90 THE HUMAN BODY. 



clover leaf; it was considered a nervous centre by the 

 ancients, perhaps because of the pain and the peculiar sen- 

 sations induced in the epigastrium by strong emotions, or 

 because they confounded the tendinous fibres with the 

 nervous tissue. 



Pleura. The cavity of the chest is lined with a serous 

 membrane, called the pleura, which forms in each half of 

 this cavity a sac without an opening. There are therefore 

 two pleurae, a right and a left. Proceeding from the edges 

 of the sternum and the costal cartilages, the pleurae cover the 

 lateral walls of the chest and a portion of the body of the 

 vertebrae. They then approach each other, leaving a space 

 between them called fas posterior mediastinum. On reaching 

 the root of the lungs, they turn from within outward, covering 

 a portion of the pericardium and of the internal surface of the 

 lungs, their posterior borders and external surface ; they then 

 penetrate the interlobular fissures, and fold back upon the 

 anterior border of the lungs and upon their internal surface 

 quite to their roots; then turning again forward, they cover 

 the sides of the pericardium, in front of which they turn back 

 to back, and then separating anew, they reach the borders 

 of the sternum from whence they sprang. The space left 

 between the pleurae behind the sternum is the anterior medi- 

 astinum, separated, as is seen, from the posterior mediastinum 

 by the heart and the root of the lungs. At the top of the chest 

 the pleurae form a conical cavity, which receives the apex of 

 the lung; at the bottom they cover the superior surface of the 

 diaphragm. In the posterior mediastinum are the oesophagus, 

 the aorta, the azygos vein, the thoracic duct, and the lower 

 portion of the trachea. In the anterior mediastinum are the 

 pericardium the envelope of the heart and the thymus 

 gland, an organ whose uses are unknown. That part of the 

 pleurae which envelops the organs of the chest, and that 

 which lines the walls of this cavity, are thus in contact with 

 each other without adhering, in a normal condition, and they 

 allow the expansion and contraction of the lungs and the 

 walls of the chest. The serous nature of the pleurae insures 

 freedom of movement, and prevents all roughness in the 

 constant friction of the surfaces. 



