66 THE ELEMENTARY NERVOUS SYSTEM 



due to the direction of the effective stroke of the cilia. 

 Each cilium moves back and forth on a fixed base. In its 

 forward or effective stroke the cilium acts in such a way 

 as to drive the supernatant fluid and its suspended par- 

 ticles along with it. In its backward stroke or recovery 

 the cilium returns to its former position, imparting as 

 little motion to the surrounding fluid as possible. Thus 

 each effective stroke moves the superimposed fluid for- 

 ward and each recovery leaves this fluid in large part 

 standing. Hence in general the fluid moves on in one 

 direction only, namely, that of the effective stroke. 



The second factor in the ciliary activity of the frog's 

 palate is shown in the sequence in which the successive 

 cilia beat. In this particular example, as in most other 

 ciliated membranes, the cilia do not beat all at the same 

 moment or synchronously, but in regular order one after 

 another, or metachronously. Thus after one cilium has 

 begun its effective stroke the next cilium in the direction 

 of this stroke takes up the activity, and so on till the wave 

 has passed over the whole field. Thus the cilia of a given 

 membrane do not act independently nor in unison but in 

 sequence, and thus exhibit a high degree of coordina- 

 tion. It is this metachronous coordination that gives to 

 the ciliated membrane the appearance of a field of grain 

 over which a wind is blowing. 



The two elements of ciliary action that have thus been 

 pointed out, though of necessity intimately associated, 

 are in truth quite independent of each other. Their in- 

 dependence is perhaps best indicated by the fact that 

 there are instances in which their directions are opposed 

 instead of being in agreement, as in the epithelium of the 

 frog's palate. One of the best of these is seen in the 

 swimming plates of the ctenophores (Fig. 15). In these 



