716 ORGANS OF TOICE AND RESPIRATION. 



pleura may be traced over the convex surface of the lung, the summit and base, 

 and also over the sides of the fissures between the lobes. It covers its anterioi' 

 surface, and front part of its root, and is reflected upon the side of the pericav- 

 dium to the :>ner surface of the sternum. Below, it covers the upper surface of 

 the Diaphragm. Above, its apex projects, in the form of a cul-de-sac, through the 

 superior opening of the thorax into the neck, extending about an inch above the 

 margin of the first rib, and receives the summit of the corresponding lung ; this 

 sac is strengthened, according to Dr. Sibson, by a dome-like expansion of fascia, 

 derived from the lower part of the Scaleni muscles. 



A little above the middle of the sternum, the contiguous surfaces of the 

 two pleurae are in contact for a slight extent ; but, above and below this point, 

 the interval left between them by their non-approximation forms the anterior 

 mediastinum. 



The inner surface of the pleura is smooth, polished, and moistened by a serous 

 fluid ; its outer surface is intimately adherent to the surface of the lung, and to the 

 pulmonary vessels as they emerge from the pericardium ; it is also adherent 

 to the upper surface of the Diaphragm ; throughout the rest of its extent it is 

 somewhat thicker, and may be separated from the adjacent parts with extreme 

 facility. 



The right pleural sac is shorter, wider, and reaches higher in the neck than 

 the left. 



Vessels and Nerves. The arteries of the pleura are derived from the intercostal, 

 the internal mammary, the phrenic, inferior thyroid, thymic, pericardiac, and 

 bronchial. The veins correspond to the arteries. The lymphatics are very 

 numerous. The nerves are derived from the phrenic and sympathetic (Luschka). 

 Kolliker states that nerves accompany the ramifications of the bronchial arteries 

 in the pleura pulmonalis. 



MEDIASTINUM. 



The Mediastinum is the space left in the median line of the chest by the non- 

 approximation of the two pleurre. It extends from the sternum in front to the 

 spine behind, and contains all the viscera in the thorax, excepting the lungs. 

 The mediastinum may be subdivided, for convenience of description, into the 

 anterior, middle, and posterior. 



The anterior mediastinum is bounded in front by the sternum, on each side by 

 the pleura, and behind by the pericardium. Owing to the oblique position of the 

 heart towards the left side, this space is not parallel with the sternum, but directed 

 obliquely from above downwards, and to the left of the median line ; it is broad 

 oelow, narrow above, verv narrow opposite the second piece of the sternum, the 

 contiguous surfaces of the two pleuras being occasionally united over a small 

 space. The anterior mediastinum contains the origins of the Sterno-hyoid and 

 Sterno-thyroid muscles, the Triangularis sterni, the internal mammary vessels of 

 the left side, the remains of the thymus gland, and a quantity of loose areolar 

 tissue, containing some lymphatic vessels ascending from the convex surface of 

 the liver. 



The middle mediastinum is the broadest part of the interpleural space. It 

 contains the heart inclosed in the pericardium, the ascending aorta, the superior 

 vena cava, the bifurcation of the trachea, the pulmonary arteries and veins, and 

 the phrenic nerves. 



The posterior mediastinum is an irregular triangular space, running parallel 

 with the vertebral column; it is bounded in front by the pericardium and roots of 

 the lungs, behind by the vertebral column, and on either side by the pleura. It 

 contains the descending aorta, the greater and lesser azygos veins and superior 

 intercostal vein, the pneumogastric and splanchnic nerves, the oesophagus, thoracic 

 duct, and some lymphatic glands. 



