The abdominal wall

The anterior abdominal wall serves many functions (Box 3.1). Its primary role is to protect the viscera from injury and infection. It not only houses the viscera within the abdomen but also maintains visceral anatomical position against gravity.

The structures of the anterior abdominal wall are shown in Figure 3.1. They consist of skin, subcutaneous tissue, rectus sheath and associated muscles, rectus muscle, transversalis fascia, and peri­toneum. The inferior epigastric vessels pass up the abdomen from their origin at the external iliac artery to travel in the transversalis fascia between the peritoneum and the rectus sheath and muscle (Figure 3.1).

The subcutaneous tissue differs above and below the umbilicus. Above the umbilicus it consists of a single sheet of fascia continuous with the other subcutaneous tissues of the body. Below the umbilicus it consists of two layers. The superficial fatty layer is called Camper's fascia and the deeper membranous layer is called Scarpa's fascia. It is between these two layers of fascia that nerves and vessels run.

There are two main groups of muscle within the anterior ab­dominal wall, which are the flat and the vertical muscles. The ver­tical muscles consist of the rectus abdominis (Figure 3.1) and pyramidalis. The two rectus abdominis muscles run either side of the midline and are split by the linea alba. Laterally the margin is called the linea semilunaris and the muscle is split by a number of tendinous intersections that give rise to the classical appearance of a muscular anterior abdominal wall (a ‘six pack'). The pyramidalis is not always present.

The three main flat muscles are the external oblique, internal oblique, and transversus abdominis muscles (Figure 3.1). These muscles have fibres that run perpendicular to each other thus re­ducing the risk of hernia and facilitating rotational abdominal movements. The muscles come together in the centre of the ab­domen forming the rectus sheath. The rectus sheath passes super­ficial and deep to the rectus muscle. Superficially it is composed of the aponeurosis of the external oblique and half of the internal oblique with the other half passing posteriorly and joining with the fibres of the transversus abdominis. Below the umbilicus all the aponeuroses move superficial to the rectus abdominis. This area of transition between where the rectus sheath has a posterior wall and where it does not is called the arcuate line (Figure 3.1).

The posterior abdominal wall is of lesser importance to the gynae­cological surgeon as the pelvis is rarely approached through it. The posterior borders of the abdomen include the ribs, vertebrae, psoas major and psoas minor, quadratus lumborum, and iliacus.

Clinical considerations

The anatomy of the anterior abdominal wall is important when con­sidering surgical incisions. In a low transverse or Pfannenstiel in­cision as might be performed for a caesarean section, the surgeon passes through only one layer of rectus sheath as the incision is below the arcuate line. A midline incision crosses the arcuate line and passes through the linea alba. A McBurney (‘grid iron') incision is commonly performed for an appendicectomy and is performed at McBurney's point, a third of the distance between the anterior su­perior iliac spine (ASIS) and the umbilicus. For this latter incision,

Box 3.1 Functions of the anterior abdominal wall

Box 3.2 Structures of the external genitalia

• Houses abdominal viscera

• Protects viscera from injury

• Protects viscera from infection

• Helps maintain position of the viscera

• Assists with increases of intra-abdominal pressure

• Forceful expiration

• Coughing

• Vomiting

• Defecation

• Micturition

the fibres of the oblique muscles are separated perpendicular to each other, reducing the risk of hernia. The risk of incisional hernia is greatest during a midline incision where only the midline structures are sutured. What is defined as ‘mass closure' involving the peri­toneum, sheath, and rectus muscles minimizes the risk of a subse­quent hernia.

The presence of the epigastric vessels is also important during laparoscopic surgery. These can normally be visualized prior to the insertion of lateral ports, thus preventing them from being ruptured.