the right superior intercostal vein. A few imperfect valves are found in this vein ; 

 but its tributaries are provided with complete valves. 



The intercostal veins on the left side, below the three upper intercostal spaces, 

 usually form two trunks, named the left lower and the left upper azygos veins. 



The left lower, or smaller azygos vein (vena azygos minor), commences in the 

 lumbar region by a branch from one of the lumbar veins (ascending lumbar] or 

 from the left renal. It passes into the thorax through the left crus of the Dia- 

 phragm, and, ascending on the left side of the spine as high as the ninth dorsal 

 vertebra, passes across the column, behind the aorta and thoracic duct, to terminate 



