158 RESPIRATION. 



ing scarcer and more irregular, until, in the smaller bronchial 

 tubes, they are represented only by minute and scattered car- 

 tilaginous flakes. And when the bronchi, by successive 

 branches, are reduced to about ^ of an inch in diameter, 

 they lose their cartilaginous element altogether, 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 epithe- 

 lium. 



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 composed 



FIG. 58. 



Ciliary epithelium of the human trachea magnified 350 diameters, a, layer of 

 longitudinally arranged elastic fibres ; b, basement-memhrane ; c, deepest cells, cir- 

 cular in form ; d, intermediate elongated cells; e, outermost layer of cells fully de- 

 veloped and bearing cilia (after Kolliker). 



of a large number of minute parts, called lobules. Each pul- 

 monary lobule may be considered a lung in miniature, consist- 

 ing, 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 mem- 

 brane 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 ir- 

 regularly 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 

 infundibulwn (Fig. 59), and the irregular oblong space in its 



