Muscles of the Thorax, Abdomen,
Neck, and Back perform Many
Functions
Muscles of the thorax (chest) have three functions: They
are involved in breathing, movement of the shoulder, and
movement of the humerus. The chest muscles involved
in breathing alter the size of the thoracic cavity. Dur-
ing inhalation (breathing in), the cavity increases in size;
during exhalation (breathing out), the cavity decreases
in size. Muscles that help move the shoulder also act to
hold the scapula in place so that it is a stable point of ori-
gin for the muscles that move the humerus. These mus-
cles are illustrated in Figure 6.16, which also provides
their respective origins, insertions, and actions.
M uscles o f th e th o ra x involved in b reath in g
• Figure 6.16
External
intercostals
Internal
intercostals
Sternum
Diaphragm
Internal-----
intercostals
External —
intercostals
Anterior superficial view
Anterior deep view
Muscle
Origin
Insertion
Action
Muscles of the thorax involved in breathing
Diaphragm
(DÏ-a-fram)
Xiphoid process
of sternum, costal
cartilages of inferior six
ribs, lumbar vertebrae,
and their intervertebral
discs
Central tendon (strong
aponeurosis near the center of
the diaphragm)
Contraction causes it to flatten and increases
the vertical (top-to-bottom) dimension of
the thoracic cavity, resulting in inhalation;
relaxation causes it to move superiorly and
decreases the vertical dimension of the
thoracic cavity, resulting in exhalation
External intercostals
Inferior border of rib
above
Superior border of rib below
Contraction elevates the ribs and increases
the anteroposterior and lateral dimensions
of the thoracic cavity, resulting in inhalation;
relaxation depresses the ribs and decreases
the anteroposterior and lateral dimensions of
the thoracic cavity, resulting in exhalation
Internal intercostals
Superior border of rib
below
Inferior border of rib above
Contraction draws adjacent ribs together to
further decrease the anteroposterior (front-to-
back) and lateral (side-to-side) dimensions of
the thoracic cavity during forced exhalation
Skeletal Muscles Are Grouped Based on Location and Action 175
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