(Integumentum Commune; Skin)
The integument (Fig. 90940) covers the body and protects the deeper tissues from injury, from drying and from invasion by foreign organisms; it contains the peripheral endings of many of the sensory nerves; it plays an important part in the regulation of the body temperature, and has also limited excretory and absorbing powers. It consists principally of a layer of vascular connective tissue, named the corium or cutis vera, and an external covering of epithelium, termed the epidermis or cuticle. On the surface of the former layer are sensitive and vascular papillæ within, or beneath it, are certain organs with special functions: namely, the sudoriferous and sebaceous glands, and the hair follicles.
|The epidermis, cuticle, or scarf skin is non-vascular, and consists of stratified epithelium (Fig. 90941), and is accurately moulded on the papillary layer of the corium. It varies in thickness in different parts. In some situations, as in the palms of the hands and soles of the feet, it is thick, hard, and horny in texture. This may be in a measure due to the fact that these parts are exposed to intermittent pressure, but that this is not the only cause is proved by the fact that the condition exists to a very considerable extent at birth. The more superficial layers of cells, called the horny layer (stratum corneum), may be separated by maceration from a deeper stratum, which is called the stratum mucosum, and which consists of several layers of differently shaped cells. The free surface of the epidermis is marked by a net-work of linear furrows of variable size, dividing the surface into a number of polygonal or lozenge-shaped areas. Some of these furrows are large, as opposite the flexures of the joints, and correspond to the folds in the corium produced by movements. In other situations, as upon the back of the hand, they are exceedingly fine, and intersect one another at various angles. Upon the palmar surfaces of the hands and fingers, and upon the soles of the feet, the epidermal ridges are very distinct, and are disposed in curves; they depend upon the large size and peculiar arrangements of the papillæ upon which the epidermis is placed. The function of these ridges is primarily to increase resistance between contact surfaces for the purpose of preventing slipping whether in walking or prehension. The direction of the ridges is at right angles with the force that tends to produce slipping or to the resultant of such forces when these forces vary in direction. 153 In each individual the lines on the tips of the fingers and thumbs form distinct patterns unlike those of any other person. A method of determining the identity of a criminal is based on this fact, impressions (finger-prints) of these lines being made on paper covered with soot, or on white paper after first covering the fingers with ink. The deep surface of the epidermis is accurately moulded upon the papillary layer of the corium, the papillæ being covered by a basement membrane; so that when the epidermis is removed by maceration, it presents on its under surface a number of pits or depressions corresponding to the papillæ, and ridges corresponding to the intervals between them. Fine tubular prolongations are continued from this layer into the ducts of the sudoriferous and sebaceous glands.|
|The epidermis consists of stratified epithelium which is arranged in four layers from within outward as follows: (a) stratum mucosum, (b) stratum granulosum, (c) stratum lucidum, and (d) stratum corneum.||The stratum mucosum (mucous layer) is composed of several layers of cells; those of the deepest layer are columnar in shape and placed perpendicularly on the surface of the basement membrane, to which they are attached by toothed extremities; this deepest layer is sometimes termed the stratum germinativum; the succeeding strata consist of cells of a more rounded or polyhedral form, the contents of which are soft, opaque, granular, and soluble in acetic acid. These are known as prickle cells because of the bridges by which they are connected to one another. They contain fine fibrils which are continuous across the connecting processes with corresponding fibrils in adjacent cells. Between the bridges are fine inter-cellular clefts serving for the passage of lymph, and in these lymph corpuscles or pigment granules may be found.||The stratum granulosum comprises two or three layers of flattened cells which contain granules of eleidin, a substance readily stained by hematoxylin or carmine, and probably an intermediate substance in the formation of keratin. They are supposed to be cells in a transitional stage between the protoplasmic cells of the stratum mucosum and the horny cells of the superficial layers.