PHYSIOLOGY OF CELLS 



63 



elements on one side bends the cilium in that direction. The cihum 

 because of the elasticity of the sheath returns to the original position 

 as soon as the contractile elements relax. The active stroke involves 

 the movement of the whole cilium while the return stroke begins at the 

 base and like a wave runs along the cilium to its tip (see Fig. 30). The 

 cilium thus presents the maximum friction in one stroke and the minimum 

 in the reverse. Since the cilia move in unison or in waves in which the 

 active beat is in the same direction, the difference in the friction of the 

 cilia in the two strokes is sufficient to 

 propel the animal. 



Many higher animals have inner sur- 

 faces covered with ciliated cells {ciliated Fig. 29. — Fibrillar structure of 



evilheliuM) the cilia of which are not used t^ ^ ^^^^^fl^^ZS^ 

 for locomotion of the body but for the 



propulsion of liquids or minute solid particles which have lodged on 

 these surfaces. Thus in the frog currents produced by cilia continually 

 carry small particles and the mucous secretion of the mouth down the 

 esophagus to the stomach. In the mammals and probably in most 

 other air breathing vertebrates solid particles inhaled with the air are 

 caught on the ciliated surfaces of the trachea and bronchi and are 

 swept up into the throat by the action of cilia. So also phlegm or 



i" 



B 



B 



I . IT 



Fig. 30. — Diagrams showing successive positions assumed by cilia in locomotion. 

 A, positions assumed from the beginning of the beat at \\ B, positions assumed during the 

 return stroke beginning at 1. {From Verworn.) 



mucus is carried upward to the back of the throat whence it is expelled 

 by "clearing the throat." 



Flagella. — Flagella are motor organs of the cell somewhat resem- 

 bling ciha. They differ from cilia chiefly in their length and in the fact 

 that when present they are few in number, there being from one to eight 

 of these organelles (rarely more) to the cell. In cells that move always 

 with the same end foremost, they are usually located at the anterior end 

 of the cell. Cilia as a rule beat in one plane forward and backward. 



