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).
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 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 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.
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).
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.