The Anterolateral Abdominal Wall

Introduction
• The abdominal wall encloses the abdominal
cavity, and can be divided into anterolateral and
posterior sections.
• Its key functions include:
– Forms a firm, flexible wall which keeps the abdominal
viscera in the abdominal cavity.
– Protects the abdominal viscera from injury.
– Maintains the anatomical position of abdominal
viscera against gravity.
– Assists in forceful expiration by pushing the abdominal
viscera upwards.
– Involved in any action (coughing, vomiting) that
increases intra-abdominal pressure.

Layers of anterolateral abdominal wall
• The anterolateral abdominal wall consists of
four main layers (external to internal):
– Skin
– superficial fascia
– muscles and associated fascia
– peritoneum.

Summary of layers anterior abdominal
wall
• There are Nine layers to the abdominal wall:
i. skin,
ii. subcutaneous tissue,
iii. superficial fascia,
iv. external oblique muscle,
v. internal oblique muscle,
vi. transversus abdominis muscle,
vii. transversalis fascia,
viii. preperitoneal adipose and areolar tissue, and
ix. peritoneum
• parietal peritoneum
• visceral peritoneum

Superficial Fascia
• The superficial fascia consists of fatty connective
tissue.
• The composition of this layer depends on its
location:
– Above the umbilicus – a single sheet of connective
tissue. It is continuous with the superficial fascia in
other regions of the body.
– Below the umbilicus – divided into two layers;
• the fatty superficial layer (Camper’s fascia) and
• the membranous deep layer (Scarpa’s fascia).
• The superficial vessels and nerves run between these two
layers of fascia.

The layers of the anterolateral abdominal wall. Below the umbilicus, there are
two layers of superficial fascia – Camper’s and Scarpa’s.

Muscles of the Abdominal Wall
• The muscles of the anterolateral abdominal
wall can be divided into two main groups:
– Flat muscles – three flat muscles, situated laterally
on either side of the abdomen.
– Vertical muscles – two vertical muscles, situated
near the mid-line of the body.

Flat Muscles
• There are three flat muscles located laterally in
the abdominal wall, stacked upon one another.
• Their fibres run in differing directions and cross
each other – strengthening the wall, and
decreasing the risk of herniation.
• In the anteromedial aspect of the abdominal wall,
each flat muscle forms an aponeurosis (a broad,
flat tendon), which covers the vertical rectus
abdominis muscle.
• The aponeuroses of all the flat muscles become
entwined in the midline, forming the linea alba (a
fibrous structure that extends from the xiphoid
process of the sternum to the pubic symphysis).

Flat muscles
1. External Oblique
• The external oblique is the largest and most
superficial flat muscle in the abdominal wall.
Its fibres run inferomedially.
– Attachments: Originates from ribs 5-12, and
inserts into the iliac crest and pubic tubercle.
– Functions: Contralateral rotation of the torso.
– Innervation: Thoracoabdominal nerves (T7-T11)
and subcostal nerve (T12).

Flat Muscles
2. Internal Oblique
• The internal oblique lies deep to the external oblique.
It is smaller and thinner in structure, with its fibres
running superomedially (perpendicular to the fibres of
the external oblique).
– Attachments: Originates from the inguinal ligament, iliac
crest and lumbodorsal fascia, and inserts into ribs 10-12.
– Functions: Bilateral contraction compresses the abdomen,
while unilateral contraction ipsilaterally rotates the torso.
– Innervation: Thoracoabdominal nerves (T6-T11), subcostal
nerve (T12) and branches of the lumbar plexus.

Flat muscles
3. Transversus Abdominis
• The transversus abdominis is the deepest of the flat
muscles, with transversely running fibres. Deep to this
muscle is a well-formed layer of fascia, known as the
transversalis fascia.
– Attachments: Originates from the inguinal ligament, costal
cartilages 7-12, the iliac crest and thoracolumbar fascia.
Inserts into the conjoint tendon, xiphoid process, linea
alba and the pubic crest.
– Functions: Compression of abdominal contents.
– Innervation: Thoracoabdominal nerves (T6-T11), subcostal
nerve (T12) and branches of the lumbar plexus

The muscles of the anterolateral abdominal wall. Note how the flat muscles
form aponeuroses medially.

Vertical Muscles
• There are two vertical muscles located in the
midline of the anterolateral abdominal wall:
– the rectus abdominis and
– pyramidalis.

Rectus Abdominis
• The rectus abdominis is long, paired muscle,
found either side of the midline in the abdominal
wall.
• It is split into two by the linea alba.
• The lateral border of the two muscles create a
surface marking, known as the linea semilunaris.
• At several places, the muscle is intersected by
fibrous strips, known as tendinous intersections.
• The tendinous intersections and the linea alba
give rise to the ‘six pack’ seen in individuals with
a well-developed rectus abdominis.

