THE NAILS AND THE HAIR. 529 



The Hair. With the exception of the palm of the hand, the sole of the foot, 

 the dorsal aspect of the third phalanges of the fingers and toes, the external 

 surface of the eyelids, the glans penis, the inner surface of the prepuce, a portion 

 of the labia, and the lips, the skin of the entire body is covered with in part large 

 and in part small hairs (lanugo) . The hair is embedded by means of its root in a 

 depression in the skin hair- follicle (Fig. 178, I) which passes obliquely through 

 the thickness of the skin, at times down into the subcutaneous connective tissue. In 

 the hair-follicle the following parts are distinguished: (i) The external fibrous layer 

 (Fig. 178, i, and Fig. 181, a), constituted of nucleated connective-tissue bun- 

 dles pursuing principally a longitudinal course, and in which the vessels and 

 nerves are distributed. (2) The inner fibrous layer (Fig. 178, 2, and Fig. 181, c), 

 which contains connective-tissue fibers pursuing especially a transverse course. 

 Toward the orifice of the hair- follicles, this layer passes over into the portion of 

 the cutis vera forming the papillae. At the bottom of the hair-follicle there is 

 formed from the inner fibrous sheath the bulbous, vascular hair-papilla compara- 

 ble to a papilla of the cutis the matrix of the hair, from which the growth of 

 the hair takes place. (3) The innermost layer of the hair-follicle proper forms, be- 

 sides, a vitreous layer (Fig. 178, 3, and Fig. 181, d). It terminates at the neck 

 of the hair-papilla; above, its prolongation passes to the junction between the 

 cutis vera and the epidermis. In addition to these layers, the hair-follicle has 

 an epithelial lining, which must be looked upon as related to the epidermis. Thus, 

 the external root-sheath (Fig. 178, 4, and Fig. 181, e), consisting of several layers 

 of soft cells of fibrillar appearance, separated by spaces, and lying in contact 

 with the vitreous layer, appears as a direct continuation of the Malpighian mucous 

 layer, and its outermost layer exhibits cells stretched laterally. At the bottom 

 of the hair-follicle it becomes narrower, and on fully developed hairs it is delimited 

 from the root of the hair itself. The horny layer of the epidermis, passing down 

 into the hair- follicle to the orifice of the sebaceous glands, retains the properties 

 that it possesses upon the external skin. Below the orifice, however, its con- 

 tinuation forms the so-called internal root-sheath. This consists (i) of the outer 

 single layer (Fig. 178, 5, and Fig. 181, /) of longitudinal, flat, homogeneous, 

 nucleated cells (Fig. 178, magnified at i) Henle's layer lying next to the outer 

 root-sheath. Internal to this, there lies (2) the layer of Huxley (Fig. 178, 6, 

 and Fig. 181, g), constituted of nucleated, rather longitudinal, polygonal cells 

 (Fig. 178, x) ; and, finally (3) the cuticula of the inner root-sheath, a layer formed 

 of cells in a manner analogous to the superficial covering of the hair, separates 

 the inner root-sheath from the hair itself. Toward the ha*lr-bulb, this triple layer 

 becomes ill defined, its cells mingling with those of the hair-bulb, without distinct 

 limitation. All hair-bulbs are provided with nerve-cells and nerve-fibers, the 

 latter having a bifid termination. 



The arrector pili muscle (Fig. 178, A) is a flat, expanded layer of unstriped 

 muscle-fibers passing from the outer fibrous layer of the bottom of the hair- 

 follicle to the upper layer of the true skin, and always subtending the obtuse 

 angle formed by the obliquely directed hair-follicle with the surface of the skin. 

 Therefore, its contraction must cause the hair to become erect (goose-flesh). As 

 a sebaceous follicle is usually present in the angle mentioned, the contraction 

 may, by pressure, cause evacuation of the secretion of the gland. In addition, 

 the muscle exerts a compressing effect upon the vessels of the papillary body. 

 Goose-flesh never occurs upon the ear, the hand, or the foot. Occasionally it 

 is only unilateral or confined to circumscribed areas. The pilomotor nerves are 

 described on p. 719. 



The arrectores pilorum receive their nerves (pilomotor nerves) from branches 

 that pass from the spinal cord and thence into the sympathetic. The irritation 

 of certain ganglia of the sympathetic has caused erection of the hair in definite 

 circumscribed areas of the skin in the ape. The muscles are stimulated by reflex 

 influences, which either extend over the entire body or remain strictly unilateral 

 or local. 



The hair, which remains firmly attached to the surface of the hair-papilla by 

 means of its swollen, lowermost portion, the head of the hair, consists of three 

 parts: (i) The medullary substance (Fig. 178, I, i), which is wanting in the 

 lanugo and in the hair of early childhood, consists of a central row of cells, from 

 two to eight in number, lying side by side (H, c). (2) Surrounding this is the 

 thicker cortical layer (h) , which consists of long, rigid, cornified hair-fiber cells 

 (H, f, f), containing the pigment-granules of the hair. Nevertheless, the hair- 

 fibers at times possess, besides, a diffuse tint. These fibers consist of minute 

 longitudinal horn-fibrils, and exhibit a longitudinal nucleus when boiled with 

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