OTHER FORMS OF CONTRACTILE TISSUE 



249 



in 



be 

 su: 



In action the cilia bend suddenly down into a hook or sickle form, and 

 then return slowly to the erect position. This 

 movement is repeated many (twelve to twenty) 

 times a second, and thus serves to move forward 

 mucus, dust, or an ovum, as the case may be. 

 The movement seems to be entirely automatic, 

 and it is quite unaffected by nerves, at any rate 

 in all the higher animals. 



There seems to be a functional connection 

 tween all the cells of a ciliated epithelial 

 urf ace, so that movement of the cilia, started in 

 one cell, spreads forward as a wave, just as, 

 when the wind blows, waves of bending pass 

 over a field of corn. 



The conditions of ciliary action are the same FIG. 99. Ciliated columnar 



epithelium from the trachea 



of a rabbit ; m\ m 2 , 



cells. 



as those for amo3boid movement of naked cells. 

 The minuteness of the object has up to now 

 prevented us from deciding whether the cilium 

 is itself actively contractile, or whether it is simply passively moved by 

 the action of the basal part situated in the hyaline border of the cell. 



m 3 , mucus-secreting 



(SCHAFER.) 



