38 DUST AND ITS DANGERS. 



mucous which that organ secretes when irri- 

 tated. 



A very considerable proportion of the in- 

 breathed foreign material gets into the mouth 

 and may be spat out or swallowed. 



The floating material which is carried past 

 the well-guarded portals of the lungs and 

 enters the windpipe and bronchial tubes and 

 lodges on their moist walls finds here a most 

 efBcient arrangement for its expulsion. Here 

 is placed, completely lining the tubes, an army 

 of thoroughfare-cleansers composed of individ- 

 uals who are not in politics, who have no vote, 

 and who present to us the unwonted, and at 

 first puzzling, spectacle of street-cleaners whose 

 business seems to be to clean the streets. 

 Completely lining the larger air-tubes like 

 a mosiac, are myriads of tiny cells shaped 

 something like a narrow short club and set 

 upon end side by side. Projecting from the 

 free ends of each one of these cells is a number 

 of very minute hairs, so that the whole cell 

 looks something like a short club with a beard 

 growing from one end (see Fig. 3). The 

 whole inner surface of these air-tubes, then, is 



