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 98-1/2 degrees
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.

250. Material Used for Clothing. It is evident that if clothing is to
do double duty in preventing the loss of heat by radiation, and in
protecting us from the hot rays of the sun, some material must be used
that will allow the passage of heat in either direction. The ideal
clothing should be both a bad conductor and a radiator of heat. At the
same time it must not interfere with the free evaporation of the
perspiration, otherwise chills may result from the accumulation of
moisture on the surface of the body.

Wool is a bad conductor, and should be worn next the skin, both in
summer and winter, especially in variable climates. It prevents, better
than any other material, the loss of heat from the body, and allows free
ventilation and evaporation. Its fibers are so lightly woven that they
make innumerable meshes enclosing air, which is one of the best of
non