OTHER FORMS OF CONTRACTILE TISSUE 249 



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



then return slowly to the erect position. This ,..,,.,., ... , ,.. n 



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 is, however, a functional connection 



between all the cells of a ciliated epithelial 



surface, 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 Fl0 -. ? 9 ;. Ciliated columnar 



epithelium from the trachea 



as those lor amoeboid movement of naked cells. of a rabbit ; mi, m 2 , m 3 , 



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. 



