THORACIC CAVITY 35 



times the eighth intercostal veins end in a common trunk called 

 the accessory hemiazygos vein, which passes behind the aorta 

 and the oesophagus to join the azygos vein ; (4) that the remain- 

 ing intercostal veins end in a common trunk called the hemi- 

 azygos vein, which also passes behind the aorta and the oeso- 

 phagus to join the azygos vein. 



Next clean the left vagus nerve as it descends along the front 

 of the left subclavian artery and across the arch of the aorta. 

 Attempt to find the small branches which pass from it to the 

 anterior pulmonary plexus on the front of the root of the lung, 

 and springing from its medial side, at the level of the aortic 

 arch, find its recurrent branch, which dips backwards below the 

 arch. Then follow the vagus to the posterior pulmonary plexus 

 on the back of the root of the left lung. Clean the plexus and 

 follow the vagus from it to the oesophagus, but not further at 

 present. Then clean the descending aorta. Now turn to the 

 interval between the left vagus and the strip of pleura covering 

 the left phrenic nerve, and dissecting carefully in the areolar 

 tissue find two small nerves, the superior cervical cardiac branch 

 of the left sympathetic trunk and the inferior cervical cardiac 

 branch of the left vagus. The sympathetic cardiac branch is 

 next the vagus and the vagus cardiac branch next the phrenic 

 nerve. When the two small cardiac nerves have been found 

 do not trace them, at present, below the lower border of the 

 aortic arch, but turn to the left subclavian artery and clean it. 

 Then clean carefully the area behind it and display the left 

 border of the oesophagus, with the left recurrent nerve running 

 along its anterior margin, and the thoracic duct ascending 

 along its posterior margin (Fig. 14, in which the recurrent nerve 

 is seen but is not labelled). Lastly, clean the pericardium 

 in front of and behind the strip of pleura covering the left 

 phrenic nerve. 



Contents of the Mediastinum and the Structures of the 

 Posterior Wall of the Thorax seen from the Eight Side. 

 After the pleura has been removed from the right side of the 

 thorax and the extra-pleural tissue has been dissected away, 

 the following structures are exposed. Below and anterior to 

 the root of the lung is the pericardium, covering the right 

 atrium of the heart. Entering the pericardium below and 

 posteriorly is the thoracic part of the inferior vena cava, and 

 entering the upper part is the superior vena cava. The 

 upper end of the superior vena cava is continuous with the 

 right innominate vein, which lies posterior to the sternal 

 end of the first costal cartilage. Arching over the root of the 

 lung, to join the superior vena cava, is the terminal part of 

 the azygos vein. Above the azygos vein and posterior to the 

 superior vena cava are parts of the trachea, the right vagus 

 nerve, and the oesophagus. On the posterior surface of the 

 root of the lung is the posterior pulmonary plexus, formed by 



