36 THE ESSENTIALS OF HISTOLOGY. 



Dilute alkaline solutions quicken the activity of cilia, or may even 

 restore it shortly after it has ceased. 



Various attempts have been made to explain the manner in which 

 cilia act, some supposing that they are themselves contractile, others 

 that their movement is a passive one, and that the real movement is at 

 their rootlets in the protoplasm of the cell. The bending-over action 

 can also be supposed to be due to the alternate flowing and ebbing 

 of hyaloplasm from the body of the cell into hollow permanent cell- 

 processes, i.e. the cilia ; if we assume that one side of each cilium is 

 less extensible than the other, it must necessarily be bent over in the 

 manner usually observed. Some cilia, however, have a spiral action 

 rather than the simple to and fro movement; in this case we may 

 assume that the line of less extensibility passes not straight along one 

 side of the cilium, but spirally round it. 



This hypothesis has the advantage that it permits ciliary motion to 

 be brought into the same category as amoeboid movements, in so far 

 that both are explicable by the flowing of hyaloplasm out of and into 

 the reticulum of spongioplasm. 



