The composition of the skin of a very black negro may be:
Black 68% Red 26%
Yellow 2% White 7%
Now the fact that in Addison's disease in which the adrenals are destroyed there occurs a coincident increase in the black in the skin, and other evidence pointing to adrenal implication in dark complexioned white people, as well as in those possessing pigmented spots, seems to indicate the adrenals as controllers of the black and white factors. Davenport has concluded that there are two double factors for black pigmentation in the full-blooded negro which are separately inheritable. The determinants of the red and yellow have still to be worked out.
The moistness of the skin, as perspiration, depends upon the number and activity of the sweat glands. It varies with the water content of the body, the state of the vegetative nervous system, and the body temperature. Thus the skin of the hyperthyroid and the subadrenal is soft and moist, because of their antagonistic effects upon the sympathetic system. The subthyroid and the hyperadrenal have dry and harsh skins for the same reason, if no other glands intervene. However, in both of the latter, if there is a persistent thymus, the skin will retain the bland quality of adolescence.
There is a curious variation among the different internal secretion types in the reaction of the skin to stroking. When the skin, especially the skin over the shoulders, the breasts and the abdomen, is stroked with some blunt object, the blood vessels react either by a greater filling up or emptying of themselves. The latter occurs most regularly in the subadrenal types, the former in the hyperthyroid. Both forms of reaction run parallel to the different check or drive effects of the vegetative apparatus. With too much drive, that is, too much thyroid, there is the flushing reaction; with too little check, that is, with too little adrenal, there is the whitening. These differences probably explain the emotional reactions of the face. In anger, for example, some people become a dead white, others a fiery red. Whether one will do one or the other may depend upon the relative predominance of the thyroid or of adrenal in the individual.
In the distribution of fat beneath and throughout the skin all of the endocrine glands appear to have a voice. The typically hyperthyroid and hyperpituitary individuals tend to be thin, as well also as those who have well-functioning or excessively functional interstitial cells. In all of these the administration of the respective internal secretions increases the burning up of material in the body, and all of them have a higher rate of tissue combustion than their confreres, with a subthyroid or subpituitary keynote in their cell chemistry, or with insufficient interstitial cell action. Generally the latter have a very dry skin, the former a moist skin. With delayed involution of the pineal, obesity results.
The elasticity of the skin is another quality that varies with the concentration in the blood of the internal secretions. Elasticity of the skin, its recoil upon being stretched like a rubber band, may be taken as a measure of the activity of all the endocrine glands. For, as can be noticed especially upon the back of the hand, the older a man grows, the less elastic becomes the skin. In older people, raising the skin upon the back of the hand will cause it to stand up as a ridge for a few seconds and then slowly to return to the level of the surrounding skin. Whereas in a youthful person it will quickly snap back into place. This quality of elasticity of the skin is due to the presence in it of the so-called yellow elastic fibres, cell products, with a resilience greater than anything devised by man. The preservation of the resilience is a function of the internal secretions. Thus, after loss of the thyroid, the ridging effect characteristic of senility can be produced in one young as measured by his years. It has been said that a man is as old as his arteries, and also that as he is as old as his skin. It might better be said that he is as old as his elastic tissue, young when he is rich in it, old when poor and losing it. And as elastic tissue and internal secretions stand in the relation of created and creators, or at least preserved and preservers, a man may be said to be as old, that is as young, fresh and active as his ductless glands.
THE HAIR
There is no characteristic of the human body, except perhaps the teeth, more influenced in its quality, texture, amount and distribution than the hair. And again, each of the glands of internal secretion plays a part, but most importantly the thyroid, the suprarenal cortex and the interstitial sex glands. All contribute their specific effect, and the blend, the sum of the additions and subtractions constituting their influences, appears as a specific trait of the individual, a trait so significant as to be used by the professionals absorbed in the study of man, the anthropologists, as a criterion of racial classifications.