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.