THORACIC CAVITY 127 



the point where the right pulmonary artery crosses anterior 

 to the stem bronchus. 



Relations of the Extra-pulmonary Part of the Left Bronchus. 

 The extra-pulmonary part of the left bronchus commences 

 and ends at the same level as the corresponding part of the 

 right bronchus, but it has further to go, because the hilum of 

 the left lung is further from the median plane than the 

 hilum of the right lung ; therefore, it is longer and less vertical 

 than the right bronchus. It gives off no branches. 



Anterior to it are the left pulmonary artery, and the upper 

 and left part of the pericardial sac which separates the 

 bronchus from the left atrium. Above it is the arch of the 

 aorta ; and posterior to it are the descending aorta, the posterior 

 pulmonary plexus, the left bronchial arteries, and the oeso- 

 phagus. 



The intra- pulmonary parts of the bronchi and their rela- 

 tions have already been examined (p. 60). Their positions 

 in relation to the heart are shown in Plate III., Fig. 64. 



The Thoracic Portions of the Vagi Nerves. The thoracic 

 parts of the vagi nerves, which are still in position, should 

 now be examined. Both vagi enter the thorax at the upper 

 aperture. 



The right vagus descends, through the superior medias- 

 tinum, posterior to the right innominate vein and the 

 superior vena cava, passing obliquely downwards and back- 

 wards (Fig. 13) along the side of the trachea, and between 

 the trachea medially, and the right pleura laterally, as far as 

 the arch of the azygos vein. Next, it passes between the trachea 

 medially, and the arch of the azygos vein laterally, and reaches 

 the posterior aspect of the root of the right lung, where it breaks 

 up into a number of branches which unite with branches of 

 the sympathetic trunk to form the posterior pulmonary plexus. 

 It emerges from the plexus usually as a single trunk which 

 runs downwards and medially, in the posterior mediastinum, 

 to the oesophagus. On the oesophagus it breaks up into 

 branches which unite with branches of the left vagus to form 

 the ccsophageal plexus (Fig. 61). At the lower end of the 

 thorax the right vagus again becomes a single trunk ; it passes 

 to the posterior aspect of the oesophagus and enters the 

 abdomen through the cesophageal orifice of the diaphragm. 



Thoracic Branches of the Right Vagus. Whilst the right 

 vagus is in the superior mediastinum it gives off a thoracic 



