EPITHELIAL TISSUES 1? 



single layer of cells placed side by side, or the membrane may be 

 several cells in thickness, in which case those cells upon the sur- 

 face usually differ in structure and appearance from those of the 

 deeper layers. When but a single layer of cells is present, the 

 tissue may be called single or pavement epithelium ; when com- 

 posed of several cell layers, the epithelium may be said to be 

 complex, compound, or stratified. 



All epithelial membranes rest upon a subjacent supporting 

 connective tissue, the tunica propria, upon the free surface of 

 which a distinct basement membrane or membrana propria is 

 usually developed. This basement membrane is formed by fine 

 reticular connective tissue fibers, many of which are elastic, and 

 are flattened connective tissue cells. 



In the stratified epithelial tissues the superficial cells those 

 nearest the free surface usually arise by cell division in the deeper 

 layers, and, if they become detached by abrasion, disintegration, 

 or by other physiological or pathological processes, they may be 

 replaced by cell reproduction occurring in the deeper layers. When 

 but a single layer of cells is present, as in the simple epithelial tis- 

 sues, abrasion or disintegration of the cells over large areas will 

 obviously become more difficult of replacement by cell division. 

 Hence it is that repair of extensively destructive pathological con- 

 ditions involving such epithelial tissues becomes exceedingly diffi- 

 cult, and often impossible. 



Each epithelial cell is to some extent a secreting cell. Some- 

 times this is its chief function, as is the case with goblet cells, 

 which might well be called " unicellular glands," and which 

 secrete an abundant sup- 

 ply of m ucus. The same 

 is true of those cells 

 which form the psuvn- 



chyma of secreting ESffii ^ Wk l[ flUf u <L/'1jfr)rQ -2 

 glands, such as the sali- 

 vary glands, kidney, and 

 liver. In many epithe- sc 



,. , . . FIG. 15. SECRETORY CAPILLARIES FROM A FUNDUS 



lia, however, secretion is GLAND OF HUMAN STOMACH> 



a subsidiary function. ^ lumen of the ffland . ^ seeretin g ce ii s ; sc , secretory 



The cells of an epi- capillaries. (After Sobotta.) 



thelial membrane are 



maintained in proper juxtaposition, one to another, by means of a 



delicate cement substance which is apparently a product of their 



