CHAP, ii.] THE CONTRACTILE TISSUES. 135 



of fatigue, the action becomes slow that the movement itself can 

 be seen; what is seen otherwise is simply the effect of the 

 movement. The movements when slow have been counted at 

 about eight (double movements) in a second; probably when 

 vigorous they are repeated from twelve to twenty times a second. 



The flexion takes place in one direction only, and all the cilia 

 of each cell, and indeed of all the cells of the same epithelium 

 move in the same direction. Moreover the same direction is 

 maintained during the whole life of the epithelium ; thus the cilia 

 of the epithelium of the trachea and bronchial passages move 

 during the whole of life in such a way as to drive the fluid lying 

 upon them upwards towards the mouth ; so far as we know in 

 vertebrates, or at least in mammals, the direction is not and cannot 

 by any means be reversed. 



The flexion is very rapid but the return to the erect position 

 is much slower ; hence the total effect of the blow, supposing the 

 cilium and the cell to be fixed, is to drive the thin layer of fluid in 

 which the cilium is working, and which always exists over the 

 epithelium, and any particles which may be floating in that fluid, 

 in the same direction as that in which the blow is given. If the 

 cell be not attached but floating free the effect of the blow may 

 be to drive the cell itself backward ; and when perfectly fresh 

 ciliated epithelium is teased out and examined in an inert fluid 

 such as normal saline solution, isolated cells or small groups of 

 cells may be seen rowing themselves about as it were by the 

 action of their cilia. 



All the cilia of a cell move, as we have just said, in the same 

 direction, but not quite at the same time. If we call the side of 

 the cell towards which the cilia bend the front of the cell and the 

 opposite side the back, the cilia at the back move a trifle before 

 those at the front so that the movement runs over the cell in the 

 direction of the movement itself. Similarly, taking any one cell, 

 the cilia of the cells behind it move slightly before, and the cilia 

 of the cells in front of it slightly after, its own cilia move. Hence 

 in this way along a whole stretch of epithelium the movement or 

 bending of the cilia sweeps over the surface in ripples or waves, 

 very much as, when the wind blows, similar waves of bending 

 sweep over a field of corn or tall grass. By this arrangement the 

 efficacy of the movement is secured, and a steady stream of fluid 

 carrying particles is driven over the surface in a uniform continued 

 direction ; if the cilia of separate cells, and still more if the 

 separate cilia of each cell, moved independently of the others, all 

 that would be produced would be a series of minute ' wobbles,' of 

 as little use for driving the fluid definitely onwards as the efforts 

 of a boat's crew all rowing out of time are for propelling the boat. 



Swift bending and slower straightening is the form of ciliary 

 movement generally met with in the ciliated epithelium of mam- 

 mals and indeed of vertebrates ; but among the invertebrates we 



