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from view and embedded in a fold of skin; the body, or uncovered part, rests on the corium and ends in a free margin. The greater part of the lateral margin is overlapped by a duplicature of skin, termed the vallum unguis or nail-wall. The nails are pink in colour, with the exception of a small semilunar area near the root, which is more opaque than the rest, and is named the lunula. The lunulæ diminish in size from the thumb towards the little finger, while the thickness of the nail diminishes towards its root and lateral margins. The corium. under the nail is highly vascular and sensitive, and presents, especially under
Eponychium Horny part of nail Stratum.
FIG. 736.-TRANSVERSE SECTION OF A NAIL.
the anterior part of the body, numerous longitudinally arranged ridges. The part of the corium under the body is termed the nail bed; that under the root, the nail matrix. The deep part of the nail consists of the stratum germinativum and stratum mucosum, while its superficial horny portion is constituted by a greatly thickened stratum lucidum, and consists of nucleated, keratinised squames. The stratum corneum is represented by the thin cuticular fold overlapping the lunula, and termed the eponychium, while the stratum granulosum can be traced only as far forwards as the nail root.
Root of nail
Pili.-Hairs are well developed on the external genitals, scalp, and margins of the eyelids, in the axilla, the vestibule of the nose, and at the entrance to the concha, and also on the face of the male. Those on the genitals and face appear about puberty. Rudimentary over the greater part of the body, they are entirely absent from the flexor surfaces of the hands and feet, the dorsal surfaces of the terminal phalanges, the glans penis, the inner surface of the prepuce, and medial surfaces of the labia. Marked variations, individual and racial, exist as to the colour of the hair, and also as to the manner of its growth; hence the terms straight, curly, woolly, etc. are used to designate it. Straight hairs are coarser than curly ones, and have, moreover, a circular or oval outline on transverse section, curly hairs being flat and riband-like.
The root of the hair is embedded in a depression of the skin, termed the hair follicle (Fig. 738); the free portion is named the scapus or shaft, and consists
from without inwards of three parts, viz., cuticle, cortex, and medulla. The cuticle is formed by a layer of imbricated scales which overlap one another from below upwards. The cortex consists of longitudinally arranged fibres made up of elongated, closely applied, fusiform cells, which contain pigment and sometimes air spaces, the latter especially in white hairs. The medulla, absent from the fine hairs of the body generally and from the hairs of young children, forms a central core, which appears black by transmitted, and white by reflected light, and is composed of polyhedral nucleated cells containing pigment, fat granules, and air spaces.
The hair follicle consists of an oblique or curved-the latter in curly hairs-invagination of the epidermis and corium, and in the case of large hairs extends into the subcutaneous tissue (Fig. 733); some little distance below its orifice the ducts of the sebaceous glands open into it. The dermic coat or portion of the follicle derived from the corium consists of a fibrous sheath of external longitudinal and internal circular connective tissue fibres, the latter being lined by a hyaline layer directly
FIG. 738.-TRANSVERSE SECTION OF HAIR FOLLICLE WITH CONTAINED HAIR (highly magnified).
continuous with the basement membrane of the corium. The parts of the follicle derived from the epidermis are named the inner and outer root sheaths. Below the orifices of the sebaceous gland ducts the outer root sheath is formed by the stratum germinativum and stratum mucosum, while above them all the epidermal strata contribute to it. The inner root sheath surrounds the cuticle of the hair, and comprises from without inwards-(a) Henle's layer, a single stratum of nucleated cubical cells; (b) Huxley's layer, a single or double layer of polyhedral nucleated cells; and (c) a delicate cuticle, composed of a single layer of flattened imbricated, cells, with atrophied nuclei. The bottom of the hair follicle is moulded on a vascular papilla, derived from the corium and capped by the bulb of the hair or expanded part of the hair root. The cells of the bulb are continuous with those of the outer root sheath, and form the different parts of the hair, as well as its inner root sheath. The vessels form capillary loops in the papilla of the hair, and send twigs into the outer layer of its fibrous sheath; the inner and outer root sheaths and the different parts of the hair are non-vascular. The nerves end in longitudinal and annular fibrils below the level of the sebaceous glands and outside the hyaline layer of the follicle.
Glandulæ Sebaceæ.-Sebaceous glands exist wherever there are hairs, and their
ducts open into the superficial parts of the hair follicles (Fig. 733); the number of glands associated with each follicle varies from one to four. On the labia minora and mammary areola they open on the surface of the skin independently of hair follicles, and in the latter situations undergo great enlargement during pregnancy. The deep extremity of each gland expands into a cluster of oval or flask-shaped alveoli, which are surrounded by a basement membrane, and filled with polyhedral cells containing oil droplets. By the breaking down of the superficial cells, their oily contents are liberated as the sebum cutaneum and discharged into the hair follicle, whilst the deeper cells undergo proliferation. The size of the gland bears no proportion to that of the hairs, since they are very large in the minute hair follicles of the foetus and newly born child, and also in the follicles of the rudimentary hairs of the nose and certain parts of the face.
