THE LUNGS 



453 



The posterior mediastinum is very delicate below the oesophagus, and usually 

 appears fenestrated; when these apertures are present, the two pleural cavities 

 communicate with each other.^ 



The pleural sacs contain a serous fluid, the liquor pleurae ; in health there is 

 only a sufficient amount to moisten the surface, hut it accunuilates rapidly after 

 death. 



Costal pleura 



Pulmonary 

 pleura 



Pulmonary 

 artery 



Bronchus 



Area of adhe- 

 sion of lungs 



Left phrenic '\ 

 nerve 



Mediastinal 

 pleura 



Mediastinal lobe 

 uf right lung 



na azygos 

 Thonu-ic duct 



Aorta 



Superior oesopha- 

 geal nerve 

 (Esophagus 

 Inferior a;sopha- 

 geal 7ierve 



Posterior vena cava 



Rigid jihrc nic 

 nerve 



Fold of pleura in- 

 closing posterior 

 vena cava and 

 right plircnic 

 nerve 



Sternum 



Fig. 360 — Cross-section of Thorax of New-borx Foal. 

 Ribs are numbered. 



THE LUNGS 



The lungs (Pulmones) occupy much the greater part of the thoracic cavity. 

 They are accurately adapted to the walls of the cavity and the other organs con- 

 tained therein. The two lungs are not alike in form or size, the right one being 

 considerably larger than the left; the difference is chiefly in width, in conformity 

 with the projection of the heart to the left. 



The lung is soft, spongy, and highly elastic. It crepitates when pressed 

 between the finger and thumb, and floats in water. When the thoracic 

 cavity of the unpreserved subject is opened, the lung collapses immediately 

 to about one-third of its original size, and loses its jjroper form; this is 

 due to its highly elastic character and the fact that the tension of the lung 



^ The apertures do not exist in the fcrtus, and are sometimes absent in the adult subject. 

 Some of them may be produced in dissection by the necessary disturbance of the parts. CHni- 

 cians state that a serous exudate formed in one pleural sac usually passes through to the other 

 side in the horse. 



