274 THE LUNGS. 



Pulriionary substance. — The substance of tlie lung is mainly com- 

 jiosed of numerous small lobules wliieli are attached to tlie ramifications 

 of the air-tubes, and are held together by those tubes, by the blood- 

 vessels, and by interlobular areolar tissue. These lobules are of 

 various sizes, the smaller uniting into larger ones ; they are bounded 

 by flattened sides, and compactly fitted to each other and to the larger 

 air-tubes and vessels of the lungs, those on the surface of the organ 

 having bases, turned outwards, from half a line to a line in diameter. 

 Though mutually adherent by means of fine areolar tissue, they are 

 quite distinct one from the other, and may be readily separated by 

 dissection in the lungs of young animals, and in those of the human 

 foetus. They may be regarded as lungs in miniature, the same elements 

 entering into their composition as form the lung itself. The structure 

 of a single lobule represents in fact that which is essential in the entire 

 organ, each being made up of the following constituents : the air-tiihes 

 and their ierminatinri air-cells, the pnlmonarij and hronchial blood- 

 vessels, with lymphatics, nerves, and interstitial areolar tissue. 



The principal divisions of the bronchi, as they pass into the lungs, 

 divide into tubes of less calibre, and these again subdivide in succes- 

 sion into smaller and smaller bronchial tubes, or bro)ichia, Avhich, 



Fig. 182. 



Fig. 182. — Portion of Transvkrse Section op a Bronchial Tube (^-inch in Diameter) 

 (F. E. Schultze). Magnified 30 Diameters. 



a, cartilage and fibrous layer with mucous glands, and, in the outer part, a little fat; 

 in tlie middle, the duct of a gland opens on the inner surface of the tube ; h, annular 

 layer of involuntary muscular fibres ; r, elastic layer, the elastic fibres in bundles which 

 are seen cut across ; (/, columnar ciliated epithelium. 



diverging in all directions, never anastomose, but terminate separately 

 in the pulmonary lobules. The prevailing form of division is dichoto- 

 raous ; but sometimes three branches arise together, and often lateral 

 branches are given off at intervals fi'om the sides of a main trunk. 

 The larger branches diverge at rather acute angles, but the more remote 

 and smaller ramifications spring less and less acutely. After a certain 

 stage of subdivision each bronchial tube is reduced to a very small size, 

 and, fomiing what has been termed a lobular bronchial tube (fig. 184;, r), 



