464 EESPIEATION. 



the opposite. An interval (mediastinum) of varying dimen- 

 sions is left between the mediastinal pleura on the one side, 

 and that on the opposite. This space has been divided into 

 the anterior, median, and posterior mediastina. The anterior 

 contains superiorly the trachea, oesophagus, anterior aorta 

 with its branches, the anterior vena cava, the thoracic duct, 

 the cardiac plexus, and the pneumogastric recurrent and 

 phrenic nerves : in the very young subject it includes the 

 thymous gland. The middle mediastinum contains the heart 

 and the roots of the large vessels, the oesophagus, the first 

 divisions of the bronchia, and the nerves above noticed. 

 The posterior mediastinum, much narrower than either of the 

 others, is traversed above by the posterior aorta, the asygos 

 vein, and the thoracic duct; a little lower by the oesophagus, 

 with the pneumogastric and left phrenic nerves. The me- 

 diastinal pleurae are attached to one another or to the struc- 

 tures in the mediastinum by a more or less lax areolar 

 tissue. The diaphragmatic pleura is that part applied on 

 the anterior aspect of the diaphragm, and the parietal that 

 covering the ribs and intercostal muscles. Both are attached 

 to the subjacent structures by areolar tissue. 



The lungs have two separate sets of blood-vessels, the 

 bronchial and pulmonary. The former are engaged chiefly 

 in the nutrition of the organ the latter in the function of 

 respiration. The bronchial arteries arise from the posterior 

 aorta, and, proceeding to the roots of the lungs, divide with the 

 bronchia, and distribute their blood to the walls of these tubes, 

 to the pleura, to the interlobular connective tissue, to the lar^e 

 vessels of the pulmonary system, and to the lymphatics. The 

 capillary plexuses of the bronchial tubes are two in num- 

 ber, a deep and a superficial. The former is composed of 

 minute vessels, running circularly round the muscular coat, 

 and the latter of still smaller vessels, running longitudinally 



