Some acquaintance with the history of the normal growth of hair is necessary to its understanding. There develops during the life of the fetus within the womb a curious sort of wooly hair everywhere over the entire body (excepting the palms and soles which remain hairless throughout life), remarkably soft and fluttery—the lanugo. At about the eighth month of intra-uterine existence, a good deal of this lanugo is lost, to be replaced on the head and eyebrows by a crop of thick, coarse, pigmented real hair. So it happens that at birth the infant's hair is a queerly irregular growth, a mixture of what is left of the general lanugo development, and the localized patches of the more human hair. Until puberty this children's hair remains the same, although at times, particularly after dentition, and after infectious diseases which undoubtedly alter the relations of the internal secretions, changes of color and texture occur. Then, with sexual ripening, there appear in males the so-called terminal hairs, over the cheeks and lips and chin, and, in both sexes, in the folds under the shoulders and over the lower abdomen, the hair which might be distinguished as the sex hair in contradistinction to the juvenile hair of the head, the extremities and the back.
Now the smoothness of the face in children is connected with the activity of the thymus and pineal glands. Among individuals in whom the juvenile thymus persists after puberty, no growth of hair occurs on the face, and in precocious involution or destruction of the pineal, hair appears on the face and in other terminal regions in children of six or less, a symptom classical in the child who suffered from a tumor of the pineal, and discussed immortality with his physicians. It is probable that these thymus and pineal effects are indirect through their action upon the sex glands. For in the types with persistent juvenile thymus there occurs a maldevelopment of the sex glands, while in those with early pineal recession the sex glands bloom simultanously with the appearance of adolescent hair and mental traits. The hastening of sexual hair by tumors of the adrenal gland may also be put down to a release from restraint of the interstitial sex cells.
There are certain spheres in the hair geography of the body, over which particular glands may be said to rule or to possess a mandate. The hair of the head seems to be primarily under the control of the thyroid. Thus in cretins reconstructed by thyroid feeding, the straight, rather animal hair becomes lustrous and fine, silken and curly. In the thyroid deficiency of adults, a prominent phenomenon often is the falling out of the hair in handfuls. Baldness is frequently associated with a progressive decrease of the concentration of thyroid in the blood. At the same time, there tends to be a thinning of the eyebrows, especially of the outer third.
The hair of the face in males, and the other terminal hairs in both males and females, is regulated by the sex glands primarily. In the female, the ovary, that is to say, the interstitial cells of the ovary, inhibit the growth of hair upon the face. In destructive disease of the ovaries, as well as in other affections of it, hair in the form of moustache, beard and whiskers may appear in female. That is why in women after the grand sex change of life, the menopause, hair often grows in the typically male regions because of loss of the inhibiting influence of the ovarian internal secretion upon them. After castration of the ovaries, the same may result. Removal of the male sex glands, or disturbances of them, will interfere with the proper development of the normal facial hair. Of the hair of the chest, the abdomen and the back, the adrenals seem to be the controllers. Adrenal types have hairy chests in males, and hair on the back in females. They have also a good deal of hair upon the abdomen. The hair on the extremities varies a good deal with the pituitary. People with hair upon hands, arms and legs, alone, are generally pituitary, or have a striking pituitary streak in their make-up.
When the adrenals increase in size in childhood, a remarkable triad follows—general hairiness, adiposity and sexual precocity. One fact should be noted. When the adrenals evoke precocity, and an early awakening of the secondary sex characteristics, it is a masculine precocity, and an approximation to the masculine even in females. There is a definite trend toward an increase of the male in the individual's composition at the expense of the female. We shall have to consider this in greater detail when we analyze the internal secretion basis of masculinity and femininity. In general, the degree of general hairiness is an index to the amount of adrenal influence upon the organism. All the endocrines which affect the hair growth also act upon the sebaceous glands which oil the skin.
THE EYES
Eyes present clues to internal secretion constitutions dependent upon influences of architecture and function. The thyroid eye is typical. It is large, brilliant and protruding. The individual is "pop-eyed." On the other hand, subthyroidized eyes tend to be sunken and lustreless. The eyes of a pituitary type are either set markedly apart, or close together, with the hair at the root of the nose so prominent as to constitute a separate bridge known as the nasal brow. The size of the pupil, and its humidity, which have so much to do with the expression of the eye, vary directly with the activities of the driving and checking divisions of the vegetative system, and are a pretty good index as to which, at the time of observation, is predominant. When the check system is in control, the pupils are large and dilated. When its antagonist and rival, the drive system, is on top, the pupils are small and contracted. The reactions of the pupils when charged by strong emotion, like fear or anger, likewise turn upon the status of check or drive internal secretions in the economy of the organism at the time the exciting agent presents itself.
MUSCLES
It would seem, at first sight, that organs like muscles, mechanical instruments for the manipulation of the organism in space, would be more or less independent of the subtler processes of internal chemistry of the blood and tissues. But no assumption would be more beside the mark. Just as much as the bones and viscera, the teeth and the hair, they show grossly how they are being influenced by all the endocrine glands. So thyroid types generally have a skeleton sparsely covered with a muscular mantle. Pituitary types have large well-developed muscles. The pineal gland has some definite relation to muscle chemistry not yet probed. Thus, it has been shown that when the pineal has been completely destroyed prematurely by lime deposits in it, there is concomitant a wasting of muscles in places. This waste is sometimes replaced by fat. Pictures and images in wood and stone of these muscle freaks dating from the fifteenth, sixteenth, and seventeenth century are in existence. Then there is the extraordinary fatigability of the muscles which occurs in the thymus types, who nevertheless have large well-rounded muscles, a paradox of contradiction between anatomy and physiology. Such a type, for instance, may be picked out by a football coach for an important position in a line-up, simply on the tremendous impressiveness of the muscle make-up, only to see him bowled over and out in the first scrimmage. The tone of muscles, the quality of resisting firmness or yielding softness, is essentially determined by the adrenal glands, especially in time of stress and strain.
Brown-Séquard was the first to show that extracts of sex glands could increase the capacity for muscular work. Whether this was a direct effect upon the muscles, or indirect through the nerves or other endocrines, no one can say. Certainly the carriage of an individual, outer symptom of the inner tonus among his muscles and tendons, may be said to be as distinctively an endocrine affair as the color of his skin. And like its variations, variations of their tone, development, reactivity, fatigability, and endurance may be traced to corresponding states of overaction, or underaction, and odd combinations of the different hormones. Much remains to be learned about them and the manner of their control. Such an affliction as flatfoot, dependent upon a laxity of the ligaments in one who seems perfectly healthy and strong, may lead the analyst back to a thymus-centered personality. That is but one example.