We must therefore look upon the organic cell as a little engine with
admirably adapted parts. Within this engine chemical activity is
excited. The fuel supplied to the engine is combined by chemical forces
with the oxygen of the air. The vigour of the oxidation is partly
dependent upon temperature, just as it is in any other oxidation
process, and is of course dependent upon the presence of fuel to be
oxidized, and air to furnish the oxygen. Unless the fuel is supplied and
the air has free access to it, the machine stops, the cell _dies_. The
energy liberated in this machine is converted into motion or some other
form. We do not indeed understand the construction of the machine well
enough to explain the exact mechanism by which this conversion takes
place, but that there is such a mechanism can not be doubted, and the
structure of the cell is certainly complex enough to give plenty of room
for it. The irritability of the cell is easily understood; for, since it
is made of very unstable chemical compounds, any slight disturbance or
stimulation on one part will tend to upset its chemical stability and
produce reaction; and this is what is meant by irritability.

Or, again, we may look upon the cell as a little chemical laboratory,
where chemical changes are constantly occurring. These changes we do not
indeed understand, but they are undoubtedly chemical changes. The result
is that some compounds are pulled to pieces and part of the fragments
liberated or excreted, while other parts are retained and built into
other more complex compounds. The compounds thus manufactured are
retained in the cell body, and it grows in bulk. This continues until
the cell becomes too big, and then it divides.

If a machine is broken it ceases to carry on its proper duties, and if
the parts are badly broken it is ruined. So with the cell. If it is
broken by any means, mechanical, thermal, or otherwise, it ceases to
run--we say it dies. It has within itself great power of repairing
injury, and therefore it does not cease to act until the injury is so
great as to be beyond repair. Thus it only stops its motion when the
machinery has become so badly injured as to be beyond hope of repair,
and hence the cell, after once ceasing its action, can never resume it
again.

There are, of course, other functions of living things besides the few
simple ones which we have considered. But these are the fundamental
ones; and if we can reduce them to an