When adrenalin is injected under the skin in sufficient quantity, it will produce paleness, trembling, erection of the hair, twitching of the limbs, quick or gasping breathing, twitching of the lips—­all the classic manifestations of fear.  These are the immediate effects of fear because they are the immediate effects of excess adrenalin in the blood upon the vegetative viscera and the muscles.  The perception by associative memory of these effects of adrenalin, the sensations arising from the organs affected, constitute the emotion of fear.  Flight follows by muscle prepared for flight, for the disturbance of the inter-muscular equilibrium tenses the flexor muscles, the muscles of flight, and relaxes the extensor muscles, the muscles of attack.

If, it would seem, the cortex secretion now pours into the blood, enough to more than overcome the effects of the medulla secretion, the inter-muscular equilibrium is disturbed in the opposite direction, for fight rather than flight, and anger results.  Or if the cortical secretion pours in an overwhelming amount of its secretion from the first into the blood there will be no fear, but anger immediately.  Habitually charging and fearless animals like the bison, bull, tiger, or lion have a relatively larger cortex in their adrenals.  Habitually fleeing and fearful animals, like the rabbit, have a small cortex and a wide medulla in their adrenals.  The reinforcing action of the thyroid is important.  The adrenal medulla reinforced by the thyroid makes for terror, the adrenal cortex reinforced by the thyroid makes for fury.

Some people are not easily frightened, others are more readily frightened, and still others are of an extremely fearful nature.  It depends upon the proportion of adrenal cortex to medulla secretion in them.  And their reaction to fear stimuli is a pretty good measure of the ratio.  These formulations apply more particularly to fear in general and anger in general.  But even in the least fearsome, i.e., an individual in whom cortex dominates medulla, there may be fear—­complexes, dating back to events and times when medulla overtopped cortex, especially childhood.  So in the coolest people, certain persons, objects, episodes, may send a wave along an old line of nerve cells and paths which lead to the adrenal medulla, and so flood him with fear, terror or even panic before his usual cortex response occurs.  Impressions during the early years of childhood, probing of the unconscious by various methods, have been shown to be the most potent in this respect.  Sometimes the episode goes further back than childhood, and one must assume an inherited conditioning of the vegetative and endocrine systems.  An animal leaping upon an ancestor in a forest during the night might account for the panic fear some people experience when alone in the dark, that nothing of their childhood history may account for.

In women, the adrenal medulla naturally tends to overtop the cortex, because the latter makes for masculinity.  Besides, the recurring cycle in the ovary, making the corpus luteum, evolves an additional stimulant to the medulla, through its irritating influence upon the thyroid.  Then the influence of the post-pituitary is anti-adrenal cortex.  So that, on the whole, a number of endocrines work to render woman naturally fearful, as we say.