PULMOXARV—VEXA CAVA SUPERIOR— TXXOMIXATE 603 



from the middle lobe of the right lung opening separately into the left auricle 

 instead of joining as usual the upper of the two right pulmonary veins. The 

 relations of the pulmonary veins to the pulmonary arteries and V^ronchi in the 

 lungs are given with the Anatomy of the Ll'ngs. At the root of the lung the 

 pulmonary veins on both sides are arranged as an upper and a lower branch, an 

 anterior descending branch of the bronchus passing between them. The upper 

 vein on the right side is larger than the lower, and usually receives the vein from 

 the middle lobe of the right lung. The lower vein on the left side is larger than 

 the upper. Both the upper and lower veins lie in front of the pulmonary artery 

 and on a lower jjlane, and run almost horizontally inwards and forwards to the 

 left auricle. As they pierce the pericardium they receive a reflexion from the 

 serous layer of that membrane. Their relations within the pericardium are given 

 with the Anatomy of the Heart. At the root of the lung their relations to the 

 surrounding structures are similar to those of the pulmonary artery (page 466). 

 A separate description is not required. 



THE VENA CAVA SUPERIOR 



The superior or descending vena cava (fig. 381a) carries to the heart the 

 l)lood returned from the head and neck and upper extremities through the right and 

 left innominate veins, and from the walls of the thorax, either directly through the 

 greater az3'gos vein, or indirectly through the innominate veins. It is formed 

 (fig. 381a) by the confluence of the right and left innominate veins at the lower 

 border of the first right costal cartilage close to the sternum, and, descending 

 from this spot in a gentle curve with its convexity to the right and in a direction 

 slightly backwards and outwards behind the sternal end of the first and second 

 intercostal spaces and second costal cartilage, terminates in the right auricle of the 

 heart on a level with the third right costal cartilage in front and the seventh 

 thoracic vertebra behind. It measures about three inches in length (7-8 cm.). 

 A little more than its lower half (4 cm.) is contained within the pericardium, the 

 serous layer of that membrane being reflected obliquely over it immediately below 

 the spot where it is joined by the vena azygos major, and on a lower level than 

 the reflexion of the pericardium on the aorta. The vena cava superior contains 

 no valve. 



Relations. — In front (fig. 322), in addition to the first and second intercostal 

 spaces and the second costal cartilage, it is covered by the remains (jf the thymus 

 gland, the interthoracic fascia, and the pericardium, and is overlapped by the right 

 pleura and lung. 



Behind (fig. 324) are the vena azygos major, the right bronchus, the right 

 pulmonary artery, and the superior right pulmonary vein; and below, the fibrous 

 layer of the pericardium. The serous layer is reflected over the front and sides of 

 the vessel, but not over its posterior part. 



To the right side are the right lung and pleura, and the phrenic nerve. 



To the left side are the innominate artery and the first or ascending portion of 

 the arch of the aorta. 



Tributaries. — In addition to the right and left innominate veins and the vena 

 azygos major, it receives small veins from the mediastinum and pericardium. 



THE INNOMINATE OR BRACHIO-CEPHALIC VEINS 



The innominate veins return the blood from the head and neck and upper 

 extremity. They are formed on each side by the confluence of the internal jugu- 

 lar and subclavian veins l^ehind the sternal end of the clavicle. They terminate 

 at the lower l)order of the first costal cartilage on the right side by uniting to form 

 the superior vena cava. The innominate veins have no valves. 



The right innominate vein (fig. 382) measures about one to one and a half 

 inches in length (2-3 cm. ), and descends from its origin behind the sternal end of tlie 

 clavicle, very slightly forwards and inwards, superficial to, and to the right of, the 



