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The most interesting part of Cannon's book consists in the investigation of the effects produced by secretion of adrenin. Adrenin is a substance secreted into the blood by the adrenal glands. These are among the ductless glands, the functions of which, both in physiology and in connection with the emotions, have only come to be known during recent years.
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. Coagulation is also hastened by heightened emotion; a wound does not bleed so freely when the wounded one is angry or excited.
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 is greatly increased by pain or excitement. The percentage of blood sugar is also greatly increased by the same causes.
Cannon found that pain, fear and rage occurred in circumstances which affected the supply of adrenin, and that an artificial injection of adrenin could, for example, produce all the symptoms of fear. He studied the effects of adrenin on various parts of the body; he found that it causes the pupils to dilate, hairs to stand erect, blood vessels to be constricted, and so on.
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."
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