Chap. Ill] 



EPITHELIAL TISSUE 



29 



and may be regarded as active prolongations of the cell-pro- 

 toplasm. The manner in which cilia move is best seen when 

 they are not acting very 

 quickly. The motion of an 

 individual cilium may be 

 compared to the lash-like 

 motion of a short-handled 

 whip, the cilium being rap- 

 idly bent in one direction. 

 The motion does not involve 

 the whole of the ciliated sur- 

 face at the same moment, 

 but is performed by the cilia 

 in regular succession, giving 

 rise to the appearance of a 

 series of waves travelling along the surface like the waves tossed 

 by the wind in a field of wheat. When they are in very rapid ac- 

 tion, their motion conveys the idea of swiftly running water. As 

 they all move in one direction, a current of much power is produced. 



Function. — Cilia have been shown to exist in almost ever}' class 

 of animal, from the highest to the lowest. In man their use is to 

 impel secreted fluids, or other matters, along the surfaces to which 

 they are attached ; as, for example, the mucus of the trachea and 

 nasal chambers, which they carry toward the outlet of these pas- 

 sages, and thus keep out foreign matter. 



2. Transitional epithelium. — This consists of two or three 

 layers of cells. The superficial cells are large and flattened, having 

 on their under surface depressions into which fit the larger ends of 



Fig. 11. — Ciliated Epithelium from 

 THE Human Trachea. (Highly magnified.) 

 a, large eUiated cell ; d, cell with two nuclei. 



Fig. 12. — Sectiox of the Transitional Epithelium lining the Bladder. 

 (Highly magnified.) o, superficial ; 6, intermediate ; c, deep layer of cells. 

 (Schafer.) 



