asma of the
blood exuded into the subjacent tissue.

[Illustration: Fig. 4.--Nerve Cells from the Gray Matter of the
Cerebellum. (Magnified 260 diameters.)]

17. Varieties of Epithelium. The squamous or pavement epithelium
consists of very thin, flattened scales, usually with a small nucleus in
the center. When the nucleus has disappeared, they become mere horny
plates, easily detached. Such cells will be described as forming the outer
layer of the skin, the lining of the mouth and the lower part of the
nostrils.

The columnar epithelium consists of pear-shaped or elongated cells,
frequently as a single layer of cells on the surface of a mucous membrane,
as on the lining of the stomach and intestines, and the free surface of
the windpipe and large air-tubes.

The glandular or spheroidal epithelium is composed of round cells or
such as become angular by mutual pressure. This kind forms the lining of
glands such as the liver, pancreas, and the glands of the skin.

The ciliated epithelium is marked by the presence of very fine
hair-like processes called cilia, which develop from the free end of the
cell and exhibit a rapid whip-like movement as long as the cell is alive.
This motion is always in the same direction, and serves to carry away
mucus and even foreign particles in contact with the membrane on which
the cells are placed. This epithelium is especially common in the air
passages, where it serves to keep a free passage for the entrance and exit
of air. In other canals a similar office is filled by this kind of
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
whos