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
There are three types of muscle tissue: skeletal,
smooth, and cardiac. Each is designed to perform a
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
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.
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.
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.
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.
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.
T h e
t r a p e z i u s
m u s c l e s
a r e
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.