A Practical Physiology | Page 6

shape found
in the loose tissue under the skin and in other parts (called
_connective-tissue cells_); C, cell having many processes or
projections--such are found in connective tissue, D, primitive cells
composed of protoplasm with nucleus, and having no cell wall. All are
represented about 400 times their real size. ]
Some cells live a brief life of 12 to 24 hours, as is probably the case
with many of the cells lining the alimentary canal; others may live for
years, as do the cells of cartilage and bone. In fact each cell goes
through the same cycle of changes as the whole organism, though
doubtless in a much shorter time. The work of cells is of the most
varied kind, and embraces the formation of every tissue and
product,--solid, liquid, or gaseous. Thus we shall learn that the cells of
the liver form bile, those of the salivary glands and of the glands of the
stomach and pancreas form juices which aid in the digestion of food.
15. The Process of Life. All living structures are subject to constant
decay. Life is a condition of incessant changes, dependent upon two
opposite processes, repair and decay. Thus our bodies are not
composed of exactly the same particles from day to day, or even from
one moment to another, although to all appearance we remain the same
individuals. The change is so gradual, and the renewal of that which is

lost may be so exact, that no difference can be noticed except at long
intervals of time.[4] (See under "Bacteria,"

Chapter XIV.
)
The entire series of chemical changes that take place in the living body,
beginning with assimilation and ending with excretion, is included in
one word, metabolism. The process of building up living material, or
the change by which complex substances (including the living matter
itself) are built up from simpler materials, is called anabolism. The
breaking down of material into simple products, or the changes in
which complex materials (including the living substance) are broken
down into comparatively simple products, is known as katabolism. This
reduction of complex substances to simple, results in the production of
animal force and energy. Thus a complex substance, like a piece of
beef-steak, is built up of a large number of molecules which required
the expenditure of force or energy to store up. Now when this material
is reduced by the process of digestion to simpler bodies with fewer
molecules, such as carbon dioxid, urea, and water, the force stored up
in the meat as potential energy becomes manifest and is used as active
life-force known as kinetic energy.
16. Epithelium. Cells are associated and combined in many ways to
form a simple tissue. Such a simple tissue is called an epithelium or
surface-limiting tissue, and the cells are known as epithelial cells.
These are united by a very small amount of a cement substance which
belongs to the proteid class of material. The epithelial cells, from their
shape, are known as squamous, columnar, glandular, or ciliated. Again,
the cells may be arranged in only a single layer, or they may be several
layers deep. In the former case the epithelium is said to be simple; in
the latter, stratified. No blood-vessels pass into these tissues; the 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