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SKIN AND EXOSKELETON

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Originally appearing in Volume V25, Page 189 of the 1911 Encyclopedia Britannica.
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SKIN AND EXOSKELETON , in See also:

anatomy. The skin (A.-S. scinn) is the covering of the whole See also: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 See also: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 See also:main layers are recognized in the - From See also:Robert Howden in See also:Cunningham's See also:Text-See also:Book of Anatomy. skin; superficially there is the See also:scarf skin or epidermis and more deeply the dermis or true skin. The epidermis under the micro-See also:scope is seen to consist of five layers. On the See also:surface is the horny layer or stratum corneum (see fig. r) composed of layers of See also:scale-like 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 See also: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 See also:negro's skin is found.

The fifth and deepest layer of the epidermis is the stratum germinativum, 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 See also:

white fibrous See also:tissue with a small number of yellow elastic See also: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 See also:fit into corresponding depressions on the deep surface ofthe epidermis. In sensitive parts like the palms and soles these papillae are specially prominent and See also:form wavy lines, each of which consists of a See also:double See also: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 See also:identification. The papillae contain leashes of See also:blood-vessels, and in some of them are See also:special tactile corpuscles in which the nerves end (see See also:NERVOUS See also:SYSTEM). In the deeper or reticular layer of the true skin the fibrous feltwork is looser and encloses pellets of See also: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 See also:present the See also:hair follicles See also: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 See also:

man on the See also:scalp, eyelids, eyebrows, armpits, pubic region, See also:vestibule of the See also:nose, See also:external auditory meatus, See also:face, ventral surface of the See also:trunk and dorsal surfaces of the See also:leg, forearm and See also:hand; indeed the only places which are quite See also: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 See also:puberty. They are of See also:great anthropological See also:interest since they differ in See also:colour and texture in different races, sometimes being straight, sometimes wavy, sometimes See also:curly. The amount and See also:distribution of See also:long hairs also vary with the See also:race. In See also:section it is only the straight hairs which are circular; wavy and curly hairs are See also:oval. In the centre of each hair is the medulla or See also:pith, though this is not always present; it is composed o'f nucleated cells containing pigment, fat and See also: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 See also:root from a hair follicle (see fig. 2), which is a tubular inpushing of the epidermis into the true skin or, in the See also:case of large hairs, deeper still into the superficial See also:fascia. It is divided into an inner and See also: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 See also:lower See also: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 See also:

side toward which it is leaning is a small See also:band of non-striated See also: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 See also:general See also:con-See also:traction of these small muscles leads to the See also:familiar See also: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 See also:fold of skin, and the corium on which it lies is known as the See also:nail See also:matrix. The body of the nail also lies on the corium, or true skin, which forms the nail See also: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 See also:horn. Near the root of each nail is a semi-lunar See also:area which is more opaque than the See also: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 See also: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 See also:

palm there are nearly 3000 to a square See also:inch, while in the skin of the back they do not reach Soo to the same area. In the armpits and groins they are very large. Each consists of a single long See also:tube, lined by columnar epithelium, and coiled up into a See also:ball or glomerulus in the subcutaneous tissue, after which it pierces the corium and epidermis to reach the surface at the See also:porus sudoriferus. Where the stratum corneum of the epidermis is thick the duct is See also:twisted like a corkscrew as it goes through. The glands of Moll in the eyelids and, the ceruminous or See also:wax glands of the See also:ear are modified sweat glands; the former, when inflamed, cause a " See also:sty." See also:EMBRYOLOGY The skin is derived partly from the ectoderm and partly from the mesoderm of the embryo. The whole of the epidermis into the corium. During the fourth and fifth months the body becomes covered by See also:fine unpigmented hairs which are known as lanugo; these begin to disappear about the eighth See also:month, but some remain until after See also: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 of embryonic See also: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 (See also:

London, 1906) ; J. C. Heisler, Text-book of Embryology (London, 1907) ; See also:Quain's Anatomy, vol. i. (London, 1908).

End of Article: SKIN AND EXOSKELETON

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