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as well as toward the hair shaft. Below, in the region of the thickened hair bulb, the root-sheaths begin to lessen in thickness, their layers becoming more and more indistinct toward the base of the hair papilla. Finally, all differentiation is lost in the region where they encircle the neck of the papilla. Toward the shaft of the hair, the root-sheath also undergoes changes. In the region into which the sebaceous glands empty, the inner root-sheath disappears, while the outer becomes continuous with the stratum germinativum of the epidermis; the outer layers of the latter-the stratum granulosum, stratum lucidum, and stratum corneum-push downward between the outer root-sheath and the hair to the openings of the sebaceous glands.

Regarding the growth of the hair, two theories are prevalent.

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Fig. 296.-Cross-section of human hair with its follicle; X about 300.

The one theory assumes that the elements destined to form the epithelial root-sheaths are derived from the epidermis by a constant process of invagination. The component parts of the hair would thus be continuous with the layers of the root-sheaths, and consequently with those of the epidermis. Thus the basal cells of the external root-sheath would extend over the papilla, and be continuous with the cells of the medulla of the hair (these relations are especially well defined in the rabbit), and the stratum spinosum (middle layer of stratum Malpighii) of the outer root-sheath would be continuous with the cortical substance of the hair. According to this theory also, the layer of Henle would correspond to the stratum lucidum of the epidermis, and at the base of the hair

would become its cuticle, while the layer of Huxley would form the cuticle of the inner root-sheath (Mertsching). The other theory assumes that the hair is derived from a matrix, consisting of proliferating cells situated on the surface of the papilla. From these germinal cells would be derived the medullary and cortical substance of the hair, its cuticle, and the inner root-sheath (Unna).

The shedding of hair is common to all mammalia, a phenomenon occurring periodically in the majority of species. In man the process is continuous. Microscopic examination shows that the hair destined to be shed becomes loosened from its papilla by a cornification of the cells of its bulb. At the same time the cortical portion of the hair bulb breaks up into a brush-like mass. Such hairs are called bulb hairs, in contradistinction to papillary hairs. In the

Nerve-
plexus of
Bonnet.

Inner root-
sheath.
Outer root-
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region of the former papilla there arises, by a proliferation of the external root-sheath, a bud which grows downward, from which a new hair with its sheaths and connective-tissue papilla is developed. The result is that the developing new hair gradually pushes the old hair outward until the latter finally drops out. The exact details of this process have given rise to considerable discussion (vid. Götte and Stieda, 87).

Adjacent to the hair follicles are bundles of smooth musclefibers, known as the arrectores pilorum. They originate from the papillary layer of the corium and extend to the lower part of the connective-tissue sheath of the hair follicles. In their course they not infrequently encircle the sebaceous glands of the follicle. Since the hair follicles have a direction oblique to the skin surface, forming with it an acute and an obtuse angle, and since the muscle is situated within the obtuse angle, its function may easily be conceived as being that of an erector of the hair. The hair papillæ are very vascular.

[graphic]

Fig. 297-Longitudinal section through hair and hair follicle of cat; X 160. Technic No. 291.

The nerve-fibers of the hair follicles have recently been studied by a number of investigators, with both the Golgi and the methyleneblue methods. It has been shown that the hair follicles receive their nerve supply from the nerve-fibers which terminate in the immediate skin area. Each follicle receives, as a rule, only one nerve-fiber, which reaches the follicle a short distance below the mouth of the sebaceous gland. The nerve-fiber, on reaching the

follicle, loses its medullary sheath and divides into two branches, which surround it in the form of a ring. From this complete or partial ring of nerve-fibers numerous varicose fibers proceed upward parallel to the axis of the follicle for a distance about equal to the cross-diameter of the follicle, to terminate, it would seem, largely outside of the glassy layer (Retzius). In certain mammalia the nerve-fibers end in tactile discs, found in the external root-sheaths of the so-called tactile hairs. The muscles of the hairs receive their innervation through the neuraxes of sympathetic neurones, which reach the periphery from the chain ganglia through the gray rami communicantes. These nerves are known as pilomotor nerves, and when stimulated, excite contraction of the erector muscles of the hairs, causing these to assume an upright position and producing the appearance termed goose skin, or cutis anserina. Langley and Sherrington have made interesting and important observations on the course and distribution of the pilomotor nerves.

