CONTRACTILE TISSUES 59 



ment is from below upward toward the throat; there is 

 some doubt as to the course of the ciliary currents in the 

 nose. Dust which might accumulate in the lungs with 

 serious results is continually cleared away and gathered 

 temporarily about the root of the tongue. Eventually 

 it is swallowed. We may not find the notion agreeable, 

 but it is a fact that all the people in a dusty place are 

 acting as vacuum cleaners, freeing the air of a part of 

 the suspended material and depositing it in their own 

 stomachs. Note that the direction of the currents in 

 the trachea is upward while in the frog's esophagus it is 

 downward. 



FIG. 10. The sea anemone. See text. 



The microscopic air-sacs to which the finest bronchial 

 tubes lead are without cilia. There is no provision for 

 the removal to the throat of dust which runs the gauntlet 

 without being arrested and arrives in these terminal 

 chambers. Such dust will remain in the tissue of the 

 lungs and may discolor those organs to a degree which 

 varies with the environment of the individual. Coal 

 miners have their lungs greatly stained with the dust 

 they have inhaled, but the portion which is retained must 

 be an exceedingly small fraction of the total which they 

 have breathed. 



Do cilia ever reverse the direction of their effective 

 stroke? The question cannot be answered surely for 

 the mammal, but a reversal has been observed in a 



