white corpuscles, seem able to make their way actively through the
tissues, as if guided by some sort of instinct.

[Illustration: Fig. 3.--Various Forms of Cells.

A, columnar cells found lining various parts of the intestines (called
_columnar epithelium_);
B, cells of a fusiform or spindle 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