idle,   it   eventually   becomes   rusty   and   useless. Similarly, a muscle cell that does not work atrophies, becoming weak and decreasing in size. 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. When a muscle dies, it becomes solid and rigid and no longer reacts. This stiffening, which occurs from 10 minutes  to  several  hours  after  death,  is  called  rigor mortis. MUSCLE TISSUES There  are  three  types  of  muscle  tissue:  skeletal, smooth,  and  cardiac.  Each  is  designed  to  perform  a specific function. Skeletal Skeletal, or striated, muscle tissues 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. 1-9). They are also called  voluntary muscles  because  they  are  under  the  control  of  our conscious   will.   These   muscles   can   develop   great power. Smooth Smooth, or nonstriated, muscle tissues are found in the walls of the stomach, intestines, urinary bladder, and blood vessels, as well as in the duct glands and in the skin. Under a microscope, the smooth muscle fiber lacks  the  striped  appearance  of  other  muscle  tissue (fig.   1-10).   This   tissue   is   also   called   involuntary muscle because it is not under conscious control. Cardiac The  cardiac  muscle  tissue  forms  the  bulk  of  the walls and septa (or partitions) of the heart, as well as the origins of the large blood vessels. The fibers of the cardiac  muscle  differ  from  those  of  the  skeletal  and smooth muscles in that they are shorter and branch into a complicated network (fig. 1-11). The cardiac muscle has the most abundant blood supply of any muscle in the body, receiving twice the blood flow of the highly vascular skeletal muscles and far more than the smooth muscles. Cardiac muscles contract to pump blood out of  the  heart  and  through  the  cardiovascular  system. Interference  with  the  blood  supply  to  the  heart  can result in a heart attack. MAJOR SKELETAL MUSCLES In   the   following   section,   the   location,   actions, origins, and insertions of some of the major skeletal muscles  are  covered.  In  figures  1-28  and  1-29  the superficial skeletal muscles are illustrated. Also note, the  names  of  some  of  the  muscles  give  you  clues  to their location, shape, and number of attachments. Temporalis The   temporalis   muscle   is   a   fan-shaped   muscle located on the side of the skull, above and in front of the ear. This muscle's fibers assist in raising the jaw and pass   downward   beneath   the   zygomatic   arch   to   the mandible (fig. 1-29). The temporalis muscle's origin is the temporal bone. It is inserted in the coronoid process (a prominence of bone) of the mandible. Masseter The masseter muscle raises the mandible, or lower jaw, to close the mouth (fig. 1-28). It is the chewing muscle in the mastication of food. It originates in the zygomatic  process  and  adjacent  parts  of  the  maxilla and is inserted in the mandible. Sternocleidomastoid The  sternocleidomastoid  muscles  are  located  on both   sides   of   the   neck.   Acting   individually,   these muscles rotate the head left or right (figs. 1-28 and 1-29).  Acting  together,  they  bend  the  head  forward toward   the   chest.   The   sternocleidomastoid   muscle originates in the sternum and clavicle and is inserted in the mastoid process of the temporal bone. When this muscle  becomes  damaged,  the  result  is  a  common condition known as a “stiff neck.” Trapezius T h e t r a p e z i u s m u s c l e s a r e a b r o a d , trapezium-shaped pair of muscles on the upper back, which  raise  or  lower  the  shoulders  (figs.  1-28  and 1-29). They cover approximately one-third of the back. They originate in a large area which includes the 12 thoracic vertebrae, the seventh cervical vertebra, and the  occipital  bone.  They  have  their  insertion  in  the clavicle and scapula. 1-19