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150 ANATOMY.
and is composed of two layers, called the external and the internal root-
sheaths.
The External Root-sheath is that portion of the hair-follicle which is
derived from the lowest or Malpighian stratum of the cuticle. It is
composed of several layers of polygonal cells with nuclei, as far as the
hair-bulb, where it is composed of but one row of cells, which become
continuous with the hair-bulb at its lowest portion.
The external root-sheath contains pigment-granules in the dark races,
and Langerhaus claims to have found in it nerve-filaments similar to
those found in the Malpighian layer of the skin.
The Internal Root-sheath is derived from the corneous layer of the
epidermis, but is not connected ^vith it. It commences just below the
orifices of the sebaceous glands, passing downward to the bottom of
the follicle, where it joins the layer of columnar cells covering the hair-
bulb.
This sheath is composed of two layers—an outermost, or layer of
Henle, and an innermost, or layer of Huxley. These two layers com-
mence as one just below the orifices of the sebaceous glands. As they
pass downward they again unite and form one layer of large polygonal
nucleated cells having no spaces between them, and finally become con-
tinuous with the hair-bulb.
The Outermost, or Layer of Henle, is composed of elongated, flat-
tened, non-nucleated cells, generally having spaces between them.
The Innermost, or Layer of Huxley, is composed of flattened, nucle-
ated scales two or three deep. The layer thus made forms the internal
lining of the follicle below the orifices of the sebaceous glands. The
innermost scales are imbricated, lapping over the superimposed layer of
cuticular imbricated scales of the hair, and thus serve to hold the hair
in position.
The Papilla of the Hair is a conical-shaped eminence in every respect
similar to the papilla of the skin—in fact, is a papilla of the skin car-
ried to a lower level than those entering into this structure, and being
continuous with the dermic layer of the follicle. It is highly vascular,
and is supplied with nerve-filaments. Its blood-vessels supply the
nourishment for its development and growth.
The bulbous expansion at the root of the hair is soft, and consists of
polyhedral epidermic cells united together by a cement-like substance.
These cells are continuous at the circumference of the bulb with the
outer root-sheath, from which they were originally derived. The base
of the bulb is attached to the bottom of the follicle, where the latter is
invaginated over the papilla, there being a depression in the base for
this purpose. The circumference of the base is attached to and contin-
uous with the lining membrane of the hair-follicle.
Extending over the surface of the papilla (above the basement-mem-
brane) is a special layer of short oval cells (hair-builders) which are
analogous to all similar V)uilding cells, such as those of the epidermis,
the nails, and the enamel of the teeth. The formation of these structures
is governed by one general law : that is, the special layer of cells at the
base and on the circumference of the papilla are constantly in an active
state of proliferation. New cells are thus formed which push the older