The muscles of the trunk include those that move the vertebral
column, the muscles that form the thoracic and abdominal walls,
and those that cover the pelvic outlet.
The erector spinae group of muscles on each side of the vertebral
column is a large muscle mass that extends from the sacrum to the
skull. These muscles are primarily responsible for extending the
vertebral column to maintain erect posture. The deep back muscles
occupy the space between the spinous and transverse processes of
adjacent vertebrae.
The muscles of the thoracic wall are involved primarily in the
process of breathing. The intercostal muscles are located in spaces
between the ribs. They contract during forced expiration. External
intercostal muscles contract to elevate the ribs during the inspiration
phase of breathing. The diaphragm is a dome-shaped muscle that forms
a partition between the thorax and the abdomen. It has three openings
in it for structures that have to pass from the thorax to the abdomen.
The abdomen, unlike the thorax and pelvis, has no bony reinforcements
or protection. The wall consists entirely of four muscle pairs,
arranged in layers, and the fascia that envelops them. The abdominal
wall muscles are identified in the illustration below.
The pelvic outlet is formed by two muscular sheets and their associated
fascia.
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