THORACIC CAVITY 13 



manubrium sterni ; (2) at the level of the third intercostal 

 space ; and (3) at the level of the fifth costal cartilage. He 

 must trace the wall of the sac in the vertical plane also. 



Commencing at the level of the third intercostal space, he 

 should place his fingers on the surface of the lung and follow 

 it anteriorly and medially until, behind the sternum, he reaches 

 the sharp anterior border, which should be pulled laterally ; 

 then, turning from the lung to the parietal pleura, he should 

 place his fingers on the inner surface of the anterior flap and 

 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 posteriorly, along the lateral boundary of the medi- 

 astinum, 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 reflection 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 this they will pass to the 

 posterior border and thence anteriorly 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 posteriorly to the 

 vertebral column, and thence laterally along the posterior parts 

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

 posterior flap to its anterior margin. 



If the dissector has followed the above instructions he 

 cannot fail to have recognised 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. n. 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 find 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 



