340
UNIT 2
Covering, Support, and Movement of the Body
10
The deep muscles of the thorax promote movements necessary for
breathing. Breathing consists of two phases—inspiration (inhaling)
and expiration (exhaling)—caused by cyclic changes in the volume of
the thoracic cavity.
Two main layers of muscles help form the anterolateral wall of
the thorax.* The thoracic muscles are very short, most extending
only from one rib to the next. On contraction, they draw the
somewhat flexible ribs closer together. The
external intercostal
muscles
, considered inspiratory muscles, form the more superficial
layer (Figure 10.11a). They lift the rib cage, which increases the
anterior to posterior and side-to-side dimensions of the thorax.
The
internal intercostal muscles
form the deeper layer and may
aid active (forced) expiration by depressing the rib cage. (However,
quiet expiration is largely passive, resulting from relaxation of the
external intercostals and diaphragm and elastic recoil of the lungs.)
The
diaphragm
, the most important muscle of inspiration, forms
a muscular partition between the thoracic and abdominopelvic
cavities (Figure 10.11b, c). In the relaxed state, the diaphragm is
dome shaped. When it contracts it moves inferiorly and flattens,
increasing the volume of the thoracic cavity, which draws air into
the respiratory passageways. The alternating rhythmic contraction
and relaxation of the diaphragm also causes pressure changes in
the abdominopelvic cavity below that facilitate the return of blood
to the heart.
In addition you can contract the diaphragm voluntarily to push
down on the abdominal viscera and increase the pressure in the
abdominopelvic cavity to help evacuate pelvic organ contents
(urine, feces, or a baby) or lift weights. When you take a deep
breath to fix the diaphragm, the abdomen becomes a firm pillar
that will not buckle under the weight being lifted. Needless to say,
it is important to have good control of the urinary bladder and
anal sphincters during such maneuvers.
With the exception of the diaphragm, which is served by the
phrenic nerves
, the muscles listed in this table are served by the
intercostal nerves
, which run between the ribs.
Forced breathing (as during exercise) calls into play a number
of other muscles that insert into the ribs. For example, during
forced inspiration the scalene and sternocleidomastoid muscles of
the neck help lift the ribs. Forced expiration is aided by muscles
that pull the ribs inferiorly and those that push the diaphragm
superiorly by compressing the abdominal contents (abdominal wall
muscles).
MUSCLE GALLERY
Table 10.5
Deep Muscles of the Thorax: Breathing
(Figure 10.11)
MUSCLE
DESCRIPTION
ORIGIN (O) AND
INSERTION (I)
ACTION
NERVE
SUPPLY
External intercostals
(in
0
ter-kos
9
talz)
(
external
5
toward the
outside;
inter
5
between;
cost
5
rib)
11 pairs lie between ribs;
fibers run obliquely (down
and forward) from each
rib to rib below; in lower
intercostal spaces, fibers are
continuous with external
oblique muscle, forming
part of abdominal wall
O—inferior border of rib
above
I—superior border of rib
below
With first ribs fixed by
scalene muscles,
pull ribs
toward one another to
elevate rib cage
; aid in
inspiration; synergists of
diaphragm
Intercostal nerves
Internal intercostals
(
internal
5
toward the
inside, deep)
11 pairs lie between ribs;
fibers run deep to and
at right angles to those
of external intercostals
(i.e., run downward and
posteriorly); lower internal
intercostal muscles are
continuous with fibers of
internal oblique muscle of
abdominal wall
O—superior border of
rib below
I—inferior border (costal
groove) of rib above
With 12th ribs fixed by
quadratus lumborum,
muscles of posterior
abdominal wall, and
oblique muscles of the
abdominal wall, they
draw ribs together
and depress rib cage
;
aid forced expiration;
antagonistic to external
intercostals
Intercostal nerves
Diaphragm
(di
9
ah-fram)
(
dia
5
across;
phragm
5
partition)
Broad muscle pierced by
the aorta, inferior vena
cava, and esophagus, forms
floor of thoracic cavity;
dome shaped in relaxed
state; fibers converge from
margins of thoracic cage
toward a boomerang-
shaped central tendon
O—inferior, internal
surface of rib cage and
sternum, costal cartilages
of last six ribs and
lumbar vertebrae
I—central tendon
Prime mover of
inspiration; flattens on
contraction
, increasing
vertical dimensions of
thorax; when strongly
contracted, dramatically
increases intra-abdominal
pressure
Phrenic nerves
*Although there is a third (deepest) muscle layer of the thoracic wall, the muscles are small and discontinuous. Additionally, their function is unclear, so they
are not included in this table.
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