cells derive their nourishment by the imbibition of the plasma 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