||The stratum lucidum appears in section as a homogeneous or dimly striated membrane, composed of closely packed cells in which traces of flattened nuclei may be found, and in which minute granules of a substance named keratohyalin are present.||The stratum corneum (horny layer) consists of several layers of horny epithelial scales in which no nuclei are discernible, and which are unaffected by acetic acid, the protoplasm having become changed into horny material or keratin. According to Ranvier they contain granules of a material which has the characteristics of beeswax.||The black color of the skin in the negro, and the tawny color among some of the white races, is due to the presence of pigment in the cells of the epidermis. This pigment is more especially distinct in the cells of the stratum mucosum, and is similar to that found in the cells of the pigmentary layer of the retina. As the cells approach the surface and desiccate, the color becomes partially lost; the disappearance of the pigment from the superficial layers of the epidermis is, however, difficult to explain.||The pigment (melanin) consists of dark brown or black granules of very small size, closely packed together within the cells, but not involving the nucleus.||The main purpose served by the epidermis is that of protection, as the surface is worn away new cells are supplied and thus the true skin, the vessels and nerves which it contains are defended from damage.||The Corium, Cutis Vera, Dermis, or True Skin is tough, flexible, and highly elastic. It varies in thickness in different parts of the body. Thus it is very thick in the palms of the hands and soles of the feet; thicker on the posterior aspect of the body than on the front, and on the lateral than on the medial sides of the limbs. In the eyelids, scrotum, and penis it is exceedingly thin and delicate.||It consists of felted connective tissue, with a varying amount of elastic fibers and numerous bloodvessels, lymphatics, and nerves. The connective tissue is arranged in two layers: a deeper or reticular, and a superficial or papillary. Unstriped muscular fibers are found in the superficial layers of the corium, wherever hairs are present, and in the subcutaneous areolar tissue of the scrotum, penis, labia majora, and nipples. In the nipples the fibers are disposed in bands, closely reticulated and arranged in superimposed laminæ.||The reticular layer (stratum reticulare; deep layer) consists of strong interlacing bands, composed chiefly of white fibrous tissue, but containing some fibers of yellow elastic tissue, which vary in number in different parts; and connective-tissue corpuscles, which are often to be found flattened against the white fibrous tissue bundles. Toward the attached surface the fasciculi are large and coarse, and the areolæ left by their interlacement are large, and occupied by adipose tissue and sweat glands. Below the reticular layer is the subcutaneous areolar tissue, which, except in a few situations, contains fat.||The papillary layer (stratum papillare; superficial layer; corpus papillare of the corium) consists of numerous small, highly sensitive, and vascular eminences, the papillæ, which rise perpendicularly from its surface. The papillæ are minute conical eminences, having rounded or blunted extremities, occasionally divided into two or more parts, and are received into corresponding pits on the under surface of the cuticle. On the general surface of the body, more especially in parts endowed with slight sensibility, they are few in number, and exceedingly minute; but in some situations, as upon the palmar surfaces of the hands and fingers, and upon the plantar surfaces of the feet and toes, they are long, of large size, closely aggregated together, and arranged in parallel curved lines, forming the elevated ridges seen on the free surface of the epidermis. Each ridge contains two rows of papillæ, between which the ducts of the sudoriferous glands pass outward to open on the summit of the ridge. Each papilla consists of very small and closely interlacing bundles of finely fibrillated tissue, with a few elastic fibers; within this tissue is a capillary loop, and in some papillæ, especially in the palms of the hands and the fingers, there are tactile corpuscles.