Most of this relatively huge size, which happens in the human alone, and not in other animals, is due to enlargement of the cortex.  Should this preponderance of the cortex over the medullary portion not occur in the human, that is, if the proportions remain like those of other animals, the brain fails to develop properly, or an entirely brainless monster is generated.  The human brain, therefore, probably owes its superiority over the animal brain, to the adrenal cortex, in development anyhow.  The growth of the brain cells, their number and complexity is thus controlled by the adrenal cortex.

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.