32 THORAX 



Posterior to the descending aorta are the left aortic inter- 

 costal arteries, the accompanying veins, and the splanchnic 

 nerves ; and still more posteriorly and laterally lie the 

 sympathetic trunk of the left side and the left intercostal 

 spaces and their contents. 



Dissection. After the structures exposed by the removal of the left 

 pleura have been located, the dissectors should direct their attention to the 

 sympathetic trunk and its branches and communications. The arrange- 

 ment of these is exactly similar to that already described on the right side 

 (see p. 26). When the dissectors have confirmed this statement they 

 should turn to the left subclavian artery, which is the most posterior of the 

 three great branches which spring from the arch of the aorta. Clean this 

 vessel without disturbing the vagus nerve, which descends along its anterior 

 border. Afterwards clean the part of the aortic arch which lies posterior to 

 the vagus nerve, and the descending aorta. Whilst cleaning the arch avoid 

 injuring the left superior intercostal vein, and, as the descending aorta is 

 being cleaned, endeavour to preserve any of the aortic branches of the 

 sympathetic which may have been found previously during the examination 

 of the sympathetic trunk. 



As the aorta is cleaned the left border of the lower part of the 

 oesophagus will be brought more clearly into view, but the dissector must 

 not attempt to clean the oesophagus at this stage. 



After the descending portion of the aorta is cleaned, the left aortic 

 intercostal arteries should be examined. They are nine in number ; they 

 emerge from the posterior aspect of the aorta, and they all pass medial to 

 the sympathetic trunk as they approach the intercostal spaces. The upper 

 arteries ascend very obliquely to gain their proper spaces. Accompanying 

 the arteries are the corresponding veins. The lower veins pass, posterior to 

 the aorta, to their terminations in the hemiazygos and accessory hemiazygos 

 veins, and will be more fully studied at a later stage ; but the veins from 

 the second and third spaces unite into a trunk called the left superior 

 intercostal vein. 



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 a 

 communication from the first and fourth spaces. It descends 

 along the medial border of the first left aortic intercostal 

 artery to the posterior end of the aortic arch, there it turns 

 anteriorly, 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. At a later 

 period of the dissection it will be traced to its termination 

 in the left innominate vein. 



Dissection. After the left superior intercostal vein has been secured 

 and studied, the dissectors should clean the region posterior to the left 

 subclavian artery, and expose thoroughly the left border of the oesophagus, 

 as that tube lies in the superior mediastinum, and the upper part of the 

 thoracic portion of the thoracic duct, which runs along the border of the 

 oesophagus. 