Bundles of non-striped muscular fibre are associated with the hair follicles, and are named the mm. arrectores pilorum. Attached to the deep part of the hair follicle, and forming with it an acute angle, they pass outwards close to the sebaceous glands, and end in the papillary layer of the corium. They are situated on the side towards which the hair slopes, so that, on contraction, they diminish the obliquity of the hair follicle and render the hair more erect, and, at the same time, compress the sebaceous glands and expel their contents. The condition of "goose-skin" is caused by the contraction of these slender muscles.
Arthur Thomson suggests that the condition of curly hair is produced by the contraction of the mm. arrectores pilorum. Straight hair is thick and cylindrical; curly hair is flat and ribbonlike. When the arrector muscle contracts, the thick rounded hair resists the tendency of the muscle to bend it, while the flat hair, not sufficiently strong to resist the strain of the muscle, becomes bent, and this is probably the explanation why the follicle assumes the curved form characteristic of the scalp of a bushman. The sebaceous gland lies in the concavity of the bend between the follicle and the muscle, and forms a mass of greater resistance, around which the follicle may be curved by the contraction of the muscle. The cells at the root of the hair accommodate themselves to the curved follicle, and, becoming more horny as they advance to the surface, retain the form of the follicle in which they are moulded.
Glandulæ Sudoriferæ.-Sudoriferous or sweat glands are found in the skin of nearly every part of the body; they are relatively few in number on the back of the trunk, but are very plentiful on the palms and soles, where they open on the summits of the curved ridges. Each consists of an elongated tube, the deeper portion of which forms its secretory part, and is coiled in the subcutaneous tissue or deep part of the corium in the form of an ovoid or spherical ball, termed the corpus glandulæ sudoriferæ (O.T. glomerulus) (Fig. 733). The superficial part of the tube, or ductus sudoriferus, extends through the corium and epidermis, and opens on the surface by a funnel-shaped orifice, the porus sudoriferus; where the epidermis is thick the duct is spirally coiled. The bodies of the glands, as a rule, vary in diameter from 0.1 to 0.5 mm., but in the axilla they are much larger, and may measure from 1 to 4 mm. Each is surrounded by a capillary network and by a capsule of connective tissue, inside which is a homogeneous basement membrane. The lumen of the tube is lined with a layer of nucleated, granular, and striated, columnar, or prismatic epithelial cells, between the deep extremities of which and the basement membrane is a layer of non-striped muscular fibres, the long axis of which is more or less parallel with that of the tube. The excretory ducts are devoid of muscular fibres, and consist of a basement membrane lined by two or three layers of polyhedral cells, which are covered, next the lumen of the duct, with a thin cuticle.
The glandulæ ciliares, at the margins of the eyelids, and the glandulæ ceruminosæ of the external acoustic meatus, are modified sudoriferous glands; the former are, however, not coiled, while the cell protoplasm of the latter contains yellowish pigment, and their gland ducts, in the foetus, open into hair follicles.
DEVELOPMENT OF THE SKIN AND ITS APPENDAGES.
Skin. The vascular and sensitive corium is developed from the mesoderm, the cells of which, immediately underlying the ectoderm, have, by the second month of fatal life, become aggregated together and flattened parallel to the surface of the embryo. By the third month they are seen to form two layers, the superficial of which becomes the
corium, and the deeper the subcutaneous tissue; the papillæ of the corium make their appearance in the fourth month. The epidermis, nails, hairs, sudoriferous and sebaceous
glands are of ectodermal origin.
The epidermis at first consists of a single layer of cells, but by the end of the second month it is duplicated, and then exhibits a superficial layer of irregular cells and a deeper layer of more or less cubical cells. By the third month three strata are seen: (a) a deep layer, consisting of a single layer of cubical cells the future stratum germinativum ; (6) a middle layer, comprising two or three strata of irregular cells—the future stratum mucosum; and (c) an outer layer, a double stratum of large cells. This outer layer appears to be homologous with a thin membrane, termed the epitrichium, first described as covering the embryo of the sloth and overlying its hairs, but since shown to be present also in birds and mammals. Over the hairy parts of the body it disappears about the sixth month; but over the free edge and root of the nails, and on the palms and soles, it develops into several layers of cells, which, in these parts, probably persist to form the thick stratum corneum. The part which persists over the root of the nail is termed the eponychium, and covers the proximal part of the lunula (vide p. 859, Fig. 737).
Nails. The first rudiment of the nails is seen about the beginning of the third month of embryonic life, and consists of a thickening of the epitrichium over the ends of the digits. Owing to the greater growth of the volar surfaces of the digits, the nail rudiment comes to be placed dorsally, and, at its proximal edge, an ingrowth of the stratum mucosum occurs to form its root, while the future nail is limited behind and at the sides by a groove. The superficial cells of the stratum mucosum become keratinised to form a thick stratum lucidum, the future nail proper, over the greater part of which the epitrichium disappears. The latter persists in the adult as the eponychium across the root of the nail, and, until fifth month, also forms a thick mass over the extremity of the nail, and is continued into the stratum corneum over the end of the digit. The future distal edge of the nail, at this stage, is continuous with the stratum lucidum in front of it; but this continuity is lost, and by the seventh month the nail presents a free border. The nails grow in length, and are renewed, in case of removal, by a proliferation of the cells of the stratum mucosum at the root of the nail, while an increase in their thickness takes place from the part of the same stratum which underlies the lunula.
