PLEURA MEDIASTINUM. 489 



and are probably continued upon the lining mucous membrane of the 

 air cells. 



The Pulmonary artery, conveying the dark and impure venous 

 blood to the lungs, terminates in capillary vessels, which form a 

 minute network upon the parietes of the bronchial cells, and then 

 converge to form the pulmonary veins, by which the arterial blood, 

 purified in its passage through the capillaries, is returned to the left 

 auricle of the heart. 



The Bronchial arteries, branches of the thoracic aorta, ramify 

 upon the bronchial tubes and in the tissue of the lungs, and supply 

 them with nutrition, while the venous blood is returned by the 

 bronchial veins to the vena azygos. 



The Lymphatics, commencing upon the surface and in the sub- 

 stance of the lungs, terminate in the bronchial glands. These glands, 

 very numerous and often of large size, are placed at the roots of the 

 lungs, around the bronchi, and at the bifurcation of the trachea. In 

 early life they resemble lymphatic glands in other situations ; but in 

 old age, and often in the adult, they are quite black, and filled with 

 carbonaceous matter, and occasionally with calcareous deposits. 



The Nerves are derived from the pneumogastric and sympathetic. 

 They form two plexuses, anterior pulmonary plexus, situated upon 

 the front of the root of the lungs, and composed chiefly of filaments 

 from the great cardiac plexus ; and posterior pulmonary plexus on 

 the posterior aspect of the root of the lungs, composed principally 

 of branches from the pneumogastric. The branches from these 

 plexuses follow the course of the bronchial tubes, and are distributed 

 to the bronchial cells. 



PLEURJE. 



Each lung is enclosed, and its structure maintained, by a serous 

 membrane the pleura, which invests it as far as the root, and is 

 thence reflected upon the parietes of the chest. That portion of the 

 membrane which is in relation with the lung is called pleura pulmo- 

 nalis, and that in contact with the parietes, pleura costalis. The 

 reflected portion, besides forming the internal lining to the ribs and 

 intercostal muscles, also covers the diaphragm and the thoracic 

 surface of the vessels at the root of the neck. 



The pleura must be dissected from off the root of the lung, to 

 see the vessels by which it is formed and the pulmonary plexuses. 



MEDIASTINUM. 



The approximation of the two reflected pleurae in the middle line 

 of the thorax forms a septum which -divides the chest into the two 

 pulmonary cavities. This is the mediastinum. The two pleurae 

 are not, however, in contact with each other at the middle line in 

 the formation of the mediastinum, but leave a space between them 

 which contains all the viscera of the chest with the exception of the 

 lungs. The mediastinum is divided into the anterior, middle, and 

 posterioi\ 



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