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 pl