176 Introduction to Anatomy and Physiology
Figure 5.18 Muscles of the Trunk
Muscle Location Primary Functions
Anterior Muscles
Pectoralis major upper chest; connects sternum, shoulder
girdle, and upper ribs to proximal
humerus
adduction and fl exion of arm
Rectus abdominis center front of abdomen; connects ribs to
pubic crest
fl exion and lateral fl exion of trunk
External oblique
front of abdomen; connects lower eight
ribs to anterior iliac crest
fl exion, lateral fl exion, and rotation to
opposite side of trunk
Internal oblique front of abdomen beneath the external
obliques;connects lower four ribs with
the iliac crest
fl exion, lateral fl exion, and rotation to
same side of trunk
Posterior Muscles
Trapezius
upper back and neck; connects skull
and thoracic vertebrae to clavicle and
scapula
extension and hyperextension of head
Erector spinae length of vertebral column; connects
adjacent vertebrae
extension, lateral fl exion, and rotation to
opposite side of vertebral column
Muscles for Breathing
Diaphragm dome-shaped muscle separating
thoracic and abdominal cavities
enlarges thoracic cavity for inhalation
Internal intercostals connect the ribs; located between them decrease thoracic cavity during forced
expiration
External intercostals connect the ribs; located between them help enlarge thoracic cavity for inhalation
serve as a protective sheath for the organs of the
thoracic and abdominal cavities. The locations
and primary functions of the major muscles of
the anterior and posterior trunk are summarized
in the table in Figure 5.18. These muscles are
showin in Figure 5.19.
Upper Limb Muscles
Because the shoulder is a ball-and-socket
joint and the most freely movable joint in the
human body, the movement capabilities of the
upper limb are impressive. This large range of
motion is achieved because the bone structure
of the glenohumeral (GLEH-noh-HYOO-mer-
al) joint provides little to no stability, rendering
it susceptible to dislocation. Therefore, it is up
to the large, powerful muscles surrounding the
shoulder to maintain the stability and integrity
of the joint.
The arm muscles enable strong, controlled
movements in sports such as gymnastics, rowing,
and archery, as well as fast, powerful movements
in weightlifting, boxing, and throwing. The
dexterity of the fi nger muscles enables precise
movements, such as typing, texting, knitting, and
playing musical instruments.
The joints of the upper limb include those
of the shoulder, elbow, wrist, and fi ngers. In this
lesson we include information about the major
muscles that cross the shoulder and elbow joints.
Because there are nine muscles that cross the
wrist and ten muscles within the hand (some