Canities

CANITIES (Latin: grayness, gray-white), graying of hair (synonyms: leucotrichia, poliosis, trichonosis cana, achromia pilorum, achromatosis pilorum). It is more correct to speak of the colorlessness of hair, since, along with the acquired form, there exists a congenital form of hair pigment deficiency. Both forms can manifest as diffuse or partial colorlessness of the hair. The latter, so-called poliosis circumscripta, occurring much less frequently than the general form, manifests as separate tufts of unpigmented hair among normally pigmented hair. Acquired graying in old age (canities senilis) is a physiological phenomenon. The time of its onset varies depending on race, heredity, the state of the endocrine organs, and the way and conditions of life. Hair on the temples turns gray first; eyebrows and eyelashes turn gray last. Graying usually begins at the root of the hair. Graying that appears frequently in young people is considered a pathological condition associated with a special constitutional predisposition (canities praesenilis, canities praematura). The main role in these cases is played by heredity, diseases of the central and peripheral nervous system, endocrine organs, infectious diseases, etc. Thus, in cases of neuralgia, limited graying has been observed in the area innervated by the affected nerve; hemicanities has been observed after hemiplegia; premature graying has been observed in epilepsy, tabes, dementia praecox, brain tumors, Basedow's disease, myxedema, diabetes, and acromegaly. Koster, by removing the superior cervical ganglion in black cats, observed the growth of white or light-gray hair on the corresponding half of the head. Steinach observed the growth of new black hair in rejuvenated old rats. Some skin diseases sometimes entail general or partial graying (alopecia areata, erysipelas). The possibility of sudden graying of hair should apparently be called into question. Graying is a slow process, the mechanism of which has not yet been finally clarified. According to some authors (Kaposi and others), it depends not on the disappearance of pigment in an already mature hair, but on the fact that the newly growing part of the hair is poorer in pigment or does not contain it at all. In the opinion of others (Landois), graying is caused, in addition to a lack of pigment, by the accumulation of air bubbles in the cortical layer of the hair, which, by reflecting light, enhance the silvery sheen of the hair. As a rule, gray hair cannot become normally colored again; only in some diseases (erysipelas, alopecia areata, migraine attacks, etc.) is temporary graying sometimes observed, which ends with the growth of pigmented hair. There is no treatment. For cosmetic purposes, hair dyeing is used. However, almost all advertised products contain toxic substances (paraphenylenediamine, etc.) and can cause dermatitis and severe general poisoning. Harmless agents are vegetable dyes—Lawsonia inermis (henna, or the Syrian shrub) and rhubarb.