ight, however, by continued use become injurious, if it were in
all cases necessarily exerted; that is to say, if by caffein the
muscles and nerves were directly spurred on to increased activity.
This is not the case, however, and just in this lies the
peculiarity of the effect in question. The muscles and the
simultaneously-acting nerves only under the influence of caffein
respond more easily to the impulse of the will, but do not develop
spontaneous activity; that is, without the co-operation of the
will.

The character of caffein action makes plain that these food
materials do not injure the organism by their caffein content, and
do not by continued use cause any chronic form of illness.

According to Dr. Hollingworth's[285] deductions, caffein is the only
known stimulant that quickens the functions of the human body without a
subsequent period of depression. His explanation for this behavior is
that "caffein acts as a lubricator for the nervous system, having an
actual physical action whereby the nerves are enabled to do their work
more easily. Other stimulants act on the nerves themselves, causing a
waste of energy, and consequently, according to nature's law, a period
of depression follows, and the whole process tends to injure the human
machine." In not a single instance during his experiments at Columbia
University did depression follow the use of caffein.

Of course, caffein, like any other alkaloid, if used to excess will
prove harmful, due to the over-stimulation induced by it. However, taken
in moderate quantities, as in coffee and tea by normal persons, the
conclusions of Hirsch[286] may be taken as correct, namely: caffein is a
mild stimulant, without direct effect on the muscles, the effect
resulting from its own destruction and being temporary and transitory;
it is not a depressant either initially or eventually; and is not
habit-forming but a true stimulant, as distinguished from sedatives and
habit-forming drugs.

_Caffein and Mental and Motor Efficiency_

The literature on the influence of caffein on fatigue has been
summarized, and the older experiments clearly pointed out, by
Rivers[287]. A summary of the most important researches which have had
as their object the determination of the influence of caffein on mental
and motor processes has been made by Hollingworth[288], from whose
monograph much of the following material has been taken.