the stratum corneum, stratum lucidum (not alwayspresent), stratum granulosum, stratum spinosum,and stratum basale.DermisThe dermis, or true skin, lies below the epidermisand gradually blends into the deeper tissues (fig. 1-30).It is a wide area of connective tissue that contains bloodvessels, nerve fibers, smooth muscles, and skinappendages.BLOOD VESSELS.—The blood vessels of thedermis can dilate to contain a significant portion of thebody's blood supply (fig. 1-30). This ability, along withthe actions of the sweat glands, forms the body'sprimary temperature-regulating mechanism. Theconstriction or dilation of these blood vessels alsoaffects blood pressure and the volume of bloodavailable to the internal organs.NERVE FIBERS.—The skin contains two typesof nerve fibers that carry impulses to and from thecentral nervous system (fig. 1-30). The nerve fibers aredistributed to the smooth muscles in the walls of thearteries in the dermis and to the smooth muscles aroundthe sweat glands and hair roots. The first type of nervefiber carries impulses to the dermal muscles andglands, while the other type carries impulses fromsensory receptors (i.e., detecting touch). Both nervefibers send messages about the external environmentto the brain.SMOOTH MUSCLES.—Smooth involuntarymuscles are found in the dermis. They are responsiblefor controlling the skin surface area. When dilated,these muscles allow for maximum skin surfaceexposure to aid heat loss. When constricted, the skinsurface exposure is decreased, thus impeding heatradiation. Repeated muscle contractions (shivering)are also a rapid means of generating body heat.Skin AppendagesThe appendages of the skin are the nails, hairs,sebaceous glands, sweat glands, and ceruminousglands.NAILS.—The nails are composed of hornyepidermal scales and are found on the dorsal surfacesof the fingers and toes. They protect the many sensitivenerve endings at the ends of these digits. Newformation of nail will occur in the epithelium of thenail bed. As a new nail is formed, the whole nail movesforward, becoming longer.HAIR.—Hair is an epithelial structure found onalmost every part of the surface of the body (fig. 1-30).Its color depends on the type of melanin present. Thehair has two components: the root below the surfaceand the shaft projecting above the skin. The root isembedded in a pit-like depression called the hairfollicle. Hair grows as a result of the division of thecells of the root. A small muscle, known as thearrector (fig. 1-30), fastens to the side of the follicleand is responsible for the gooseflesh appearance of theskin as a reaction to cold or fear. Each hair follicle isassociated with two or more sebaceous glands.SEBACEOUS GLANDS.—Sebaceous glandsare found in most parts of the skin except in the soles ofthe feet and the palms of the hand (fig. 1-30). Theirducts open most frequently into the hair follicles andsecrete an oily substance that lubricates the skin andhair, keeping them soft and pliable and preventingbacterial invasion.SWEAT GLANDS.—Sweat glands are found inalmost every part of the skin (fig. 1-30). They arecontrol mechanisms to reduce the body's heat byevaporating water from its surface. The perspirationsecreted is a combination of water, salts, amino acids,and urea. Normally, about one liter of this fluid isexcreted daily. However, the amount varies withatmospheric temperature and humidity and the amountof exercise taken. When the outside temperature ishigh, or upon exercise, the glands secrete largeamounts to cool the body through evaporation. When1-23HM3F0130SHAFT OFHAIROPENING OFSWEAT DUCTNERVEFIBERSSWEATDUCTSWEATGLANDSEBACEOUSGLANDHAIRFOLLICLEARRECTORMUSCLEBLOODVESSELSDERMISEPIDERMISFigure 1-30.—Cross section of the skin.
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