vii CILIA 287 



becomes sickle-shaped instead of straight, and then more 

 slowly straightens again, both movements, however, being 

 extremely rapid and repeated about ten times or more 

 every second. These two movements are of course 

 antagonistic ; the bending drives the water or fluid in which 

 the cilium is placed in one direction, while the straightening 

 drives it back again. Inasmuch, however, as the bending 

 is much more rapid than the straightening, the force ex- 

 pended on the water in the former movement is greater 

 than in the latter. The total effect of the double move- 

 ment therefore is to drive the fluid in the direction towards 

 which the cilium is bent : that is, of course, if the cell on 

 which the cilia are placed is fixed. If the cell be floating 

 free, the effect is to drive or row the cell backwards ; for 

 the cilia may continue their movements even for some 

 time after the epithelial cell, with which they are connected, 

 is detached from the body. And not only do the move- 

 ments of the cilia thus go on independently of the rest of 

 the body, but they appear not to be controlled by the action 

 of the nervous system. Each cilium is comparable to one 

 of the mobile processes of a white corpuscle. A ciliated 

 cell differs from an amoeboid cell in that its contractile 

 processes are permanent, have a definite shape, and are 

 localised in a particular part of the cell, and that the 

 movements of the processes are performed rhythmically 

 and always in the same way. But the exact manner in 

 which the movement of a cilium is brought about is not 

 as yet thoroughly understood. 



Although no other part of the body has any control 

 over the cilia, and though, so far as we know, they have 

 no direct communication with one another, yet their action 

 is directed towards a common end — the cilia, which cover 

 extensive surfaces, all working in such a manner as to 

 sweep whatever lies upon that surface in one and the 

 same direction. Thus, the cilia which are developed 

 upon the epithelial cells, which line the greater part 

 of the nasal cavities and the trachea, with its rami- 



