"tendencies to reaction", carries about the right
meaning, namely that the individual, because of his internal state,
tends towards a certain action. "Determining tendencies" (perhaps
better, "directive tendencies") is a term that has been much used in
psychology, with the meaning that the inner tendency determines or
directs behavior. Much used also are "adjustment" and "mental set",
the idea here being to liken the individual to an adjustable machine
which can be set for one or another sort of work. Often "preparation"
or "readiness for action" is the best expression.

Organic States that Influence Behavior

Beginning at the lowest of our three levels, let us observe not even
the simplest animal, but a single muscle. If we give a muscle electric
shocks as stimuli, it responds to each shock by contracting. To a weak
stimulus, the response is weak; {73} to a strong stimulus, strong. But
now let us apply a long series of equal shocks of moderate intensity,
one shock every two seconds. Then we shall get from the muscle what is
called a "fatigue curve", the response growing weaker and weaker, in
spite of the continued equality of the stimuli. How is such a thing
possible? Evidently because the inner condition of the muscle has been
altered by its long-continued activity. The muscle has become
fatigued, and physiologists, examining into the nature of this
fatigue, have found the muscle to be poisoned by "fatigue substances"
produced by its own activity. Muscular contraction depends on the
oxidation of fuel, and produces oxidized wastes, of which carbon
dioxide is the best known; and these waste products, being produced in
continued strong activity faster than the blood can carry them away,
accumulate in the muscle and partially poison it. The "organic state"
is here definitely chemical.

[Illustration: Fig. 22.--Fatigue curve of a muscle. The vertical lines
record a series of successive contractions of the muscle, and the
height of each line indicates the force of the contraction. Read from
left to right.]

This simple experiment is worth thinking over. Each muscular
contraction is a response to an electric stimulus, but the force of
the contraction is determined in part by the internal state of the
muscle. Fatigue is an _inner_ state of the muscle that _persists_ for
a time (till the blood carries away the wastes), and that
_predisposes_ the muscle _towards_ a certain kind of response, namely,
weak response. Thu