The muscles of the abdominal wall
1. Anterolateral
a. Two vertical muscles
i. Rectus abdominus
ii. Pyramidalis
b. Three thin muscle layers that alternate in in their fiber direction. Their
aponeurotic extensions form a sheath for the rectus abdominus and
pyramidalis muscles.
i. External Abdominal Oblique
ii. Internal Abdominal Oblique
iii. Transversus Abdominus
2. Posterior: Quadratus Lumborum
3. The external abdominal oblique arises form the lower border and outer surface of
the lower eight ribs (5th through 12th). The upper five interdigitate with the
serratus anterior and the lower three interdigitate with the costal attachments of
the latissimus dorsi.
a. The muscle fibers arising from the last two ribs descend vertically and and
attach to the anterior half of the outer lip of the iliac crest. The posterior
margin helps to form the lumbar triangle or Triangle of Petit
i. The posterior margin of the external abdominal oblique
ii. The lateral border of the latissimus dorsi muscle that attaches to
the iliac crest
iii. The iliac crest
b. The remaining six are directed obliquely downward and anteriorly to end
in the external Oblique aponeurosis. The muscle fibers terminate along a
line that begins adjacent to the anterior superior iliac spine and ascends
obliquely to the costal margin.
c. The aponeurosis
i. ultimately forms the linea alba with the help of the fibers from the
other side.
ii. fuses with that of the internal abdominal oblique to form the sheath
for the rectus abdominus muscle at its lateral border above the
umbilicus
iii. Form the inguinal ligament. This is a thickened rolled under border
that stretches form the ASIS to the pubic tubercle. The fascia lata
attaches to the inferior edge. The lateral one third of the inguinal
ligament is attached internally to the iliacus fascia as this fascia
passes into the thigh on the iliacus muscle.
4. Internal Abdominal Oblique
a. Arises from the posterior layer of the thoracolumbar fascia, the anterior
two thirds of the iliac crest, the lateral two thirds of the inguinal ligament
and by deeper fibers from the iliacus fascia for 5-6 cm medial to the ASIS
b. From this posterior and inferior line of origin, the muscle fans out into a
very extensive insertion.
c. The posterior fibers ascend vertically to insert on the inferior borders of
the lower three to four ribs and appear to be in continuity with the internal
intercostal muscles.
d. The other fibers spread out like a fan and end in an aponeurosis as they
approach the linea semilunaris. The ultimate insertion is the linea alba and
the pubic bone.
e. In the upper three-fourths of the abdomen the aponeurosis splits at the
linea semilunaris sending one sheet anterior and one posterior to the rectus
abdominus muscle. In the lower one-fourth the aponeurosis fails to split
and passes to the median line entirely anterior to the rectus abdominus
muscle forming an Arcuate Line
5. The Transversus Abdominus Muscle
a. This is the third and innermost flat muscle of the abdomen
b. It arises from the
i. deep surface of the costal margins of the lower six ribs and its
fascicles interdigitate with the diaphragm
ii. the fusion of the posterior and anterior layers of the thoracolumbar
fascia
iii. from the anterior three-fourths of the iliac crest
iv. the lateral third of the inguinal ligament
v. and from the iliacus fascia
c. the muscle fibers end in the aponeurosis and combine with that of the
internal abdominal oblique aponeurosis to form the posterior layer of the
rectus abdominus sheath. But midway between the umbilicus and the
symphysis pubis the aponeurosis of all three flat muscles pass anterior to
the rectus abdominus muscle forming an Arcuate Line.
6. The rectus sheath is divided into
a. The upper three-fourths where the
i. aponeurosis of the internal oblique splits to form an anterior and
posterior layer around the rectus abdominus and the
ii. the aponeurosis of the transversus abdominus helps to form the
posterior aspect of the sheath
b. The lower one-fourth
i. There is no split of the internal obdominal oblique aponeursois and
the
ii. Transversus abdominus goes anterior to the muscle leaving on the
transversalis fasica and peritoneum to cover its inside