Anterior Abdominal Wall Anatomy
The skin and subcutaneous tissue of the anterior abdominal wall overlie
four muscles that move the trunk, control intra-abdominal pressure and support
the abdominal contents. The main nerves and blood vessels lie in the
neurovascular plane, deep to all but one of the muscles. Deep to the muscles
are the transversalis fascia, extraperitoneal fat and the parietal peritoneum.
The midline umbilicus marks the site of former attachment
of the umbilical cord. In a lean person, it usually lies midway between the
xiphisternum and the symphysis pubis, at the level of the fourth lumbar
vertebra, but its position is variable.
The subcutaneous tissue has an outer fatty layer, which
is particularly thick in obese individuals, and a deeper membranous layer,
which lies on the external oblique muscle (Fig. 4.12).
Although thin over most of the abdominal wall, the membranous layer becomes
substantial inferiorly. Laterally, it descends into the thigh and attaches to
the fascia lata, while medially it continues around the external genitalia into
the perineum (p. 244). In the event of a rupture of the male urethra, urine can
escape not only into the subcutaneous tissues of the perineum but may track
into the abdominal wall deep to the membranous layer, but not into the thigh
(p. 227).
The subcutaneous tissue receives its blood from small
branches of the arteries that supply the abdominal muscles. The superficial
veins drain either upwards towards the axilla or downwards to the groin. In
portal hypertension, the superficial veins may dilate and become visible,
radiating from the umbilicus (caput medusae; p. 185). The nerve supply to the
skin is segmental and is provided by cutaneous branches of the lower thoracic
spinal nerves and the first lumbar nerve (Figs 4.2 & 4.3).
Muscles
On each side of the midline, there are four principal
muscles. Three of these are flat muscles, arranged in layers in the lateral
part of the abdominal wall. External oblique is the most superficial,
internal oblique lies deep to it and the deepest layer is transversus
abdominis. As each of these muscles is traced anteriorly and medially, its
fleshy part gives way to an aponeurosis (Fig. 4.12).
The aponeuroses of the flat muscles form a sheath around the fourth muscle,
rectus abdominis. In the midline, the aponeuroses from both sides interdigitate
to form the linea alba, which bares an obvious scar, the umbilicus. All three
aponeuroses attach inferiorly to the pubic crest. These muscles are
innervated by the lower six thoracic nerves and the first lumbar nerve (p.
145).
Immediately above the groin, the inguinal canal traverses
the lowest part of the abdominal wall and transmits the spermatic cord in the
male and the round ligament of the uterus in the female (p. 146). The umbilicus
and linea alba are potential sites for hernias. Umbilical hernias are common in
infants, due to weakness of the umbilical scar. In later life, weakening of the
linea alba near the umbilicus can result in a paraumbilical hernia.
The muscle fibres of external oblique slope downwards and
forwards (Fig. 4.13). Superiorly, a series of fleshy
slips attaches to the outer surfaces of the lower eight ribs, the upper slips
interdigitating with serratus anterior, the lower ones with latissimus dorsi.
The most posterior fibres attach inferiorly to the iliac crest; elsewhere, the
fibres give way to the aponeurosis, which passes medially in front of rectus
abdominis to reach the linea alba. The aponeurosis possesses a free lower
border that extends from the anterior superior iliac spine to the pubic
tubercle and forms the inguinal ligament (Figs 4.1 & 4.12), which marks the
boundary between the abdominal wall and the anterior aspect of the thigh.
Immediately above the medial end of the inguinal ligament, the external oblique
aponeurosis presents an aperture, the superficial inguinal ring, which is the
medial opening of the inguinal canal (p. 146).
Internal oblique
Internal oblique attaches to the lateral two-thirds of
the inguinal ligament, to the anterior part of the iliac crest and to the
thoracolumbar fascia (Fig. 4.15), through which it is anchored to the lumbar
vertebrae. Most of its fibres slope forwards and upwards. The uppermost fibres
attach to the costal margin between the ninth and twelfth ribs, while the
remainder give way to the aponeurosis of the muscle (Fig. 4.14).
