EPITHELIUM. 239 



Ciliated Conoidal Epithelium. 



The cells of this variety of conoidal epithelium agree pre- 

 cisely in form, size, and arrangement with those of the first 

 described subdivision, the only difference being, that they are 

 possessed of the singular addition of vibratile cilia?.* (See 

 Plate XXI. fig. 3.) 



The cilia? taper from base to apex, are attached to the 

 thickened margins of the summits of the cells, ten or twelve 

 of them belonging to each. In the frog they would appear 

 to be not merely attached to a circular line, but also to the 

 segment of the cell described within this. (See Plate XXI. 



fig- i.) 



During life the cilia? are in a constant state of activity : the 

 power by which their motions are effected is, however, in- 

 volved in the greatest obscurity : it can scarcely be the re- 

 sult of muscular structure, as some have supposed, since the 

 entire cilia is many times smaller than the smallest mus- 

 cular fibre. The idea has been put forth that the cilia? are 

 hollow ; that they communicate with a vessel which runs 

 along their bases, containing fluid ; and that they are moved 

 by the successive injection and expulsion of this fluid. 



One fact has been observed, which affords countenance to 

 the above explanation of the motion of the cilia?, viz., that 

 this takes place in a determined direction: commencing in the 

 cilia? on one side, it runs along them to the opposite, the 

 several cilia? being thus successively called into action. It is 

 this peculiar character of the motion of the cilia? which has 

 led to its comparison with the waving of a corn-field over 

 which the wind passes in successive gusts. The motion, in 

 whatever way effected, is singularly beautiful, and, strange to 

 say, exhibits many of the characters of volition : thus it will 



* To Purkinje and Valentin especially belong the honour of making 

 known in all its extent the phenomenon of ciliary motion, and which 

 before that time had been observed only in some few of the lower animals, 

 and concerning- the nature of which many errors prevailed. They dis- 

 covered it in the respiratory and female genital organs in 1834. (Muller, 

 Archiv. 1834, p. 391.) 



