STRUCTURE OF THE LUNGS. 201 



Each lung is partially subdivided into separate portions, 

 called lobes ; the right lung into three lobes, and the left 

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

 Fig. S 8. 



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 bronchial tube, of air-cells, bloodvessels, nerves, and 

 lymphatics, with a sparing amount of areolar tissue. 



On entering a lobule, the small bronchial tube divides 

 and subdivides; its walls, at the same time, becoming 

 thinner and thinner, until at length they are formed only 

 of a thin membrane of areolar and elastic tissue, lined by 

 a layer of squamous epithelium, not provided with cilia. At 

 the same time, they are altered in shape ; each of the 

 minute terminal branches widening out funnel-wise, and 

 its walls being pouched out irregularly into small saccular 

 dilatations, called air-cells (fig. 59). Such a funnel-shaped 

 terminal branch of the bronchial tube, with its group of 

 pouches or air-cells, has been called an in) 



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

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

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

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

 cilia (after Kolliker.) 



