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Skin And Exoskeleton

SKIN AND EXOSKELETON, in anatomy. The skin (A.-S. scinn) is the covering of the whole body, and is continuous at the different orifices with the mucous membrane. It acts firstly as a protective layer, secondly as a regulator of the temperature, thirdly as an excretory organ and fourthly as a tactile and sensory organ in which nerves end.

The skin varies in thickness from -5 mm. in the eyelids to 4 or more mm. in the palms and soles; it is also very thick over the back of the body. Two main layers are recognized in the Stratum lucidum Blood-vessels and nerves From Robert Howden in Cunningham's Text-Book of Anatomy, FIG. I. Vertical section of Epidermis and Papillae of Corium (highly magnified).

skin; superficially there is the scarf skin or epidermis and more deeply the dermis or true skin. The epidermis under the microscope is seen to consist of five layers. On the surface is the horny layer or stratum corneum (see fig. i) composed of layers of scalelike cells, the walls of which are turned into the horny substance keratin. Deep to this is a thin layer of scale-like cells without keratin known as the stratum lucidum. Deeper still is a layer, the stratum granulosum, in which the cells are not so flattened and contain granules of a substance known as eleidin. In the fourth layer, stratum mucosum or stratum Malpighii, the cells are polygonal and are connected together by delicate prickle-like processes. It is in the deeper layers of these cells that the pigment of the negro's skin is found. The fifth and deepest layer of the epidermis is the stratum germinalivum, in which there is only one layer of columnar cells. The whole of the epidermis is non-vascular, and it will be noticed that as the different layers approach the surface the cells become more and more flattened. The true skin, dermis or corium is composed of a felted network of white fibrous tissue with a small number of yellow elastic fibres interspersed. It is divided into two layers.

The superficial or papillary layer lies next to the epidermis and is raised into a number of papillae or conical projections which fit into corresponding depressions on the deep surface of the epidermis. In sensitive parts like the palms and soles these papillae are specially prominent and form wavy lines, each of which consists of a double row between which the ducts of the sweat glands pass on their way to the surface. So large are the papillae in these situations that the epidermis is also raised into ridges, and these in the fingers form the characteristic whorls so valuable for purposes of identification. The papillae contain leashes of blood-vessels, and in some of them are special tactile corpuscles in which the nerves end (see NERVOUS SYSTEM).

In the deeper or reticular layer of the true skin the fibrous feltwork is looser and encloses pellets of fat. It also contains a network of blood-vessels and nerves, and in some places a layer of striped or unstriped muscle. Where hairs are present the hair follicles lie in this deeper layer, which gradually merges with the subcutaneous fatty tissue (see fig. 2).

As appendages of the skin are found the hairs, the nails and the sebaceous and sweat glands.

Hair. The hairs are found in man on the scalp, eyelids, eyebrows, armpits, pubic region, vestibule of the nose, external auditory meatus, face, ventral surface of the trunk and dorsal surfaces of the leg, forearm and hand; indeed the only places which are quite free from them are the palms of the hands, soles of the feet and the glans penis. In some places, such as the armpits, pubic region and the face of the male they grow to a considerable length at and after puberty. They are of great anthropological interest since they differ in colour and texture in different races, sometimes being straight, sometimes wavy, sometimes curly. The amount and distribution of long hairs also vary with the race. In section it is only the straight hairs which are circular; wavy and curly hairs are oval. In the centre of each hair is the medulla or pith, though this is not always present; it is composed of nucleated cells containing pigment, fat and air spaces. Outside this is the fibrous layer or cortex, also containing pigment and air spaces, while most superficially is the cuticle made up of overlapping scales. The hair grows at its root from a hair follicle (see fig. 2), which is a tubular inpushing of the epidermis into the true skin or, in the case of large hairs, deeper still into the superficial fascia. It is divided into an inner and outer root sheath, the former representing the more superficial layers of the epidermis, the latter the deeper layers. At the bottom of the follicle the hair enlarges to form the bulb, and into the lower part of this a vascular papilla projects from the true skin. The cells of the hair are derived from, and are continuous at the bulb with those of the outer root sheath, and therefore with the deeper layers of the epidermis.

