EPITHELIUM. 



39 



presents itself under four principal forms, the characters 

 of each of which are distinct enough in well-marked ex- 

 amples ; but when, as frequently happens, a continuous 

 surface possesses at different parts two or more different 

 epithelia, there is a very gradual transition from one to 

 the other. 



(i.) The first and most common variety is the squamous 

 or tesselated epithelium (figs, i and 2), which is composed 

 of flat, oval, roundish, or polygonal nucleated cells, of 

 various size, arranged in one, or in many superposed 

 layers. Arranged in several superposed layers this form of 



Fig. i.* 



Fig. 2.f 



epithelium covers the skin, where it is called the Epidermis, 

 and is spread over the mouth, pharynx, and oesophagus, 

 the conjunctiva covering the eye, the vagina, and entrance 

 of the urethra in both sexes ; while, as a single layer the 

 same kind of epithelium lines the interior of most of the 

 serous and synovial sacs, and of the heart, blood-vessels 

 and lymph-vessels. 



(2.) Another variety of epithelium named spheroidal, from 



* Fig. i. Fragment of epithelium from a serous membrane (peri- 

 toneum) ; magnified 410 diameters, a. cell ; b. nucleus ; c. nucleoli 

 (Henle). 



f Fig. 2. Epithelium scales from the inside of the mouth; magnified 

 260 diameters (Henle). 



