38 THE THORACIC VISCERA, BUCCAL CAVITY, 



The right lung is divided into four lobes. The 

 most posterior of these, the infra-cardiac lobe, pro- 

 jects between the heart and the diaphragm, enclosed 

 in a special fold of the right pleura. 



3. The Pleurae. Each side of the thorax is lined 

 by a delicate membrane, the parietal pleura, which 

 at the anterior end of the thorax bends backward 

 along the great blood-vessels to the root of the lungs, 

 and thence is reflected over the external surface of the 

 lungs, forming the visceral pleura. In the unopened 

 thorax the lungs fill the whole cavity, and the 

 parietal and visceral layers of the pleura come into 

 contact, forming a smooth si i ding-surface for the ex- 

 pansion and collapse of the lungs. The right and 

 left pleural sacs meet in the mid-line to form a double- 

 walled septum, the mediastimim. The portion of this 

 extending from the heart to the sternum is the an- 

 terior mediastinwn. The portion between the dorsal 

 surface of the heart and the vertebral column is the 

 posterior mediastimim, it encloses between its layers 

 the oesophagus, the descending aorta, the lower por- 

 tion of the trachea, etc. 



4. The Phrenic Nerves, one on each side, lying be- 

 tween the heart and the lung. Follow each to its 

 termination in the diaphragm. 



5. The Trachea can be seen passing into the thorax 

 from the neck. Its further dissection should be made 

 after that of the heart. 



6. The (Esophagus is a wide muscular tube lying 

 along the dorsal wall of the thorax. It is easily seen 

 by lifting up the right lung. 



