SKIN STRUCTURE The  EPIDERMIS  is  the  outer  skin  layer.  It is  made  up  of  tough,  flat,  scalelike  epithelial  cells. Four different sublayers of epidermal cells have been  identified.  The  uppermost  is  called  the  horny layer (stratum corneum). It is composed of scaly dead cells that form a protective surface and are gradually  sloughed  off  naturally  or  by  irritation (e.g.,  sunburn)  or  abrasion.  This  scaly  layer,  if unbroken, can block the passage of almost every known  type  of  germ;   however,   its   protective powers  are  reduced  if  the  skin  is  not  cleansed regularly. Two middle layers of cells may be pres- ent in a particular area of skin, depending on its thickness  (the  soles  of  the  feet  are  the  thickest skin, the eyelids the thinnest). In the innermost sublayer, the stratum germinativum, new epider- mal  cells  are  constantly  being  produced  to  replace the  sloughed  off  cells.  These  newly  formed  cells push  the  older  cells  outward.  As  they  approach the surface, they become drier or more scalelike. Because of this constant activity of the deeper cells of the epidermis, any injury of the outer layer of the skin is repaired in a few days without leaving a  scar.  Skin  pigment,  called  melanin,  which  is responsible  for  skin  color,  is  found  here  in  this deepest  sublayer.  The  color  and  quantity  of  the melanin are the chief factors in determining one’s complexion. The pigment can be darkened by ex- posure to the ultraviolet rays of the sun (tanning). Freckles  are  patches  of  melanin. The  DERMIS,  or  true  skin,  lies  below  the epidermis  and  gradually  blends  into  the  deeper tissues. It is a wide area of connective tissue that contains  blood  vessels,  hair  follicles,  nerve  end- ings, smooth muscles, and sweat and oil glands. The blood vessels of the dermis can dilate to contain  a  significant  portion  of  the  body’s  blood supply. This ability, along with the actions of the sweat glands, forms the body’s primary tempera- ture  regulating  mechanism.  The  constriction  or dilation of these blood vessels also affects blood pressure and the volume of blood available to the internal organs. The skin contains nerve endings that carry im- pulses  to  and  from  the  central  nervous  system. The nerves 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. Through these nerves, messages about the  external  environment  are  carried  to  the  brain. 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 ex- posure  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. 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 for- mation of nail will occur in the epitheliums of the nail  bed.  As  new  nail  is  formed,  the  whole  nail moves  forward,  becoming  longer. HAIR is an epithelial structure found on al- most  every  part  of  the  surface  of  the  body.  Its color  depends  on  the  type  of  melanin  present.  The hair has two components: the root below the sur- face and the shaft projecting above the skin. The root  is  embedded  in  a  pitlike  depression  called  the hair  follicle.  Hair  grows  as  a  result  of  the  divi- sion of the cells of the root. A small muscle, the arrector, 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 folli- cle is associated with two or more sebaceous (oil) glands. SEBACEOUS  GLANDS  are  found  in  most parts  of  the  skin  except  in  the  soles  of  the  feet and the palms of the hand. Their ducts open most frequently into the hair follicles and secrete an oily substance  that  lubricates  the  skin  and  hair,  keep- ing  them  soft  and  pliable  and  preventing  bacterial invasion. SWEAT GLANDS are found in almost every part of the skin. They are control mechanisms to reduce  the  body’s  heat  by  evaporation  of  water from  its  surface.  The  perspiration  secreted  is  a combination of water, salts, fatty acids, and urea. Normally, about one liter of this fluid is excreted daily.  However,  the  amount  varies  with  atmos- pheric  temperature  and  humidity  and  the  amount of exercise taken. When the outside temperature is high, or upon exercise, the glands secrete ex- cessive  amounts  to  cool  the  body  through  evap- oration. When evaporation cannot handle all the sweat that has been excreted, the sweat collects in  beads  on  the  surface  of  the  skin. CERUMINOUS   GLANDS   are   modified sweat  glands  found  only  in  the  auditory  canal. 3-19