Rectus Abdominis
• Attachments: Originates from the crest of the
pubis, before inserting into the xiphoid process of
the sternum and the costal cartilage of ribs 5-7.
• Functions: As well as assisting the flat muscles in
compressing the abdominal viscera, the rectus
abdominis also stabilises the pelvis during
walking, and depresses the ribs.
• Innervation: Thoracoabdominal nerves (T7-T11)

Pyramidalis
• This is a small triangular muscle, found
superficially to the rectus abdominis. It is
located inferiorly, with its base on the pubis
bone, and the apex of the triangle attached to
the linea alba.
– Attachments: Originates from the pubic crest and
pubic symphysis before inserting into the linea
alba.
– Functions: It acts to tense the linea alba.
– Innervation: Subcostal nerve (T12).

Rectus Sheath
• The rectus sheath is formed by the aponeuroses
of the three flat muscles, and encloses the rectus
abdominis and pyramidalis muscles.
• It has an anterior and posterior wall for most of
its length:
– The anterior wall is formed by the aponeuroses of the
external oblique, and of half of the internal oblique.
– The posterior wall is formed by the aponeuroses of
half the internal oblique and of the transversus
abdominis.

Rectus Sheath
• Approximately midway between the umbilicus
and the pubic symphysis, all of the aponeuroses
move to the anterior wall of the rectus sheath.
• At this point, there is no posterior wall to the
sheath; the rectus abdominis is in direct contact
with the transversalis fascia.
• The area of transition between having a posterior
wall, and no posterior wall is known as the
arcuate line.

Surface Anatomy
• The abdominal cavity contains numerous
organs – many of which can be palpated
through the abdominal wall, or their position
can be visualised by surface markings.
• The umbilicus is the most visible structure of
the abdominal wall, and is the scar of the site
of attachment of the umbilical cord. It is
usually located midway between the xiphoid
process and the pubis symphysis.

Surface Anatomy
• The rectus abdominis muscle gives rise to
abdominal markings.
• The lateral border of this muscle is indicated
by the linea semilunaris, a curved line running
from the 9th rib to the pubic tubercle.
• The linea alba is a fibrous line that splits the
rectus abdominis into two.
• It is visible as a vertical groove extending
inferiorly from the xiphoid process.

Surface Anatomy
• The abdomen is a large area, and so it split
into nine regions – these are useful
clinically for describing the location of pain,
location of viscera and describing surgical
procedures.
• The nine regions are formed by two horizontal
and two vertical planes

Surface Anatomy
• Horizontal planes:
– Transpyloric plane – Horizontal line halfway between
the xiphoid process and the umbilicus, passing
through the pylorus of the stomach.
– Intertubercular plane – Horizontal line that joins the
iliac crests.
• Vertical planes – run from the middle of the
clavicle to the mid-inguinal point (halfway
between the anterior superior iliac spine of the
pelvis and the pubic symphysis). These planes are
the mid-clavicular lines.

The nine regions of the abdomen

Clinical Relevance: Surgical Incisions in
Abdominal Wall
Vertical Incisions
• Median
– An incision that is made through the linea alba. It can be
extended the whole length of the abdomen, by curving
around the umbilicus.
– The linea alba is poorly vascularised, so blood loss is
minimal, and major nerves are avoided.
– All can be used in any procedure that requires access to
the abdominal cavity.
• Paramedian
– Similar to the median incision, but is performed laterally to
the linea alba, providing access to more lateral structures
(kidney, spleen and adrenals).
– This method ligates the blood and nerve supply to muscles
medial to the incision, resulting in their atrophy.

Transverse Incisions
• Transverse
– This incision is made just inferior and laterally to the umbilicus.
– This is a commonly used procedure, as it causes least damage to
the nerve supply to the abdominal muscles, and heals well.
– The incised rectus abdominis heals producing a new tendinous
intersection.
– It is used in operations on the colon, duodenum and pancreas.
• Suprapubic (Pfannenstiel)
– Suprapubic incisions are made 5cm superior to the pubis
symphysis.
– They are used when access to the pelvic organs is needed.
– When performing this incision, care must be taken not the
perforate the bladder (especially if it is not catheterised), as the
fascia thins around the bladder area.

Transverse Incisions
• Subcostal
– This incision starts inferior to the xiphoid process, and
extends inferior parallel to the costal margin.
– It is mainly used on the right side to operate on the
gall bladder and on the left to operate on the spleen.
• McBurney
– This is a ‘grid iron’ incision, because it consists of two
perpendicular lines, splitting the fibres of the muscles
without cutting them – this allows for excellent
healing.
– McBurney incision is performed at McBurney’s point
(1/3 of the distance between the ASIS and the
umbilicus). It is mostly used in appendectomies.

