CHAP. II.] 



EPITHELIAL TISSUE. 



11 



squeeze in between them. Columnar epithelium is found in its 

 most characteristic form lining the mucous 

 membrane of the intestinal canal. 



Glandular epithelium is found in the re- 

 cesses of secreting glands. The cells are of 

 many different shapes, and are usually set 

 round a tubular or saccular cavity, into 



FIG. 7. SIMPLE COL- 

 UMNAR EPITHELIUM, a, 

 the cells ; b, intercellular 



which the secretion is poured. The proto- f ubstanc Between the 



lower end of cells. 



plasm of these cells is generally filled by 



the materials which the 



gland 

 secretes. 



In ciliated epithelium the cells, 

 which are generally columnar in 

 shape," bear at their free extrem- 

 ities little hair-like processes 

 which are agitated incessantly 

 with a lashing or vibrating mo- 

 tion. These minute and delicate 

 processes are named cilia, and 

 may be regarded as active prolon- 

 gations of the cell-protoplasm. 



FIG. 8. - GLANDULAR EPITHELIUM, The maimer in which cilia move 

 WITH THE CELLS SET ROUND A SIMPLE 



SACCULAR GLAND. (Highly magnified.) is best seen when they are not 



acting very quickly. The mo- 

 tion 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, giv- 

 ing rise to the appearance of 

 a series of waves travelling 

 along the surface like the 

 waves caused by the wind 

 in a field of wheat. When 

 they are in very rapid action, their motion conveys the iilea of 

 swiftly running water. 



F IG> 9. CILIATED EPITHELIUM FROM THK 

 HUMAN TRACHEA. (Highly magnified.) a, 



large ciliated cell ; d, cell, with two nuclei. 



