20 THORAX 



follow it medially. He will find, at a certain point posterior 

 to the sternum, and to the left of the median plane, that his 

 fingers cease to pass towards the opposite side but are 

 carried backwards on the mediastinal part of the parietal 

 pleura, along the lateral boundary of the mediastinum, until 

 they come to the big blood-vessels and the air tube of the 

 lung, which collectively form its root. Along the front of the 

 vessels his fingers will now pass laterally, following the reflec- 

 tion of the pleura on the front of the vessels, to the medial 

 surface of the lung, and then anteriorly to its anterior border. 

 Round the anterior border they will arrive at the lateral 

 surface of the lung ; along that surface they will pass to the 

 posterior border and thence forwards along the posterior 

 part of the medial surface to the posterior surface of the root, 

 where they will feel, distinctly, the hard outline of the 

 bronchus. Following the posterior surface of the root medially, 

 they will reach the posterior part of the lateral boundary of 

 the mediastinum, along which they will pass backwards to the 

 vertebral column, and thence laterally along the posterior parts 

 of the ribs, and finally forwards along the inner surface of the 

 posterior flap to its anterior margin. 



If the dissector has followed the instructions given above 

 he cannot have failed to recognise that the pleural sac is in- 

 vaginated by the lung, which in its growth laterally from 

 the mediastinal septum has invaginated and expanded a part 

 of the medial wall of the sac. The dissector should now 

 examine a transverse section of a hardened thorax, or, if 

 that is not available, the diagram on p. 18. The study of 

 either will convince him that the lung, carrying the in- 

 vaginated part of the wall of the pleural .sac on its surface, 

 has expanded until it has practically obliterated the cavity of 

 the sac ; and he will understand that the invaginated pleura 

 on the surface of the lung, which is called the visceral pleura, 

 is everywhere in close apposition with the non- invaginated 

 portion, which is termed the parietal pleura ; all that inter- 

 venes between the two portions, in ordinary circumstances 

 during life, is a thin stratum of fluid, sufficient to lubricate 

 the surfaces and prevent friction during the movements of 

 the lung and the chest wall. 



After the dissector has grasped the facts noted above he 

 should follow the inner surface of the pleura in the transverse 

 plane at the level of the fifth costal cartilage, that is, below 