Common surgical incisions

The Posterior Abdominal Wall

The Posterior Abdominal Wall
• It is formed by the lumbar vertebrae, pelvic
girdle, posterior abdominal muscles and their
associated fascia.
• Major vessels, nerves and organs are located
on the inner surface of the posterior
abdominal wall.

Posterior Abdominal Muscles
• There are five muscles in the posterior abdominal
wall:
– the iliacus,
– psoas major,
– psoas minor,
– quadratus lumborum and
– the diaphragm
• NB: The posterior aspect of the diaphragm is
considered to be part of the posterior abdominal
wall

Quadratus Lumborum
• The quadratus lumborum muscle is located laterally in
the posterior abdominal wall.
• It is a thick muscular sheet which is quadrilateral in
shape.
• The muscle is positioned superficially to the psoas
major.
– Attachments: It originates from the iliac crest and
iliolumbar ligament. The fibres travel superomedially,
inserting onto the transverse processes of L1 – L4 and the
inferior border of the 12th rib.
– Actions: Extension and lateral flexion of the vertebral
column. It also fixes the 12th rib during inspiration, so that
the contraction of diaphragm is not wasted.
– Innervation: Anterior rami of T12- L4 nerves

Quadratus Lumborum

Psoas Major
• The psoas major is located near the midline of
the posterior abdominal wall, immediately
lateral to the lumbar vertebrae.
– Attachments: Originates from the transverse
processes and vertebral bodies of T12 – L5. It then
moves inferiorly and laterally, running deep to the
inguinal ligament, and attaching to the lesser
trochanter of the femur.
– Actions: Flexion of the thigh at the hip and lateral
flexion of the vertebral column.
– Innervation: Anterior rami of L1 – L3 nerves.

Psoas Minor
• The psoas minor muscle is only present in 60%
of the population. It is located anterior to the
psoas major.
– Attachments: Originates from the vertebral bodies
of T12 and L1 and attaches to a ridge on the
superior ramus of the pubic bone, known as the
pectineal line.
– Actions: Flexion of the vertebral column.
– Innervation: Anterior rami of the L1 spinal nerve.

Iliacus
• The iliacus muscle is a fan-shaped muscle that is
situated inferiorly on the posterior abdominal
wall. It combines with the psoas major to form
the iliopsoas – the major flexor of the thigh.
– Attachments: Originates from surface of the iliac fossa
and anterior inferior iliac spine. Its fibres combine
with the tendon of the psoas major, inserting into the
lesser trochanter of the femur.
– Actions: Flexion of the thigh at the hip joint.
– Innervation: Femoral nerve (L2 – L4).

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Clinical Relevance: Psoas Sign
• The psoas sign is a medical sign that indicates
irritation to the iliopsoas group of muscles.
• The sign is elicited by flexion of the thigh at the
hip.
• The test is positive if the patient reports lower
abdominal pain.
• A right sided psoas sign is an indication of
appendicitis.
• As the iliopsoas contracts, it comes into contact
with the inflamed appendix, producing pain.

Fascia of the Posterior Abdominal
Wall
• A layer of fascia (sheet of connective tissue) lies
between the parietal peritoneum and the
muscles of the posterior abdominal wall.
• This fascia is continuous with the transversalis
fascia of the anterolateral abdominal wall.
• Whilst the fascia is one continuous sheet, it is
anatomically correct to name the fascia according
to the structure it overlies:
– Psoas Fascia
– Thoracolumbar fascia

Psoas Fascia
• The psoas fascia covers the psoas major
muscle.
• It is attached to the lumbar vertebrae
medially, continuous with the thoracolumbar
fascia laterally and continuous with the iliac
fascia inferiorly

Thoracolumbar fascia
• The thoracolumbar fascia consists of the three layers;
– Posterior
– middle and
– anterior.
• Muscles are enclosed between these layers:
• Quadratus lumborum – between the anterior and middle
layers.
• Deep back muscles – between the middle and posterior
layers.
• The posterior layer extends between the 12th rib and the iliac
crest posteriorly.
• Laterally the fascia meets the internal oblique and transversus
abdominis muscles, but not the external oblique.
• As it forms these attachments it covers the latissimus dorsi.

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• The anterior layer attaches to the anterior aspect
of the transverse processes of the lumbar
vertebrae, the 12th rib and the iliac crest.
• Laterally the fascia is continuous with the
aponeurotic origin of the transversus abdominis
muscle.
• Superiorly the fascia thickens to become the
lateral arcuate ligament, which joins the
iliolumbar ligaments inferiorly.

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