THE LOWER OPENING 



899 



side, ill addition, there is a small ])()rti()n of the a])ex of the lung with its pleural 

 covering. 



The lower opening, or base, is limited in front l)y the ensiform cartilage, he- 

 hind hy the twelfth thoracic vertehra, and laterally l)y the twelfth rihs. 



The diaphragm arches upwards to form a vaulted floor to the intervening space. 

 It is aliout one inch higher on the right side than on the left, and its flattened 

 central portion is lower than either of the lateral arches. The thoracic cavity is 

 mucli dee]icr l)ehind than in front. 



The following structures i)ass through the /out/- opening of the thorax: — (1) The 

 aorta, vena azygos major, thoracic duct, the trunks of the eflerent lymphatics from 

 the lower inten-ostal spaces, and, occasionally, the" left sym])athetic nerve pass 



Fig. 521. 



-Anterior View of the Thorax, with Outlines of the 

 Diaphragm and Lungs. 



through the aortic opening. (2) The oesophagus, pneumogastric nerves, and some 

 small I esophageal arterial branches emi-rge through the a^sophageal o]iening. 



(o) The vena cava inferior ascends through the caval opening, and the branches 

 of the phrenic nerves pierce the diajjhragm to reach its lower surface. 



(4) The splanchnic nerves, the vena azygos minor, and the symi»athetic trunk 

 of each side pass tlirough the crura. 



The viscera contained within the thoracic cavity are: the heart, wliieh is en- 

 closed in the ])ericar<lium; and the lungs, with tlieir ]ileural investments. 



The heart lies between the lungs in the so-called mediastinal space. This is 

 l)Ounded on each side by the reth'xions of the ]>leune, which jiassfrom the front to 

 the back of the thoraci*- cavity; ami tin; connuon space is further subdivided into 

 a superior mediastinum above the heart sac, an anterior and a posterior resiK'C- 



