RESPIRATION. 



and their walls thinner; the cartilaginous rings, especially 

 becoming scarcer and more irregular, until, in the smaller 

 bronchial tubes, they are represented only by minute and 

 scattered cartilaginous flakes. And when the bronchi, by 

 successive branches, are reduced to about -^ of an inch 

 in diameter, they lose their cartilaginous element alto- 

 gether, and their walls are formed only of a tough, fibrous^ 

 elastic membrane, with traces of circular muscular fibres; 

 they are still lined, however, by a thin mucous membrane, 

 with ciliated epithelium. 



Each lung is partially subdivided into separate portions, 

 called lobes ; the right^ lung, jnto three lobes, and the left 

 lung into two (fig. 57). Each of these lobes, again, is 



Pig. 58.* 



-composed of a large number of minute parts, called lobules. 

 Each pulmonary lobule may be considered a lung in 

 miniature, consisting, as it does, of a branch of the bron- 

 chial tube, of air-cells, blood-vessels, nerves, and lymphatics, 

 with a sparing amount of areolar tissue. 



On entering a lobule, the small bronchial tube divides 



* Fig. 58. Ciliary epithelium of the human trachea magnified 350 

 diameters, a, Layer of longitudinally arranged elastic fibres ; ft, Base- 

 ment membrane ; c, Deepest cells, circular in form ; d, Intermediate 

 elongated cells ; e, Outermost layer of cells fully developed and bearing 

 -cilia (from Kolliker). 



