STRUCTURE OF LUXG. 335 



wards, establishing the division of the mass into lobules: where it sepa- 

 rates the lobules it is named interlobular tissue, and is free from fat. 



Bronchial branches in the limy. When a bronchus is followed into 

 the pulmonary structure it is found to divide generally in a binary order, 

 and to diminish in size at each subdivision, until one terminal offset ap- 

 pertains to a lobule. In the lobule the tube has a diameter of ^ to B ^ of 

 an inch. When this last degree of diminution is reached, the tube gives 

 origin to the air cells. 



The larger bronchial branches have the same composition as the trachea, 

 but they are round in the lung, instead of being semi-elliptical as in the 

 trachea. The smallest branches want some of the elements found in the 

 larger bronchi ; and those from which the cells spring are irregular in 

 shape, appearing to be spaces amongst the cells rather than tubes with 

 continuous walls. 



Changes in the bronchi. The modifications of the component parts of 

 the bronchi are the following: The pieces of cartilage are broken up in 

 the smaller bronchial tubes, and are scattered over the wall as irregular 

 fragments. Becoming thinner and smaller as the subdivision of the air 

 tube proceeds, they at last disappear, -and are absent, from the terminal 

 brandies. Thejibrous and elastic tissues of the bronchial tubes are con- 

 tinued to the air cells, but in the small cell-bearing branches, the bundles 

 of elastic tissue are diffused, and, much diminished in strength, blend with 

 the fibrous or areolar tissue to form the wall. The muscular fibres are 

 diffused over the inner surface of the smaller bronchi, where they have an 

 annular arrangement; they extend beyond the limit of the pieces of carti- 

 lage, but they cease where the cells begin to be formed. The mucous 

 membrane becomes thinner as it extends onwards in the bronchial pieces, 

 and is finally continued to the cells, where it is transparent. Its epithe- 

 lium is columnar and ciliated in the bronchial tubes, but is changed to 

 squamous or laminar in the air cells. 



Lobules and lobes. A lobule is a cluster of air cells around a terminal 

 branch of the air tube. Varying in size and shape, each lobule is invested 

 by areolar tissue, and possesses its own offset of the air tube, as well as 

 distinct branches of vessels and nerves. The lobes are produced by the 

 aggregation of the lobules. 



The air cells are the little recesses or dilatations connected with the 

 smallest branches of the air tube. They are polyhedral in form, except 

 on the surface of the lung, and are distinct one from another, save through 

 the channel of the air passage. The cells are clustered in groups around 

 the terminal branches of the air tube, with which they communicate by 

 large orifices. These small spaces are about yj^- of an inch across, but 

 they are larger on the surface and at the edges than in the deeper parts of 

 the lung. The cell wall is formed by areolar and elastic tissue, and is 

 lined by a transparent mucous membrane possessing laminar epithelium. 

 Beneath the mucous lining is a network of capillaries of the pulmonary 

 vessels. 



VESSELS OF THE LUNG. Two sets of vessels are furnished to the lung, 

 one being concerned in its function, the other in the nutrition. The ves- 

 sels conveying blood to the lung to be aerated, and carrying that fluid 

 away after it has been subjected to the respiratory process, are named 

 pulmonary. The vessels connected with the nutrition of the texture are 

 called bronchial. 



The pulmonary artery divides like the bronchus, which it accompanies 