||Development.The epidermis and its appendages, consisting of the hairs, nails, sebaceous and sweat glands, are developed from the ectoderm, while the corium or true skin is of mesodermal origin. About the fifth week the epidermis consists of two layers of cells, the deeper one corresponding to the rete mucosum. The subcutaneous fat appears about the fourth month, and the papillæ of the true skin about the sixth. A considerable desquamation of epidermis takes place during fetal life, and this desquamated epidermis, mixed with sebaceous secretion, constitutes the vernix caseosa, with which the skin is smeared during the last three months of fetal life. The nails are formed at the third month, and begin to project from the epidermis about the sixth. The hairs appear between the third and fourth months in the form of solid downgrowths of the deeper layer of the epidermis, the growing extremities of which become inverted by papillary projections from the corium. The central cells of the solid downgrowths undergo alteration to form the hair, while the peripheral cells are retained to form the lining cells of the hair-follicle. About the fifth month the fetal hairs (lanugo) appear, first on the head and then on the other parts; they drop off after birth, and give place to the permanent hairs. The cellular structures of the sudoriferous and sebaceous glands are formed from the ectoderm, while the connective tissue and bloodvessels are derived from the mesoderm. All the sweat-glands are fully formed at birth; they begin to develop as early as the fourth month.||The arteries supplying the skin form a net-work in the subcutaneous tissue, and from this net-work branches are given off to supply the sudoriferous glands, the hair follicles, and the fat. Other branches unite in a plexus immediately beneath the corium; from this plexus, fine capillary vessels pass into the papillæ, forming, in the smaller ones, a single capillary loop, but in the larger, a more or less convoluted vessel. The lymphatic vessels of the skin form two net-works, superficial and deep, which communicate with each other and with those of the subcutaneous tissue by oblique branches.||The nerves of the skin terminate partly in the epidermis and partly in the corium; their different modes of ending are described on pages 1059 to 1061.||The Appendages of the SkinThe appendages of the skin are the nails, the hairs, and the sudoriferous and sebaceous glands with their ducts.||The Nails (ungues) (Fig. 90943) are flattened, elastic structures of a horny texture, placed upon the dorsal surfaces of the terminal phalanges of the fingers and toes. Each nail is convex on its outer surface, concave within, and is implanted by a portion, called the root, into a groove in the skin; the exposed portion is called the body, and the distal extremity the free edge. The nail is firmly adherent to the corium, being accurately moulded upon its surface; the part beneath the body and root of the nail is called the nail matrix, because from it the nail is produced. Under the greater part of the body of the nail, the matrix is thick, and raised into a series of longitudinal ridges which are very vascular, and the color is seen through the transparent tissue. Near the root of the nail, the papillæ are smaller, less vascular, and have no regular arrangement, and here the tissue of the nail is not firmly adherent to the connective-tissue stratum but only in contact with it; hence this portion is of a whiter color, and is called the lunula on account of its shape.||The cuticle as it passes forward on the dorsal surface of the finger or toe is attached to the surface of the nail a little in advance of its root; at the extremity of the finger it is connected with the under surface of the nail a little behind its free edge. The cuticle and the horny substance of the nail (both epidermic structures) are thus directly continuous with each other. The superficial, horny part of the nail consists of a greatly thickened stratum lucidum, the stratum corneum forming merely the thin cuticular fold (eponychium) which overlaps the lunula; the deeper part consists of the stratum mucosum. The cells in contact with the papillæ of the matrix are columnar in form and arranged perpendicularly to the surface; those which succeed them are of a rounded or polygonal form, the more superficial ones becoming broad, thin, and flattened, and so closely packed as to make the limits of the cells very indistinct. The nails grow in length by the proliferation of the cells of the stratum mucosum at the root of the nail, and in thickness from that part of the stratum mucosum which underlies the lunula.||Hairs (pili) are found on nearly every part of the surface of the body, but are absent from the palms of the hands, the soles of the feet, the dorsal surfaces of the terminal phalanges, the glans penis, the inner surface of the prepuce, and the inner surfaces of the labia. They vary much in length, thickness, and color in different parts of the body and in different races of mankind. In some parts, as in the skin of the eyelids, they are so short as not to project beyond the follicles containing them; in others, as upon the scalp, they are of considerable length; again, in other parts, as the eyelashes, the hairs of the pubic region, and the whiskers and beard, they are remarkable for their thickness. Straight hairs are stronger than curly hairs, and present on transverse section a cylindrical or oval outline; curly hairs, on the other hand, are flattened. A hair consists of a root, the part implanted in the skin; and a shaft or scapus, the portion projecting from the surface.