STRUCTURE OP THE LUNGS. 551 



Their posterior border is rounded and broad, the anterior sharp, and 

 marked by one or two deep fissures, and the inferior border which 

 surrounds the base is also sharp. The colour of the lungs is pinkish 

 grey, mottled, and variously marked with black. The surface is 

 figured with irregularly polygonal outlines, which represent the lobules 

 of the organ, and the area of each of these polygonal spaces is crossed 

 by lighter lines. 



Each lung is divided into two lobes, by a long and deep fissure, 

 which extends from the posterior surface of the upper part of the 

 organ, downwards and forwards to near the anterior angle of its base. 

 In the right lung the upper lobe is subdivided by a second fissure, 

 which extends obliquely forwards from the middle of the preceding to 

 the anterior border of the organ, and marks off a small triangular 

 lobe. 



The right lung is larger than the left, in consequence of the inclina- 

 tion of the heart to the left side. It is also shorter, from the great 

 convexity of the liver, which presses the diaphragm upwards upon the 

 right side of the chest considerably above the level of the left; and lias 

 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, backwards, on both sides, viz. 



Pulmonary veins, 

 Pulmonary artery, 

 Bronchus. 



From above, downwards, on the right 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 disposed on the two sides : 



Right. Left. 



Bronchus, Artery, 



Artery, Bronchus, 



Veins. Veins. 



Structure. The lungs are composed of the ramifications of the 

 bronchial tubes which terminate in bronchial cells (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 areolar tissue, constitute the parenchyma. The parenchyma 

 of the lungs, when examined on the surface or by means of a section, 

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

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



