438 MOVEMENTS. 



they are analogous to the ciliary movements, the cause of 

 which is equally obscure. 



Ciliary Movements. The epithelium covering certain 

 of the mucous membranes is provided with little hair-like 

 processes upon the free portion of the cells, called cilia. 

 These are in constant motion, from the beginning to the end 

 of life, and produce currents on the surfaces of the mem- 

 branes to which they are attached, the direction being always 

 from within outward. In many of the infusoria, the ciliary 

 motion serves as a means of progression, effects the intro- 

 duction of nutriment into the alimentary canal, and, indeed, 

 is almost the sole agent in the performance of the func- 

 tions involving movement. Even in higher classes, as the 

 mollusca, the movements of the cilia are of great impor- 

 tance. In man, and the warm-blooded animals generally, 

 the ciliated or vibratile epithelium is of the variety called 

 columnar, conoidal, or prismoidal. The cilia are attached to 

 the thick ends of the cells, and form on the surface of the 

 membrane a continuous sheet of vibrating processes. 



It is unnecessary to describe in detail the size and form of 

 the cells provided with cilia, as their variations in different 

 situations have been and will be considered in connection with 

 the physiological anatomy of different parts. In general 

 structure, the ciliary processes are entirely homogeneous, and 

 gradually taper from their attachment to the cell to an ex- 

 tremity of excessive tenuity. Although anatomists, from 

 time to time, have described striae at the bases of the cilia, 

 and have attempted to explain their motion by a kind of 

 muscular action, no well-defined structure has ever been 

 actually demonstrated in their substance. 



Certain currents were observed in the infusoria, mollusca, 

 and other of the lower order of animals, long before the 

 structure of the cilia had been accurately described ; but in 

 1835, Purkinje and Yalentin, in a very elaborate memoir, 

 described these structures fully, and noted the situations 



