at these processes of adaptation and heredity
in the human body have been going on for _countless millions of years_,
you can readily understand how it is that the human body of today is
made up of more than thirty different kinds of cells, each having its
special function.

[Sidenote: Functions of Different Human Cells]

We have muscle cells, with long, thin bodies like pea-pods, who devote
their lives to the business of contraction; thin, hair-like connective
tissue cells, whose office is to form a tough tissue for binding the
parts of the body together; bone cells, a trades-union of masons, whose
life work it is to select and assimilate salts of lime for the upkeep of
the joints and framework; hair, skin, and nail cells, in various shapes
and sizes, all devoting themselves to the protection and ornamentation
of the body; gland cells, who give their lives, a force of trained
chemists, to the abstraction from the blood of those substances that are
needed for digestion; blood cells, crowding their way through the
arteries, some making regular deliveries of provisions to the other
tenants, some soldierly fellows patrolling their beats to repel invading
disease germs, some serving as humble scavengers; liver cells engaged in
the menial service of living off the waste of other organs and at the
same time converting it into such fluids as are required for digestion;
windpipe and lung cells, whose heads are covered with stiff hairs, which
the cell throughout its life waves incessantly to and fro; and, lastly,
and most important and of greatest interest to us, brain and nerve
cells, the brain cells constituting altogether the organ of objective
intelligence, the instrument through which we are conscious of the
external world, and the nerve cells serving as a living telegraph to
relay information, from one part of the body to another, with the
"swiftness of thought."

Says one writer, referring to the cells of the inner or true skin: "As
we look at them arranged there like a row of bricks, let us remember two
things: first, that this row is actually in our skin at this moment;
and, secondly, that each cell is a living being--it is born, grows,
lives, breathes, eats, works, decays and dies. A gay time of it these
youngsters have on the very banks of a stream that is bringing down to
them every minute stores of fresh air in the round, red corpuscles of
the blood, and a constant stream of suitable food in the serum. But it
is