t 

 THE LUNGS 1197 



red blood in the pulmonary veins. The lungs are composed nf an external serous coat, a sub- 

 serous areolar tissue, and the pulmonary substance or parenchyma. 



The serous coat is thin, transparent, and invests the entire organ as far as the root. It is 

 composed of a layer of endothelial cells resting upon a thin layer of fibroelastic tissue, and is 

 continuous with the subserous tissue. It is known as the pulmonary pleura (p. 1183). 



The subserous areolar tissue contains a large proportion of elastic fibres; it invests the 

 entire surface of the lung, and extends inward between the lobules. 



The parenchyma is composed of lobules which, although closely connected by an 

 interlobular areolar tissue, are quite distinct from one another. The lobules vary in size from 

 0.3 cm. to 3 cm. (g inch to 1^ inches); those on the surface are large, of pyramidal form, with 

 the bases turned toward the surface; those in the interior are smaller and of various forms. 

 Each lobule is composed of one of the ramifications of a bronchiole (about 1 mm. in diameter) 

 and its terminal air cells, and of the ramifications of the pulmonary and bronchial vessels, lym- 

 phatics, and nerves, all of these structures being connected by fibroelastic tissue. 



The bronchus upon entering the lung divides like the ducts of a gland, the terminal tubules, 

 about 1 mm. in diameter, constituting the bronchioles. Each bronchiole forms a lobule that is 

 separated and closed from its neighbor. The bronchiole diminishes in diameter to about 0.5 mm. 

 and divides into several tubules (0.3 to 0.4 mm. in diameter), the respiratory bronchioles, upon 

 the sides of which some minute depressions, alveoli, may be seen. Each respiratory bronchiole 

 further subdivides into several alveolar ducts (0.2 mm. in diameter), along which alveoli are 

 numerous. The alveolar ducts lead into individual dilated spaces, the alvei, air sacs, or infun- 

 dibula (0.3 to 5 mm. in diameter), upon the sides of which are alveoli or saccules (0.05 to 0.1 mm. 

 in diameter). The alveoli are partially separated from one another by low partitions or septa, 

 of the same structure as the alveolar wall. 



Changes in Structure of the Bronchus and Its Divisions. Each main bronchus resembles 

 the trachea in structure. Within the lung the bronchial tubes are circular in outline. As the 

 bronchus divides and redivides monopodially, changes occur in the various coats. The mucous 

 coat presents a corrugated appearance and the cartilage in the submucosa changes to a com- 

 plete investing layer composed of a number of individual plates. With this change the smooth 

 muscle tissue, internal to the cartilage, increases so as to form a complete layer. As the smaller 

 tubes are approached the cartilage decreases in amount, while the muscle increases relatively. 

 The mucous glands in mucosa disappear in those tubules of 1 mm. in diameter, while the car- 

 tilage persists until a diameter of about 0.5 mm. is reached. 



Each bronchiole consists of a layer of simple columnar and goblet epithelial cells resting upon 

 a basement membrane and a fibroelastic tunica propria; external to this is found a layer of 

 smooth muscle tissue, and beyond this a layer of white fibrous tissue containing the main vessels 

 and nerves of the tubules. The respiratory bronchiole is lined by simple ciliated cells, that 

 gradually give way to nonciliated columnar and cuboidal cells, and lastly flattened epithelium 

 (the respiratory epithelium). Some muscle tissue is still present, and the elastic tissue increases 

 in quantity. The alveolar ducts are lined by simple epithelial plates resting upon a basement 

 membrane supported by elastic tissue. At the end of this tubule the muscle tissue disappears. 

 The air sacs are composed of the alveoli or saccules lined by respiratory epithelium that rests 

 upon a basement membrane supported by a meshwork of elastic tissue containing the densest 

 capillary plexus of the body. This elastic tissue allows the alveoli to increase from two to three 

 times their ordinary diameters. Separating one alveolus from another is a low septum that 

 has the same structure as the alveolar wall. 



Vessels of the Lungs. The pulmonary artery (Figs. 905 and 906) conveys the venous 

 blood to the lungs; it divides into branches which accompany the bronchial tubes, and terminates 

 in a dense capillary network upon the walls of the air cells. In the lung the branches of the 

 pulmonary artery are usually above and in front of a bronchial tube, the vein below. The 

 pulmonary arteries are the nutrient vessels of the respiratory epithelium. 



The pulmonary capillaries form plexuses which lie immediately beneath the epithelium of the 

 walls and septa of the air cells and of the alveoli. In the septa between the air cells the capillary 

 network forms a single layer. The capillaries form a very minute network, the meshes of which 

 are smaller than the vessels themselves; 1 their walls are also exceedingly thin. The arteries 

 of neighboring lobules are independent of one another, but the veins freely anastomose. 



The pulmonary veins commence in the pulmonary capillaries, the radicles coalescing into 

 larger branches, which run along through the substance of the lung, independently from the 

 minute arteries and bronchi. After freely communicating with other branches they form large 

 vessels, which ultimately come into relation with the arteries and bronchial tubes, and accom- 

 pany them to the hilum of the organ. Finally, they open into the left auricle of the heart, con- 

 veying oxygenated blood to be eventually distributed to all parts of the body by the aorta. 



The bronchial arteries supply blood for the nutrition of bronchial tubes and vessels of the 

 lung. The thoracic aorta usually gives off two left bronchial arteries. The single right bron- 



i The meshes are only 0.002'" to 0.008'" in width, while the vessels are 0.003'" to 0.005'" (Kolliker, Human 

 Microscopic Anatomy). 



