EPITHELIAL TISSUE. 



HISTOLOGY. 



Histology deals with the minute structure, (and especially 

 with the charactersj of the cells and the tissues arising from 

 them. In the adult occur cells varying in shape and size, and 

 adapted for various functions ; those cells which are alike 

 grouped together into tissues. A tissue, then, may be defined 

 as an aggregate of similar cells, together with a varying amount 

 of intercellular substances, usually produced by the cells them- 

 selves. The cells themselves are the living portions of the 

 tissue ; the intercellular substance, by its amount and character, 

 being directly influential in determining the nature of the 

 tissue. Tissues may be solid or fluid ; may form thin sheets 

 or thick masses. All tissues can be grouped under four heads,, 

 — epithelial, nervous, muscular, and connective. 



Epithelial tissues are the primitive tissues. An epithelium 

 is a layer of cells covering any free surface on or in the body. 

 Thus in the gastrula both ectoderm and entoderm are epithelia, 

 since the one covers the outside, while the other lines the 

 archenteron. The mesothelium is also epithelial ^ since it lines 

 the cavity of the coelom. Epithelia are classified according to^ 

 shape, arrangement, or 

 characterof the cells. In 

 cubical or columnar epi- 

 thelium the cells have 

 shapes corresponding 

 to their names ; in 

 pavement epithelium the 

 cells are greatly flat- 

 tened, so that each 

 one, while very thin, covers, comparatively, a large amount 

 of surface. EpitheUa are simple when the cells are arranged 

 in a single layer ; stratified when they form several layers. 

 In some cases the epithelial cells may bear on their free sur- 



1 The term epithelium is sometimes restricted to those layers on the outer surface of 

 the body, or, lilce the epithelium of the lungs and stomach, connected with the exterior. The 

 similar cells in the closed cavities, like the body cavity or the blood-vessels, are then called 

 endothelia. The distinction is of little importance. 



Fig. 8. Epithelia; A, columnar; B, pave- 

 ment, in perspective ; C, cubical ; D, stratified. 



