188 EPITHELIAL GLOBULES. 



a circuraductory movement in such a manner as to describe 

 a sort of funnel, or else curve like a whip-lash (flagellum 

 of the infusoria, tail of the spermatozoids), or simply oscillate, 

 however, always more towards one side than the other, so 

 as to produce, at length, in the fluid or mucus in which they 

 are immersed, a progressive movement which is always in 

 the same direction (Fig. 57, upper arrow). Their rapidity 

 of motion renders observation of them often extremely diffi- 

 cult, as they make, at times, from 200 to 250 movements in 

 a second. 



When examined with a less powerful magnifier, the result 

 of these movements gives the epithelial surface in which they 

 take place the aspect of a field of wheat moved by the wind, 

 or of a river glistening in the sun. Small bodies (coal-dust), 

 placed on this surface, move upon it in a decided direction. 

 These phenomena may be easily observed in the frog : in the 

 mucous of its trachea the motion is seen to proceed from the 

 lower to the upper part, that is, from the lung to the mouth ; 

 in the pharyngeal and casophageal mucous, on the contrary, 

 it proceeds from the mouth to the stomach. These currents 

 are explained by the fact that the vibrations of these fila- 

 ments are made in a peristaltic form ; that is, in those of the 

 O3sophagus, for instance (in the frog), the movement or undu- 

 lating wave begins in the cilia of the cells of the tongue, and 

 continues in those which are situated lower in the pharyngeal 

 duct ; the nervous system, meanwhile, has nothing to do with 

 the co-ordination of these movements, and on a piece of de- 

 tached mucous we may by the regular direction of the move- 

 ment even distinguish the buccal extremity from the ceso- 

 phageal extremity of the fragment ; we see the cilia, also, 

 fall and rise ten or twelve times in a second, and, like oars, 

 hold out their thin edge as they rise, and strike with their flat 

 surface as they sink, by which means they advance. (The 

 process may be reversed, according to the peculiarity of the 

 animal under consideration.) 



If the surface be scraped, and the cells detached from it, 

 we find that the cilia with which they are still provided, 

 continue to move, but in an irregular manner: while the cell 

 floating in the fluid, being displaced by the movements of 

 the cilia, eddies about at random. Michael Foster com- 

 pares it,- under these circumstances, to " a boat without a 

 rudder, manned by mad sailors." It is thus probable that, 

 when the cells are in their accustomed place, the movements 

 of the vibratile cilia (those of the mouth in their relation to 



