STRUCTURE OF THE HUMAN MECHANISM 11 



important arteries and veins. In both cavities the lining 

 membrane (pleura or peritoneum) is folded back over the 

 organs ; that is to say, the organs do not really lie in the 

 cavities, but only fill them as the hand would fill a bladder 

 one wall of which it pushes in against the other. The sur- 

 faces of the organs, like the walls of the cavity, are conse- 

 quently smoothly covered and glide over one another with 

 very little friction. The 

 preservation of these pleu- 

 ral and peritoneal linings 

 in their normal condition 

 is a matter of great impor- 

 tance ; when inflamed or 

 otherwise injured their sur- 

 faces become roughened, 

 and adhesions of connec- 

 tive tissue often develop 

 between them which fas- 

 ten the organs together or 

 to the walls of the cavity, FlG - 3 - Cross section of the chest ante- 



so that surgical interfer- 

 ence is sometimes neces- 



rior to the branching of the trachea 



A, a. vertebra of the spinal column ; B, spinal 

 cord ; (7, the pleural cavity (which is exag- 



\ Sary. Pleurisy is Such an gerated for the sake of clearness, the sur- 



\ . a , r , i i face of the lung being actually in contact 



\ inflammation Of the pleura, wit h the body wall). The oesophagus, tra- 



\ peritonitis of the peritO- chea together with several large arteries 



, , , and veins, are shown in the mediastinum ven- 



\neum ; and both are Very tral to the vertebra and in the order named 



serious conditions. 



7. Attachment of the organs to the walls of the pleural 

 and peritoneal cavities. The pleural cavity is completely 

 divided by a median partition of connective tissue (the 

 mediastinum), within which are found the trachea, the 

 oesophagus, the great blood vessels, and lying within a 

 special cavity of its own the heart. Approximately half- 

 way from the anterior to the posterior border of the medi- 

 astinum the trachea divides within that membrane into two 



