Besides its action upon the sex cells and the brain cells, the internal secretion of the adrenal cortex acts upon the pigment cells of the skin, blunting their sensitiveness to light. In degeneration of the interior of the gland, which destroys the medulla, but not the cortex, the color of the skin is left unmodified. If, however, the cortex is invaded, as happens most often in the classical tuberculosis of the adrenals which drew the attention of the Englishman Addison to them, then a darkening of the skin, which may go on to a negroid bronzing, follows. That means an increased sensitiveness of the pigment cells of the skin to light. Skin color control may therefore be looked upon as an adrenal cortex function.
So much is known about the adrenal cortex. Upon the medulla, the interior gland of the gland, there has been lavished an amount of attention beside which the cortex is to be classed as a neglected wall-flower. Nearly everything that possibly could be determined about an internal secretion has in its case been settled or plausibly guessed at. The cells manufacturing the secretion, its exact chemistry and function, its action upon the blood, the liver and spleen, the heart and lungs, the brain and nervous system, have been minutely investigated, studied and charted. Its source in the food, its fate in the body, its place in the history of the individual and the species, its importance as a weapon in the struggle for existence, and the survival of the fittest have been made the subject of an astonishing number of researches, considering the short period of scarce three decades that intensive science has centered its barrage upon it.
In the first place, the medulla contains numerous nerve cells, belonging to the vegetative, also called the sympathetic nervous system. But these nerve cells are merely minor notes of the symphony. The motif is settled by a majority of large, granular cells, which stain a distinctive yellowish-brown when the gland is fixed in a solution of bichromate of potash. All chromium salts, in fact, stain the therefore labelled chromaffin cells. The characteristic staining power appears to be dependent upon, or correlated with, the presence of the internal secretion of the medulla of the adrenal, adrenalin. For the content of adrenalin, as calculated chemically, and the depth of stain as seen under the microscope, rise and fall together. Chromaffin reaction and adrenalin content go together. The poisonous skin glands of the toad have been found to give a marked chromaffin reaction, and to contain a large amount of adrenalin. Other masses of cells in the human body, especially along the course of the sympathetic nervous system, have been shown to give the reaction and to contain adrenalin.
The erratic Brown-Séquard pounded and hammered away for more than thirty years on the importance to life of the adrenal glands, since death occurred so quickly after their removal. But it was not until Schaefer, the Scotch physiologist, (who has done more than any other living man to stimulate study of the internal secretions) found that an extract of them, when injected into a vein, produced a remarkable though temporary rise of the blood pressure, that a real enthusiasm for its investigation was generated. As the upshot, a number of other significant properties besides the first of blood-pressure raising, have been put down to its credit. Chemical tests demonstrated that it originated in the medulla. The exact amount of it present in the medulla, in the blood issuing from the adrenals and in the circulation in general have been determined. The concentration in the blood is about one part in twenty million, while there is about a hundred thousand times as much stored in the gland as reserve. In infections and intoxications, after muscular exertion, and with profound emotions, there is a decrease of it in the gland and an increase in the blood. Pain and excitement, especially fear and rage, will bring about its discharge from the gland. With its entry into the blood, there is a tremendous heightening of the tone, a tensing, of the nervous system. The nerve cells become more sensitive to stimuli, more sugar is poured into the blood from the liver, more red blood corpuscles are squeezed into the circulation from the blood lakes of the liver and spleen. There is a redistribution of the whole blood mass, a good deal of it being withdrawn from the internal viscera, and hurried to the skeleton muscles and the brain. The heart beats more strongly, the eye sees more clearly, the ear hears more distinctly, and the breathing is more rapid. The temperature rises, the hair of the head and the body becomes erect, the skin gets moist and greasy. It will help a fatigued muscle to regain its normal tone. In short, it has a reinforcing action upon the nutritive properties of the blood, the tone of the muscles, and the activity of the brain and the vegetative nerves.
Chemists set themselves the task of discovering just what was the substance possessed of such extraordinary and hitherto unimagined properties. The pure adrenalin was isolated, capable of evoking all the reactions of the impure adrenal extract mixtures. The final triumph was the preparation of it artificially in the laboratory, its synthesis. When a substance can be synthesized in the chemist's laboratory, it means that its composition has become thoroughly understood. Here at last was an example of those mysterious internal secretions, the existence of which had indeed been postulated and proven, but which had never actually been inspected by the eye of mortal man. To have it in a test-tube, indeed to possess it in large quantities in bottles, to be able to manipulate and examine it without fear of the co-action of admixed impurities, to see it with the eye, and to taste it with the tongue, was truly a marvel. The miracle aroused at once scores of researches.
THE GLAND OF COMBAT AND FIGHT
Considering its effects, one is reminded at once of the similarity to the expression of a primitive emotion like anger or fear. So, by turning a relation upside down, it was argued that if artificial adrenalin could produce all these effects of an emotion like fear, the emotion itself should produce an increase of the natural adrenalin in the blood. This was found to be the case. Cannon of Harvard has built up an entire theory of the adrenal as the gland of emergencies upon the basis of these effects. In the facing of crises the adrenal functions as the gland of combat. And indeed, as I have mentioned, the more combative and pugnacious an animal, the more adrenal it has, while the timid and meek and weak have less.
The Glands of Combat, the glands of emergency energy, the glands of preparedness,—such are the adrenal glands when viewed from the adrenalin standpoint. A picture of its activity in the evolutionary scheme of struggle and survival is something like the following: meeting an enemy, the animal is put in danger. It must fight or flee for its life. In either case, certain conditions must be fulfilled, if the body of the animal endangered is to be saved. To prevent injury to itself, and to do as much injury as possible to the foe—that becomes its immediate urge and necessity. Of the two animals, if in one the heart should begin to beat more strongly, the blood pressure to rise, the blood to flow more rapidly through the attacking instruments, the muscles, the teeth and claws, the brain and its eyes, while the other animal experiences none of these, the former will be the victor in fight or flight. Adrenalin may be looked upon as the invention for the mobilization at a moment's notice, or as we say, after generations of use, by instinct, of all these visceral and blood advantages in the struggle of combat or flight.
The nature of instinct, in its relation to the glands of internal secretion, is a problem for another chapter. But we may note that the James-Lange theory of an emotion regards it as a consciousness of the very changes in the organism adrenalin causes. Since adrenalin is the starter of the whole process, and since McDougal has defined emotion as the feeling aspect of an instinct, just as an instinct may be defined as the motor aspect of an emotion, the adrenals as emotion-genetic, and instinct-genetic, play a part in the most profound processes of the subconscious and unconscious.