e may be only secondary, induced by a
pain springing from quite another source than retarded digestion.

Professor Cannon's experiments are most interesting as he traces the
variations of the flow of adrenal secretion induced by emotion and then
retraces the effect of the chemical changes so produced upon bodily and
mental states. The secretion of adrenin[10] is greatly increased by pain
or excitement. The percentage of blood sugar is also greatly increased
by the same causes. The heaviness of fatigue is due, as we know, to
poisonous uneliminated by-products resulting from long continued or
over-taxing exertion of any sort. Under the influence of fatigue the
power of the muscles to respond to any kind of stimulus is greatly
reduced. (It is interesting to note, however, that muscular fibre
detached from the living organism and mechanically stretched and relaxed
shows after a period the same decrease in contractability under
stimulation.) On the other hand any increase in adrenal secretion
results in renewed sensitiveness to stimulation, that is by an increased
power of the muscle to respond. Falling blood pressures diminish
proportionately the power of muscular response. Rising blood pressure is
effective "in largely restoring in fatigued structures their normal
irritability" and an increase of adrenin seems to raise blood pressure
by driving the blood from the interior regions of the body "into the
skeleton muscles which have to meet, by extra action, the urgent demands
of struggle or escape."

[Footnote 10: I follow Cannon in the form of this word.]

Adrenin is of real use in counteracting the effects of fatigue or in
enabling the body to respond to some unusual call for effort. The
coagulation of the blood is also affected by the same agent, that is, it
coagulates very much more rapidly.[11] Coagulation is also hastened by
heightened emotion; a wound does not bleed so freely when the wounded
one is angry or excited. A soldier, then, in the stress of combat is not
only rendered insensible to fatigue and capable of abnormal activity,
but his wounds are really not so dangerous as they would otherwise be.
There are here suggestions of elemental conditions having to do with
struggle and survival, conditions which play their very great part in
the contests of life.

[Footnote 11: Cannon thinks, however, that this effect is produced
indirectly.]

Emotions set free, as has been said, larger percentages of sugar which
ar