22 



INTRODUCTION 



it has merely to form the thinnest of coverings, while in the case of 

 sensory structures it is modified (sensory epithelium) to receive the 

 stimuli from without. 



The usual classification of epithelia is based on the shapes and 

 arrangements of the cells. Thus in cubical epithelium (fig. lo, A) 



Fig. 13. — Epithelia: A, cubical; B, squamous; C, cylindrical; D, stratified cylindrical 

 ciliated at E; F, stratified squamous. The deeper tissues indicated by oblique lines. 



the cells are about as high as broad; in columnar (C) their height 

 exceeds their diameter; while in squamous epithelium, the cells are 

 thin and flat, covering the largest amount of surface with the least 

 amount of material (B). Sometimes the epithelial cells are in a 



Fio. 14. — Different types of glands; A, to D, tubular; E, F, acinous; A, simple; B, 



coiled; C-F, branched. 



single layer, forming simple epithelium {A, B, C); in other places 

 there are several layers — the epithelium is stratified {D, E, F). 



Frequently epithelia, usually the columnar variety, are called 

 upon to move fluids slowly; then the free surface is covered with 