The hair follicle always projects somewhat obliquely into the skin, and attached to the side toward which it is leaning is a small band of non-striated muscular fibres called arrector pili. When this acts it diminishes the obliquity of the hair and so makes it " bristle " or " stand on end," while a general contraction of these small muscles leads to the familiar condition of " gooseflesh."

Nails. The nails are specially thickened parts of the epidermis, and are divided into a root and a body. The former is concealed by a fold of skin, and the corium on which it lies is known as the nail matrix. The body of the nail also lies on the corium, or true skin, which forms the nail bed and is very sensitive. This body of the nail is formed by the stratum germinativum and stratum mucosum in its deeper part, and more superficially by the stratum lucidum, which is here very much thickened and converted into keratin or horn. Near the root of each nail is a semi-lunar area which is more opaque than the rest and forms the white lunula.

Sweat Glands. Sebaceous glands are found wherever there are hairs, however rudimentary, and open by their ducts into the superficial part of the hair follicle (see fig. 2). Their deeper or secreting part divides into a number of bag-like alveoli composed of cells, which secrete oil droplets. There may be two or three glands to each hair follicle, and their size does not vary directly with that of the hair, since they are very large in the nose, where the hairs are quite rudimentary. They are also found on the labia minora and nipples, where no hairs are. Sudoriparous or sweat glands (see fig. 2) are found all over the surface of the body, but are specially numerous on the palms and soles. It is estimated that in the palm there are nearly 3000 to a square inch, while in the skin of the back they do not reach 500 to the same area. In the armpits and groins they are very large. Each consists of a single long tube, lined by columnar epithelium, and coiled up into a ball or glomerulus in the subcutaneous tissue, after which it pierces the corium and epidermis to reach the surface at the porus sudoriferus. Where the stratum corneum of the epidermis is thick the duct is twisted like a corkscrew as it goes through.

The glands of Moll in the eyelids and the ceruminous or wax glands of the ear are modified sweat glands; the former, when inflamed, cause a " sty."

EMBRYOLOGY The skin ft derived partly from the ectoderm and partly from the mesoderm of the embryo. The whole of the epidermis Duct of sweat gland Hair Papillae of corium Hair follicle Glomerulus Oblique section through Papilla of hair a Pacinian corpuscle From Robert Howden, in Cunningham's Text-Book of Anatomy.

FIG. 2. Vertical section of the Skin (schematic).

and its appendages are ectodermal, and in the early embryo consist of a single layer of cells ; later on this becomes double, and the superficial layer is called the epitrichium, which, after the- sixth month is cast ofif and mixes with the secretion of the large sebaceous glands to form the soapy vernix caseosa with which the foetus is coated at birth. In the meantime the cells of the deeper layer divide and form the various layers of the epidermis already enumerated. It is held, however, by some observers that part of the epitrichium remains as the stratum corneum. The mesodermal cells belong to the mesenchyme, and form the fibrous tissue of the true skin as well as the arrectores pilorum muscles and, in the scrotum, the dartos layer of unstriped muscle. In the sixth month fatty tissue appears in the deeper parts, and so the fat of the superficial fascia or subcutaneous tissue is formed. The nails are said to appear as thickenings of the epidermis at about the ninth week, quite at the tips of the digits. Later on they shift to the dorsal side, and in doing so carry the nerves in the nail bed with them. This is trie only explanation available of the fact that the ventral nerves to the tips of the fingers encroach on the dorsal area. By about the twelfth week the nails are perfectly formed, but tney do not reach the level of the finger tips until the eighth month The hairs are developed in the third month of foetal life by ingrowths of the stratum mucosum ol the epidermis into the conum. During the fourth and fifth months the body becomes covered by fine unpigmented hairs which are known as lanugp; these begin to disappear about the eighth month but some remain until after birth. On the scalp, however, the hair at birth is often more deeply pigmented than that which succeeds it. The sebaceous and sweat glands, like the hair follicles, are ingrowths of the stratum mucosum of the epidermis into the corium. The former become very large in the later months oi embryonic life, and secrete a large part of the above-mentioned vernix caseosa. The development of the mammary gland from modified sebaceous glands has already been referred to (see MAMMARY GLAND).

