GENERAL ANATOMY. 89 



and are therefore separated by the epithelia ; these are the 

 glandular epithelia. Further, all influences of the external 

 world chiefly impress the surface of the body, causing sen- 

 sations ; hence also certain epithelia are of the greatest im- 

 portance for the reception of sensory perceptions, and serve 

 for hearing, seeing, smelling, tasting, and touching. Such 

 areas of epithelium are called sensory epithelia. 



Our attention will be directed chiefly to the ordinary 

 covering epithelium, so far as this serves exclusively for 

 protection, or only incidentally has excretory and sensory 

 functions. 



Covering Epithelium. The covering epithelium con- 

 sists of cells which, in order to serve the function of the 

 tissue, are united by a small quantity of cementing sub- 

 stance. We speak of one-layered or of many-layered 

 epithelia, according as we find in sections running perpen- 

 dicularly to the surface one or several superimposed layers 

 (Figs. 24, 25). 



One-layered Epithelium. - - Exclusively one-layered 

 epithelia are found in all invertebrated animals and in 

 Amphioxus; in the vertebrates, on the other hand, they 

 are limited to the internal cavities of the body, and even 

 here are occasionally, as always in the skin, replaced by 

 a many layered epithelium. According to the shape of 

 the cells we distinguish cuboidal or pavement, flat, and 

 columnar epithelium. In the case of pavement epithelium 

 (Fig. 24, b] the cells are all developed about equally in all 

 directions of space, and because they have become com- 

 pressed by lateral pressure have the appearance of cubical 

 blocks or paving-stones. In columnar epithelium the long 

 axis, the distance from the central to the peripheral end of 

 the cell, is especially great (Fig. 24, c) ; finally, in flat or 

 squamous epithelium the longitudinal axis is greatly short- 

 ened (Fig. 24, a) and the separate cells have become changed 

 into thin scales. 



Flagellated and Ciliated Epithelia. Further differ- 

 ences which obtain in the three above-mentioned kinds of 

 epithelium are conditioned by the presence or absence of 



