DISSECTION OF THE BACK AND THORAX. 



103 



when taken from its centre than from its sides. It is in consequence of 

 this configuration of the diaphragm that the liver, the stomach, and 

 other abdominal organs lie under cover of the ribs. The vertex of the 

 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 blood vessels of the fore limb and head, and many 

 important nerves are transmitted through this opening. On transverse 

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



Fig. 0. 



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

 pleura (parietal); 6. Pulmonic pleura (visceral); A. (C) Abdominal cavity ; D. (C) Diaphragm ; 

 P. (A) Pericardial sac ; R. (A) Root of lung ; V. (C) Fold of right pleural membrane enveloping 

 posterior vena cava. 



The diagrams are intended to show the arrangement of each pleura) membrane— A at the level of 

 the heart, B in front of that organ, and O beliind it. With this object each pleural sac has been 

 represented as an actual space, although, as explained in the text, the cavity is only a potential one 

 in the normal chest of the live subject. 



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

 be described as having a roof, formed by the dorsal vertebras 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 im- 

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