Hairs. The hair rudiments appear about the third month of embryonic life as solid downgrowths of the stratum mucosum, which pass obliquely into the subjacent corium. The deep end of this column of cells expands to form the hair bulb, and is moulded on a papilla derived from the corium; the epidermis immediately overlying the papilla becomes differentiated into the hair and its inner root sheath, while the peripheral cells form its outer root sheath. The surrounding corium is condensed to form the fibrous sheath of the hair follicle, the hyaline layer of which is continuous with the basement membrane covering the corium. The hair gradually elongates, and, reaching the neck of the follicle, its extremity lies at first under the epitrichium, but becomes free on the disappearance of the latter. This takes place about the fifth month of fœtal life, and the first crop of hairs constitutes the lanugo, and is well developed by the seventh month. The lanugo consists of very delicate hairs, some of which are shed before, the remainder shortly after birth-the last to drop out being those of the eyelashes and scalp and are replaced by stronger hairs. Shedding and renewal of the hairs take place during life; prior to the shedding of a hair active growth and proliferation of the cells of the hair bulb cease, and the papilla becomes atrophied, while the hair root, gradually approaching the surface, at last drops out. New hairs arise from epidermic buds, which extend downwards from the follicle, and their development is identical with that of the original hairs.
Sebaceous Glands. These appear about the fifth month as solid outgrowths from the sides of the hair follicles, and consist of epidermal offshoots continued from the cells of the outer root sheath. Their deep ends become enlarged and lobulated, to form the secreting part of the gland, while the narrow neck connecting this with the follicle forms its duct. The sebaceous secretion, together with the cast-off epidermal cells, is collected on the surface of the body during the last months of intra-uterine life, and forms a layer of varying thickness, termed the vernix caseosa or smegma embryonum.
Sudoriferous Glands.-These, like the hairs, arise as solid downgrowths of the stratum mucosum. They descend, however, perpendicularly, instead of obliquely, and are of a yellowish colour; they appear on the palms and soles early in the fifth month, but much later over the hairy parts of the body. The downgrowths extend through the corium, and, on reaching the subcutaneous tissue, become coiled up to form the body or secreting part of the gland. The ducts of the glands do not open on the surface until the seventh month.
ENDINGS OF NERVES OF GENERAL SENSATIONS.
The peripheral endings of the nerves associated with the special senses have been described in the preceding pages. Under this heading will be considered the terminations of those sensory nerves which are widely distributed throughout the body and are associated with the muscular sense and the senses of pressure, heat, cold, and pain. These nerves may end as fine ramifications of the axis cylinders lying free amongst the tissues, or in special end organs where the terminations of the axis cylinders are surrounded by connective tissue capsules.
Free nerve-endings are found chiefly in the epithelium covering the skin or the mucous membranes. The nerve-fibres, after subdividing in the sub-epithelial connective tissue, lose successively their medullary and primitive sheaths and are continued as naked axis cylinders, which, if stained with gold chloride, are seen to consist of fine varicose filaments. The axis cylinders subdivide and form primary and secondary plexuses, and from the latter numerous fibrillæ pierce the sub-epithelial basement membrane and ramify between the overlying epithelial cells where they end in minute knobs of flattened discs. In the epidermis the nerve fibrillæ are limited to the stratum mucosum, a, b, primary plexus in connective tissue of cornea; c, branch but in the cornea they reach the passing to sub-epithelial plexus e; f, intra-epithelial plexus; d, terminations of fibrils.
surface layers of epithelium (Fig. 739). Free nerve-endings
also occur around the sudoriferous glands, in the papillæ and root sheaths of the hair follicles, in the sub-epithelial and intermuscular connective tissues, and in
FIG. 739.-VERTICAL SECTION OF CORNEA STAINED WITH
SPECIAL END ORGANS.
The special end organs vary greatly in size and form, but in all of them the termination of the axis cylinder is enclosed within a connective tissue capsule or sheath of varying thickness. The following are the more important special end organs.
(1) End Bulbs of Krause (Fig. 742).-These are minute cylindrical or oval bodies which are found in the conjunctiva, in the mucous membrane of the lips, and in
FIG. 740.-ENDING OF NERVE IN TACTILE DISCS OF THE
Modifications of free nerve-endings are seen in the tactile discs or cells of Merkel; here the neuro-fibrillæ end in the deeper layers of the epidermis in crescentic or cup-shaped expansions, in contact with large, modified epithelial cells. These tactile discs are well marked in the pig's snout (Fig. 740).