interlace in a thick network called a plexus. The cervical  plexus  is  located  in  the  neck,  and  the brachial  plexus  is  in  the  shoulder.  In  the  pelvic region   are   the   lumbar,   sacral,   and   pudendal plexuses. AUTONOMIC  NERVOUS  SYSTEM The  autonomic  nervous  system,  as  its  name implies,  functions  automatically.  It  helps  to regulate   the   smooth   muscles,   cardiac   muscle, digestive tube, blood vessels, sweat and digestive glands,  and  certain  endocrine  glands.  It  is  not directly  under  the  control  of  the  brain  but  usu- ally works in harmony with the nerves that are underthebrain’s  control.  The  autonomic  nervous system  is  divided  into  the  sympathetic  and parasympathetic systems (see table 3-1). Sympathetic Nervous System Numerous  ganglia  (nerve  centers)  located  just outside the spinal cord, beside the vertebrae, are the   basis   of   the   sympathetic   (thoracolumbar) system.  These  nerve  centers  connect  with  the thoracic  and  lumbar  regions  of  the  spinal  cord and, through the spinal nerves, with the muscles, organs,  and  glands  they  affect. Because  one  function  of  the  sympathetic system is to increase the activity of the body to enable  it  to  meet  danger  or  undergo  strenuous physical activity, it has been called the “fight or flight” nervous system. The sympathetic nerves, when  stimulated,  usually  discharge  as  a  unit,  and the  effects  can  be  noticed  especially  under  cir- cumstances  of  fright  or  rage;  for  example,  the heart  beats  faster,  blood  pressure  rises,  the  spleen discharges  red  blood  cells  into  the  blood,  the blood sugar level rises, the pupils dilate, and the peripheral blood vessels constrict. These changes prepare  the  body  for  a  stressful  situation. Parasympathetic  System The ganglia of the parasympathetic system are located  in  the  midportion  of  the  brain,  the medulla  oblongata,  and  the  sacral  regions.  For this   reason   the   parasympathetic   system   is sometimes  called  the  craniosacral  system.  The ganglia  in  the  midbrain  and  medulla  oblongata send  impulses  out  along  cranial  nerves  (oculo- motor, facial, glossopharyngeal, and vagus). The sacral  ganglia  stem  from  the  second,  third,  and fourth  sacral  nerves. The  parasympathetic  nerves  do  not  all discharge  at  once.  They  aim  more  toward  con- serving and restoring energy. Their actions slow the heart beat, lower the blood pressure, stimulate gastrointestinal  movements  and  secretion,  aid  ab- sorption,  constrict  the  pupils,  dilate  peripheral blood  vessels,  and  empty  the  bladder  and  rectum. Overall they promote the autonomic restoration of body systems to normal functioning after sym- pathetic stimulation. The  sympathetic  and  parasympathetic  systems counterbalance  each  other  to  preserve  a  har- monious balance of body functions and activities. THE  SENSORY  SYSTEM The sensory system functions to inform areas of the cerebral cortex of changes that are taking place within the body or in the external environ- ment. The special sensory receptors are designed to respond only to a special individual stimulus such  as  sound  waves,  light,  taste,  smell,  pressure, heat,  cold,  pain,  or  touch.  Positional  changes, balance,  hunger,  and  thirst  sensations  are  also detected  and  passed  on  to  the  brain. SMELL Odor  is  perceived  upon  stimulation  of  the receptor  cells  in  the  olfactory  membrane  of  the nose. The olfactory receptors are very sensitive, but they are also easily fatigued. This explains why odors  that  are  initially  very  noticeable  are  not sensed  after  a  short  time.  Smell  is  not  as  well developed  in  man  as  in  other  mammals. TASTE The taste buds are located in the tongue. The sensation  of  taste  is  limited  to  sour,  sweet,  bit- ter, and salty. Many foods and drinks tasted are actually  smelled,  and  their  taste  depends  upon their odor. This can be demonstrated by pinching the nose shut when eating onions. Sight can also affect  taste.  Several  drops  of  green  food  color- ing  in  a  glass  of  milk  will  make  it  all  but  un- palatable, even though the true taste has not been affected. SIGHT The  eye,  the  organ  of  sight,  is  a  specialized structure for the reception of light. It is assisted in its function by accessory structures, such as the 3-34