GENERAL HISTOLOGY. 73 



I. Epithelial Tissues. 



Morphology of Epithelial Tissues.— On several grounds the 

 epithelia must bo considered first. They are the oldest tissues; 

 they are the first to ap2)ear in the animal kingdom, there being 

 animals which consist only of epithelia. Further, each separate 

 organism during the first stages of embryonic life consists only of 

 cpitlielia] layers, the germ-layers. With this is also connected tlie 

 fact that in epithelial tissues the cells have undergone the least 

 degree of histological change, and that the formation of plasmic 

 products is subordinated. 



Function of Epithelium. — The most important purpose of the 

 epithelium is to form a protecting and excluding covering over 

 surfaces, equally valuable whether the surfaces are external (surface 

 of the body) or caused by cavities in the interior of the body (the 

 body cavity, lumen of the gut and blood-vessels). The importance 

 of the epithelia in this respect is shown by the fact that if the 

 protecting layer be removed, inflammation arises and continues 

 until the epithelium is regenerated. Only exeeirtionally do areas 

 occur which are free from epithelium; the teeth of vertebrates, the 

 antlers of stags, are parts of the body which, on account of their 

 hardness, can exist, at least for a more or less considerable time, 

 without epithelial covering. 



Glandular and Sensory Epithelia. — By their superficial posi- 

 tion epithelia are suited for presiding over two other functions: all 

 substances which ought to be removed from the body — some 

 because they have become useless, and consequently injurious 

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

 they have to perform important functions (secreta) — must jiass the 

 surface, and are therefore separated by the epithelia; these are the 

 rjlimdular epithelia. Further, all influences of the external world 

 chiefly impress the siirface of the body, causing sensations; hence 

 also certain epithelia are of the greatest importance for the recei>- 

 tion of sensory stimuli, and serve for hearing, seeing, smelling, 

 tasting, and touching. Such areas of epithelium are called sensoi-ij 

 epilliclia. 



Covering Epithelium. — The covering epithelium consists of 

 cells which, in order to serve the function of the tissue, are united 

 bv a small quantity of cementing substance. We sjieak of simple 

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

 perpendicularly to the surface one or several superimposed layers 

 (figs. 25, 20, 27). 



