AIR-TUBES OF THE LUNGS. 



803 



bronchia. But the cartilages are absent in bronchia the diameter of 

 which is less than a quarter of a line; the walls of such tubes are en- 

 tirely membranous. A fibrous coat and longitudinal elastic fibres are 

 present in all the tubes, down to the very smallest. The muscular 

 fibres of the unstriped kind, are no longer limited to the posterior 

 portion of the tube, but, arranged in circular bundles, form a layer ex- 

 ternal to the cartilages ; they extend to the smallest ramifications of 

 the bronchia. The mucous membrane lining the bronchia is thinner 

 than that of the bronchi and trachea, with which it is continuous ; it 

 is covered with a ciliated columnar epithelium. The walls of the 

 bronchi and larger bronchia, are provided with mucous glands. 



The terminal bronchial offsets distributed to the lobules, or the lob- 

 ular bronchial tubes, divide within the lobules, from four to nine times, 

 according to the size of the lobule. The branches thus formed be- 

 come gradually smaller, being at length reduced to g^th, g^th, or ^th 

 of an inch in diameter. They finally terminate in the so-called inter- 

 cellular passages, or air-sacs, which offer a marked contrast, both in 

 form and structure, to the tubes ; for instead of a cylindrical form, 

 they appear like irregular passages, traversing the substance of the 

 lobule at various angles, and communicating freely with each other ; 

 at the same time they no longer present either longitudinal, elastic, or 

 circular muscular fibres. Moreover, the diameter of the intercellular 

 passages is somewhat greater than that of the finest bronchial tubes 

 from which they proceed, and it increases a little at each division, 

 whilst the tubes, in this respect, as already mentioned, diminish in 

 size as they divide. Finally, the sides of the intercellular passages, 

 at first smooth, like those of the lobular 

 bronchial tubes, soon become recessed by 

 numerous closely-set, sharply-defined, cup- 

 shaped depressions; these* are the so- 

 called air-vesicles or air-cells. An inter- 

 cellular passage may, indeed, be regarded 

 as a space between the air cells, which 

 surround it on all sides. 



The air-cells or air-vesicles (Fig. 112, 

 2, 3), the ultimate recesses to which the 

 minutely subdivided air gains access, 

 measure from 5 J<jth to ^th of an inch in 

 diameter. They are smaller in the inte- 

 rior of the lungs, larger on the surface, 

 and largest at the apices and thin edges 

 of those organs. They are larger in the 

 male than in the female, and gradually 

 increase in size as life advances. Those 

 cells which occupy the central portions of 

 a lobule, appear like polyhedral alveoli, 

 separated from each other by delicate 

 septa. By some, it is said that the sides 

 of the cells are frequently perforated or 

 deficient, so that neighboring cells corn- 



112. 



Fig. 112. Magnified diagrammatic view 

 of groups or clusters of air cells, one be- 

 ing laid open. 1, small bronchial tube, 

 dividing into others which are mem- 

 branous. 2, vesicular portion of lobule, 

 with air-cells on its surface. 3, the same 

 laid open, to show the recesses or air- 

 cells in its interior. 



