Figure 3-25.Smooth muscle fibers.
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.
3-25). 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 (partitions) of the
heart, as well as the origins of the great 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 com-
plicated network (fig. 3-26). The cardiac muscle
has the most abundant blood supply of any mus-
cle in the body, receiving twice the blood flow of
the highly vascular skeletal muscles and far more
than the smooth muscles. Cardiac muscles con-
tract 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
IMPORTANT FUNCTIONAL MUSCLES
These muscles, shown in figures 3-27 and 3-28,
are described below.
The MASSETER muscle raises the mandible,
or lower jaw, to close the mouth. It is the chewing
Figure 3-26.Cardiac muscle fibers.
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 TEMPORAL muscle assists the masseter
and draws the mandible backward. It has its origin
in the temporal fossa and is inserted in the cor-
onoid process of the mandible.
The STERNOCLEIDOMASTOID muscles
are located on both sides of the neck. Acting in-
dividually, these muscles rotate the head left or
right. Acting together, they bend the head for-
ward toward the chest. The sternocleidomastoid
muscle originates in the sternum and clavicle and
is inserted in the mastoid process of the temporal
bone. This muscle is commonly affected in cases
of stiff neck.
The TRAPEZIUS muscles are a broad,
trapezium-shaped pair of muscles on the upper
back, which raise or lower the shoulders. They
cover approximately one-third of the back. They
originate in a large area, which includes the 12
thoracic vertebrae, the seventh cervical vertebra,