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
120,000. The hair possesses great durability, as is evinced by its
endurance of chemical processes, and by its discovery, in the tombs of
mummies more than two thousand years old. The hair is remarkable for its
elasticity and strength. Hair is found to differ materially from horn in
its chemical composition. According to Vauquelin, its constituents are
animal matter, a greenish-black oil, a white, concrete oil, phosphate of
lime, a trace of carbonate of lime, oxide of manganese, iron, sulphur,
and silex. Red hair contains a reddish oil, a large proportion of
sulphur, and a small quantity of iron. White hair contains a white oil,
and phosphate of magnesia. It has been supposed that hair grows after
death, but this theory was probably due to the lengthening of the hair
by the absorption of moisture from the body or atmosphere.

The _nails_ constitute another class of appendages of the skin. They
consist of thin plates of horny tissue, having a root, a body, and a
free extremity. The root, as well as the lateral portion, is implanted
in the skin, and has a thin margin which is received into a groove of
the true skin. The under surface is furrowed, while the upper is
comparatively smooth. The nails grow in the same manner as the cuticle.

* * * * *

CHAPTER X.

PHYSIOLOGICAL ANATOMY.

SECRETION.

The term _Secretion_, in its broadest sense, is applied to t