DUST AND ITS DANGERS. 



39 



lined with these delicate hairs which are called 

 cilia. 



Now, these myriads of cilia, year in and year 

 out, day and night, while life lasts, are con- 

 stantly swinging their free ends back and forth, 

 bending as they recover, and then with a quick 



FIG. 3. CILIATED CELLS FROM THE LARGE AIR-TUBES OF THE 

 HUMAN LUNGS, SEEN FROM THE SIDE. HIGHLY MAGNIFIED. 



snap forward so that any small object which 

 lodges on the walls of the larger air-tubes 

 since all the cilia act in rhythm is swept up- 

 wards toward the mouth, away from the peril- 

 ously delicate and sensitive lungs. 



The movement of these cilia is less vigorous 

 when the body is quiet, as in sleep, increasing 



