THE LUNGS. 



973 



the superior vena cava, with the vena azygos major opening into it, the bifurcation 

 of the trachea and the two bronchi, the pulmonary artery dividing into its two 

 branches and the right and left pulmonary veins, the phrenic nerves, and some 

 bronchial lymphatic glands. 



Left Inom- Left Carotid Thymus 

 inate Vein. Artery. Gland. 



Pneumogastric 

 Nerve. 



Pneumogastric 

 Nerve. 

 "s 



Internal Mammary 

 Artery. 



Vertebral 



Artery.,- 

 Left Sublcavian, 

 Artery. 



(Esophagus.--" 



2nd Rib. 



FIG. 537. Transverse section through the upper margin of the third dorsal vertebra. (Braune.) 



The posterior mediastinum (Fig. 536) 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 from the lower border of the 

 fourth dorsal vertebra, and on either side by the pleura. It contains the descend- 

 ing thoracic aorta, the greater and lesser azygos veins, the pneumogastric and 

 splanchnic nerves, the oesophagus, thoracic duct, and some lymphatic glands. 



THE LUNGS. 



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

 placed one on each side 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 surfaces (Fig. 538). 



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, which separates the right lung from the upper surface of the right lobe of 

 the liver and the left lung from the upper surface of the left lobe of the liver, the 

 stomach, and spleen; its circumference is thin, and projects for some distance into 

 the phrenico-costal sinus of the pleura, between the lower ribs and the costal 

 attachment of the Diaphragm, extending lower down externally and behind than in 

 front. 



The external or thoracic surface is smooth, convex, of considerable extent, 

 and corresponds to the form of the cavity of the chest, being deeper behind than 

 in front. 



The inner surface is concave. It presents in front a depression corresponding 



