38 THE, THORACTC VISCERA, BUCCAL “CA VIZ ¥, 
The right lung is divided into four lobes. The 
most posterior of these, the z/ra-cardzac lobe, pro- 
jects between the heart and the diaphragm, enclosed 
in a special fold of the right pleura. 
3. The Pleurz. Each side of the thorax is lined 
by a delicate membrane, the farzetal 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 vzsceral 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 sliding-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 medzastinum. The portion of this 
extending from the heart to the sternum is the az- 
tertor mediastinum. The portion between the dorsal 
surface of the heart and the vertebral column is the 
postertor mediastinum, it encloses between its layers 
the cesophagus, the descending aorta, the lower por- 
Hon Oltne tracked, eve. 
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 Gsophagus is a wide muscular tube lying 
along the dorsal wall of the thorax. It is easily seen 
by lifting up the right lung. 
