EPITHELIAL CELLS. 



669 



Tesselated epithe- 

 lium is the simplest 

 form, and, as its name 

 implies, resembles flags 

 of pavement, over- 

 lapping each other 

 at their edges. They 

 assume, more or less, 

 the polygonal form, 

 and their size varies in 

 the different mem- 

 branes. The cells of the 

 pericardium, or cover- 

 ing membrane of the 

 heart, are much smaller 1, 

 than those of the cover- 

 ing membrane of the 

 lungs, &c. On some 

 surfaces we have many 

 layers — in the skin, for 

 instance ; if a verti- 

 cal section of such 

 be made, and viewed 

 under the microscope, 

 it will be seen to be 

 composed of number- 

 less layers, shown in 

 fig. 304. The skin 

 taken from the sole 

 of the foot, in con- 

 sequence of the con- 

 tinued pressure there 



Fig. 302. 



Simple isolated cells containing reproductive' 

 granules. 2, Mucous membrane of stomach, 

 showing nucleated cells. 3, One of the 

 tubular follicles from a pig's stomach. 4, Sec- 

 tion of a lymphatic, magnified 50 diameters. 



Fig. 303. (,]) 



(1) a is a diagram of a portion of the involuted mucous membrane, showing 

 the continuation of its elements in the follicles and villi, with a nerve entering 

 its submucous tissue. The upper surface of one villus is seen covered with 

 cylindrical epithelium ; the other is denuded, and with the dark line of base- 

 ment membrane only running round it ; i, pavement epithelium scales, sepa- 

 rated and magnified 200 diameters ; in the centre of each is a nucleus, with a 

 smaller spot in its interior, called the nucleoli, c, pavement epithelium scales, 

 from the mucous membrane of the bronchial or air tubes of the lung ; d 

 represents another form of epithelium, termed the vibratlle or ciliated ; the 

 nuclei are visible, with cilia at their upper or free surfaces, magnified 250 

 diameters. 