For further details see J. P. M'Murrich, Development of the Human Body (London, 1906) ; J. C. Heisler, Text-book of Embryology (London, !9O7) ; Quain's Anatomy, vol. i. (London, 1908).

COMPARATIVE ANATOMY In the larval (gastrula) stage of the Amphioxus (lancelet) cilia are present on the surface, and in the superficial epidermal cells of some fishes and amphibian larvae there is a striated layer on the free edge which is looked upon as a relic of ancestral cilia.

Skin Glands. The skin glands of the Cyclostomata (hags and lampreys) and fishes are generally unicellular and secrete slime which protects the surface of the body ; the amount of slime poured cut by some of the cyclostomes is enormous. Many of these slime cells, from their shape, are spoken of as goblet cells. Some of the teleostean fish have poison glands at the bases of their dorsal fins and opercula.

In the mud fish (Dipnoi) and amphibians multicellular spherical glands appear as involutions of the ectoderm. .2 Sometimes, as in the so-called parotids of the toad, these form large masses. Reptiles and birds are singularly wanting in *o skin glands, though the latter have a large uropygial gland at , the root of the tail which secretes oil to lubricate the feathers; it is the chief constituent of the " parson's nose " of the fowl. In mammals, except the Cetacea, the sebaceous and sudoriparous glands already described in man are found ; some of the former sometimes attain a large size, as in the interdigital gland of the sheep, Miiller's gland at the back of the pig's knee and the suborbital gland of ruminants. In addi3 tion to these, special scent-producing glands are often found o in different parts, the most remarkable of which, perhaps, are the scent glands beneath the tail of the skunk, while in male monotremes there is a special poison gland in the leg which is connected with a spur in the foot.

Pigment. Pigment cells are present both in the dermis and epidermis of fishes and amphibians, and the pigment may be either intra- or extra-cellular. In many cases it is under the control of the nervous system, so that forms like the flat-fish and the common frog can adapt their coloration to that of their g background. In animals permanently excluded from the light, J pigment is absent. In reptiles movable pigment cells are often , found, as in the chameleon, while in birds the pigment is some^ times of great brilliancy in the necks and wattles. In mam7 mals, as in man, the pigment is confined to the cells of the stratum mucosum layer of the epidermis. 5 Scales. In the elasmpbranch fishes scales are found com- J posed of enamel superficially, and of dentine and bone deeply. _, They are developed from the epidermis and dermis, and in "* almost every way resemble the teeth of these animals, which are only modifications of them. The bony basal part of each scale is plate-like, hence this kind of scale is known as placoid. In the ganoid fishes, such as the sturgeon, much larger plaques called ganoid scales form a complete armature. In the teleostean fishes the scales overlap like tiles and are either cycloid, having a smooth border, or ctenoid, in which the free posterior border is serrated. Existing amphibians are usually remarkable for absence of any skin armour, though in fossil forms (Stegocephala) it was very complete. The reptilian class is. specially noticeable for the production of epidermal scales, which undergo many modifications. In the Ophidia they are cast off periodically in one mass as the snake's slough, while in the Chelonia they form the different varieties of tortoise-shell. Bony structures, developed in the dermis, may underlie these epidermal horny thickenings, and are very strongly developed in the dorsal and ventral bony shields of the Chelonia (carapace and plastron), which secondarily fuse with the true endoskeleton. The armadillo is the only mammal which has a true bony exoskeleton.

Feathers. Birds are remarkable for the possession of feathers, which are highly modified scales. The embryonic or down feathers are simple, and consist of a brush of hair-like barbs springing from a basal quill or calamus. From the whole length of each barb a series of smaller barbules comes off like branches of a shrub. The adult or contour feathers are formed at the bottom of the same follicles which lodge the down feathers and, by their growth, push these out. At first they are nothing more than enlarged down feathers, but soon one of the barbs grows enormously, and forms a main shaft or rachis to which the other barbs are attached on either side. From the sides of the barbs grow the barbules, just as in the down feathers, and these, in the case of the large wing feathers (remiges) and tail leathers (rectrices), are connected by minute hooks so that the feather vane, as opposed to the shaft, has a more resistant texture than it has in the feathers of the back or breast. The bird's moult is comparable to the casting of the scales in the reptiles.

