the  stratum corneum,  stratum lucidum  (not always present),   stratum  granulosum,   stratum  spinosum, and  stratum basale. Dermis The dermis, or true skin, lies below the epidermis and gradually blends into the deeper tissues (fig. 1-30). It is a wide area of connective tissue that contains blood vessels,   nerve   fibers,   smooth   muscles,   and   skin appendages. BLOOD  VESSELS.—The  blood  vessels  of  the dermis can dilate to contain a significant portion of the body's blood supply (fig. 1-30). This ability, along with the   actions   of   the   sweat   glands,   forms   the   body's primary   temperature-regulating   mechanism.   The constriction  or  dilation  of  these  blood  vessels  also affects   blood   pressure   and   the   volume   of   blood available to the internal organs. NERVE FIBERS.—The skin contains two types of  nerve  fibers  that  carry  impulses  to  and  from  the central nervous system (fig. 1-30). The nerve fibers are distributed to the smooth muscles in the walls of the arteries in the dermis and to the smooth muscles around the sweat glands and hair roots. The first type of nerve fiber   carries   impulses   to   the   dermal   muscles   and glands,  while  the  other  type  carries  impulses  from sensory  receptors  (i.e.,  detecting  touch).  Both  nerve fibers send messages about the external environment to the brain. SMOOTH  MUSCLES.—Smooth   involuntary muscles are found in the dermis. They are responsible for  controlling  the  skin  surface  area.  When  dilated, these   muscles   allow   for   maximum   skin   surface exposure to aid heat loss. When constricted, the skin surface   exposure   is   decreased,   thus   impeding   heat radiation.  Repeated  muscle  contractions  (shivering) are also a rapid means of generating body heat. Skin Appendages The  appendages  of  the  skin  are  the  nails,  hairs, sebaceous   glands,   sweat   glands,   and   ceruminous glands. NAILS.—The   nails   are   composed   of   horny epidermal scales and are found on the dorsal surfaces of the fingers and toes. They protect the many sensitive nerve   endings   at   the   ends   of   these   digits.   New formation of nail will occur in the epithelium of the nail bed. As a new nail is formed, the whole nail moves forward, becoming longer. HAIR.—Hair is an epithelial structure found on almost every part of the surface of the body (fig. 1-30). Its color depends on the type of melanin present. The hair has two components: the root below the surface and  the  shaft  projecting  above  the  skin.  The  root  is embedded   in   a   pit-like   depression   called   the   hair follicle. Hair grows as a result of the division of the cells   of   the   root.   A  small   muscle,   known   as   the arrector (fig. 1-30), fastens to the side of the follicle and is responsible for the gooseflesh appearance of the skin as a reaction to cold or fear. Each hair follicle is associated with two or more sebaceous glands. SEBACEOUS  GLANDS.—Sebaceous   glands are found in most parts of the skin except in the soles of the feet and the palms of the hand (fig. 1-30). Their ducts open most frequently into the hair follicles and secrete an oily substance that lubricates the skin and hair,  keeping  them  soft  and  pliable  and  preventing bacterial invasion. SWEAT GLANDS.—Sweat glands are found in almost  every  part  of  the  skin  (fig.  1-30).  They  are control   mechanisms   to   reduce   the   body's   heat   by evaporating  water  from  its  surface.  The  perspiration secreted is a combination of water, salts, amino acids, and  urea.  Normally,  about  one  liter  of  this  fluid  is excreted   daily.   However,   the   amount   varies   with atmospheric temperature and humidity and the amount of  exercise  taken.  When  the  outside  temperature  is high,   or   upon   exercise,   the   glands   secrete   large amounts to cool the body through evaporation. When 1-23 HM3F0130 SHAFT OF HAIR OPENING OF SWEAT DUCT NERVE FIBERS SWEAT DUCT SWEAT GLAND SEBACEOUS GLAND HAIR FOLLICLE ARRECTOR MUSCLE BLOOD VESSELS DERMIS EPIDERMIS Figure 1-30.—Cross section of the skin.