782 ORGANS OF VOICE AND RESPIRATION. 



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 inter- 

 costal, 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 pleura3. It extends from the sternum in front to the 

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

 The mediastinum is 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 below, narrow above, very 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 origin of the Sterno- 

 hyoid and Sterno-thyroicl muscles, the Triangularis Sterni, the internal mam- 

 mary vessels of the left side, the remains of the thymus gland, and a quantity 

 of loose areolar tissue, in which some lymphatic vessels are found 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 triangjar 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 lelt 

 superior intercostal vein, the pneumogastric and splanchnic nerves, the ojsopha- 

 gus, thoracic duct, and some lymphatic glands. 



THE LUNGS. 



The Lungs are the essential organs of respiration ; they are two in number ; 

 placed one in each of the lateral cavities of the chest, separated from each other 

 by the heart and other contents of the mediastinum. Each lung is conical in 

 shape, and presents for examination an apex, a base, two borders, and two sur- 

 faces (see Fig. 418, p. 756). 



The apex forms a tapering cone, which extends into the root of the neck, 

 about an inch to an inch and a half above the level of the first rib. 



The base is broad, concave, and rests upon the convex surface of the Dia- 

 phragm; its circumference is thin, and fits into the space between the lower 

 ribs and the costal attachment of the Diaphragm, extending lower down exter- 

 nally and behind than in. front. 



