THORACIC CAVITY 131 



similar form during life when empty and flaccid, but becomes 

 more circular when solids or fluids are passing along it ; 1 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 



FIG. 68. 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 varies with the length of the thorax but averages 

 from 17-5 to 20 cm. (seven to eight inches). In the upper 

 1 It may be dilated after death, see Figs. 47, 48, 54. 