Some aponeurotic fibres reach the linea alba by passing anterior
to rectus abdominis, while others pass behind the rectus (p. 143). The lowest
fibres arch medially and downwards, contributing to the roof of the inguinal
canal. They unite with the underlying fibres of transversus to form the
inguinal falx (conjoint tendon), which descends to the pecten pubis (pectineal
line) on the pubic bone.
The upper part of this muscle arises from the inner
aspects of the lower six costal cartilages (Fig. 4.17) by fleshy slips, which
interdigitate with the costal attachments of the
diaphragm. The middle part of the muscle fuses with the thoracolumbar fascia
while the lowest fibres attach to the iliac crest and the lateral half of the
inguinal ligament. Most of the fibres run horizontally forwards and are replaced,
near the lateral border of rectus, with an aponeurosis (Figs
4.16 & 4.18). The upper part of the aponeurosis reaches the linea alba
by passing posterior to rectus abdominis while the inferior part passes
anterior to it (see below).
The lowest fibres of transversus abdominis attach to the
lateral part of the inguinal ligament and arch over the inguinal canal and,
fusing with those of the overlying internal oblique, contribute to the inguinal
falx.
Rectus abdominis runs vertically on each side of the
linea alba, from the pubis to the front of the chest wall (Fig.
4.16). The inferior attachment is to the anterior aspect of the pubic
symphysis and to the pubic crest. The muscle widens superiorly and attaches to
the anterior surfaces of the fifth, sixth and seventh costal cartilages. Its
gently convex lateral border forms a surface feature called the linea
semilunaris. Rectus abdominis is characterized by transverse tendinous intersections,
usually at the levels of the xiphisternum, the umbilicus and midway between the
two.
Rectus sheath
Rectus abdominis is enclosed in a sheath formed by the
aponeuroses of the
flat abdominal muscles.
The anterior wall of the
sheath, which is anchored to the tendinous intersections, covers the entire
length of the muscle (Fig. 4.14). By contrast, the posterior wall is not
attached to the muscle and falls short of its superior and inferior
extremities. Superiorly, the posterior wall of the sheath terminates at the
costal margin, above which rectus is in direct contact with the costal
cartilages. Inferiorly, the posterior wall continues only a short distance
below the umbilicus, where it thins out or ends abruptly. In the latter case,
the posterior wall has a recognizable inferior margin, the arcuate line (Fig.
4.18), below which the posterior surface of rectus is in direct contact with
the transversalis fascia.
In addition to rectus abdominis, the rectus sheath
contains the small triangular pyramidalis muscle, the superior and inferior
epigastric vessels (Figs 4.16 & 4.18) and the
terminal parts of the lower six intercostal nerves that supply rectus and the overlying
skin.
The abdominal muscles flex the lumbar spine, rectus
abdominis being particularly powerful in this action. Lateral flexion and rotation
of the trunk are produced by coordinated contraction of the oblique muscles on
both sides of the midline. Acting collectively, the abdominal muscles increase
intra-abdominal pressure and, if the respiratory passages are open, the
diaphragm is pushed upwards as in forced
expiration, sneezing and coughing. Increased abdominal pressure with the airway
closed (straining) occurs when lifting heavy objects and during defecation,
childbirth and vomiting.
The deep surfaces of transversus and rectus abdominis are
covered by the transversalis fascia, which forms part of a complete fascial
sheet lying deep to the muscles surrounding the peritoneal cavity. Several
names are given to this continuous fascial sheet and are derived from the
muscles to which the fascia relates. For example, the iliac fascia and psoas
fascia cover the iliacus and psoas muscles, respectively. Above the midpoint of
the inguinal ligament, an aperture in the transversalis fascia (the deep
inguinal ring) forms the lateral opening of the inguinal canal.
Deep to the transversalis fascia is the extraperitoneal
fat, which contains four vestigial structures
converging on the umbilicus. Descending from the liver is the round ligament of
the liver (liga- mentum teres hepatis; Figs 4.30 & 4.59), the remnant of
the left umbilical vein. Ascending in the midline from the urinary bladder is
the median umbilical ligament or urachus (Fig. 4.18). Inclining upwards from
each side of the pelvis is the occluded part of the umbilical artery.
