46 THE TISSUES. 



portion of glands. The cells are in general spheroidal or 

 polyhedral, but vary in shape in different situations. 



Specialized epithelium : In some situations epithelium-cells 

 become modified in various ways different from those above 

 mentioned, as the pigmented epithelium of the retina and else- 

 where, and the so-called neuro-epithdmm occurring in certain 

 sensory organs, the retina, ear, tongue, and olfactory mem- 

 brane. 



Endothelium : The closed internal cavities of the body 

 the vascular channels, serous cavities (as the pleura, peri- 

 toneum, and pericardium), synovial membranes, and lymph- 

 spaces are lined with endothelium (Fig. 15). Endothelium 



FIG. 15. 



.-, 



Endothelial lining of small vein and capillary (Engelmann). 



is distinguished from epithelium mainly in being of meso- 

 blastic origin. Morphologically it is similar to simple 

 squamous epithelium, consisting of a single layer of polygonal 

 flat nucleated cells, smooth and shiny, which meet edge to 

 edge by oftentimes sinuous or serrated margins. Endothelium- 

 cells merge into the flattened type of connective-tissue cells, 

 with which they are genetically related. 



