1044 



VEETEBEATED ANIMALS 



nature, its cells (fig. 778) usually form but a single layer ; and are 

 so deficient in tenacity of mutual adhesion that they cannot be de- 

 tached in the form of a continuous membrane. Their shape varies 

 greatly. Sometimes they are broad, flat, and scale-like, and their 

 edges approximate closely to each other, so as to form what is 

 termed a ' pavement ' or ' tessellated ' epithelium : such cells are 

 observable on the web of a frog's foot or on the tail of a tadpole ; 

 for, though covering an external surface, the soft moist cuticle of 

 these parts has all the characters of an epithelium. In other cases 

 the cells have more of the form of cylinders, standing erect side by 

 side, one extremity of each cylinder forming part of the free surface, 

 whilst the other rests upon the membrane to which it serves as a 

 covering. If the cylinders be closely pressed together, their form is 

 changed into prisms; and such epithelium is often known as 

 prismatic 1 ." On the other hand, if the surface on which it rests be 

 convex, the bases or lower ends of the cvlinders become smaller than 



FIG. 778. Detached epithelium-cells: 

 a, with nuclei //, and nucleoli c, 

 from mucous membrane of the 

 mouth. 



FIG. 779. Ciliated epithelium : 

 a, ivucleated cells resting on 

 their smaller extremities; l>, 

 cilia. 



their free extremities; and thus each has the form of a truncated 

 cone rather than of a cylinder, and such epithelium (of which 

 that covering the villi of the intestine is a peculiarly good ex- 

 ample) is termed ' conical.' But between these primary forms of 

 epithelial cells there are several intermediate gradations ; and one 

 often passes almost insensibly into the other. Any of these forms 

 of epithelium may be furnished with cilia ; but these appendages are 

 more commonly found attached to the elongated than to the 

 flattened forms of epithelial cells (tig. 77'.'). Ciliated epithelium is 

 found upon the lining membrane of the air-passages in all air- 

 breathing Vertebrata ; and it also presents itself in many other 

 situations, in which a propulsive' power is needed to prevent an ac- 

 cimiulal ion of mucous or ot her secret ions. < hying to the very slight 

 attachment that usually exists between the epithelium and the 

 membranous surface \\hei-eon it lies, there is usually no difficulty 

 whatever in examining it, nothing more being necessary than to 

 scrape the surface of the membrane with a knife and to add a little 

 water to what has been thus removed. The ciliary action will 

 generally be found to persist for some hours or even days after 

 death if the animal has been previously in full vigour: and the 

 cells that bear the cilia, when detached from each other, will 



