68 



HUMAN ANATOMY. 



Squamous epithelium, when occurring as a single layer, is composed of 

 flattened polyliedral nucleated plates which, when viewed from the surface, present 

 a regular mosaic, sometimes described by the terms "pavement" or "tessellated." 

 Such arrangement of the squamous type is unusual in the human body, the lining 

 of the alveoli of the lungs, the posterior surface of the anterior capsule of the 

 crystalline lens, the membranous labyrinth, and a few other localities being the 

 chief places where a single layer of squamous cells occurs. 



The far more usual arrangement of such cells is several superimposed layers, 

 this constituting the important group of stratified squamous epithelia. When 



Fig. 8o. 



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Simple squamous epithelium from 

 anterior capsule of crystalline lens. 

 X 400. 



!©«©!i) f" 



Section of stratified squamous epithelium from anterior surface of 

 cornea. X 500. 



seen in section, the deepest cells are not scaly, biit irregularly columnar, resting 

 upon the basement membrane by slightly expanded bases. The surface of the un- 

 derlying connective tissue supporting this variety of epithelium is beset with minute 

 elevations or papillae, which serve as advantageous positions for the terminations of 

 the blood-vessels, as well as specialized nerve-endings. Owing to the more favored 

 nutrition of the deepest stratum, the cells next the connective tissue exhibit the 

 greatest vitality, and often are the exclusive source of the new elements necessary 



Fig. 82. 



Isolated surface cells from epithe- 

 lium lining the mouth. X 350. 



Fig. 83. 



^ \Sj^/lir /' .' f- " A^^^' ^ 



if. 



fr^ 



K'^i 



y '.^ 







Epithelial cells from epider- 

 mis, showing intercellular bridges. 

 X675. 



to replace the old and effete cells which are continually being removed at the free 

 surface ; this loss is due not only to mechanical abrasion, but also to the displace- 

 ment of the superficial elements by the new cells formed within the deeper layers. 



Passing from the basement membrane towards the free surface, the form of the 

 cells undergoes a radical change. The columnar type belongs to the deepest layer 

 alone ; the superimposed cells assume irregularly polyhedral forms and then gradu- 

 ally expand in the direction parallel to the free surface to become, finally, converted 

 into the large, thin scales so characteristic of the superficial layers of stratified 

 epithelium. The position of the nucleus also varies with the situation of the cells, 