The deepest layer of the abdominal wall is the parietal peritoneum
(Fig. 4.18). Although the peritoneum and the abdominal musculature are adherent
in most areas, they are only loosely attached between the pubis and umbilicus. The
distended bladder intervenes between the parietal peritoneum and the abdominal
wall (Fig. 4.30) and can be accessed through a lower abdominal incision without
opening the peritoneum.
The skin, muscles and parietal peritoneum of the anterior
abdominal wall are innervated by the lower six thoracic nerves and the first
lumbar nerve.
At the costal margin, thoracic nerves 7–11 leave their
intercostal spaces and enter the neurovascular plane of the abdominal wall between transversus abdominis and internal oblique (Fig.
4.17). The seventh and eighth nerves slope upwards, the ninth runs horizontally
and the tenth and eleventh incline downwards. The nerves pierce rectus
abdominis and the anterior layer of the rectus sheath to emerge as anterior
cutaneous branches that supply the overlying skin (Fig. 4.12).
The subcostal nerve (T12) takes the line of the twelfth
rib across the posterior abdominal wall (p. 201). It continues around the flank
in the neurovascular plane and terminates in a similar manner to the lower
intercostal nerves.
The seventh to twelfth thoracic nerves give off lateral
cutane- ous nerves, which further divide into anterior and posterior branches.
The anterior branches supply skin as far forwards as the lateral edge of rectus
abdominis while the posterior branches supply skin overlying latissimus dorsi.
The lateral cutaneous branch of the subcostal nerve is distributed to the skin
on the side of the buttock.
First lumbar nerve
The first lumbar nerve divides into upper and lower
branches, the iliohypogastric and ilioinguinal nerves (Figs 4.102 & 4.103).
The iliohypogastric nerve reaches the neurovascular plane in the loin and
divides just above the iliac crest into two terminal branches. The lateral
cutaneous branch supplies the side of the buttock and the anterior cutaneous
branch supplies the suprapubic region. The ilioinguinal nerve leaves the
neurovascular plane by piercing internal oblique above the iliac crest (Fig.
4.14). It continues between the two oblique muscles and accompanies the
spermatic cord or round ligament of the uterus in the inguinal canal (Figs 4.21
& 4.24). Emerging from the superficial inguinal ring (Fig. 4.20), it gives
cutaneous branches to skin on the medial side of the root of the thigh, the
proximal part of the penis and front of the scrotum or the mons pubis and the
anterior part of the labium majus.
Blood vessels
The blood supply to the abdominal wall is provided by the
superior and inferior epigastric arteries, supplemented by the musculophrenic
artery and the lower posterior intercostal arteries. The superior epigastric
artery descends behind rectus abdominis and may anastomose with the inferior
epigastric artery (Fig. 4.18). The latter vessel arises from the external iliac
artery immediately above the inguinal ligament and inclines upwards and
medially, passing just medial to the deep inguinal ring (Figs 4.18 & 4.23).
The inferior epigastric artery enters the rectus sheath by passing in front of
its posterior wall at the arcuate line. From the anterior ends of the lower two
or three intercostal spaces, posterior intercostal arteries continue forwards
in the neurovascular plane.
Venous drainage of the deeper layers of the abdominal
wall is via venae comitantes of the respective arteries. Blood from the
superficial tissues drains into veins, lying in the subcutaneous tissue, which
run towards the axilla and groin. Dilatation of the subcutaneous veins is an
important clinical sign in patients with obstruction of venous flow within the
abdomen, for example, within the inferior vena cava or the liver. Dilatation of
the superficial veins is an important clinical sign in patients with portal
vein obstruction (p. 185).
Lymphatics
Lymph from the abdominal wall above the level of the
umbilicus drains upwards. Lymphatics
from the skin
and subcutaneous tissue accompany
the subcutaneous veins and drain into the axillary nodes, while those from
the deeper tissues follow the course of the superior epigastric artery to the
internal thoracic nodes. The superficial lymphatics of the lower half of the
abdominal wall pass to the superficial inguinal nodes, while the deeper lymph
vessels follow the course of the inferior epigastric artery to reach the external
iliac nodes.
