HISTOLOGY. 17 
nervous, muscular and connective tissues; the members of each group 
having certain fundamental points in common. 
Epithelial Tissues. 
Epithelia are the covering tissues, and occur on any free surface, 
internal or external, of the body. Both comparative anatomy and 
embryology shew them to be the primitive tissues, for there are many 
lower animals which are made up entirely of epithelia, while in the 
vertebrates the embryo consists solely of epithelia until the mesenchyme 
appears. Epithelia may come from any of the germ layers, in rare 
cases (synovial cavities) even from mesenchyme. 
Cc 
iS 
Fic. ro.—Epithelia: A, cubical; B, squamous; C, cylindrical; D, stratified cylindrical, 
ciliated at EZ; F, stratified squamous. 
The character of epithelium varies according to the character of 
the work it has to perform. That on the outside of the body is largely 
protective, hence it is often thickened and strengthened in different 
ways to afford resistance against external injuries. In other places, 
as glands, it has to elaborate and to allow the passage outward of 
material from within. In the body cavity and in the blood-vessels 
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. 10, A) 
the cells are about as high as broad; in columnar (C) their height 
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