DISSECTION OP THE BACK AND THORAX. 



101 



cone lies in front, and is bounded by the body of the 1st dorsal vertebra 

 above, and at the sides by the 1st ribs, which meet below. The 

 trachea, the oesophagus, the bloodvessels of the fore limb and head, 

 and many important nerves ai-e transmitted through this opening. On 

 transverse section, the thorax is not circular, as a cone is, but gives a 

 heart-shaped outline. It looks as if it had been squeezed laterally ; and 

 it might be described as having a roof, formed by the dorsal vertebrae 

 and the ribs as far as their angles ; a floor, much less extensive, formed 

 by the sternum ; and lateral walls, formed by the ribs and intercostal 

 muscles. 



Contents of the Cavity. — In point of size, the lungs are the most 

 important organs in the thorax, the heart coming next. Besides these. 



1. Chest-wall; 2. Pleural cavity or sac; 3. Lung; 4. Mediastinal pleura (parietal); 5. Costal 

 pleura (parietal) ; 6. Pulmonic pleura (visceral); A. (No. 3) Abdominal cavity ; D.(No. 3) Diaphragm ; 

 P. (No. 1) Pericardial sac; R. (No. 1) Root of lung ; V. (No. 3) Fold of right pleural membrane 

 enveloping posterior vena cava. 



the cavity lodges the main arterial and venous trunks, the thoracic duct, 

 the trachea, the oesophagus, and many important nerves, all of which 

 will be examined in due course. 



The Pleurae. Each half of the thorax possesses a serous membrane 

 termed the pleura. Like other serous membranes, the pleura is arranged 

 in the form cf a shut sac, and consists of a visceral and a parietal 



