circulation, digestion, and even such functions as speaking and seeing. At  one  end  of  some  muscles  are  long  white TENDONS that attach the muscles to bone. The point of fixed attachment of a muscle to bone is called   the   ORIGIN.   The   more   flexible   at- tachments, especially to a movable bone, is termed the   INSERTION. Muscle tissue has a highly developed ability to contract. CONTRACTIBILITY enables a mus- cle to become shorter or thicker, and this ability, along  with  interaction  with  other  muscles,  pro- duces  movement  in  internal  and  external  body parts.  Muscle  contraction  in  a  tissue  or  organ  pro- duces  motion  and  provides  power  and  speed  for body activity. A contracting muscle is referred to as a PRIME MOVER. A muscle that is relaxing while a prime mover is contracting is called the ANTAGONIST.  Muscular  tone,  or  TONICITY, is a continual state of partial contraction that gives muscles a certain firmness. ISOMETRIC muscle contraction occurs when the muscle is stimulated and shortens, but no movement occurs, as when a  person  tenses  his  or  her  muscles  against  an immovable  object.  Muscles  are  also  capable  of stretching when force is applied (EXTENSIBIL- ITY) and regaining their original form when that force   is   removed   (ELASTICITY). All types of muscles respond to stimulus. This property   is   called   EXCITABILITY   or   IR- RITABILITY.  The  mechanical  muscular  action of  shortening  or  thickening  is  activated  by  a stimulus sent through a motor nerve. All muscles are linked to nerve fibers that carry messages from the central nervous system. The chemical action of muscle fibers consists of  two  stages, CONTRACTION   and   RE- COVERY.  In  the  contraction  stage,  two  protein substances  (actin  and  myosin)  react  to  provide energy  through  the  breakdown  of  glycogen  into lactic  acid.  In  the  recovery  stage,  oxygen  reacts with  lactic  acid  to  release  carbon  dioxide  and water. When  a  muscle  contracts,  it  produces  chemical waste  products  (carbon  dioxide,  lactic  acid,  and acid phosphate), which make the muscle more ir- ritable.  If  contraction  is  continued,  the  muscle  will finally  cramp  up  and  refuse  to  move.  This  con- dition is known as fatigue. If it is carried too far, the muscle cells will not recover and permanent damage will result. Muscles, therefore, need rest to  allow  the  blood  to  carry  away  the  waste materials and bring in fresh glucose, oxygen, and protein to restore the muscle protoplasm and the energy that was used. The  importance  of  exercise  for  normal  mus- cle activity is clear, but excessive muscle strain is damaging.  For  example,  if  a  gasoline  motor stands   idle,   it   eventually   becomes   rusty   and useless. Similarly, a muscle cell that does not work becomes weak and flabby. On the other hand, a motor that is never allowed to stop and is forced to run too fast or to do too much heavy work soon wears  out  so  that  it  cannot  be  repaired.  In  the same way, a muscle cell that is forced to work too hard without proper rest will be damaged beyond repair.  Violent  exercise  is  never  good.  Exercise should  be  adapted  to  the  individual  and  should never be carried to the point of extreme fatigue. During   exercise,   massage,   or   ordinary   ac- tivities, the blood supply of muscles is increased. This brings in fresh nutritional material, carries away waste products more rapidly, and enables the muscles to build up and restore their efficiency and  tone. When a muscle dies, it becomes solid and rigid and  no  longer  reacts.  This  stiffening,  which  oc- curs from 10 minutes to several hours after death, is  called  RIGOR  MORTIS. Muscles seldom act alone; they usually work in muscle groups held together by sheets of a white fibrous tissue call FASCIA. There are three types of  muscle  tissue:  skeletal,  smooth,  and  cardiac. Each  is  designed  to  perform  a  specific  function. SKELETAL  MUSCLES  are  attached  to  the bones  and  give  shape  to  the  body.  They  are responsible  for  allowing  body  movement.  This type  of  muscle  is  sometimes  referred  to  as STRIATED because of the striped appearance of the muscle fibers under a microscope (fig. 3-24). They   are   also   called   VOLUNTARY   muscles because  they  are  under  the  control  of  our  con- scious   will.   These   muscles   can   develop   great power. SMOOTH,   or   NONSTRIATED,   muscle tissues are found in the walls of the stomach, in- testines,  urinary  bladder,  and  blood  vessels,  as Figure  3-24.—Striated muscle fibers. 3-15