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 show 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. 



FIG. 10. Epithelia: A, cubical; B, squamous; C, cylindrical; D, stratified cylindrical, 



ciliated at E; 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 



