494 BRONCHIAL TUBES. 



cf the liver, which presses the diaphragm upwards upon the right side of 

 the chest considerably above the level of the left ; and has three lobes 

 The left lung is smaller, has but two lobes, but is longer than the right. 



Each lung is retained in its place by its root, which is formed by the 

 pulmonary artery, pulmonary veins and bronchial tubes, together with the 

 bronchial vessels and pulmonary plexuses of nerves. The large vessels 

 of the root of each lung are arranged in a similar order from before, back- 

 wards, on both sides, viz. 



Pulmonary veins, 

 Pulmonary artery, 

 Bronchus. 



From above, downwards, on the light side, this order is exactly re- 

 versed ; but on the left side, the bronchus has to stoop beneath the arch 

 of the aorta, which alters its position to the vessels. They are thus dis- 

 posed on the two sides : 



Right. Left. 



Bronchus, Artery, 



Artery, ,Bronchus, 



Veins. Veins. 



Structure. The lungs are composed of the ramifications of the bron- 

 chial tubes which terminate in intercellular passages and air-cells, of the 

 ramifications of the pulmonary artery and veins, bronchial arteries and 

 veins, lymphatics and nerves; the whole of these structures, being held 

 together by areolo-fibrous tissue, constitute the parenchyma. The paren- 

 chyma of the lungs, when examined on the surface or by means of a sec- 

 tion, is seen to consist of small polyhedral divisions, or lobules, which are 

 connected to each other by an inter-lobular areolar tissue. These lobules 

 again consist of smaller lobules, and the latter are formed by a cluster of 

 air-cells, in the parietes of which the capillaries of the pulmonary artery 

 and pulmonary veins are distributed. 



Bronchial Tubes. The two bronchi proceed from the bifurcation of the 

 trachea to their corresponding lungs. The right takes its course nearly 

 at right angles with the trachea, and enters the upper part of the right 

 lung ; while the left, longer and smaller than the right, passes obliquely 

 beneath the arch of the aorta, and enters the lung at about the middle of 

 its root. Upon entering the lungs they divide into two branches, and 

 each of these divides and subdivides dichotomously to their ultimate ter- 

 mination in the intercellular passages and air-cells. 



According to Mr. Rainey,* the bronchial tubes continue to diminish in 

 size until they attain a diameter of - 5 ^ to -^ of an inch, and arrive within 

 J of an inch of the surface of the lung. They then become changed in 

 structure, and are continued onwards in the midst of air-cells, under the 

 name of intercellular passages. Lastly, the intercellular passages, after 

 several bifurcations, terminate, each by a cascal extremity or air-cell. The 

 intercellular passages are at first cylindrical like the bronchial tubes, but 

 soon become irregular in shape from the great number of air-cells which 

 open into them on all sides. The air-cells are small, irregular in shape, 

 and, most frequently, four-sided cavities, separated by thin septa, and 

 communicating freely with the intercellular passages, and sparingly with 



* Medico-Chirurgical Transactions, vol. xxviii. 