C. THE NAILS.

The

The nails are a peculiar modification of the epidermis. external arched portion is called the body of the nail; that area upon

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Fig. 298.-Longitudinal section through human nail and its nail groove
(sulcus); X 34.

which the latter rests, the nail bed, or matrix; and the two folds of epidermis which overlap the nail, the nail walls. The groove which exists between the nail wall and nail bed is known as the sulcus of the matrix, and the proximal imbedded portion of the nail as the nail root, since all growth of the nail takes place in this region. The nail bed consists of the corium, which is here made up of a dense felt-work of coarse connective-tissue fibers. Immediately beneath the nail the corium is raised into a number of more or less symmetric longitudinal ridges, which again become continuous with the connective-tissue papillæ of the skin at the line where the nail projects beyond its bed.

The depressions between the ridges are occupied by epidermal cells, which also form a thin covering over the ridges themselves.

These cells correspond here to the basilar layer of the stratum Malpighii. The stratum granulosum is not uniformly present, although occurring as isolated areas in the region of the nail root and lunula, the white area of demilunar shape at the proximal portion of the nail. Unna has demonstrated that the pale color of the lunula and root of the nail is due to the presence of keratohyalin. Formerly, this peculiarity was attributed to a difference in the distribution of the vessels in the various portions of the nail bed. The body of the nail, with the exception of the lunula, is transparent-a condition which may be explained by the fact that the elements of the nail correspond to those of the stratum lucidum. As a consequence, the vessels of the matrix shine through, except at the lunula, where the keratohyalin granules render the nail opaque.

The nail itself consists of elements homologous to those of the stratum lucidum. They are flat, transparent cells, closely approximated, and all contain nuclei. The cells overlie each other like tiles, and are so arranged that each succeeding lower layer projects

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Fig. 299.-Transverse section through human nail and its sulcus; X 34.

a little further distalward than the preceding. At the period when the nails are formed, about the fourth month of fetal life, sulci are already present. The first trace of the nail is seen as a marked thickening of the stratum lucidum in the region which later becomes the body of the nail; in this stage the structure is still covered by the remaining layers of the stratum corneum, constituting the eponychium. The embryonal nail then spreads in all directions until it finally reaches the sulcus. Henceforward the growth is uniform. The eleidin normally present in the stratum lucidum of the skin also occurs in the nail, and is derived, as we have already seen, from the keratohyalin. It may readily be conceived that later, when growth is confined to the root of the nail, keratohyalin is also present. As soon as the nail begins to grow forward, in the ninth month, the greater part of the eponychium is thrown off; but during the entire extrauterine life, a portion of the eponychium is retained at the nail wall, and as hyponychium on the anterior and under surface of the nail.

D. THE GLANDS OF THE SKIN.

The glands in the skin are of two kinds-sweat-glands and sebaceous glands. A modification of the latter is seen in the mammary glands.

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Basement

membrane.

Nonstriated

muscle-cell.

1. The Sweat-glands. The sweat-glands, or sudoriparous glands, are distributed throughout the entire skin, but are especially numerous in certain regions-as, for instance, the axilla, palm of the hand, and sole of the foot. They lie imbedded either in the adipose tissue of the true skin, or still deeper in the subcutaneous connective tissue

[graphic]

(axilla). To this group of glands belong also the ceruminous glands of the ear, the glands of Moll in the eyelid, and the circumanal glands.

Gland-cell.

Fig. 300. Cross-section of tubule of coiled portion of sweat-gland from human axilla. Fixation with sublimate; X 600.

The sweat-glands are simple tubular in type, and their secreting portion is coiled; hence the name coil-glands. The coil is, as a

Nucleus of
nonstriated
muscle-cell.

Nucleus of
gland-cell.

Fig. 301. Tangential section through coiled portion of sweat gland from human axilla. Sublimate fixation; X 700.

rule, 0.3 or 0.4 mm. in diameter, but in the axilla reaches from 3 to 7 mm. The excretory duct (the sudoriferous duct) is nearly straight during its course upward through the corium, and always enters the epidermis between two papillæ of the corium. From here on, its course is spiral, and it should be borne in mind that in its passage through the epidermis it has no other wall than the epidermal cells of the various layers through which it passes, although these cells are arranged concentrically around the lumen

[graphic]

of the duct. The lining of the secretory or coiled portion of the sweatgland consists of cubical cells with finely granular protoplasm and round or oval nuclei possessing one or two nucleoli. In the excre

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