THORACIC VISCERA 311 



The lungs lie quite free in the thoracic cavity, 

 except at their roots, where the blood-vessels and 

 bronchi enter them. The left lung is divided into 

 two lobes ; and the right into four, of which the 

 smallest and hindmost lies in the median plane 

 behind the heart, closely applied to the oesophagus. 



4. The diaphragm is a thin partition, separating the thoracic 



cavity from the much larger abdominal cavity. Its 

 marginal portion is muscular, the fibres arising from 

 the inner surface of the hinder ribs and from the 

 vertebral column, and converging to be inserted into 

 the thinner, tendinous, central portion. The muscular 

 margin is specially developed in the dorsal region, 

 forming two stout pillars arising from the lumbar 

 vertebrae. 



The diaphragm is arched so as to present a very 

 convex surface towards the thorax : by contraction 

 of its muscular portion, in the act of inspiration, this 

 convexity is diminished and the thoracic cavity 

 thereby enlarged. The small size of the thoracic 

 relatively to the abdominal cavity should be noticed. 



5. The phrenic nerves are a pair of white cords lying 



between the heart and the lungs, and readily seen 

 on pressing these apart. Posteriorly they divide into 

 branches supplying the muscular portion of the 

 diaphragm. 



Their origin from the fourth cervical nerves wil 

 be seen in the dissection of the neck (p. 338). 



6. The pleurae. Each side of the thorax is lined by a 



glistening membrane, the pleura, which is reflected 

 over the lung at its root in the same way as the 

 peritoneum is reflected over the abdominal viscera. 

 The right and left pleural sacs are separated from 

 each other by the mediastinal cavity, the greater part 

 of which is occupied by the heart and pericardium, 

 the posterior part remaining vacant. 



During life the lungs are distended so as to 

 almost fill the thoracic cavity, the visceral layer of 