||The root of the hair (radix pili) ends in an enlargement, the hair bulb, which is whiter in color and softer in texture than the shaft, and is lodged in a follicular involution of the epidermis called the hair follicle (Fig. 90944). When the hair is of considerable length the follicle extends into the subcutaneous cellular tissue. The hair follicle commences on the surface of the skin with a funnel-shaped opening, and passes inward in an oblique or curved directionthe latter in curly hairsto become dilated at its deep extremity, where it corresponds with the hair bulb. Opening into the follicle, near its free extremity, are the ducts of one or more sebaceous glands. At the bottom of each hair follicle is a small conical, vascular eminence or papilla, similar in every respect to those found upon the surface of the skin; it is continuous with the dermic layer of the follicle, and is supplied with nerve fibrils. The hair follicle consists of two coatsan outer or dermic, and an inner or epidermic.||The outer or dermic coat is formed mainly of fibrous tissue; it is continuous with the corium, is highly vascular, and supplied by numerous minute nervous filaments. It consists of three layers (Fig. 90945). The most internal is a hyaline basement membrane, which is well-marked in the larger hair follicles, but is not very distinct in the follicles of minute hairs; it is limited to the deeper part of the follicle. Outside this is a compact layer of fibers and spindle-shaped cells arranged circularly around the follicle; this layer extends from the bottom of the follicle as high as the entrance of the ducts of the sebaceous glands. Externally is a thick layer of connective tissue, arranged in longitudinal bundles, forming a more open texture and corresponding to the reticular part of the corium; in this are contained the bloodvessels and nerves.||The inner or epidermic coat is closely adherent to the root of the hair, and consists of two strata named respectively the outer and inner root sheaths; the former of these corresponds with the stratum mucosum of the epidermis, and resembles it in the rounded form and soft character of its cells; at the bottom of the hair follicle these cells become continuous with those of the root of the hair. The inner root sheath consists of (1) a delicate cuticle next the hair, composed of a single layer of imbricated scales with atrophied nuclei; (2) one or two layers of horny, flattened, nucleated cells, known as Huxleys layer; and (3) a single layer of cubical cells with clear flattened nuclei, called Henles layer.||The hair bulb is moulded over the papilla and composed of polyhedral epithelial cells, which as they pass upward into the root of the hair become elongated and spindle-shaped, except some in the center which remain polyhedral. Some of these latter cells contain pigment granules which give rise to the color of the hair. It occasionally happens that these pigment granules completely fill the cells in the center of the bulb; this gives rise to the dark tract of pigment often found, of greater or less length, in the axis of the hair.||The shaft of the hair (scapus pili) consists, from within outward, of three parts, the medulla, the cortex, and the cuticle. The medulla is usually wanting in the fine hairs covering the surface of the body, and commonly in those of the head. It is more opaque and deeper colored than the cortex when viewed by transmitted light; but when viewed by reflected light it is white. It is composed of rows of polyhedral cells, containing granules of eleidin and frequently air spaces. The cortex constitutes the chief part of the shaft; its cells are elongated and united to form flattened fusiform fibers which contain pigment granules in dark hair, and air in white hair. The cuticle consists of a single layer of flat scales which overlap one another from below upward.||Connected with the hair follicles are minute bundles of involuntary muscular fibers, termed the Arrectores pilorum. They arise from the superficial layer of the corium, and are inserted into the hair follicle, below the entrance of the duct of the sebaceous gland. They are placed on the side toward which the hair slopes, and by their action diminish the obliquity of the follicle and elevate the hair (Fig. 90944). 154 The sebaceous gland is situated in the angle which the Arrector muscle forms with the superficial portion of the hair follicle, and contraction of the muscle thus tends to squeeze the sebaceous secretion out from the duct of the gland.