120 ANATOMY OF UPPER EXTREMITY, ETC. 



days it will become so stiffened and hardened that it may be opened 

 and the interior examined, by windows cut into the cavities, without 

 its walls collapsing; the valves will be preserved so nearly in their 

 natural condition as to convey a much clearer idea of their character 

 and relations than can be otherwise obtained. 



DISSECTION IV. 



POSTERIOR MEDIASTINUM. 



To examine the posterior mediastinum, the left lung should be lifted 

 from its pleural cavity and turned over to the opposite side, where it 

 should be confined, or held resting in the space that was occupied by 

 the heart. It may be desirable, in order better to get at the parts to 

 be dissected, to remove the anterior halves of the ribs on one side ; 

 this can be accomplished by the saw, or by bone forceps. 



The POSTERIOR MEDIASTINUM is bounded in front by the 

 posterior surface of the pericardium, behind by the verte- 

 bral column, and on each side by the pleura. It contains 

 the thoracic aorta and vena azygos, the oesophagus and 

 thoracic duct, and the pneumogastric and sympathetic 

 nerves. 



The continuity of the pleura, as it is reflected from the 

 base of the lung to the vertebrae, ribs, and diaphragm, will 

 now be well seen, and the present, perhaps, affords the best 

 opportunity of appreciating the manner in w r hich one part 

 of a serous membrane is attached to the parietes of a cavity, 

 and the other to the organ contained in it ; the organ being 

 in reality outside of it, and merely pushing inward the part 

 covering it. 



The left pleura is to be carefully dissected from the subjacent parts, 

 the whole length of the thorax. This will expose the contents of the 

 mediastinum. 



The PNEUMOGASTRIC NERVES will be found lying, the 

 left upon the anterior surface of the oesophagus ; the right 

 upon the posterior surface. They give off numerous fila- 

 ments to the oesophagus as they descend upon it to the 

 stomach, where they terminate in gastric branches. They 

 also give off pulmonary and cardiac branches, and form a 

 large plexus behind the root of the lung, with which are 

 connected some filaments from the gangliated cord of the 

 sympathetic. In the upper part of the thorax they send ol 



