THORACIC CAVITY 



103 



more circular when solids or fluids are passing along it ; and 

 (2) that it is somewhat constricted at the level of the left 

 bronchus. 



An inch or more of the upper part of the posterior 

 mediastinal portion of the tube should be removed and dis- 

 sected under water in a cork-lined tray. It will be found to 

 possess from without inwards the following coats: (i) an 

 external fibrous sheath; (2) a muscular coat; (3) a sub- 

 mucous coat ; and (4) a mucous internal lining. The 

 submucous coat forms a loose connection between the 

 muscular and mucous coats ; consequently, when the muscular 



Pericardium 



Thoracic aorta 



Vena cava inferior 



Pleura 



Thoracic duct 

 Vena azygos 



Pleura 



FIG. 50. Tracing of section through the Posterior Mediastinum 

 at the level of the eighth thoracic vertebra. 



coat is contracted the mucous lining is thrown into longi- 

 tudinal folds. The muscular coat consists of an external layer 

 of longitudinal fibres and an internal layer of circular fibres. 



Aorta Descendens. The descending aorta commences 

 at the termination of the aortic arch, at the lower border of 

 the left side of the fourth thoracic vertebra. It passes down- 

 wards, through the posterior mediastinum, and it leaves the 

 thorax by passing through the aortic aperture of the diaphragm, 

 opposite the lower border of the twelfth thoracic vertebra. 

 Its length is about seven inches. In the upper part of its 

 extent it lies to the left of the vertebral column ; but in the 

 lower part it lies anterior to the column, in the median plane. 



Branches, Branches spring from both the anterior and the 

 posterior aspects of the descending aorta. Those from the 

 anterior aspect are the two left bronchial arteries, four 



ii 7 c 



