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 are immediately utilized by the muscles in heightened or fatiguing effort. All these experiments point very clearly to reservoirs of power, both physical and mental, upon which we may draw in times of stress and under emotional excitement.[12] Such emotionally induced chemical actions and reactions as have been indicated release these stored energies, render us for the time being unconscious of fatigue and even guard us against the too rapid exhaustion of vital power. Whatever heightens emotion, therefore, modifies the very chemical structure of the body.