CILIATED CELLS. 



107 



Cilia 



Cell 



The air vesicles, with their supplying air tubes and their 

 surrounding blood tubes, are bound together by elastic tissue, 

 which fills up most of the intervening space. 



The windpipe has in its walls C-shaped cartilages, with the 

 open part of the C on the dorsal surface. These cartilages 

 continue in the bronchi, and so on until in the smaller twigs 

 they finally disappear. 



The cartilages are held together, and the dorsal gap of the 

 cartilages (the gap would be like that of a series of horse- 

 shoes piled on top of each other) bridged, by tough fibrous 

 tissue, with much elastic tissue, and with plain muscle fibers ; 

 these last-mentioned structures are very abundant in the 

 smaller air tubes. 



The lining of 

 the trachea is a 

 Mucous Membrane. 

 It pours out on 

 its surface a sub- 

 stance somewhat 

 like white-of-egg, 

 called Mucus. 

 This keeps the 



air moist, and catches particles of dust, etc., that are in the 

 inspired air. There is a constant slow current of mucus 

 toward the throat, whence it is, from time to time, hawked up. 

 This current of mucus is caused by the Cilia projecting from 

 the lining cells of tin trachea. They are little hairlike pro- 

 jections, in countless numbers, like a field of grass, each stalk 

 having the power of bending back and forth, making a quick 

 stroke toward the throat, then a slower recover stroke. Thus 

 the united wavelike action of the myriads of lashing cilia 

 paddles the mucus headward. (See page 388.) 



All the cavities and passages in the body to which the air 



Nucleus 



Fig. 41. Ciliated Cells Lining the Air Tubes. 



