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.