GENERAL HISTOLOGY 65 



injurious (excreta), and others, as, for example, the digestive fluids, because 

 they have to perform important functions (secrela, — must pass the surface 

 and are therefore exuded by the epithelia; these are the glandular epithclia. 

 Further, all external influences chiefly impress the surface of the body, 

 causing sensations; hence certain epithelia are of the greatest importance 

 for the reception of sensory stimuli, and serve for hearing, seeing, smelling, 

 tasting, and touching. Such areas of epithelium are called sensory 

 epithelia. 



Covering Epithelium. — The covering epithelium consists of cells 

 which are united by a small quantity of cementing substance. We speak 

 of simple or of stratified epithelia, according as we find, in sections running 

 perpendicularly to the surface, one or several superimposed layers of cells 

 (figs. 26, 27, 28). 



Simple Epitheliiim. — Only one-layered epithelia are found in all in- 

 vertebrated 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 pavement epiilielium 

 (fig. 26, b) the cells are developed about equally in all directions of space, 

 and because they have become compressed by lateral pressure have the 

 appearance of cubical blocks or paving-stones. In eoliimnar epithelium the 

 long axis, the distance from the deeper to the peripheral end of the cell, 

 is especially great (fig. 26, c) ; finally, in flat or squamous epithelium this 

 is greatly shortened (fig. 26, a) and the separate cells form thin plates. 



Flagellated and Ciliated Epithelia. — Further differences in these 

 three kinds of epithelium are caused by the presence or absence of pro- 

 cesses {cilia, or flagella) on the peripheral end of the cells. These are fine 

 threads which arise from the body of the cell, extend above the surface 

 and maintain an extremely lively motion. In flagellated epithelium 

 (fig. 26, d) each cell has only one vibratile projection, but this is strongly 

 developed; in ciliated epithelium (fig. 26, e), the surface of each cell is 

 covered with a thick forest of shorter threads moving in unison. 



Cuticle. — The majority of the one-layered epithelia are covered by a 

 cuticle, a membrane secreted by the epithelial cells which hence very fre- 

 quently shows the impression of the cells as polygonal markings. In 

 many cases thin and inconspicuous, it may in other instances become 

 thickened into a very considerable layer, much thicker than the matrix 

 layer of epithelium which secretes it. The cuticle is composed of layers 

 parallel with the surface, and forms a more effective protection for the 

 body than does epithelium; it becomes a protective armor, as shown, 

 5 



