42 THORAX 



passes upwards behind to the root of the lung, turns forwards 

 above its superior border, at the level of the fifth thoracic 

 vertebra, and joins the superior vena cava, on its posterior 

 aspect, immediately above the point where the latter enters 

 the pericardium, at the level of the second costal cartilage 

 (see Figs. 13, 15). As it turns forwards the vein lies im- 

 mediately to the right side of the oesophagus, trachea and 

 vagus nerve (Figs. 13, 31). 



The tributaries of the vena azygos are : ( i ) The right 

 superior intercostal vein, which drains blood from the 

 greater part of the second and third intercostal spaces. 

 (2) The lower eight intercostal veins and the subcostal vein 

 of the right side. (3) The vena hemiazygos, and frequently 

 (4) the vena hemiazygos accessoria. Both the latter enter it 

 from the left. In many cases the accessory hemiazygos vein 

 joins the hemiazygos vein. (5) Two or more bronchial veins 

 from the right lung. (6) Some veins from the oesophagus. 

 (7) Some minute pericardial veins. 



The vena azygos commences in the abdomen, where it 

 anastomoses either with the ascending lumbar veins (see 

 p. 408) or directly with the inferior vena cava. Thus, it 

 forms a more or less direct anastomosis between the two 

 venae cavse. 



The intercostal veins and the accompanying arteries and 

 nerves, on both sides, and the hemiazygos and accessory 

 hemiazygos veins will be studied at a later period of the 

 dissection (see p. 140). 



The Left Superior Intercostal Vein. This vein is formed 

 by the intercostal veins from the second and third intercostal 

 spaces of the left side, and it not uncommonly receives 

 communicating veins from the first and fourth spaces. It 

 descends along the medial border of the first left aortic inter- 

 costal artery to the posterior end of the aortic arch ; there it 

 turns forwards, along the left side of the aortic arch, and, 

 passing at the same time obliquely upwards, it crosses lateral 

 to the left vagus and medial to the left phrenic nerve 

 (Fig. 14). At a later period of the dissection it will be 

 traced to its termination in the left innominate vein. 



The Posterior Intercostal Membranes are medial to the 

 internal intercostal muscles and on a more posterior plane. 

 Each is attached, medially, to the anterior costo-transverse 

 ligament, which passes from the neck of the rib below to the 



