CHAPTER III. 

 THE EPITHELIAL TISSUES. 1 



I. ENDOTHELIUM. 



General Characters. (1) The cells possess thin membranous bodies, 

 except at the site of the nucleus, to enclose which the cell-body is 

 thickened. (2) The intercellular substance is minimal in amount ; 

 clear and homogeneous in character. (3) The cells are arranged, 

 edge to edge, in a single layer. The wavy or denticulate edges of 

 neighboring cells fit into each other, being separated by a mere line 

 of the intercellular substance which in this tissue has received the 

 name of " cement-substance " (Fig. 23). 



Endothelium forms a thin membranous tissue composed almost 

 exclusively of cells. It occurs in its most isolated form in the cap- 

 illary bloodvessels, the walls of which are simply tubes of endo- 

 thelinm, supported externally by the surrounding tissues and fluids 

 and internally by the enclosed blood. It also covers the tissues 

 surrounding the serous cavities of the body, where it serves both as 

 a lining to the cavities and a smooth covering to the organs, dimin- 

 ishing the friction resulting from their movements against each 

 other. It does not occur in any situation where it would be exposed 

 directly to the external world. 



The cells of endothelium vary somewhat in size and shape. They 

 may be polygonal, diamond, or stellate in form, and during life are 

 soft and extensible so that their sizes may be modified by stretching 

 or tension in one or more directions. The cell-bodies, or cytoplasm, 

 are usually clear and apparently structureless or only slightly granu- 

 lar, but occasionally some of the cells are smaller and more granular 

 than the majority. This is especially marked in the cells surround- 

 ing minute apertures that are found here and there in the endo- 



1 The term "epithelial" is used here in its most inclusive sense to designate 

 those tissues which cover surfaces, whether those surfaces are exposed to the outer 

 world, as, for example, the skin and the mucous membranes, or are wholly enclosed, 

 as are the inner surfaces of the bloodvessels, lymphatics, and serous surfaces. 



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