idle, it eventually becomes rusty and useless.Similarly, a muscle cell that does not work atrophies,becoming weak and decreasing in size. On the otherhand, a motor that is never allowed to stop and is forcedto run too fast or to do too much heavy work soon wearsout so that it cannot be repaired. In the same way, amuscle cell that is forced to work too hard withoutproper rest will be damaged beyond repair.When a muscle dies, it becomes solid and rigid andno longer reacts. This stiffening, which occurs from 10minutes to several hours after death, is called rigormortis.MUSCLE TISSUESThere are three types of muscle tissue: skeletal,smooth, and cardiac. Each is designed to perform aspecific function.SkeletalSkeletal, or striated, muscle tissues are attached tothe bones and give shape to the body. They areresponsible for allowing body movement. This type ofmuscle is sometimes referred to as striated because ofthe striped appearance of the muscle fibers under amicroscope (fig. 1-9). They are also called voluntarymuscles because they are under the control of ourconscious will. These muscles can develop greatpower.SmoothSmooth, or nonstriated, muscle tissues are found inthe walls of the stomach, intestines, urinary bladder,and blood vessels, as well as in the duct glands and inthe skin. Under a microscope, the smooth muscle fiberlacks the striped appearance of other muscle tissue(fig. 1-10). This tissue is also called involuntarymuscle because it is not under conscious control.CardiacThe cardiac muscle tissue forms the bulk of thewalls and septa (or partitions) of the heart, as well asthe origins of the large blood vessels. The fibers of thecardiac muscle differ from those of the skeletal andsmooth muscles in that they are shorter and branch intoa complicated network (fig. 1-11). The cardiac musclehas the most abundant blood supply of any muscle inthe body, receiving twice the blood flow of the highlyvascular skeletal muscles and far more than the smoothmuscles. Cardiac muscles contract to pump blood outof the heart and through the cardiovascular system.Interference with the blood supply to the heart canresult in a heart attack.MAJOR SKELETAL MUSCLESIn the following section, the location, actions,origins, and insertions of some of the major skeletalmuscles are covered. In figures 1-28 and 1-29 thesuperficial skeletal muscles are illustrated. Also note,the names of some of the muscles give you clues totheir location, shape, and number of attachments.TemporalisThe temporalis muscle is a fan-shaped musclelocated on the side of the skull, above and in front of theear. This muscle's fibers assist in raising the jaw andpass downward beneath the zygomatic arch to themandible (fig. 1-29). The temporalis muscle's origin isthe temporal bone. It is inserted in the coronoid process(a prominence of bone) of the mandible.MasseterThe masseter muscle raises the mandible, or lowerjaw, to close the mouth (fig. 1-28). It is the chewingmuscle in the mastication of food. It originates in thezygomatic process and adjacent parts of the maxillaand is inserted in the mandible.SternocleidomastoidThe sternocleidomastoid muscles are located onboth sides of the neck. Acting individually, thesemuscles rotate the head left or right (figs. 1-28 and1-29). Acting together, they bend the head forwardtoward the chest. The sternocleidomastoid muscleoriginates in the sternum and clavicle and is inserted inthe mastoid process of the temporal bone. When thismuscle becomes damaged, the result is a commoncondition known as a “stiff neck.”TrapeziusT 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 and1-29). They cover approximately one-third of the back.They originate in a large area which includes the 12thoracic vertebrae, the seventh cervical vertebra, andthe occipital bone. They have their insertion in theclavicle and scapula.1-19
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