Hairs. Hairs are only found in the mammalian class, and are divided into the long tactile bristles or vibrissae and the smaller hairs which maintain the warmth of the body. In some animals the hair, of the body is composed of long, stiff hairs, which are probably specialized for protective purposes, and short, soft hairs, which form the fur and keep in the warmth. Sometimes these long hairs are greatly enlarged and hardened to form protective spines as in the porcupine, hedgehog, spiny mouse and spiny ant-eater (Echidna).

Horns. Horns are of three kinds: (l) antlers, (2) hollow horns and (3) hairy horns of the rhinoceros.

Antlers are growths of true bone and, except for their very vascular covering of skin (velvet), are not exoskeletal structures. They grow with great rapidity, and in the deer family are renewed each year. As soon as their growth is finished the skin covering dries up and strips off. The small horns of giraffes are also bony structures though permanent.

The hollow horns of the ruminants (Bovidae) are cases of hardened epidermis which fit over a bony core and are permanent. They are found in both sexes, and in this differ from the antlers of the deer, which, except in the reindeer, are confined to the male. In the prongbuck (Antilocapra) the hollow horns are shed periodically.

The hairy horns of the rhinoceros are a mass of hairs cemented together by cells. The hairs grow from dermal papillae, but differ from true hairs in not being sunk into hair follicles.

Claws and Hoofs. These are modifications of nails, but whereas in nails and claws the structures are confined to the dorsal aspect of the digits, in hoofs they spread to the plantar surface as well. It has been shown in the embryological section of this article that the nail appears at the very tip of the digit, and in this position it remains in many amphibians, e.g. giant salamander, while in hoofed mammals it develops both ventrally and dorsally. In the Felidae the claws are retractile, but the real movement occurs between the middle and terminal phalanges of the digits.

Spurs. Spurs are quite distinct from nails and claws; they are very common in birds as horny epidermal sheaths covering bony outgrowths of the radial side of the carpus, metacarpus or metatarsus. The spur- winged goose has a carpal spur; in the screamers (Palamedea and Chauna) the spur or spurs are metacarpal, while in many gallinaceous birds (e.g. common fowls and pheasants) metatarsal spurs are found. In the mammals the male monotremes (Echidna and Ornithorhynchus) have spurs attached to an extra (? sesamoid) bone in the hind leg, perforated for the duct of the already mentioned poison gland.

Beaks. Certain fishes belonging to the family Mormyridae have a fleshy prolongation of the lower lip, and are hence termed beaked fishes. In the Amphibia Siren and the tadpoles of most Anura (frogs and toads) have small horny beaks. In the Reptilia horny beaks are found in the Chelonia, while in birds beaks are constant and replace the teeth in modern species. In mammals a horny beak is found in Ornithorhynchus, though it coexists with true teeth in the young and with horny pads in adult specimens. In all these cases the beaks are formed from cornified epidermal scales.

Baleen. The baleen which is found in the mouths of the Balaenidae or whalebone whales is a series of flattened triangular horny plates arranged on either side of the palate. The inner edges and apices of these are frayed out into long fibres which act as strainers. In Balaena mysticetus, the Greenland whale, there are nearly four hundred of these plates, the longest of which often exceed 10 ft. In its development baleen resembles rhinoceros horn in that it consists of a number of epidermal hair-like fibres cemented together and growing from dermal papillae, though not from true hair follicles.

For further details and literature see R. Wiedersheim, Comparative Anatomy of Vertebrates, translated by W. N. Parker (London, 1907) ; S. H. Reynolds, TheVertebrate Skeleton (Cambridge, 1897).

(F. G. P.)

