STRUCTUEE OF THE LUNGS. 273 



EOOT OF THE LUHG. 



The root of each lung is composed of the bronchus and the large 

 blood-vessels, together with the nerves, lymphatic vessels, and glands, 

 connected together by areolar tissue, and enclosed in a sheath of the 

 pleura. 



The root of the right lung lies behind the superior vena cava and 

 part of the right auricle, and below the azjgos vein, which arches over 

 it to enter the superior cava. That of the left lung passes below the 

 arch of the aorta, and in front of the descending aorta. The phrenic 

 nerve descends in front of the root of each lung, and the pneumogastric 

 nerve behind, whilst the ligamentura latum pulmonis is continued from 

 the lower border. The bronchus, together with the bronchial arteries 

 and veins, the lymphatics and lymphatic glands, arc placed on a plane 

 230sterior to the great blood-vessels ; the pulmonary artery lies more 

 forward than the bronchus, and to a great extent conceals it, whilst the 

 pulmonary veins are placed still farther in advance. The pulmonary 

 plexuses of nerves lie on the anterior and posterior aspect of the root, 

 beneath the pleura, the posterior being the larger of the two. OJLn% \ 



The order of position of the great an--tube and pulmonary vessels ^^ 



from above downwards differs on the two sides ; for whilst on the right-^ ^. t\ V 

 side the bronchus is highest and the pulmonary artery next, on the left,, 

 the air-tube, in passing obliquely beneath the arch of the aorta, is ^ r\ 'S' V 

 depressed below the level of the left pulmonary artery, which is the 

 highest vessel. On both sides the pulmonary veins are the lowest of 

 the three. 



Before entering the substance of the lung, the bronchus divides into 

 two branches, an upper and a lower, one for each lobe. The lower 

 branch is the larger of the two, and on the right side gives off a third 

 small branch which enters the middle lobe of that lung. 



The pulmonary artery also divides, before penetrating the lung to 

 which it belongs, into two branches, of which the lower is the larger 

 and supplies the inferior lobe. On the right side the upper of these 

 two branches gives the branch to the middle lobe. A similar arrange- 

 ment prevails in regard to the right pulmonary veins, the upper one 

 of which is formed by branches proceeding from the superior and 

 middle lobes of the right lung. 



STRUCTURE OF THE LUNGS. 



Coverings. — Beneath the serous covering, already noticed, there is 

 placed a thin layer of suiseroifs areolar tissue mixed with a large number 

 of elastic fibres. It is continuous with the areolar tissue in the interior 

 of the lung, and has been described as a distinct coat under the name 

 of the second or deeper layer of tlie pleura. In the lungs of many 

 animals, such as the lion, seal, and leopard, this subserous layer forms 

 a very strong membrane, composed principally of elastic tissue ; in 

 others, for instance the guinea-pig, a network of plain muscular fibres 

 is found which have a general radiating direction from the apex. A 

 close plexus of lymjihatic vessels is also met with in this sub-pleural 

 tissue : these vessels communicate on the one side by means of stomata 

 with the pleural cavity, and on the other, as will be afterwards noticed, 

 with a network of similar vessels in the interalveolar septa of the lunirs 

 (Klein). 



