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Encyclopedia Britannica - Main :: SIV-SOU |
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SKIN AND EXOSKELETON , in anatomy. The skin (A.-S. scinn) is the covering of the whole body
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
- From Robert Howden in Cunningham's Text-Book of Anatomy. skin; superficially there is the scarf skin or epidermis and more deeply the dermis or true skin. The epidermis under the micro- scope is seen to consist of five layers. On the surface is the horny layer or stratum corneum (see fig. r) composed of layers of 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 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 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 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 ofthe 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
special
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
great
interest
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 con-traction 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 Soo 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 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 fine unpigmented hairs which are known as lanugo; these begin to disappear about the eighth month, but some remain until after birth
birth
become very large in the later months of 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, 1907) ; Quain's Anatomy, vol. i. (London, 1908). End of Article: SKIN AND EXOSKELETON If you wish, you can link directly to this article.
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