192 



NORMAL HISTOLOGY. 



off in addition. Although the number of branches of the air-tubules is 



much increased in the third quarter, it is in the last one and towards the 



periphery of the lobule throughout, that the tubules break up into a 



profusion (50-100) of truly terminal bronchioles. Each terminal bronchiole 



communicates by its slightly dilated 

 distal extremity with from three to six 

 spherical cavities, the atria, each of 

 which, in turn, communicates with a 

 group of larger irregular cavities, the 

 alveolar sacs, into which directly open 

 the ultimate air-spaces, the pulmonary 

 alveoli. The latter open not only into 

 the alveolar sacs but also into the atria 

 and even the distal part of the terminal 

 bronchiole, which is beset with scattered 

 alveoli. The mass of pulmonary tissue 

 connected with each terminal bronchiole, 

 including the air-spaces and accompany- 

 ing blood-vessels and nerves, constitutes 

 a lung-unit, by the aggregation of which 

 the lobule is formed. The lobules are 

 separated by distinct tracts of interlobular 

 connective tissue, in which the air-tubes 

 and accompanying blood-vessels course 

 until they enter the lobules. 



The Bronchioles. After the 

 bronchus begins to give off branches 



within the lung, the cartilage-rings gradually decrease in size and thickness 



until replaced by irregular angular plates, which appear at increasingly longer 



intervals until they finally cease, cartilage being seldom present in bronchioles 



of less than i mm. in diameter. As the cartilage tends to disappear, the 



unstriped muscle broadens into a 



continuous layer, which, in turn, 



becomes thinner as the air-tube 



diminishes and extends only as far 



as the terminal bronchioles. The 



muscle is arranged as a sphincter- 



like band around the openings 



by which the terminal bronchioles 



communicate with the atria. 



The walls of bronchioles of 



medium size (2-3 mm. ) consist 



of three coats, which from with- 



out in are: (i) an external fibrous 



iuniCi composed of fibre-elastic 



tissue, which encloses the cartilage 



FIG. 238. Diagram illustrating relations of 

 terminal divisions of air-tubes. 3, bronchiole 

 ending in terminal bronchi ( TB)\ latter divide 

 into atria (A), each of which communicates 

 with several air-sacs, (s) into which open the 

 alveoli (a); PA, branch of pulmonary artery 

 following bronchiole ; PV, pulmonary vein at 

 periphery of lung-unit. ( Miller.) 



FIG. 239. Corrosion-preparation of lung, showing 

 lung-units; a, minute bronchus ending in terminal 

 bronchi (6); c, atria; d, air-sacs; <?, alveoli. X 8. 



(often elastic in type) and accom- 



panying blood-vessels and blends 



with the adjoining lung-tissue; (2) 



a thin layer of unstriped muscle, sometimes incomplete and composed of 



circularly disposed bundles; and (3) the mucous membrane, thrown into 



longitudinal folds and consisting of ciliated columnar epithelium, with nu- 



merous goblet-cells, and a tunica propria made up chiefly of meshes of elastic 



