858 HUMAN ANATOMY. 



with the thymus gland or the fatty tissue which replaces it, and is overlapped by 

 the right pleura and lung. Behind, it crosses the origin of the right bronchus and 

 the structures at the root of the right lung, from which it is separated by numerous 

 lymphatic nodes ; to the right it is in contact with the pleura covering the inner surface 

 of the right lung and with the right phrenic nerve ; and to the left it lies alongside the 

 ascending portion of the aortic arch. 



In its lower intrapericardial portion it has to the left the systemic aorta: anteriorly , 

 the right auricle; posteriorly, the right pulmonary artery, the right superior pul- 

 monary vein, and the right bronchus, while upon the right it is free. 



The vena cava superior contains no valves. 



Tributaries. — In addition to the right and left innominate veins, by the union 

 of which it is formed, the v^ena cava superior receives the vena azygos major and 

 small veins from the mediastinum and pericardium. 



Variations. — Cases have been recorded in which the vena cava superior received the right 

 internal mammary or the right superior intercostal vein which normally open into the right 

 innominate vein. It may also receive the vena thyreoidea ima, a vein only occasionally present 

 and draining the territory supplied by the art. thyreoidea ima. 



A more remarkable and rarer variation is the union with the superior vena cava of a com- 

 paratively large vein which issues from the right lung. A similar condition has been observed 

 in connection with the innominate veins, and its probable significance will be considered in 

 connection with the variations of those vessels. 



Practical Considerations. — The superior vena cava would be involved in a 

 stab-wound passing through either the first or the second intercostal space on the 

 right side, close to the sternum. The vessel is subject to compression in aneurism 

 of the ascending aorta {q.v.), producing venous congestion in the veins of the neck 

 and of the upper extremities. 



The Innominate Veins. 



The innominate or brachio-cephalic veins (vv. anonymae) (Fig. 751) are two in 

 number, a right and a left. They are situated in the upper portion of the thoracic 

 cavity, being formed by the union of the internal jugular and subclavian veins, and 

 terminate by uniting opposite the first costal cartilage of the right side to form the 

 vena cava superior. The union' of the internal jugular and subclavian vein takes 

 place on each side opposite the sternal end of the clavicle ; but, since the vena cava 

 superior lies entirely to the right of the median line of the body, the left innominate 

 vein has a much greater distance to traverse in order to reach its point of termination 

 than has the right one, and consequently it will be necessary to describe each vein 

 separately. 



The right innominate vein has a length of 2-4 cm. (^-1% in.) and an 

 almost vertical course, opening directly downward into the vena cava superior. It 

 lies behind the inner end of the right clavicle, from which it is separated by the lower 

 portions of the sterno-hyoid and sterno-thyroid muscles, and a little lower it is behind 

 the first right costal cartilage. To the right it is in relation with the inner surface 

 of the right pleura and with the right phrenic nerve, to the left with the brachio- 

 cephalic artery and right pneumogastric nerve, and behind with the pleura. 



The left innominate vein has a length almost double that of the right, meas- 

 uring 5-9 cm. (2-2,14 in.) from its origin behind the sternal end of the left clavicle 

 to its union with the right vein to form the vena cava. Its course is transverse 

 from left to right and at the same time slightly downward, and it extends completely 

 across the uppermost part of the thoracic cavity, resting below upon the aortic arch, 

 and passing in front of the left subclavian and common carotid arteries, the trachea, 

 the brachio-cephalic artery, and the pneumogastric nerve. It is separated from the 

 manubrium sterni by the insertion of the sterno-hyoid and sterno-thyroid muscles and 

 by the fatty tissue representing the thymus gland, and, being on a level with or 

 slightly above the upper border of the manubrium, it can usually be felt in the supra- 

 sternal fossa. 



Neither of the innominate veins possesses valves. The left is of somewhat greater 

 diameter than the right, owing to the greater number of tributaries which it receives. 



