RESPIRATION 



381 



frog. Air passes through the nares, the glottis, and into the 

 windpipe. This cartilaginous tube, the top of which may easily 

 be felt as the Adam's apple 

 of the throat, divides into 

 two bronchi. The bronchi 

 within the lungs break up 

 into a great number of 

 smaller tubes, the bronchial 

 tubes, which divide some- 

 what like the small branches 

 of a tree. This branching 

 increases the surface of the 

 air tubes within the lungs. 

 The bronchial tubes, indeed 

 all the air passages, are 

 lined with ciliated cells. 

 The cilia of these cells are 

 constantly in motion, beat- 

 ing with a quick stroke toward the outer end of the tube, that is, 



toward the mouth. Hence 



/-f-j 



Air passages in the human lungs; a, larynx; h, tra- 

 chea (or windpipe); c, c?, bronchi; e, bronchial 

 tubes; /, cluster of air cells. 



-^^, 



Bronchiole 



Diagram of two air cells, showing the capillary 

 network which covers them, and at a the 

 structures which intervene between the air 

 and the blood are indicated; /, mucous 

 membrane of the air cell; 2, submucous 

 mesh work; 3, wall of capillary; 4, plasma 

 in capillary; 5, red blood corpuscle. 



if any foreign material should 

 get into the windpipe or bron- 

 chial tubes, it will be expelled 

 by the action of the cilia. It 

 is by means of cilia that phlegm 

 is raised from the throat. 

 Such action is of great impor- 

 tance as it prevents the filling 

 of the air passages with foreign 

 matter. The bronchi end in 

 very minute air s.xs called 

 alveoli; these are little pouches 

 having elastic walls. It is 

 into these pouches that air is 

 taken when we inspire or take 

 a deep breath. Thus we see 

 the lung of man gets a very 



