342
UNIT 2
Covering, Support, and Movement of the Body
10
Unlike the thorax, the anterior and lateral abdominal wall has no
bony reinforcements (ribs). Instead, it is a composite of four paired
muscles, their investing fasciae, and aponeuroses.
Three broad flat muscle pairs, layered one atop the next,
form the lateral abdominal walls: The fibers of the
external
oblique muscle
run inferomedially and at right angles to
those of the
internal oblique
, which immediately underlies it
(Figure 10.12a, b). The fibers of the deep
transversus abdominis
muscle
run horizontally across the abdomen at an angle to both.
This alternation of fascicle directions is similar to the construction
of plywood (which is made of sheets with different grains) and
provides great strength.
These three muscles blend into broad insertion aponeuroses
anteriorly. The aponeuroses, in turn, enclose a fourth muscle pair
medially, the straplike
rectus abdominis muscles
, and then fuse,
forming the
linea alba
(“white line”), a tendinous raphe (seam)
that runs from the sternum to the pubic symphysis (Figure 10.12a,
c). The snug enclosure of the rectus abdominis muscles within
the aponeuroses, the so-called
rectus sheath
, prevents them from
“bowstringing” (protruding anteriorly) when they contract. Table
10.4 describes the quadratus lumborum muscles of the
posterior
abdominal wall.
The abdominal muscles protect and support the viscera most
effectively when they are well toned. When weak or severely
stretched (as during pregnancy), they allow the abdomen to
become pendulous (i.e., to form a potbelly). Additional functions
include lateral flexion and rotation of the trunk and flexion of the
trunk against resistance (as in sit-ups).
During inspiration, the abdominal muscles relax, allowing the
descending diaphragm to push the abdominal viscera inferiorly.
When these abdominal muscles contract, several different activities
may occur. For example, when they contract simultaneously, they
pull the ribs inferiorly and compress the abdominal contents.
This forces the visceral organs upward on the diaphragm, aiding
forced expiration. When the abdominal muscles contract with
the diaphragm and the glottis is closed (the Valsalva maneuver),
the increased intra-abdominal pressure promotes urination,
defecation, childbirth, vomiting, coughing, screaming, sneezing,
burping, and nose blowing. (Next time you perform one of these
activities, feel your abdominal muscles contract under your skin.)
These muscles also contract during heavy lifting— sometimes so
forcefully that hernias result. Contracting the abdominal muscles
as the deep back muscles contract helps prevent hyperextension of
the spine and splints the entire body trunk.
MUSCLE GALLERY
Table 10.6
Muscles of the Abdominal Wall: Trunk Movements and Compression
of Abdominal Viscera
(Figure 10.12)
MUSCLE
DESCRIPTION
ORIGIN (O) AND
INSERTION (I)
ACTION
NERVE
SUPPLY
MUSCLES OF THE
ANTERIOR AND LATERAL
ABDOMINAL WALL
Four paired flat muscles; important in supporting and protecting abdominal viscera; promote lateral flexion and
flexion of vertebral column
Rectus abdominis
(rek
9
tus ab-dom
9
ı˘-nis)
(
rectus
5
straight;
abdom
5
abdomen)
Medial superficial muscle
pair; extend from pubis
to rib cage; ensheathed
by aponeuroses of lateral
muscles; segmented by
three tendinous inter-
sections
O—pubic crest and
symphysis
I—xiphoid process and
costal cartilages of ribs
5–7
Flex and rotate lumbar
region of vertebral
column
; fix and depress
ribs, stabilize pelvis
during walking, increase
intra-abdominal pressure;
used in sit-ups, curls
Intercostal nerves
(T
6
or T
7
–T
12
)
External oblique
(o-ble
-
k
9
)
(
external
5
toward outside;
oblique
5
running at
an angle)
Largest and most superficial
of the three lateral muscles;
fibers run downward and
medially (same direction
outstretched fingers take
when hands are in pants
pockets); aponeurosis turns
under inferiorly, forming
inguinal ligament
O—by fleshy strips from
outer surfaces of lower
eight ribs
I—most fibers insert into
linea alba via a broad
aponeurosis; some insert
into pubic crest and
tubercle and iliac crest
When pair contract
simultaneously,
flex
vertebral column and
compress abdominal
wall
and increase intra-
abdominal pressure;
acting individually,
aid muscles of back in
rotating trunk and flexing
laterally
; used in oblique
curls
Intercostal nerves
(T
7
–T
12
)
Internal oblique
(
internal
5
toward the
inside; deep)
Most fibers run upward
and medially; however, the
muscle fans so its inferior
fibers run downward and
medially
O—lumbar fascia, iliac
crest, and inguinal
ligament
I—linea alba, pubic crest,
last three or four ribs,
and costal margin
As for external oblique
Intercostal nerves
(T
7
–T
12
) and L
1
Transversus abdominis
(trans-ver
9
sus)
(
transverse
5
running
straight across)
Deepest (innermost) muscle
of abdominal wall; fibers
run horizontally
O—inguinal ligament,
lumbar fascia, cartilages
of last six ribs; iliac crest
I—linea alba, pubic crest
Compresses abdominal
contents
Intercostal nerves
(T
7
–T
12
) and L
1
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