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). It appears probable that the 

 connection between neighboring cells is really much more intimate, 

 and that what appears to be a homogeneous cement-substance is in 

 reality a fluid derived from the lymph, and that the cells are con- 

 nected with each other by exceedingly delicate cytoplasmic projec- 

 tions which join each other, the tissue fluid lying in the spaces between 

 these cytoplasmic bridges. This arrangement is analogous to that 

 described in connection with the prickle-cells which have for a long 

 time been known to exist in the epidermis (see Stratified Epithelium). 



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- 

 thelium, 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- 

 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. Some- 

 times all these tissues are called epithelium and the term endothelium is discarded. 

 Other authors use endothelium to designate only the cells lining the bloodvessels 

 and lymphatics and similar cells occurring in connective tissue. The term endothe- 

 lium is retained here to distinguish cells derived from the mesoblast, from the epi- 

 thelium arising from the epiblast and hypoblast, and because there are morpho- 

 logical differences between these groups of tissues in the adult. 



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