||The Sebaceous Glands (glandulæ sebaceæ) are small, sacculated, glandular organs, lodged in the substance of the corium. They are found in most parts of the skin, but are especially abundant in the scalp and face; they are also very numerous around the apertures of the anus, nose, mouth, and external ear, but are wanting in the palms of the hands and soles of the feet. Each gland consists of a single duct, more or less capacious, which emerges from a cluster of oval or flask-shaped alveoli which vary from two to five in number, but in some instances there may be as many as twenty. Each alveolus is composed of a transparent basement membrane, enclosing a number of epithelial cells. The outer or marginal cells are small and polyhedral, and are continuous with the cells lining the duct. The remainder of the alveolus is filled with larger cells, containing fat, except in the center, where the cells have become broken up, leaving a cavity filled with their debris and a mass of fatty matter, which constitutes the sebum cutaneum. The ducts open most frequently into the hair follicles, but occasionally upon the general surface, as in the labia minora and the free margin of the lips. On the nose and face the glands are of large size, distinctly lobulated, and often become much enlarged from the accumulation of pent-up secretion. The tarsal glands of the eyelids are elongated sebaceous glands with numerous lateral diverticula.||The Sudoriferous or Sweat Glands (glandulæ sudoriferæ) are found in almost every part of the skin, and are situated in small pits on the under surface of the corium, or, more frequently, in the subcutaneous areolar tissue, surrounded by a quantity of adipose tissue. Each consists of a single tube, the deep part of which is rolled into an oval or spherical ball, named the body of the gland, while the superficial part, or duct, traverses the corium and cuticle and opens on the surface of the skin by a funnel-shaped aperture. In the superficial layers of the corium the duct is straight, but in the deeper layers it is convoluted or even twisted; where the epidermis is thick, as in the palms of the hands and soles of the feet, the part of the duct which passes through it is spirally coiled. The size of the glands varies. They are especially large in those regions where the amount of perspiration is great, as in the axillæ, where they form a thin, mammillated layer of a reddish color, which corresponds exactly to the situation of the hair in this region; they are large also in the groin. Their number varies. They are very plentiful on the palms of the hands, and on the soles of the feet, where the orifices of the ducts are exceedingly regular, and open on the curved ridges; they are least numerous in the neck and back. On the palm there are about 370 per square centimeter; on the back of the hand about 200; forehead 175, breast, abdomen and forearm 155, and on the leg and back from 60 to 80 per square centimeter. Krause estimates the total number at about 2,000,000. The average number of sweat glands per square centimeter of skin area in various races as shown by the fingers is as follows: 155|
|They are absent in the deeper portion of the external auditory meatus, the prepuce and the glans penis. The tube, both in the body of the gland and in the duct consists of two layersan outer, of fine areolar tissue, and an inner of epithelium (Fig. 90946). The outer layer is thin and is continuous with the superficial stratum of the corium. In body of the gland the epithelium consists of a single layer of cubical cells, between the deep ends of which and the basement membrane is a layer of longitudinally or obliquely arranged non-striped muscular fibers. The ducts are destitute of muscular fibers and are composed of a basement membrane lined by two or three layers of polyhedral cells; the lumen of the duct is coated by a thin cuticle. When the cuticle is carefully removed from the surface of the corium, the ducts may be drawn out in the form of short, thread-like processes on its under surface. The ceruminous glands of the external acoustic meatus and the ciliary glands at the margins of the eyelids are modified sudoriferous glands.|
|Note 153. Professor Arthur Thomson, of Oxford, suggests that the contraction of these muscles on follicles which contain weak, flat hairs will tend to produce a permanent curve in the follicle, and this curve will be impressed on the hair which is moulded within it, so that the hair, on emerging through the skin, will be curled. Curved hair follicles are characteristic of the scalp of the Bushman. [back]|
|Note 154. Clark and Lhamon, Anatomical Record, 1917, xii. [back]|
|Note 155. Journal of Anatomy and Physiology, vol. xliv. [back]|