A, membrane of the hair-follicle, cells with nuclei and pigmentary granules; B, external lining of the root sheath; C, internal lining of the root sheath; D, cortical or fibrous portion of the hair shaft; E, medullary portion (pith) of shaft; F, hair-bulb, showing its development from cells from A. The golden rule of all bathing is that it must never be followed by a chill. If even a chilliness occur after bathing, it must immediately be broken up by some appropriate methods, as lively exercise, brisk friction, hot drinks, and the application of heat. Swimming is a most valuable accomplishment, combining bathing and exercise. Bathing of the feet should never be neglected. Cleanliness of the hair is also another matter requiring strict attention, especially in children. 248. Care of the Hair and Nails. The hair brush should not be too stiff, as this increases the tendency towards scurfiness of the head. If, however, the hair is brushed too long or too hard, the scalp is greatly stimulated, and an increased production of scurf may result. If the head be washed too often with soap its natural secretion is checked, and the scalp becomes dry and scaly. The various hair pomades are as a rule undesirable and unnecessary. The nails should be kept in proper condition, else they are not only unsightly, but may serve as carriers of germs of disease. The nails are often injured by too much interference, and should never be trimmed to the quick. The upper surfaces should on no account be scraped. The nail-brush is sufficient to cleanse them without impairing their smooth and polished surfaces. Illustration: Fig. 107.—Longitudinal Section of a Finger-Nail. A, last phalanx of the fingers; B, true skin on the dorsal surface of the finger; C, epidermis; D, true skin; E, bed of the nail; F, superficial layer of the nail; H, true skin of the pulp of the finger. 249. Use of Clothing. The chief use of clothing, from a hygienic point of view, is to assist in keeping the body at a uniform temperature. It also serves for protection against injury, and for personal adornment. The heat of the body, as we have learned, is normally about 98½° F. This varies but slightly in health. A rise of temperature of more than one degree is a symptom of disturbance. The normal temperature does not vary with the season. In summer it is kept down by the perspiration and its rapid evaporation. In winter it is maintained by more active oxidation, by extra clothing, and by artificial heat. The whole matter of clothing is modified to a great extent by climatic conditions and local environments,—topics which do not come within the scope of this book.