ETHNOLOGY The colour of the human skin has always held an important place among physical criteria of race. Physiology explains colour as a consequence of climate and even diet. The pigment or colouring matter under the epidermis, or rather under the second or Malpighian skin, is not peculiar to the Negroid and other coloured races, but is common to all human beings. It is simply more abundant in certain peoples, and this abundance is attributed to the stimulating action of the solar heat, combined with moisture and an excess of vegetable food, yielding more carbon than can be assimilated, the character being then fixed by heredity. Theodor Waitz quotes examples proving " that hot and damp countries favour the darkening of the skin," and that the same race inclines to be darker in low marshy districts than on the hills. C. R.' Lepsius asserts that the hotter the climate the darker the negro, pointing out that if you-follow the line of greatest heat from Africa into Asia, it is in those regions of the latter continent that the darkest Asiatics are found. Many apparent exceptions to this general law occur, but they may be explainable as due to local causes. Thus Schweinfurth (Heart of Africa) believes that the reddish tint of the Bongos and other of the peoples inhabiting the hot, moist White Nile district is due to the ferruginous nature of the laterite soil: the hue of the A-Zandeh (Niam-Niam) of the Welle valley being possibly explicable in the same way. In South America all shades of complexion intermingle. Thus in Bolivia the coppery Maropas, the dark brown Aymaras, the yellowish Moxos, and the light Mosetenos, Siriones, and Guarayos are, so to speak, neighbours. In Australasia there is the contrast between the yellow-brown Malays and the sooty-black Tasmanians. Such deviations from the colour-law may be attributed'to descent (dark peoples migrating to cold, light to tropical countries), or to such varied causes as dryness, moisture, food and the vegetable peculiarities of the land, by all of which the complexion may be affected, and the influence of temperature mitigated.

The colour of the human skin cannot, then, be regarded as an entirely trustworthy racial test, even blackness not being an exclusively negro characteristic. It serves, however, to divide Man into three fundamental types corresponding to the three great ethnic groups, viz. the White, the Yellow and the Black man. The first predominate in Europe, the second in Asia, while the third have their chief centres in Africa and Melanesia. Interbreeding and, in a lesser degree, the influence of environment have caused the occurrence within the three main groups of almost every shade and tint of complexion. Thus the colour of the skin affords a faulty basis of ethnological classification, since in the same ethnic group it varies so widely and races of one group resemble in this particular races of another. The so-called Red Indians are usually classified as a fourth group, but they are not really red-skinned. The name has come about through their custom of smearing their faces with red ochre. But among the American aborigines, side by side with the yellow, olive brown or even black (e.g. the Charruas of Uruguay) , there are tribes of reddish-yellow or coppery hue. This tint is found also in certain African tribes. The palms of the hands and the soles of the feet of negroes are never black, but always yellowish, and in all coloured races the back of the body is a shade darker than the front.

It is noteworthy that the skin of the coloured races is always of a lighter tint in the newly-born than in the adult ; the negro baby is born a light grey colour, and the dark pigment is absent in the negro foetus. On the eighth day, sometimes as early as the third, the negro infant changes its colour to a hue nearly as dark as that of its parents. It would seem as if the blackness is associated with the general thickening of the skin and is an accompaniment of the :neral organic adaptation of the negro to his hot malarious climate, he effects of sunburn vary with different races. It is with the races having intermediate pigmentation, such as the dark Europeans and the yellow peoples, that the effect is most noticeable. With the former the Sun burns the skin uniformly, making them of the tint of mulattoes. The colour so acquired is merely temporary. It diminishes in winter, and disappears entirely on their return to a cold temperate climate. With the Asiatics the Sun causes different tints. The skins of the Indo-Chinese and the Malays become dark olive. The Fuegians and Galibis turn brick-colour or dull red. The Chinese skin turns darker in winter and paler in summer. Among certain peoples whose skins are naturally dark the parts of the body exposed to the light and air are often lighter than those covered by their clothes. This is the case with the Fuegians and the Sandwich Islanders. The fair European skin reddens under the Sun, passing from pale red to brick red or to patches of deep red.

Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)

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