EPITHELIUM 



227 



sometimes in several superimposed layers (laminated epithelium). 

 Each cell has within it a portion of denser substance called the 

 nucleus. 



The different varieties of epithelia are classified according to 

 the form and arrangement of their cells, or according to the 

 functions which these cells perform. 



Those epithelial structures which consist of flat scale-like cells 

 are known as squamous, or tesselated epithelia. In these the 

 cells are either cemented together by some kind of intercellular 

 substance, or they are united by partial fusion, or by the inter- 



Fig. 219. Squamous Epithelium 

 of the Peritoneum. Magnified 

 400 diameters. 



Fig. 220. Overlapping Squamous 

 Cells from the Inside of the 

 Mouth. Magnified 260 diame- 

 ters. 



locking or overlapping of their edges. The epidermis (fig. 150) 

 is of this character ; and it will be remembered that, in this 

 instance, there are numerous superimposed layers of cells. The 

 epithelial lining of the heart, blood-vessels, lymphatics, and all 

 serous cavities, consists of very flat squamous cells, often arranged 

 in a single layer, and to this the special term endothelium is 

 more commonly applied. 



In the linings of the ducts and cavities of glands we meet 

 with spheroidal cells, which have often become polygonal by 

 mutual pressure ; and as these cells are the active agents by which 

 the various secretions are prepared from the blood that circulates 

 in the neighbouring capillaries, the epithelial tissue which they 

 form is often spoken of as secreting or glandular epithelium. 

 Examples of this variety may be seen in the peptic glands of the 

 stomach (fig. 101), in the tubules of the kidney, and in the 

 salivary glands. 



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