epithelium.

18. Functions of Epithelial Tissues. The epithelial structures may be
divided, as to their functions, into two main divisions. One is chiefly
protective in character. Thus the layers of epithelium which form the
superficial layer of the skin have little beyond such an office to
discharge. The same is to a certain extent true of the epithelial cells
covering the mucous membrane of the mouth, and those lining the air
passages and air cells of the lungs.

[Illustration: Fig. 5.--Various Kinds of Epithelial Cells

A, columnar cells of intestine;
B, polyhedral cells of the conjunctiva;
C, ciliated conical cells of the trachea;
D, ciliated cell of frog's mouth;
E, inverted conical cell of trachea;
F, squamous cell of the cavity of mouth, seen from its broad surface;
G, squamous cell, seen edgeways.
]

The second great division of the epithelial tissues consists of those
whose cells are formed of highly active protoplasm, and are busily engaged
in some sort of secretion. Such are the cells of glands,--the cells of the
salivary glands, which secrete the saliva, of the gastric glands, which
secrete the gastric juice, of the intestinal glands, and the cells of the
liver and sweat glands.

19. Connective Tissue. This is the material, made up of fibers and
cells, which serves to unite and bind together the different organs and