the follicle determines their cylindrical growth, the shaft
of the hair. So closely are these cells welded to form the cylinder, that
even under a microscope the hair presents only a fibrous appearance,
except in the center, where the cells are larger, forming the
medulla, or pith (Fig. 106).

The medulla of the hair contains the pigment granules or coloring matter,
which may be of any shade between a light yellow and an intense black. It
is this that gives the great variety in color. Generally with old people
the pigment is absent, the cells being occupied by air; hence the hair
becomes gray or white. The thin, flat scales on the surface of the hair
overlap like shingles. Connected with the hair-follicles are small bundles
of muscular fibers, which run obliquely in the skin and which, on
shortening, may cause the hairs to become more upright, and thus are made
to "stand on end." The bristling back of an angry cat furnishes a familiar
illustration of this muscular action.

[Illustration: Fig. 100.--Hair and Hair-Follicle.

A, root of hair;
B, bulb of the hair;
C, internal root sheath;
D, external root sheath;
E, external membrane of follicle;
F, muscular fibers attached to the follicle;
H, compound sebaceous gland with its duct;
K, L, simple sebaceous gland;
M, opening of the hair-follicle.
]

Opening into each hair-follicle are usually one or more sebaceous, or
oil, glands. These consist of groups of minute pouches lined with
cells producing an oily material which serves to oil the hair and keep the
skin moist and pliant.

238. The Nails. The nails are also formed of epidermis cells
which have undergone compression, much like those forming the shaft of a
hair. In other words, a nail is simply a thick layer of horny scales built
from the outer part of the scarf skin. The nail lies upon very fine and
closely set papillae, forming its matrix, or bed. It is covered at its
base with a fold of the true skin, called its root, from beneath
which it seems to grow.

The growth of the nail, like that of the hair and the outer skin, is
effected by the production of new cells at the root and under surface. The
growth of each hair is limited; in time it falls out and is replaced by a
new one. But the nail is kept of proper size simply by the removal of its
free edge.

239. The Sweat Glands. Deep in the substance of the true skin, or in
the fatty tissue beneath it, are the sweat glands. Each gland
con