44 



THORAX 



is the superior vena cava, and above and behind it, the vena 

 azygos. Whilst above the root of the left lung is the aortic 

 arch, and behind it, the descending aorta (Figs. 12 and 13). 

 Bronchi. There are two primary bronchial tubes, one 

 for each lung. They spring from the termination of the 

 trachea, and each passes downwards and laterally, in the 

 root of the corresponding lung, to the hilus, through which 

 it enters the lung. After passing through the hilus it descends, 

 in the substance of the lung, to the base, lying nearer the 

 posterior than the anterior border. In the root of the lung 

 the bronchus is crossed anteriorly by the pulmonary artery, 



First thoracic 

 vertebra 



Fourth thoracic 



vertebra j"". 



Superior 

 mediastinum 

 Manubrium sterni 



Posterior 

 mediastinum 



Middle mediastinum 

 FIG. 19. Diagram of the Mediastina. 



which afterwards descends on the postero-lateral aspect of the 

 intra-pulmonary part of the bronchial tube. 



The relations of the bronchi are considered later (p. 96). 



The Mediastinum and its Contents. It has been pointed 

 out already that the mediastinum is the interval which 

 extend^ ftom the sternum to the vertebral column between 

 the -two pleural sacs; that it is occupied by some of the 

 most important viscera, vessels and nerves in the body, 

 i.e. the heart enclosed in the pericardium ; the aorta and 

 its great branches ; the great vessels which carry the blood 

 t& and from the heart; the oesophagus and trachea; the 

 vagi and phrenic nerves ; and the thoracic duct. It was 



