_Hairs_ are horny appendages of the skin, and, with the exception of the
hands, the soles of the feet, the backs of the fingers and toes, between
the last joint and the nail, and the upper eyelids, are distributed more
or less abundantly over every part of the surface of the body. Over the
greater part of the surface the hairs are very minute, and in some
places are not actually apparent above the level of the skin; but the
hair of the head, when permitted to reach its full growth, attains a
length of from twenty inches to a yard, and, in rare instances, even six
feet. A hair may be divided into a middle portion, or _shaft_, and two
extremities; a peripheral extremity, called the _point;_ and a central
extremity, inclosed within the hair sac, or follicle, termed the _root_.
The root is somewhat greater in diameter than the shaft, and cylindrical
in form, while its lower part expands into an oval mass, called the
_bulb_. The shaft of the hair is not often perfectly cylindrical, but is
more or less flattened, which circumstance gives rise to waving and
curling hair; and, when the flattening is spiral in direction, the
curling will be very great. A hair is composed of three different layers
of cell-tissues: a loose, cellulated substance, which occupies its
center, and constitutes the _medulla_, or pith; the fibrous tissue,
which incloses the medulla, and forms the chief bulk of the hair; and a
thin layer, which envelops this fibrous structure, and forms the smooth
surface of the hair. The medulla is absent in the downy hairs, but in
the coarser class it is always present, especially in white hair. The
color of hair is due partly to the granules and partly to an
inter-granular substance, which occupies the interstices of the granules
and the fibers. The quantity of hair varies according to the proximity
and condition of the follicles. The average number of hairs of the head
may be stated at 1,000 in a superficial square inch; and, as the surface
of the scalp has an area of about one hundred and twenty superficial
square inches, the average number of hairs on the entire head is